Spectroscopy of supergiants with infrared excess: results of 1998-2018

TL;DR

This study presents two decades of spectroscopic analysis of peculiar supergiants with infrared excess, revealing chemical composition changes, s-process element enhancements, and stellar ejections, advancing understanding of late stellar evolution.

Contribution

It provides new observational evidence of chemical and kinematic phenomena in post-AGB supergiants, including first detections of s-process element ejection into circumstellar envelopes.

Findings

Detection of s-process element excesses in seven post-AGB stars.

First discovery of s-process heavy metal ejection into circumstellar envelopes.

Lithium excess observed in two peculiar supergiants.

Abstract

The results of our second stage (1998-2018) of the detailed spectroscopy of peculiar supergiants identified with galactic infrared sources, performed mainly at the 6-meter BTA telescope are summarized. The main aspect of the program is a search for the evolutionary variations in the chemical composition of stars, past the AGB stage and the TDU, as well as an analysis of spectral manifestations of kinematic processes in their extended, often unstable, atmospheres and in the envelopes. The most significant result is detection of the s-process element excesses in seven single post-AGB stars, which confirms the theory of evolution of this type of stars. In three of these stars we for the first time discovered the ejection of the s-process heavy metals to the circumstellar envelopes. A lithium excess was found in the atmospheres of two peculiar supergiants V2324 Cyg and V4334 Sgr. The results of investigation of the kinematical state of atmospheres and envelopes will clarify the equilibrium of matter produced by stars in the AGB and post-AGB stages and delivered to the interstellar medium.

Full text

1 INTRODUCTION

Peculiar supergiants we study possess a large infrared flux excess and primarily belong to the evolutionary stage after the asymptotic giant branch, further referred to as post-AGB. Our results of research of a sample of supergiants obtained at the first stage of spectroscopic observations at the 6-meter telescope are presented in the survey [1]. Here we complement this survey, adding the results obtained and published over 1998–2018. We shall briefly recall the main features of post-AGB stars, also called the protoplanetary nebulae (PPN), then give a rationale for the relevance of a detailed study of these objects and describe the main results obtained by the program in the previous decade.

The PPN stage hosts far evolved stars of intermediate masses, with initial masses on the Main Sequence (MS further in the text) of less than 8$\div 9\mathcal{M}_{\odot}$. At the previous evolution stage, the asymptotic giant branch (AGB), these stars are observed as cool (their effective temperature is T${}_{\rm eff}\approx 3000$ K) supergiants. The AGB stage, being significantly shorter than the previous stages of evolution is very important due to a large mass loss and the formation of a circumstellar shell [2]. Also note that in this very mass range, near the MS, about 10% of stars possess large magnetic fields. Using the method of measuring circular polarization of maser lines, magnetic fields were also detected in a number of their far evolved descendants, PPN and PN [3, 4].

After the helium depletion in the core of an AGB star, a degenerate C–O core surrounded by alternative energetically active shells of burning helium and hydrogen is formed. Most of the time the energy release is provided by the hydrogen burning, while the helium shell adjacent to the degenerate C–O core remains inert (for details and references, see the survey [5]). Between these major energy-release layers a thin shell is formed, the so called “He intershell”, in which, as it gets compressed and as the temperature rises, the helium ignites and carbon accumulates, which creates conditions for the reaction of prime importance, 13C($\alpha$, n)16O. This reaction is the main source of neutrons providing the subsequent synthesis of the heavy metal nuclei (a detailed description of these processes and the necessary references are available in the papers [6, 7]). An additional energy release in the “He intershell” changes the configuration of the internal structure of the star and leads to an instability with a development of efficient convective mixing of matter in the layers. Due to the penetrating convection, the stellar matter, recycled in nuclear fusion accompanying these processes of energy release [8], outflows into the atmosphere of the star. It is common practice to call the mixing of matter due to the repeatedly recurring alternation of thin, energetically active layers of burning hydrogen and helium “the third dredge-up” (hereinafter – TDU).

The primary gas in the early Universe was a mixture of hydrogen and helium admixed with light elements (Li, Be, B). Later, in the course of evolution and explosions of massive first-generation stars there began formation of nuclei of heavier chemical elements (C, O, Ne, Mg, Si and Fe). The entire variety of chemical abundances that we observe now is due to nuclear fusion results in the evolution of several stellar populations. An increased interest in AGB stars is due to the fact that the inside of these stars that are on the short-term (characteristic time of $10^{3}$ years) evolutionary stage, form ideal physical conditions for the synthesis of heavy metal nuclei and for the transport of accumulated nuclear reaction products into the stellar atmospheres. The calculations show that AGB stars are the main suppliers of heavy metals (over 50% of all elements are heavier than iron) into the interstellar medium, while the most efficient suppliers are AGB stars with initial masses not exceeding 3 $\mathcal{M}_{\odot}$. The synthesis of heavy metal nuclei is carried out due to the so-called $s$-process, the essence of which is in a slow (compared with the $\beta$-decay) neutronization of nuclei [9].

The seed nuclei for a chain of $s$-process reactions are the Fe nuclei. In stars with the initial masses below 3 $\mathcal{M}_{\odot}$ the necessary neutron flux is provided by the 13C($\alpha,\,n)^{16}$O reaction, and in the case of more massive stars with initial masses greater than $4\div 5\mathcal{M}_{\odot}$, a similar reaction takes course in the 22Ne nuclei. These more massive AGB stars may also be sources of Li. A description of the evolution of stars near the AGB and the results of modern calculations of the synthesis and outflow of elements are given in [10, 11].

The matter of an AGB star is lost through two processes: firstly, carbon and oxygen produced during the nucleosynthesis join the degenerate C–O core (which changes the character of energy release), secondly, the matter is lost owing to wind from the surface of the star. As a result, the AGB star loses from 50% to 85% of its initial mass [12]. Due to the matter loss, an optically thick envelope is formed around the star, and the star becomes difficult to observe in the optical range. At some point in the AGB star lifetime its mass loss rate suddenly increases dramatically. One of the reasons for the large mass loss rate increase can be the pulsating instability of supergiants, which is inherent, according to [12], to the stars on this evolutionary stage in a fairly wide range of fundamental parameters. Two physical phenomena: the synthesis of heavy metals and the pulsation activity characteristic of stars on the AGB stage and after it, determine the main interest in these objects.

The complexity of the theoretical study of stellar nucleosynthesis is caused by the fact that the efficiency of synthesis and the ejection of fresh nuclei onto the stellar surface depends on a large number of factors: the original mass of a star on the MS, its metallicity, the parameters of the matter outflow, the features of the nucleosynthesis process and uncertainties of numerical description of the mixing process. Therefore, besides the classical problem of studying the chemical composition of stars on nontrivial stages of evolution, of particular interest is also the possibility of studying the processes of exchange of matter between the stellar atmosphere and the circumstellar dust envelope, and also the search for mechanisms explaining the peculiarities of chemical composition of the atmospheres of stars surrounded by dust envelopes. It is generally believed that the conditions of a gas and dust envelope can be conducive to quite effective processes of selective depletion of metal nuclei onto the dust grains. In the course of formation of dust particles, the elements with a high temperature condensation predominantly depart from the gas component. In particular, this selective process distorts the pattern of the TDU, increasing the C/O, C/Fe, N/Fe, O/Fe ratios and decreasing the $s$/Fe ratio. The main argument confirming the operation of the mechanism of selective depletion of metal nuclei on the dust particles is a dependence of the chemical element abundances on the temperature of condensation on the dust particles [13].

The abundance of Fe, Mg, Si, Ca in the atmospheres of such stars is reduced by several orders of magnitude, while the CNO, S, and even the iron group element Zn have solar abundances. It is worth emphasizing that this pattern is similar to the behavior of the chemical element abundances in the gas component of the ISM. The stellar atmosphere should obviously be stable enough so that the mixing or the stellar wind would not distort the chemical abundance pattern. However, there are evidences that the atmospheres of post-AGB stars are unstable: most of these objects manifest pulsations, the outflow of matter, which is manifested in the presence of a variable emission, mainly in the lines of hydrogen. The selective depletion process efficiency, its dependence on the effective temperature and metallicity of the star are illustrated in the data of Rao et al. [14].

A theoretical study of the evolution of stars near the AGB is difficult due to the complexity of this system including at least two components: the central star continuing to evolve and a cooling envelope that may have quite a complex structure. A lot of ambiguity remains in the understanding of stellar wind, convection processes, and, therefore, the outflow of matter. However, in general, the idea has come up that along with the star’s initial mass, the rate of mass loss during its evolution is the most important parameter which does not only determine the final mass after the AGB phase, but also the internal structure of the star and the characteristic times of evolution in the final phases, as well as the changes in the chemical composition of the stellar atmosphere.

The Infrared Astronomical Satellite, or the IRAS, performed a survey of about 95% of the entire sky. One of the results of this IR survey was identification at high Galactic latitudes of the sources representing the circumstellar envelopes with temperatures from 200 to 1000 K. Subsequently, some of these objects were identified with high luminosity stars presumably at the AGB and post-AGB evolutionary stages. A small part of selected objects is available for the high spectral resolution spectroscopic observations in the optical range, providing an empirical base in the stellar nucleosynthesis studies. After the new type of far evolved stars with a large excess of infrared flux was identified, programs of the spectroscopy of post-AGB and AGB stage candidate stars were initiated at several large telescopes of the world. Their purpose is to investigate the features of anomalous spectra, the kinematic state of the outflowing atmospheres and extended circumstellar envelopes and, above all, to study the process of stellar nucleosynthesis and release of its products into the stellar atmosphere.

2 SPECTROSCOPY OF POST-AGB CANDIDATES: RESULTS OF 1998–2018

This kind of an observational program under the direction of the author of the present paper is also performed at the 6-m BTA telescope coupled with modern echelle spectrographs. In the course of longterm observations (spectroscopy of each of the objects was carried out repeatedly) we have obtained and published several fundamentally new results. The chemical composition was studied in about twenty supergiants with an IR-excess, located mainly in the galactic field. For more details, see the survey by Klochkova [1]. Since 1998, the observations are carried out with the NES echelle spectrograph [15, 16] in combination with the CCD chip sized $1{\rm K}\times 1{\rm K}$, since 2000 –$2{\rm K}\times 2{\rm K}$, and since 2011 – $2{\rm K}\times 4{\rm K}$. The result is a collection of high-quality spectra, which were intended primarily to search for the chemical composition anomalies due to the nuclear fusion of chemical elements in the depths of stars of small and intermediate masses and a subsequent outflow of the synthesis products into the stellar atmospheres. This observational material is also used to search for the peculiarities of spectra of post-AGB candidates, to analyze the velocity field in the atmospheres and envelopes of stars with mass outflow, as well to search for the expected long-term variability of spectral features. To reduce the losses at the entrance slit of the spectrograph, the observations are performed with an image slicer, so every echelle order is repeated thrice.

In general, based on the studied sample of PPN candidates using the published results of other authors, we can talk about the heterogeneity of the chemical composition of the sample. An excess of the $s$-process elements expected for stars in the post-AGB stage as a consequence of the previous evolution of the star and the process of the TDU, is so far extremely rarely found. For example, we have revealed an excess of carbon and $s$-process elements in the atmosphere of the first post-AGB star we chose to study, CY CMi (IRAS 07134+1005) [17]. By now, a significant excess of $s$-process elements was found only for eleven post-AGB stars, seven of which were examined from the BTA spectra: IRAS 04296+3429 [18], IRAS 07134+1005 [17], IRAS 20000+3239 [19], IRAS 22272+5435 [20], IRAS 23304+6147 [21], RAFGL2688 [22], as well as a virginide V1 K307 [23] in the globular cluster M 12. In addition, conclusions about the excess of heavy metals are published for the following PPN candidates: HD 158616 [24], HD 187885 [24], IRAS 22223+4327 [25], IRAS 02229+6208 [26] and IRAS 05341+0852 [25]. In the spectrum of the nameless and faint in the optical range post-AGB star with an emission at 21 $\mu$, identified with an IR source IRAS 20000+3239, an overabundance of heavy elements was detected [19]. For example, the concentration of La nuclei is increased by an order of magnitude with respect to the stellar metallicity. In this regard, the La II line is so strong that its intensity is comparable to the intensity of the H$\alpha$ line.

Let us emphasize that all of these stars, selected based on their chemical composition and the presence of a band at 21 $\mu$ are single objects, as confirmed by the authors of [27]. Currently, 27 stars are known with an emission at 21 $\mu$, and they all belong to the post-AGB stage only [28]. The presence of a feature at 21 $\mu$ in the spectra of post-AGB stars with carbon-rich shells, allows to suggest some kind of a complex molecule containing the carbon atoms as the main agent. However, other, but not confirmed, alternatives of identifying the features at 21 $\mu$ were proposed. For example, nanoparticles of iron oxide FeO [29], capable of, when heated, producing radiation at the wavelength of 21 $\mu$, and titanium carbide TiC [30] were considered as agents.

Based on the first facts of detection of heavy metals overabundance in the papersof Klochkova [1], Decin et al. [31] made a conclusion about the relationship of emissions at 21 $\mu$ in the IR spectra of stars at the post-AGB stage and manifestations of heavy metals excess in their atmospheres. Our results obtained for AFGL 2688 strengthened this conclusion, since this object is characterized by both a slight excess of $s$-process elements, and a 21 $\mu$ band, practically not revealed on the background of a monotonously increasing IR continuum. So far, this result remains one of the most significant (but not explained) in the pattern of stellar nucleosynthesis manifestations at the AGB and post-AGB stages. Van Winckel and Reyniers [24] reiterated a study of all six stars with a feature at 21 $\mu$ known by that time, and on a uniform spectroscopic material confirmed a conclusion about the effectiveness of the $s$-process for this group of objects. Specifically, these authors have obtained a strict correlation between the magnitude of neutron exposure estimated from the observations as an [h$s$/l$s$] ratio (an excess of the $s$-process heavy nuclei relative to the lighter ones), and [$s$/Fe], as well as a less pronounced anti-correlation between [h$s$/l$s$] and metallicity [Fe/H].

As a rule, in the atmospheres of PPN candidates there is an overdeficiency (relative to their metallicity) of heavy nuclei, the existence of which in the atmospheres of low-mass supergiants at the post-AGB stage finds no definite explanation as yet. There is a number of physical (a hydrogen deficiency in the atmospheres; overionization of atoms with a low second ionization potential) and methodical (parameter errors) effects that could explain the observed overdeficiency of the $s$-process elements in the atmospheres of low mass supergiants. None of the suggested explanations is consistent with the full chemical abundances picture of these objects. A low occurrence of this type of atmosphere enrichment is a consequence of differences in the evolution of AGB stars, that vary in their initial masses, since it is the initial mass of the star that affects the possible manifestations of nucleosynthesis and the matter loss rate. It is currently established that the surface layers of the less massive AGB stars (with initial masses below $4\,\mathcal{M}_{\odot}$) are enriched with carbon and heavy metals—this is how C-rich stars appear, while more massive stars at the subsequent stages of their evolution remain O-rich stars.

2.1 Spectral mimicry of supergiants

An important property that determines the reliability of results of studying the AGB- and post-AGB stars, is a high quality of the high-resolution spectra obtained on the echelle spectrographs of the BTA. For the majority of stars of the program the data of such quality has been obtained for the first time, which allowed us to make a number of new conclusions regarding the evolutionary status of individual objects, spectral features, the velocity field and chemical composition of the atmospheres. First of all, note our conclusion about the heterogeneity of the studied sample of objects in general. As we collected the spectra and the results, it became clear that the original sample of stars with a large excess of IR flux, in addition to AGB and post-AGB stars, includes massive high luminosity stars with extended and structured circumstellar envelopes. Therefore, the necessary and the most time consuming point of our research is determination of luminosity and mass of a star, and therefore recording of its evolutional stage. When studying individual stars, we repeatedly found a similarity and even a complete reiteration of spectral features in objects of different nature: LBV stars, stars with a B[e] phenomenon that are likely to be binary intermediate-mass systems soon after the stage of a rapid matter exchange; white and yellow hypergiants and low mass supergiants at the post-AGB stage with a large excess of infrared flux.

Paradoxically, the high luminosity stars, fundamentally differing in mass (the most massive stars with initial masses exceeding $20\div 40\leavevmode\nobreak\ \mathcal{M}_{\odot}$ and intermediate mass stars 3–9 $\mathcal{M}_{\odot}$) and evolutional stage, have close observed properties: the features of the optical and radio spectra, a large excess of IR flux, a complex and time-varying velocity field, which testifies to the instability of extended atmospheres and expanding gas and dust envelopes. This similarity of the observed properties of two types of objects predetermines the problem of spectral mimicry [32].

Distinctive features of the optical spectrum of a hypergiant are considered to be a powerful emission (often possessing two peaks) in the lines of the Balmer and Paschen series of hydrogen, having broad wings due to Thomson scattering and the presence of forbidden emissions of metals. However, it must be borne in mind that a combination of these features is also present in the spectra of other types of high luminosity stars (see Fig. 1). To get acquainted with the properties of these massive hot supergiants you can use the papers [33-35] having good examples of optical spectra obtained with high spectral resolution. A characteristic feature of the spectra of B[e] stars are the emissions in the lines of an IR triplet Ca II and in the forbidden lines of Ca[II] 7291 and 7324 Å. These profiles are in a good agreement with the hypothesis of the presence of rotating circumstellar disks in the systems of B[e] stars. For a more detailed study of the spectra of B[e] stars, the atlas of spectra of a B[e] star MWC 314 and a hypergiant V1302 Aql [36] may also be useful.

It should be borne in mind that only a combination of an intense (often two-peak) H I emission with forbidden emissions of metals does not give grounds for classifying a star as a hypergiant. The rationale for the high luminosity and a large matter loss rate at a slow velocity of its outflow is principal here. As important is a detailed analysis of the radial velocity pattern measured from the lines of different nature. A good illustration of this problem is given by the already mentioned above spectral atlas [35], which compares the spectra of the star MWC 314 (Sp = B3 Ibe) with a B[e] phenomenon and the spectra of a hypergiant V1302 Aql (its Sp = F5 Ia*+*e at the times of observations used in the atlas). These stars with different masses, luminosities and spectral classes have similar spectra. In particular, powerful hydrogen emission lines have two-peak profiles. The spectra contain plenty of permitted and forbidden metal emission lines and features with the P Cyg-type profiles.

The most famous object, which was for many years considered as a probable star in the post-AGB stage is V1302 Aql, having numerous spectral peculiarities. For a long time the evolutionary status of the supergiant V1302 Aql, associated with a powerful source of IR radiation IRC+10420, was unclear. A manifold of observable properties allowed to consider it as a star at the post-AGB stage or as a very massive star, past the stage of the red supergiant. Obviously, depending on the adopted status, and hence the luminosity of the object, the estimate of its distance from the observer may vary by several times. However, the information obtained over the last decades through various observational experiments as well as the NES BTA monitoring results [37-39] leave no doubt as to the fact that V1302 Aql belongs to extremely rarely observed objects, yellow hypergiants. Moreover, V1302 Aql, whose luminosity is $L\approx 5\times 10^{5}\leavevmode\nobreak\ L_{\odot}$, is now considered as the most undisputed massive object in the Galaxy (with an initial mass of $20\div 40\leavevmode\nobreak\ \mathcal{M}_{\odot}$) with a record mass loss rate, undergoing a short evolutionary transition from the massive red supergiants to the Wolf–Rayet stars [40].

2.2 A peculiar supergiant V510 Pup

Another program object, a supergiant V510 Pup is a clear example of spectral mimicry. The luminosity of this star, associated with an IR-source IRAS 08005$-$2356 barely reaches the luminosity level of a normal supergiant, while its wind parameters are close to the hypergiant wind parameters and even those of an LBV [41]. In the spectrum of V510 Pup the absorptions of Y II and other $s$-process elements are abnormally strong. All the absorption components are shifted to the shortwave spectral region, what indicates the outflow of stellar matter. The circumstellar envelope of the supergiant manifests itself in the optical spectrum, in particular, in the form of molecular bands of the carbon-containing molecules C2 and CN. The emission in H$\alpha$ by manifold exceeds the continuum level, what testifies to the powerful stellar wind. The $V_{r}$ differences within one spectrum reach 100 km s*-1* mainly due to the differential shift of emission and absorption components, but also due to the systematic velocity variations with the intensity and line wavelength. Such large differential line shifts clearly indicate the velocity gradient in the layers of the atmosphere of V510 Pup, where these lines are formed, and many P Cyg-type profiles are indicative of the expansion of the atmosphere.

2.3 A variable star V2324 Cyg

For the first time, high spectral resolution optical spectroscopy of a variable star V2324 Cyg (Sp = F3 Ie) associated with an IR source IRAS 20572+4919 was performed at the BTA coupled with the NES spectrograph. The results are published by Klochkova et al. [42]. Over 200 absorptions were identified (mainly Fe II, Ti II, Cr II, Y II, Ba II) within the wavelength range from 4549 to 7880 Å. The spectral class F0 III and rotation velocity $V\sin i=69$ km s*-1* were determined. The lines of neutral hydrogen and the Na I D-lines have a complex P Cyg-type profile. Neither the systematic course of radial velocity with the depth of the photospheric line, nor the temporal variability of Vr were detected. The average value of the heliocentric radial velocity is V${}_{r}=-16.8\pm 0.6$ km s*-1*. Velocities determined from the cores of absorption components of the H$\beta$ and Na I wind lines range from $-140$ to $-225$ km s*-1* (the velocities of expansion of the corresponding layers amount from about 120 to 210 km s*-1*). The maximum expansion velocity obtained from the blue component of the split H$\alpha$ absorption is V${}_{r}=450$ km s*-1* as of December 12, 1995. Using the model atmosphere method the following parameters were determined: effective temperature T${}_{\rm eff}=7500$ K, surface gravity $\log g=2.0$, microturbulence velocity V${}_{t}=6.0$ km s*-1* and a metallicity close to solar.

The main feature of the chemical composition of the star is a lithium and sodium excess. An excess of lithium was also found in the spectra of a post-AGB star associated with the source IRAS 04296+3429 [18]. It should be noted that the Li I 6707 Å line has a close neighbor line Ce II 6708.1 Å. Therefore, it can be blended in the spectra of stars with a large excess of the $s$-process heavy metals. In particular, the authors of [43] explained the lithium excess, discovered in [44] in the atmosphere of the central star of IRAS 05341+0852 by this blending.

Based on the results obtained, the belonging of V2324 Cyg to post-AGB stars is being questioned. The low luminosity of the star is not consistent with the post-AGB stage: the spectral classification indicates the luminosity class III. The H$\alpha$ line profile and very high wind velocities, usually inherent to supergiants are also not combined with the status of a post-AGB star. The aforementioned lithium excess in the atmosphere of the low luminosity star suggests an alternative evolutionary phase for V2324 Cyg. The phenomenon of lithium overabundance in the atmospheres of F-giants with an IR excess is already known. A part of these Li-rich giants who have not yet reached the AGB stage and are observed on the red giant branch, has a low mass, not exceeding 2.5$\mathcal{M}_{\odot}$. The so-called 11cool bottom process” is suggested as a lithium production mechanism for such giants, it is based on the synthesis of beryllium, a transfer of its nuclei to the base of the convective shell and a subsequent destruction to the lithium nuclei in the near-surface layers of the star. It will be appropriate here to refer to thepaper [45], the authors of which identified the Li abundance for a sample of G–K giants and subgiants with solar metallicity, and where the largest values of Li abundance are obtained for the fast-rotating stars V$\sin i>30$ km s*-1*.

The belonging of V2324 Cyg to giants is consistent with the low luminosity of the star M${}_{V}\approx 0\aas@@fstack{m}1$, obtained using its parallax of $\pi=1.55$ mas according to the Gaia DR2 data and the color excess E(B-V)=$0\aas@@fstack{m}79$, according to [46]. However, the spectral features of V2324 Cyg in the form of strong H I emissions do not allow to finally accept for V2324 Cyg the status of a red giant. The H$\alpha$ profile has a P Cyg-type, its emission peak is 3-4 times higher than the continuum level [42]. The H$\alpha$ profile mapping in the spectra taken at different times indicates their variability: the shape and intensity of the emission and the depth of the absorption component vary. The position of the emission peak and the absorption also varies. The spectral features rather indicate a high luminosity, and therefore a greater distance to V2324 Cyg. Note that the authors of [46], via modeling the energy distribution, calculated the total radiation flux, having at that obtained a high luminosity $\log L/L_{\odot}=3.78$ and distance $d=4.4$ kpc. To clarify the nature of this star we further require accumulation of more data not only about the object itself, but also about the stars in the direction of the Cas OB7 association, a member of which V2324 Cyg apparently is.

2.4 Detection of a new phenomenon in the spectra of post-AGB stars

As we emphasized above, an important result of the spectroscopic research of post-AGB stars is that based on the sample studied so far, we can talk about its chemical composition heterogeneity. An excess of the $s$-process elements expected for stars in the post-AGB stage as a consequence of the previous evolution of the star and the process of the TDU, is extremely rare. Over two decades of the search, an excess of the $s$-process elements has only been detected in seven post-AGB stars (about fifty objects were studied in details). An analysis of the totality of observed properties of the post-AGB objects, in the atmospheres of whose central stars large excesses of carbon and heavy metals are revealed, led to the conclusion that the circumstellar envelopes of these selected stars have a complex morphology [47]. In addition, their envelopes are also enriched with carbon, which is manifested in the IR, radio and optical spectra, in the presence of molecular lines and the C2, C3, CN, CO bands. Figure 2 shows the Swan system band of the C2 molecule in the spectra of the RAFGL 2688 nebula [22] according to the observations at different nights at the 6-m telescope with the PFES spectrograph [48] and with the NES spectrograph [15], providing the spectral resolution of $R=15\,000$ and $R=60\,000$, respectively.

The collection of spectra obtained via the long-term spectroscopy at the BTA with high spectral resolution served as the basis for the study of the variability of spectra and the variability of the velocity field in the atmosphere and envelope of the star. This variability is explained by the outflow of extended atmospheres in combination with the pulsations of deeper atmospheric layers. A detailed study [49] of the optical spectra of a post-AGB star HD 56126 (IRAS 07134+1005) in the wavelength region from 4012 to 8790 Å, obtained with the echelle spectrographs of the 6-m telescope, led to a detection of a complex and variable profile shape of strong lines (H I and the Fe II, Y II, Ba II, etc. absorptions), forming in the expanding atmosphere (in the wind base) of the star. The authors of [49] have shown that for studying the kinematics of the atmosphere one has to measure the velocities from the individual features of these profiles. Based on the metal lines and molecular features, differential shifts of lines were found, reaching $\Delta V_{r}=15$–$30$ km s*-1*. The atmosphere of the star simultaneously manifests both the expanding layers and layers infalling on the star. A comparison of data for different times of observations leads to the conclusion on the variability of radial velocity and the velocity pattern in general. The position of the molecular spectrum is temporally stable, which indicates the stability of the process of expansion of the circumstellar envelope in HD 56126, observed in the C2 and Na I lines.

For several selected objects of the program we found a phenomenon, previously unknown for the spectra of post-AGB stars, namely, an asymmetry and/or splitting of the strongest absorptions with a low excitation potential of the lower level. Let us take a closer look at this effect on the example of a spectrum of a post-AGB supergiant V5112 Sgr, studied by Klochkova [50]. V5112 Sgr has an envelope enriched by the TDU, an emission at the wavelength of 21 $\mu$, and a powerful and structured IR envelope. The discovered absorption splitting effect is maximal for the Ba II ion lines, in the profiles of which in the case of V5112 Sgr three components are observed (see, for example, Fig. 3). At that, the shape of the profiles of the split lines and the position of the atmospheric (“stellar”) component changes over time, while the position of circumstellar components is almost stable. The analysis of the velocity pattern, performed using the radio spectroscopy data of the object, allowed to make a conclusion that the shortwave components of the split Ba II absorptions are formed in the structured circumstellar envelope expanding at velocities of $V_{\rm exp}\approx 20$ and $30$ km s*-1*. Detection of the envelope components of heavy metals indicates an effective transport into the envelope of the matter synthesized in the previous stage of stellar evolution. Thus, an enrichment of a shell of a star with heavy metals synthesized at the AGB stage was discovered for the first time.

A good example is also the object IRAS 23304+6147. In the atmosphere of the faint in the optical range central star of this source previously revealed an excess of carbon and heavy metals [21]. According to the observations with high spatial resolution at the Hubble Space Telescope [51], the circumstellar envelope in this system has a complex structure including a multipole and an extended halo with arcuate features. The subsequent spectral monitoring at the BTA + NES has led to new results [52]. In particular, a comparison of the radial velocity $V_{r}=-25.7$ km s*-1*, obtained from numerous symmetric absorptions of a weak and moderate intensity, with the previously published data indicates the absence of significant velocity variations and its coincidence with the systemic velocity based on the radio data. High spectral resolution allowed to measure the positions of 24 rotational lines of the (0;0) $\lambda$5165 Å Swan system band of the C2 molecule. As a result, an expansion velocity of the circumstellar envelope was determined, $V_{\rm exp}=15.5$ km s*-1*, which is typical for stars in the post-AGB stage. In addition, the Swan system (0;1) 5635 Å band revealed a complex emission-absorption profile. For the first time asymmetry of strong absorption profiles of ionized metals  (Y II, Ba II, La II, Si II) was detected in the optical spectrum of the source IRAS 23304+6347, due to the presence in these lines of a shortwave component forming in the circumstellar envelope. The position of the long-wave component in the profiles of these lines corresponds to the velocity determined from the symmetric absorptions and matches the systemic velocity. The position of the shortwave component, on the other hand, corresponds to the velocity based on the Swan band molecule C2 and coincides with the position of the circumstellar component of the Na D1 profile. The silicon excess, the synthesis of which is possible due to the ‘hot bottom process’ in the hot layers of the convective shell in massive AGB stars, allows to talk about the belonging of the studied object to stars with initial masses above 4 $\mathcal{M}_{\odot}$.

To date, the $s$-process heavy metals have been revealed in the structured circumstellar envelopes of only three post-AGB stars, the atmospheres of which reveal products of the TDU. Hence, the result of the multi-year spectral monitoring at the BTA is a discovery of a channel through which an enrichment by the $s$-process heavy metals of structured envelopes of stars at the post-AGB stage is taking place.

New features in the spectrum were also found as a result of analysis of multiple observations with high spectral resolution of a post-AGB star V448 Lac [53]. A study of the behavior of the profiles of spectral features and the velocity field in the atmosphere and the circumstellar envelope allowed to detect an asymmetry and a temporal variability of profiles of the strongest absorptions with the excitation potential of the lower level $\chi_{\rm low}<1$ eV. Such features are manifested primarily in the profiles of the resonant lines Ba II, Y II, La II, Si II. The peculiarity of profiles can be explained by a superposition of stellar absorptions and envelope emissions. For the first time in the spectrum of V448 Lac, an emission was found in the Swan system band  (0;1) 5635 Å of the C2 molecule. Based on the H$\alpha$ core, a radial velocity variability with an amplitude of $\Delta V_{r}\approx 8$ km s*-1* was found. The average velocity variability based on the weak metal lines with a smaller amplitude $\Delta V_{r}\approx 1$–$2$ km s*-1* can be a manifestation of low-amplitude pulsations of the atmosphere. Apart from that, time-variable differential shifts of lines in the interval of $\Delta V_{r}=0$–$8$ km s*-1* were found. The position of the molecular spectrum is stable in time, which indicates the constancy of the circumstellar envelope registered from the 2 and Na I lines: $V_{\rm exp}=15.2$ km s*-1*. The expansion velocity magnitude is typical of post-AGB stars.

For the first time the splitting of the strongest absorptions with the low-level excitation potential of $\chi_{\rm low}<1$ eV was found in the optical spectra of a post-AGB star V354 Lac, also obtained with a spectral resolution of $R=60\,000$ at the 6-m BTA telescope [54]. An analysis of the kinematic pattern showed that the shortwave component of the split line is formed in the powerful gas and dust envelope of the star. Neglecting the splitting of strong lines in the calculation of the chemical composition leads to the overestimated excesses of $s$-process elements (Ba, La, Ce, Nd) in the stellar atmosphere. A strong absorption profile variability was detected. Unlike the above data for V448 Lac, for V354 Lac, no velocity variation was recorded over 15 years of observations, which suggests the stability of the velocity field in the atmosphere and the circumstellar envelope of V354 Lac.

2.5 Structured envelopes of post-AGB stars

Having obtained new results regarding the details of the chemical composition of the sample of post-AGB stars, surrounded by complex envelopes, we tried to find an interrelation of chemical composition features of their atmospheres with the structural features of the circumstellar shells. The stellar wind of the objects at the AGB stage usually possesses a spherical symmetry, which determines mainly the spherical shape of the slowly expanding envelopes of these stars. However, their closest descendants, post-AGB stars, are the objects whose central star is surrounded, as a rule, by a non-spherical envelope of a complex structure (a bipolar and even a quadrupole shape). A transition to this complex morphology from the predominantly spherical shells of AGB stars occurs exactly at the post-AGB stage, which necessitates a detailed and comprehensive study of these objects. A qualitatively new level of understanding the structure and kinematics of envelopes around the far evolved stars is provided by their observations with the Hubble Space Telescope [51, 55]. These observations have indeed revealed a bipolar structure and jets in many objects, previously considered to be only the point objects. A heterogeneous asymmetric nebula RAFGL 2688, surrounding an F-supergiant V1610 Cyg, faint in the optical range, has a large spatial extent (the main features of its structure can even be observed with a ground-based telescope) and shows a hierarchy of features of different scales, with different velocities, very well illustrates the complexity of morphology of the envelopes in post-AGB stars.

As the detailed images were accumulated, it turned out that the so-called arcs observed in the envelopes of several post-AGB stars indicate occasional mass-loss-rate variations occurring at the AGB stage. The fact that the arcs are not closed and are asymmetrical with respect to the central star, testifies that mass loss occurred during several episodes of wind intensification with a predominance of ejections at various solid angles.

Based on the observations of a representative sample of stars at the BTA with a high spectral resolution, Klochkova [47] considered the features of optical spectra of a sample of post-AGB stars with the atmospheres enriched in carbon and $s$-process heavy metals, and with carbon-rich circumstellar envelopes. As a result, it was concluded that the line profile peculiarities (the presence of an emission component in the D-lines of a Na I doublet, the nature of molecular features, asymmetry and splitting of strong absorption profiles with a low lower-level excitation potential) are related to kinematic and chemical properties of the circumstellar envelope and to the type of its morphology. In particular, the variability of the observed profiles of the absorption-emission line H$\alpha$ and lines of metals, as well as the type change (absorption/emission) of Swan bands of the C2 molecule, observed in several objects is caused by the variations in the structure of the circumstellar envelope. The H$\alpha$ profile type (pure absorption, pure emission, P Cyg or inverse P Cyg-type, with two emission components) is not associated with the chemical composition of the atmosphere of the central star. The main factors, influencing the type of the H$\alpha$ profile and its variability are the matter loss rate, stellar wind velocity, kinematics and optical thickness of the envelope.

The above-mentioned splitting into the components of the profiles of strongest absorptions of heavy metals in the spectra of supergiants V5112 Sgr, V354 Lac and the central star of the IR source IRAS 23304+6147 suggests that the formation process of a structured circumstellar envelope is accompanied by its enrichment with the stellar nucleosynthesis products. Attempts to find the relationship between the features of the optical spectrum and the morphology of the circumstellar environment is hampered by the fact that the observed structure of the envelope depends heavily on the declination of the axis of symmetry to the line of sight and on the degree of angular resolution of the equipment, both spectral and that used to produce direct images.

2.6 Cool AGB supergiants

The far evolved stars at the AGB stage, observed at the short evolutionary phase of the transition to a planetary nebula, are low-mass cores with a mass of about 0.6 $\mathcal{M}_{\odot}$. Degenerate cores are surrounded by extended gas-dust envelopes that were formed owing to a significant loss of matter of a star at the previous stages of evolution. The presence of the circumstellar gas and dust manifests itself in the peculiarities of the IR, radio, and optical spectra of AGB. The optical spectra of these low-mass supergiants differ from the spectra of classical massive supergiants by the presence of molecular bands superimposed on the spectrum of a cool supergiant, anomalies of the absorption-emission profiles of the H I, Na I and He I, lines as well as the presence of emission lines of some metals. In addition, all these spectral features are often variable in time. In general, we see in the optical spectra of AGB supergiants (and their descendants) the following features distinguishing them from the spectra of massive supergiants: complex line profiles of neutral hydrogen that contain the variable in time absorption and emission components; absorption or emission bands of molecules, predominantly carbon bearing; envelope components of the Na I and K I resonance lines; narrow forbidden or permitted emissions of metals that are formed in the envelope.

R Sct is a semi-regular variable of the RV Tau-type. Variable RV Tau-type supergiants are in the instability band for the population II stars. R Sct is the brightest and unstable representative of these stars, has a variability period of about 142 days. Kipper and Klochkova [56], using the R Sct spectra, obtained at the BTA + NES and also using the spectra from the archive of the NARVAL spectropolarimeter of the Peak du Midi observatory TBL telescope, determined the fundamental parameters, studied the characteristics of the spectrum and the kinematic pattern, and also calculated a detailed chemical composition of the atmosphere. The metallicity of the star is ${\rm Fe/H}\approx-0.5$, at a very large excess of carbon ${\rm[C/Fe]}=+0.84$. No excess of the $s$-process heavy metals was detected, therefore an assumption was made that the star has not yet experienced the TDU. The resulting effective temperature T${}_{\rm eff}=4500$ K also indicates that R Sct is still at the AGB stage.

The spectrum of R Sct revealed the splitting of the strongest absorptions as well as the presence of weak and variable emissions in the Fe I, Ti I lines [56]. Near the brightness minimum, emission is also observed in the H$\alpha$, H$\beta$ lines. Radial velocity measured from the weak symmetrical absorption, varies with a large amplitude. The pattern of radial velocities measured from different types of spectral features varies with time, and a velocity stratification is observed for selected phases.

A cool supergiant V1027 Cyg is associated with an IR source IRAS 20004+2955. Kwok [57] included this star in his now widely known list of candidates in protoplanetary nebulae. However, V1027 Cyg so far remains poorly studied, there is no consensus on the evolutionary status of this star. Arkhipova et al. [58] based on the UBV-photometry and low-resolution spectroscopy classified this star as a semi-regular variable with the brightness variation amplitude increasing from the $V$ band to $U$ band from $0\aas@@fstack{m}4$ to $0\aas@@fstack{m}8$. Later, measuring radial velocities with a correlation spectrometer, Arkhipova et al. [59] found a radial velocity variability in the range from $-10$ to $+20$ km s*-1* and noted a strengthening of the Ba II lines. Klochkova et al. [60], using the spectra taken with the echelle spectrographs of the 6-m BTA telescope, and applying the method of model atmospheres determined the main parameters of V1027 Cyg (effective temperature $T_{\rm eff}=5000$ K, surface gravity $\log g=1.0$), metallicity ${\rm[Fe/H]}_{\odot}=-0.2$ and the abundances of sixteen elements in the atmosphere of the star. A close to solar metallicity combined with a modest radial velocity of the star suggest that V1027 Cyg belongs to the Galactic disk population.

A more detailed study of the characteristics of the spectrum and the velocity field in the atmosphere of V1027 Cyg was carried out based on the high spectral resolution observations with the NES echelle spectrograph of the 6-m telescope [61]. From the symmetric absorptions of a low and moderate intensity, slight variations in radial velocity $V_{r}$ (abs) were revealed with an amplitude of about 5 km s*-1* occurring due to pulsations. In the red spectral region, numerous weak lines of the CN molecule were identified and the K I 7696 Å line, which has a P Cyg-type profile. An agreement of radial velocities, measured from the symmetric absorptions of metals and from the CN lines, indicates the formation of the CN spectrum in the stellar atmosphere. Numerous interstellar bands, DIBs, were identified in the spectrum, the position of which in the spectrum, V${}_{r}{\rm(DIBs)}=-12.0$ km s*-1*, corresponds to the velocity of the interstellar medium in the Local Arm of the Galaxy.

The spectrum of V1027 Cyg reveals a long-wave and temporally variable H$\alpha$ profile shift due to the distortion of the core and the shortwave line wing. The splitting of the strongest metal absorptions and their ions (Si II, Ni I, Ti I, Ti II, Sc II, Cr I, Fe I, Fe II, Ba II) was discovered in the spectrum of the star for the first time. A broad profile of these lines contains in its core a stably located weak emission, the position of which can be considered as the systemic velocity $V_{\rm sys}=5.5$ km s*-1*. A more specific interpretation of the radial velocity pattern, isolation of the circumstellar and photospheric components on the Na I D-line profiles, and, consequently, a refinement of the distance to the star, and defining its evolutionary status yet requires spectroscopy with extremely high spectral resolution.

IR source RAFGL 5081 (IRAS 02441+6922) was registered in the earliest balloon IR surveys. Later, Kwok et al. [62] added this source to the list of candidate objects in the AGB stars. Currently, there is no information about the central star of the RAFGL 5081 source. The SIMBAD database contains no observational data for this object, both spectral and photometric, in the visible wavelength range. Publications about the observations of the IR source RAFGL 5081 in other wavelength ranges are rather scarce. After completing the infrared observations of RAFGL 5081 at the CFHT, having adopted the data of the IRAS satellite, Kwok et al. [62] attributed this object to a group of sources with a maximum IR radiation in the region of $\lambda<10\leavevmode\nobreak\ \mu$, but could not say anything sure about its status. No radiation in the CO, OH, H2O bands is registered [63, 64].

The optical spectra of the weak in the visible region central star of the IR source RAFGL 5081 were for the first time studied [65] based on a long-term spectral monitoring with high spectral resolution conducted at the 6-m telescope with the NES spectrograph over 2001–2015. From a comparison with related objects the spectral class of the star is determined to be close to G5-8 II, what is consistent with the effective estimate of the temperature of the star T${}_{\rm eff}=5400\pm 100$ K, which is made according to spectral criteria. From the equivalent width of the oxygen triplet O I 7773 Å W(O I) = 0.84 Å in the spectrum of the companion of RAFGL 5081, a preliminary luminosity estimate is done: M${}_{\rm bol}=-4^{\rm m}$. A low luminosity allows to exclude the option of a massive red supergiant. In the spectrum of the companion of RAFGL 5081 an absorption Li I 6707 Å with the equivalent width of $W\approx 0.1$ Å is reliably identified, which indicates that the object belongs to more massive AGB stars with an initial mass of $M_{\rm init}\geq 4\leavevmode\nobreak\ \mathcal{M}_{\odot}$.

An unusual spectral phenomenon has been detected in the spectra of the star: broad absorption profiles of medium and low intensity are splitted and, unlike the wind absorptions, are stationary. The stationarity of absorptions eliminates the possibility of explaining the anomalous profiles by the spectroscopic binarity of the star. Radial velocities for the wind components of the Na I D-line and H$\alpha$ profiles reach the values of $-250$ km s*-1* and $-600$ km s*-1*, respectively (see Fig. 4). These profiles contain narrow components whose number, depths and positions vary with time. The temporally variable multicomponent structure of the Na I D-line, and H$\alpha$ profiles indicates non-uniformity and instability of the circumstellar envelope of RAFGL 5081.

For all dates of observations, heliocentric radial velocities $V{\rm r}$ were measured corresponding to the positions of all components of metal absorptions, as well as those of the Na I D-lines and H$\alpha$. An analysis of multicomponent profiles of Na I (1) lines revealed the presence of components with the velocity of V${}_{r}\rm{IS}=-65$ km s*-1*, based of which it was concluded that RAFGL 5081 is located behind the Perseus Arm, i.e. not closer than 2 kpc.

2.7 Detection of spectral binarity (SB2) of the star BD$-$6 $\degr$1178

A poorly studied cool star BD$-6\degr 1178$ is identified with IRAS 05238$-$0626, an IR-source at the galactic latitude of $b\approx-22\degr$. Due to the presence of a radiation excess in the region of 12–60 $\mu$ and taking into account the position on the IR color diagram, this object is considered to be a candidate in protoplanetary nebulae [66]. Some signs of photometric variability were found for the object [67]: the average magnitude in the system close to the $R$ filter varies in the $[10\aas@@fstack{m}78;10\aas@@fstack{m}87]$ interval with an average error of about $0\aas@@fstack{m}01$. As for the spectroscopy of BD$-6\degr 1178$, before the observations at the BTA, the literature contained only the data about the low-resolution spectra, approximately 5 Å/pixel, based on which the spectral class was estimated as F2 II [68]. From the high spectral resolution spectra obtained with the BTA + NES on arbitrary dates, spectral classification was carried out, and absorption profiles and the velocity field in the atmosphere of the star were thoroughly studied [69]. The results of the quantitative spectral classification of the star BD$-6\degr 1178$ led to a conclusion on the spectroscopic binarity of the star. Both components are F stars, F5 IV–III + F3V, with close rotation velocities: 24 and 19 km s*-1*. For four times of observations heliocentric radial velocities of both components of the binary system were measured. This is a double-lined spectroscopic binary with rather narrow and well resolved lines. The maximum recorded mutual shift of the component spectra is about 120 km s*-1*.

In general, no grounds were found for the classification of BD$-6\degr 1178$ as a post-AGB star. The coordinates of BD$-6\degr 1178$ and the distance to it of $d\approx 450\pm 37$ pc suggest its belonging to the Ori OB1 association. Therefore, BD$-6\degr 1178$ may be a young object of the Galactic disk on the stage prior to MS. Note that earlier Torres et al. [70], exploring a sample of T Tau-type candidate stars, included in it BD$-6\degr 1178$ as well. Based on the analysis of spectral and photometric data these authors found 17 new T Tau-type stars and 13 new Ae/Be Herbig-type stars. However, they were unable to include BD$-6\degr 1178$ in any of these groups, attributing it to the mixed-type object group (“miscellaneous”).

Detecting spectroscopic binarity of the SB2 type also gives us reason to doubt the classification of BD$-6\degr 1178$ as a post-AGB star, since among the known stars on this evolutionary stage there are no SB2 binaries. A number of post-AGB stars are binary systems, but of SB1-type. The nature of their invisible companion is unknown, since its features are not observed in the spectra of binary post-AGB stars. Either a white dwarf, or a low-mass MS star can be a companion.

The features of post-AGB stars in the binary systems are presented in van Winckel’s surveys [71, 72]. Note that as a result of a long-term monitoring, Hrivnak et al. [73] did not find any binary systems among the PPN with a 21 $\mu$ feature and with the atmospheres enriched with heavy metals.

2.8 Stars approaching the PN stage

Our program also includes a number of hot stars, which have closely approached the stage of a planetary nebula. An IR source IRAS 01005+7910 (hereinafter referred to as IRAS 01005) located high above the Galactic plane, its latitude is ${b=16\aas@@fstack{\circ}6}$. In the optical range, the object is associated with a nameless peculiar B-supergiant. The source’s position on the IRAS color diagram is consistent with the stage of the protoplanetary nebula. Photometric variability of IRAS 01005 was studied by several groups. Hrivnak et al. [74] have noted the brightness variability of an object on a scale which is very short for the protoplanetary nebulae, less than a few days. Arkhipova et al. [75] during the long-term photometric monitoring conducted in the UBV-bands for several hot objects, including IRAS 01005, found a fast and irregular variability of their brightness. Based on these observations of a low-amplitude variability, the authors of [75] came to a conclusion on the variations in the stellar wind parameters of these stars and (or) about the presence of micro pulsations with characteristic periods of several hours.

The principal results for the central star of IRAS 01005 are obtained in the paper [76], whose authors, by defining the fundamental parameters of this supergiant (T${}_{\rm eff}=21\,500$ K, surface gravity $\log g=3.0$, metallicity ${\rm[Fe/H]}=-0.31$ and abundance of a number of chemical elements), confirmed for it the post-AGB stage. A high galactic latitude combined with reduced metallicity indicates that the IRAS 01005 belongs to the old population of the Galaxy. An important result of this work is a detection of a carbon excess ($\rm C/O>1$) in the atmosphere of the central star in the IR spectrum of the circumstellar envelope of which spectral features of a carbon-containing molecule, fullerene C60 were detected [77].

After detecting the variability of the spectrum of IRAS 01005 at the 6-m telescope, its central star was monitored with high spectral resolution, $R=60\,000$. Based on 23 spectra, the spectral class of the central star was determined as ${\rm B}1.5\pm 0.3$, the luminosity class Ib, numerous spectral features were identified, including many forbidden ones, the variability of their profiles and radial velocity was considered [78]. Based on the position of symmetric and stable forbidden emission profiles [N I], [N II], [O I], [S II] and [Fe II], systemic velocity V${}_{\rm sys}=-50.5$ km s*-1* was determined. The presence of forbidden emissions [N II] and [S II] indicates the onset of ionization of the circumstellar envelope and the proximity of the planetary nebula stage. The differences in radial velocity Vr determined from the line cores, of about 34 km s*-1*, are partly caused by the deformations of profiles by variable emissions. The Vr difference from the wings of lines is smaller, it is approximately equal to 23 km s*-1*, and may be due to the pulsations and/or a hidden star binarity. Profile deformations of the absorption-emission lines may be associated with the variations in their absorption (photospheric) components with variations of geometry and kinematics at the base of the wind (see Fig. 5). Our material suggests that over two days the velocity variations reach a confidently measured value.

As can be clearly seen in Fig. 6, H$\alpha$ lines in the spectrum of the central star IRAS 01005 possess P Cyg III0-type wind profiles. It is shown that the deviations of the wind from the spherical symmetry are small. We recorded a variable wind velocity (in the interval of 27–74 km s*-1* for different times of observations) and a high intensity of the long-wave emission (the continuum level in excess up to seven times), which is not typical for the classical supergiants, but rather for hypergiants. For the five main components of Na I D-lines the heliocentric radial velocities are: $V_{r}=-72.5$, $-65.3$, $-52.2$, $-27.7$ and $-10.2$ km s*-1*, which, within the error limits, coincides with the published data. A weak component, $V_{r}=-52.2$ km s*-1*, is formed in the stellar atmosphere, two more long-wave components are interstellar, forming in the Local Arm. The presence of the $V_{r}=-65.3$ km s*-1* component, apparently originating in the interstellar medium of the Perseus Arm, allows us to consider $d=2.5$ kpc as the bottom distance estimate to IRAS 01005. The most shortwave component, $V_{r}=-72.5$ km s*-1*, can be formed in the circumstellar envelope expanding with the velocity $V_{\rm exp}\approx 22$ km s*-1* typical for the PPN.

In cooperation with a group of specialists from the Moscow University Sternberg Astronomical Institute a complex research (long-term photometry, spectroscopy of a moderate and high resolution) was conducted of several related hot stars, close to the PN phase: V1853 Cyg (IRAS 20462+3416) [79], V886 Her (IRAS 18062+2410) [80], StH$\alpha$62 (IRAS 07171+1823) [81]. Photometric parameters of these objects vary with an amplitude approximately equal to $0\aas@@fstack{m}2$–$0\aas@@fstack{m}3$. Their optical spectra are somewhat different, due to a slight difference in the degree of approximation to the planetary nebula phase. The spectrum of the B-supergiant V886 Her includes complex H II lines with powerful emission components, light metal ion absorption lines, forbidden emissions, as well as a rich nebular spectrum of the envelope [80].

The same methods were used to study the properties of a high-latitude planetary nebula G 199.4+14.3 [81]. The spectrum of its central star StH$\alpha$62 in the range of $\lambda$ 4330–7340 Å contains absorption lines, corresponding to the spectral class Sp = B0.5, in combination with the emission lines of the gas envelope. Figure 7 in a way of example demonstrates a fragment of the StH$\alpha$62 spectrum obtained at the 6-m telescope with the PFES spectrograph [48], containing the [S II] $\lambda$ 6716.45 and 6730.85 Å forbidden lines. The presence of the [N II] and [S II] forbidden emissions allowed the authors of [81] to estimate the density and temperature of the gas envelope of the star, the values of which are usual for the low excitation planetary nebulae within the range of formation of the [S II] and [N II] lines, where the gas ionization is just starting. The rate of expansion of the outer shell from the forbidden lines amounted to 12–13 km s*-1*. Assuming that the star is a low-mass ($\mathcal{M}=0.55\pm 0.05\leavevmode\nobreak\ \mathcal{M}_{\odot}$) protoplanetary object, a distance estimate of $d=5.2\pm 1.2$ kpc was found. As shown in [81], the main characteristics and features of the optical spectrum of V1853 Cyg are close to those of StH$\alpha$62.

HD 331319 is a post-AGB candidate, an optical component of the IR source IRAS 19475+3119. New results for it were for the first time obtained with the echelle spectrograph of the 6-m telescope. The fundamental parameters and a detailed chemical composition of its atmosphere were determined using the model atmosphere method [82]. A particularly significant result is detection of helium lines in the spectrum of this object with an effective temperature T${}_{\rm eff}=7200$ K. An excess of helium in the observed layers of the atmosphere is interpreted as a consequence of the synthesis of this element during the previous evolution of the star. An excess of nitrogen and oxygen is also detected: ${\rm[N/Fe]}_{\odot}=+1.30$ dex, ${\rm[O/Fe]}_{\odot}=+0.64$ dex at a slight excess of carbon. The metallicity of the stellar atmosphere ${\rm[Fe/H]}_{\odot}=-0.25$ is slightly different from the solar value. The $s$-process metals abundance is underestimated relative to the metallicity: for Y, Zr ${\rm[X/Fe]}_{\odot}=-0.68$. The barium abundance is also underestimated with respect to metallicity: ${\rm[Ba/Fe]}_{\odot}=-0.47$. The Li 6707 Å line was not found in any of the obtained spectra of the star. In general, the abundance of chemical elements raises doubts about the belonging of the object IRAS 19475+3119 to the post-AGB evolutionary stage. Metallicity combined with the radial velocity V${}_{r}=-3.4$ km s*-1* and galactic latitude $|b|=2\aas@@fstack{\circ}7$ of the object indicate its belonging to the population of the disk of the Galaxy. According to the position of the absorption bands formed in the circumstellar envelope, the expansion velocity of the shell was determined, V${}_{\exp}\approx 21$ km s*-1*.

2.9 Spectral atlases

Working with high quality spectral material we obtained at the BTA for the supergiants of various types (different spectral classes, luminosity classes, evolutional stages, chemical composition features), we considered creation of spectral atlases as a necessary stage of our work. The following atlases, relevant for this context are published: the spectra of hypergiants and supergiants [83], the spectra of a canonical post-AGB star HD 56126 [84]. The atlas [85] presents a comparison of spectra of a low-mass post-AGB supergiant BD$+48\degr 1220$ (IRAS 05040+4820), a peculiar supergiant 3 Pup (IRAS 07418$-$2850) and a massive supergiant $\alpha$ Cyg (=IRC+50337).

A detailed study of the spectrum of a post-AGB supergiant BD$+48\degr 1220$ was done in [86], the authors of which, applying the method of model atmospheres determined the main parameters of the star and the chemical composition of its atmosphere, slightly different from that of the Sun. Given a small excess of the iron peak elements, ${\rm[Met/H]}_{\odot}=+0.20$, a mild excesses of carbon and nitrogen was found ${\rm[C/Fe]}_{\odot}=+0.25$, ${\rm[N/Fe]}_{\odot}=+0.27$. The $\alpha$-process elements Mg, Si and Ca have a small excess, on the average of $[\alpha/{\rm H}]_{\odot}=+0.12$. A large excess of sodium was revealed, ${\rm[Na/Fe]}_{\odot}=+0.75$, which is probably a consequence of the outflow of matter, processed in the NeNa cycle, into the atmosphere.

Obviously, the spectral atlases published in the accessible form, contribute to the preservation of observational data and undoubtedly facilitate the subsequent work of spectroscopists, including the beginners. Our atlases contain spectral material both in the graphical representation, as well as tables with a detailed identification of all spectral features. A major part of these atlases is available in digital form on the Internet.

2.10 Interstellar and circumstellar features

A fundamentally new result was also obtained in relation to the formation of mysterious spectral features identified with the so-called diffuse interstellar bands (DIBs). It turned out that in the spectrum of V5112 Sgr the radial velocity determined from the DIBs coincides with the velocity from the shortwave envelope component of the Na I D-lines, which allowed to conclude on the formation of DIBs in the circumstellar envelope [50]. There are several programs aimed at finding the DIBs in the circumstellar envelopes, but all attempts were unsuccessful: the discovered envelope features were not subsequently confirmed. Based on the extensive and high-quality spectral material providing the high accuracy of positional measurements in the spectra, as well as having an extensive experience with the spectra, we have for the first time obtained a reliable positive result in the long-term search for the DIB analogues in the circumstellar medium.

Note that Klochkova et al. [52] measured the positions of five absorptions, which can be identified with the DIBs in the spectrum of the central star of IRAS 23304+6147, namely: 5797, 6196, 6203, 6207, 6613 Å. The average velocity value for them is V$\rm{}_{r}(DIBs)=-15$ km s*-1*. If we discard the 6613 Å band, which is blended in the spectrum of the studied supergiant with a strong Y II line, we then obtain an average velocity of V$\rm{}_{r}(DIBs)=-14.0\pm 1.3$ km s*-1*, close to the velocities measured from the longest-wave component of the Na I D-lines, V${}_{r}=-13.2$ km s*-1*. Thus, we conclude that in the case of IRAS 23304+6147 the DIBs are formed in the interstellar medium. Hence, the star V5112 Sgr still remains the only star at the post-AGB stage, in the spectrum of which the DIBs, forming in the circumstellar envelope are found [50].

3 SPECIAL REPRESENTATIVES OF POST-AGB-STAR FAMILIES

Let us briefly discuss some of the special types of stars observed at the post-AGB stage. The first step for us in learning the post-AGB candidates were the F-supergiants at high galactic latitudes. The prototype star for them is a peculiar supergiant UU Her. In the 1980s these strange objects received a lot of attention, which was due to the internal contradiction of the original idea about them as the classical supergiants residing at such high latitudes of the Galaxy. However, the subsequent study of the chemical composition and other observable properties of these stars showed that both UU Her, and related objects are in fact the far evolved low mass stars belonging to the old population of the Galaxy. The contradiction between the high luminosity of UU Her-type stars and their location at high latitudes of the Galaxy has thus been resolved.

A high-latitude supergiant LN Hya belongs to the semi-regular variables that are the far evolved stars in the post-AGB stage. The observations of LN Hya, carried out at the 6-m telescope with a high spectral resolution in 2003–2011, allowed us to study in detail the features of its optical spectrum and the velocity field in the atmosphere [87]. Radial-velocity variations from date to date with an amplitude of up to 3 km s*-1* were found from the weak symmetric photospheric absorptions, which is a consequence of weak pulsations. For a long time, an emission in H$\alpha$ was considered to be the only peculiar feature of the LN Hya spectrum. However, as a result of spectral monitoring with a high spectral resolution we discovered a peculiarity and variability of the profiles of strong lines of Fe I, Fe II, Ba II, Si II, and others. For the first time an asymmetric shape of the profiles of these lines was revealed: their shortwave wings are elongated, and the nuclei are either split or distorted by emission. The observational season of 2010 was the most intriguing, when from one spectrum to another the position and depth of the H$\alpha$ absorption component, the intensity of the shortwave and longwave emission components, as well as the ratio of their intensities had been varying. In the spectrum of June 1, 2010, there appeared weak emissions of neutral atoms (V I, Mn I, Co I, Ni I, Fe I), which is clearly seen in the spectral fragment, presented in Fig. 8. These features of the stellar spectrum, registered for the first time give us grounds to assume that in 2010 we managed to register short-term variations in the physical state in the upper layers of the LN Hya atmosphere.

3.1 Close binaries with a large hydrogen deficiency

A special subgroup of stars at the post-AGB stage is a very small group of binary systems with a large hydrogen deficiency (HdBs). The prototype of these close binaries is a bright enough star $\upsilon$ Sgr (HD 181615). Only four HdBs-type stars are known up to date, and these objects possess very close effective temperatures, about 10 000 K. It is assumed that the post-AGB companion in these binary systems is in the helium burning stage in the layer surrounding the degenerate C-O–core. The lifetime of the helium supergiant is only about $10^{4}$ years, which explains the rare occurrence of HdBs stars. The nature of the second, invisible system component remains unknown. A keen interest to these objects is due to the fact that, according to modern concepts, they are considered to be the predecessors of the SN Ia supernovae.

Currently, the $\upsilon$** Sgr and KS Per** A-supergiants are very well studied. The main features of the supergiant $\upsilon$ Sgr (Sp = A2 Ia) are a strong and variable emission in H$\alpha$ and a large excess of the IR flux. According to the spectra obtained at the BTA with the NES spectrograph, using the data from the archive of the Pic du Midi Observatory 2-m telescope, Kipper and Klochkova [88] determined a detailed chemical composition of the supergiant’s atmosphere in the $\upsilon$ Sgr system. The hydrogen abundance is reduced by five orders of magnitude with a slight iron deficiency ${\rm[Fe/H]}=-0.8$ dex. Interestingly, a similar hydrogen deficiency was obtained for the star KS Per (IR source IRAS 04453+4311). The atmospheres of both stars revealed a large nitrogen excess and a noticeable excess of heavy metals. Figure 9 presents a fragment of the spectrum of $\upsilon$ Sgr, saturated with the anomalously strong lines of the N I atom. A rare for this stage of evolution feature of the $\upsilon$ Sgr spectrum are numerous emissions of metals and their ions, the place of formation of which can be an envelope around the binary system or the disk.

3.2 Stars at the late helium flash stage

A famous supergiant FG Sge over the past 100 years shows a high rate of evolution: throughout the live of one generation of observers, the star has crossed over the Hertzsprung–Russel diagram. Bloecker and Schoenberner, modeling the evolutionary behavior of stars of small and intermediate masses, devoted a separate study [89] to the evolution of FG Sge. We refer the interested readers to this publication itself, without considering it in detail here. Van Genderen and Gautschy [90], having gathered the FG Sge photometry data over 100 years of observations, restored the course of its evolution from the state of a hot star Sp = O3 in 1880 (the central star of a planetary nebula He 1–5) up to a cool star of spectral class K2 in 1992. Given that, the radius of the star increased more than two orders of magnitude from 1 to 184 $R_{\odot}$. Iben and MacDonald [91] called the behavior of a few objects of this type a “born again behavior”. As a result of a “very late” He-shell flash in the central star of the planetary nebula layer instead of a movement to the white dwarf phase the star returns to the AGB stage. Recall that a “very late” He-shell flash is a flash observed in a far evolved star, up to the state of a PN nucleus, after the final burnout of the fuel in hydrogen shell [89]. This recovery to the AGB is accompanied by an increase in the luminosity of the star to $4^{\rm m}$ and a decrease of effective temperature by tens of thousands degrees. Moreover, such serious changes in the parameters of the star occur over a very short time of only about 5–15 years! Wherein, the stellar atmosphere gets hydrogen-depleted and enriched with the CNO-group elements, and the $s$-process elements [91].

In February 1996, a Japanese amateur astronomer Y. Sakurai registered a flash of a peculiar object, which was then named in his honor the Sakurai’s object (V4334 Sgr). During 1996 the magnitude of the object has increased from $12\aas@@fstack{m}5$ to $11\aas@@fstack{m}2$. Being classified by the rate of brightness variations during the flash as a slow nova, it was referred to peculiar objects after its first spectra were obtained at the ESO on the 3.6-m telescope, which did not corresponded to the expected spectrum of novae. In March 1996, direct CCD images of the Sakurai’s object taken with the ESO 0.9-m telescope revealed a planetary nebula around the object. Duerback and Benetti [92], having examined the low-resolution spectra, concluded on a significant weakening of the neutral hydrogen lines at the presence of strong lines of carbon and oxygen. Considering the totality of the observed features they attributed the flared object to R CrB-type stars. The first high-resolution spectra of V4334 Sgr were obtained at the MacDonald Observatory 2.7-m telescope [93] and at the BTA in June 1996. As a result of analysis of these spectra and calculating the chemical composition by the model atmospheres [93, 94] a decrease of the hydrogen abundance by 3 dex was discovered (thereat, it decreased by 0.7 dex only from May to October 1996), and a carbon excess, and large excesses of Li and Sr, Y, Zr heavy metals too. Kipper [95] thoroughly considered the variations in the parameters, the spectrum and chemical composition of V4334 Sgr according to the results of spectroscopy of the star at the BTA after the flash. The detected deficiency of hydrogen and metallicity, the excess of heavy metals and a high radial velocity indicate that the object belongs to the old protoplanetary nebulae in the bulge of the Galaxy.

It is now assumed that the Sakurai’s object has experienced a very late He-shell flash, after which it entered into a fast final evolutionary phase at the post-AGB stage [89]. Therefore, the study of the Sakurai’s object gives another opportunity of testing the theoretical modeling of the evolution of the planetary nebulae nuclei and circumstellar envelopes, the loss of matter, convection, stellar nucleosynthesis and variations of the surface chemical composition. In this regard, the Sakurai’s object can be put on a par with FG Sge.

According to the model of evolution of the star [91], the result of an He-shell flash should be an up to 10 times luminosity increase and a Teff variation from 40 000 K to 6300 K over 17 years. Model parameters obtained by Asplund et al. [93] and Kipper and Klochkova [94], allow us to state that V4334 Sgr evolves faster (within six months its temperature dropped by 600 K). Despite the proximity of evolutionary phases, the rate of evolution of FG Sge and V4334 Sgr differs significantly. FG Sge has gone through a “very late” He-shell flash over about 100 years, and V4334 Sgr – 10$\div$15 times faster. Lawlor and MacDonald [96] based on the model calculations of evolution studied the phenomenon of “very late” He-shell flashes and concluded that the difference in the evolution rates of FG Sge and V4334 Sgr can be explained by assuming that FG Sge is observed on a slow repeated comeback to the AGB, while V4334 Sgr—on the fast and the first one. Lawlor and MacDonald [96] emphasize that a repeated slow comeback to the AGB is possible under the condition of a low convective mixing efficiency. A further very long monitoring of both objects is required to check this explanation. Clayton et al. [97] believe that the “born again” star V605 Aql, based on its spectral properties, the light curve and the evolution of the dust envelope is closer to V4334 Sgr than to FG Sge.

In as little as a year after the flare, the formation of a dense dust shell, which has significantly weakened the visible flux, started around V4334 Sgr and the object became inaccessible for the high-resolution spectroscopy in the optical range. According to the results of radio observations, this newly formed envelope has a bipolar structure, in contrast to the spherical shape, previously observed [98].

3.3 A star at the stage of evolution before AGB

The list of protoplanetary nebula candidates includes a star V534 Lyr (HD 172324), located at a high galactic latitude ($b=18\aas@@fstack{\circ}58$). In addition to a high latitude, an additional impetus for us to study V534 Lyr was the emission in the H I lines, which was discovered in the early work [99] and confirmed later in [100]. This star was further studied repeatedly using different photometric systems and spectral methods, but so far none of the available publications has any definite conclusion about its evolutionary status.

Based on multiple observations at the 6-m telescope with the NES echelle spectrograph, we thoroughly studied the features of the optical spectrum of V534 Lyr and the temporal behavior of the heliocentric radial velocity corresponding to the position of all the metal absorption components as well as the Na I D-lines and the H$\alpha$ line [101]. The analysis of the velocity field of spectral features of various nature revealed a low-amplitude variability of Vr from the lines with a high excitation potential, which form in deep layers of the stellar atmosphere, and allowed to estimate the systemic velocity V${}_{\rm lsr}\approx-105$ km s*-1*. The distance estimate of $d\approx 6$ kpc for a high-latitude star leads to the absolute magnitude value of M${}_{v}\approx-5\aas@@fstack{m}3$, which is consistent with its spectral classification. A previously unknown for this star spectral phenomenon was discovered: a splitting of profiles of selected metal absorptions at separate points of observation. In all cases when splitting is present in the spectrum, it reaches large values: $\Delta V_{r}=20$–$50$ km s*-1*.

The spectral class of the star is close to A0 Ib, and the effective temperature T${}_{\rm eff}\approx 10\,500$ K. Metallicity, reliably determined by the method of model atmospheres using the iron group metal lines, slightly differs from the Fe abundance: ${\rm[Met/Fe]}_{\odot}=+0.06$. A nitrogen and helium excess indicates an advanced evolutionary stage of the star. A reduced iron group metal abundance in combination with a high radial velocity indicates that the star belongs to the thick disk of the Galaxy. A set of observable features of V534 Lyr: the presence of probable pulsations in deep layers of the atmosphere, a splitting of metal absorption profiles with a low lower-level excitation potential observed at certain times, a low metallicity, the type and variability of the H$\alpha$ and H$\beta$ emission-absorption profiles (see Fig. 10), allows to refer the star to variable population II stars above the HB. In the course of subsequent evolution, it may approach the instability band, and then the AGB stage. The authors [101] eventually concluded on the discrepancy of the V534 Lyr belonging to the post-AGB stage.

4 CONCLUSIONS AND NEW TASKS

The paper briefly discusses the most important issues and data concerning the final stages of evolution of stars of different masses and the nucleosynthesis processes during the evolution of these objects. We presented the results obtained performing the spectroscopic study of a sample of peculiar supergiants, identified with galactic IR sources at the 6-m BTA telescope. The initial list of post-AGB candidates was introduced by Kwok in the survey [57]. From this list we have studied all the stars reachable by the coordinates that have a visible brightness of V$\leq 13\aas@@fstack{m}1$. The main aspects of our program is a search for the evolutionary changes of chemical composition of the stars that have passed the AGB stage and the TDU, as well as an analysis of spectral manifestations of the kinematic processes in their extended atmospheres and envelopes. The most significant result of the program is the detection of an $s$-process element excess in seven post-AGB stars, which empirically confirms the theory of evolution of stars of this type. In three of these stars we have for the first time detected an outflow of $s$-process heavy metals into the circumstellar envelopes.

The expected lithium excess is registered in the atmospheres of peculiar supergiants V2324 Cyg and V4334 Sgr (the Sakurai’s object). The atmosphere of the “born-again” star V4334 Sgr is enriched in carbon and the $s$-process heavy metals. A lithium excess, suspected for two central stars of IR sources RAFGL 5081 and IRAS 04296+3429, weak in the optical range, may be a consequence of blending of the Li I 6707 line with the Ce II line.

The results of study of the kinematics of atmospheres and shells will serve to clarifying the amount of matter produced by the stars at the AGB and post-AGB stages and supplied to the ISM. Up to now the models of chemical evolution of the Galaxy use an approximation where in the spherically symmetric envelopes of stars the matter moves only in one direction.

The results obtained during the PPN spectroscopy program allow us to distinguish the priority directions in the further study of objects at the stage of transition of a star to a planetary nebula and related objects. First of all, a further investigation of the ‘spectroscopic mimicry’ problem is required, which allows peculiar low-mass supergiants to disguise themselves as the most massive stars of high luminosity. Secondly, we believe that we should further focus on the spectral monitoring of selected variable and rapidly evolving objects. Thirdly, a detailed study of the structure of circumstellar envelopes is required, attracting the spectropolarimetry method too to clarify the mechanisms of the outflow and accretion of matter.

It must be emphasized that the program of monitoring the supergiants of various nature is limiting for the high spectral resolution spectroscopy even on the largest telescopes. From the observational point of view, the task is complicated by the need of multiple observations of variable objects, as well as high requirements for the stability of equipment, and, in particular, for the high accuracy of positional measurements. A new aspect of spectroscopy of high-luminosity stars with an infrared excess, consisting in the study of circumstellar envelopes, including molecular spectroscopy, implies, among other things, requirements to increase the share of high-resolution spectropolarimetry (its first experience is already published [102]) and provide a extremely high resolution $R\geq 10^{5}$. The importance of increasing the spectral resolution is illustrated in Figs. 2 and 3. Figure 2 clearly shows that the rotational structure of a molecular band can be identified only with a resolution of at least 60 000. Therefore, our program stimulates the further development and sophistication of the spectroscopic equipment of the BTA, as well as the systems for the reduction of spectroscopic data and methods of its analysis.

Acknowledgements.

The author thanks the Russian Foundation for Basic Research (11-02-00319 a, 14-02-00291 a and 18–02–00029 a) for the financial support. The author is grateful to the co-authors who participated in the implementation of the supergiant spectroscopy program at the BTA and in the preparation of joint publications. The work made use of the SIMBAD, SAO/NASA ADS, Gaia DR2, AAVSO and VALD astronomical databases.