FUSE Spectroscopic Analysis of the Slowest Symbiotic Nova AG Peg During Quiescence

TL;DR

This study analyzes the far ultraviolet spectrum of the symbiotic nova AG Peg during quiescence, revealing a hot white dwarf with a temperature of 150,000 K and discussing the spectral contributions of the white dwarf and potential accretion disk.

Contribution

First detailed FUV spectral modeling of AG Peg during quiescence, identifying a hot white dwarf as the dominant source of UV flux and evaluating accretion disk presence.

Findings

White dwarf temperature ~150,000 K

White dwarf radius ~0.06 Rsun at 800 pc

Accretion disk models less favored than WD photosphere

Abstract

We present a far ultraviolet spectroscopic analysis of the slowest known symbiotic nova AG Peg (M3/4III giant + hot white dwarf; P = 818.4 days) which underwent a nova explosion in 1850 followed by a very slow decline that did not end until 1996, marking the beginning of quiescence. The 19 years of quiescence ended in June 2015, when AG Peg exhibited a Z And-type outburst with an optical amplitude of 1.5 magnitudes. We have carried out accretion disk and WD photosphere synthetic spectral modeling of a Far Ultraviolet Spectroscopic Explorer (FUSE) spectrum obtained on June 5.618, 2003 during the quiescence interval 12 years before the 2015 outburst. The spectrum is heavily affected by ISM absorption as well as strong emission lines. We de-reddened the FUSE fluxes assuming E(B-V) = 0.10, which is the maximum galactic reddening in the direction of AG Peg. We discuss our adoption of the pre-Gaia distance over the Gaia parallax. For a range of white dwarf surface gravities and surface temperatures we find that the best-fitting photosphere is a hot WD with a temperature T = 150,000 K, and a low gravity log(g)~6.0-6.5. For a distance of 800 pc, the scaled WD radius is about 0.06 Rsun, giving log(g) = 6.67 for a 0.65 Msun WD mass. The Luminosity we obtain from this model is 1729 Lsun. The hot photosphere models provide better fits than the accretion disk models which have FUV flux deficits toward the shorter wavelengths of FUSE, down to the Lyman Limit. Given the uncertainty of the nature of a true symbiotic accretion disk, and, while a very hot low gravity degenerate star dominates the FUV flux, the presence of a steady-state (standard) accretion disk cannot be summarily ruled out.

Tables

Table 1. Table 1: Orbital and Physical Parameters of AG Peg

Parameter	Value	References
Period	818.4 d	Fekel et al. (2000)
Inclination	${50}^{\circ }$	Kenyon et al. (1993)
Distance	800 pc	Kenyon et al. (1993)
Separation	$2.5\pm 0.1$ AU	Kenyon et al. (1993)
$E\left(B-V\right)$	0.10$\pm$0.05	Kenyon et al. (1993)
RG Spectral Type	M3/4 III	Kenyon et al. (1993)
WD Mass	$0.65\pm 0.1M_{\odot }$	Kenyon et al. (1993)
WD Radius (quiescence)	$\sim 0.05-0.06R_{\odot }$	Skopal et al. (2017)
WD Temperature (quiescence)	95k K, $\sim$160k K	Mürset et al. (1991); Skopal et al. (2017)
RG Mass	$2.6\pm 0.4M_{\odot }$	Kenyon et al. (1993)
RG Temperature	3,500 K	Richichi et al. (1999); van Belle et al. (1999)

Table 2. Table 2: Single hot component fitting results

$T_h$	$R_h$	$R_h$	$M_h$	$Log\left(g\right)$	$L$	$L$
(1000 K)	(1000 km)	($R_{\odot }$)	($M_{\odot }$)	(cgs)	($L_{\odot }$)	(erg/s)
100	55.5	0.0798	0.4	6.23	572	$2.19\times {10}^{36}$
100	55.5	0.0798	0.65	6.44	572	$2.19\times {10}^{36}$
100	55.5	0.0798	1.0	6.63	572	$2.19\times {10}^{36}$
150	42.9	0.0615	0.4	6.46	1729	$6.64\times {10}^{36}$
150	42.9	0.0615	0.65	6.67	1729	$6.64\times {10}^{36}$
150	42.9	0.0615	1.0	6.86	1729	$6.64\times {10}^{36}$
180	39.6	0.0570	0.4	6.53	3056	$1.17\times {10}^{37}$
180	39.6	0.0570	0.65	6.74	3056	$1.17\times {10}^{37}$
180	39.6	0.0570	1.0	6.93	3056	$1.17\times {10}^{37}$

Full text

We have carried out accretion disk and WD photosphere synthetic spectral modeling of a Far Ultraviolet Spectroscopic Explorer (FUSE) spectrum obtained on June 5.618, 2003 during the quiescence interval $\sim$12 years before the 2015 outburst. The spectrum is heavily affected by ISM absorption as well as strong emission lines. We de-reddened the FUSE fluxes assuming $E(B-V)=0.10$, which is the maximum galactic reddening in the direction of AG Peg. We discuss our adoption of the pre-Gaia distance over the Gaia parallax. For a range of white dwarf surface gravities and surface temperatures we find that the best-fitting photosphere is a hot WD with a temperature $T_{\rm wd}=150,000$ K, and a low gravity $log(g)\sim 6.0-6.5$. For a distance of 800  pc, the scaled WD radius is $R_{\rm wd}\sim 0.06\times R_{\odot}$, giving $log(g)=6.67$ for a $0.65M_{\odot}$ WD mass. The Luminosity we obtain from this model is $L=6.64\times 10^{36}{\rm erg/s}=1729L_{\odot}$. The hot photosphere models provide better fits than the accretion disk models which have FUV flux deficits toward the shorter wavelengths of FUSE, down to the Lyman Limit. Given the uncertainty of the nature of a true symbiotic accretion disk, and, while a very hot low gravity degenerate star dominates the FUV flux, the presence of a steady-state (standard) accretion disk cannot be summarily ruled out.

— novae, cataclysmic variables — stars: white dwarfs — stars: individual (AG Peg)

††slugcomment: Submitted to The Astrophysical Journal††facilities: FUSE††software: IRAF (v2.16.1, Tody (1993)), Tlusty (v203) Synspec (v48) Rotin(v4) Disksyn (v7) (Hubeny & Lanz, 2017a, b, c), PGPLOT (v5.2), Cygwin-X (Cygwin v1.7.16), xmgrace (Grace v2), XV (v3.10)

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Accepted version \setwatermarkfontsize1.1in

1 Introduction

AG Peg is the slowest known symbiotic nova (M3/4III giant + hot white dwarf; $P_{\rm orb}=818.4$ days) which underwent a nova explosion in 1850 followed by $\sim$165 years of decline and quiescence. A number of studies (Kenyon et al., 2001; Yoo, 2006, 2008; Kim & Hyung, 2008; Sanad & Bobrowsky, 2017) document the very slow decline, since the 1850 nova explosion, and the transition into quiescence (up to $\sim$1995). From roughly 1997 to June 2015, the brightness of AG Peg appears to be fairly constant: its visual magnitude (e.g. AAVSO light curve - LC) shows variation between about $\sim$9.3 and $\sim$8.3. Its photometric U-LC clearly exhibits a modulation with the orbital phase (Skopal et al., 2017, ; attributed to the nebular component). This led Skopal et al. (2017, and references therein) to confirm that AG Peg had at last reached its quiescence. In 2015, AG Peg unexpectedly transitioned to a weaker, sharper outburst with an optical amplitude of $\sim$1.5 magnitudes and short duration characteristic of Z And-type symbiotic variables. This new type of outburst may be due to gravitational energy release driven by (disk?) accretion and unlikely to be of thermonuclear origin like the 1850 outburst. This kind of transition of a symbiotic nova to Z And-type outbursts is unprecedented.

In order to shed light on the nature of the hot component (the ”central engine”), we present here an analysis of a FUSE spectrum of AG Peg obtained in quiescence. In this far ultraviolet wavelength range down to the Lyman Limit, the contribution of the nebular continuum produced by the photoionization of the red giant wind (which flattens the slope) is totally negligible. In this region, we are probing the flux of the innermost disk and white dwarf photosphere.

The published orbital and physical parameters of AG Peg are listed in Table 1 with their references. Its Gaia (DR2, Prusti et al., 2016; Brown et al., 2018) parallax is $\varpi\sim 0.380298..\pm 0.081982..$ mas, giving a distance is $d=2629_{-462}^{+725}$pc. However, the standard uncertainty is $\sigma_{\varpi}\sim 0.02$ for a 9 mag star (e.g. Schaefer, 2018) with such a parallax ($\varpi\sim 0.4$ mas), or four times smaller. The uncertainty in distance $(d=1/\varpi)$ will have a subtantially non-Gaussian shape when $\sigma_{\varpi}$ grows to a substantial fraction of the parallax $\varpi$. The problem becomes non-trivial for cases where $\sigma_{\varpi}/\varpi\sim 0.2$ or larger (Bailer-Jones, 2015). The parallax of AG Peg has a significance of 4.6 sigma, and from its quality flags (e.g. goodness of fit, number of visibility periods) it appears that the Gaia parallax for AG Peg is actually unreliable (see e.g. Eyer et al., 2018; Luri et al., 2018, ; see also https://gea.esac.esa.int/archive/documentation/GDR2/Data_processing/ ). One of the explanations for the unreliability of AG Peg parallax is the fact that a long-period binary orbit will cause the center-of-light to wobble with a shift comparable to the parallaxe. As the binary separation is about 2 AU with a period of almost 2 yr, over a 6 month period, the components move a distance of the order $\sim 1$ AU (depending on the exact mass ratio and inclination).

Therefore, we mainly present results based on the widely accepted pre-Gaia distance to AG Peg of $\sim$800 pc.

In section 2, we provide the details of the FUSE observation including the complexities encountered and how we corrected for them, particularly the molecular hydrogen absorption that is pervasive in the FUSE wavelength range along with other ISM absorption features (see subsection 2.1). In subsection 2.2, we describe our suite of modeling codes for both NLTE high gravity model atmospheres and model accretion disks: the Wade & Hubeny (1998) grid of standard, steady state disk models and accretion disk models we have generated from scratch. Our grid has non-standard accretion disk models in the form of (inner and outer) disk truncation, and covers parameter space outside of the Wade and Hubeny grid. In section 3, we present the results of our synthetic spectral analysis of the FUSE data using accretion disk models and white dwarf models. Finally, in section 4 we summarize our conclusions.

2 Observations and Method of Analysis

2.1 The FUSE Spectrum.

AG Peg was observed with FUSE on June 5.618, 2003 (exposure start time MJD 52795.61791667) and data from the AAVSO indicate it had a magnitude $m_{v}=8.5$. The orbital phase at the time of the FUSE observation was $\phi=0.31$, which we derived using the orbital ephemeris of Fekel et al. (2000) 2 447 165.3($\pm$48)+ 818.2($\pm$1.6) x E (itself based upon the inferior conjunction of the M3/4 giant). We retrieved the FUSE data (ID Q1110103) from the MAST archive. The instrument was set in time-tag and the data were obtained through the MDRS aperture with an exposure time of 2116 seconds (one single FUSE orbit). The data were calibrated through the pipeline with the final version of CalFUSE (Dixon et al., 2007) and further processed using our suite of FORTRAN programs, unix scripts, and IRAF procedures especially written for this purpose (Godon et al., 2012). The FUSE data come in the form of eight spectral segments (SiC1a, SiC1b, SiC2a, SiC2b, LiF1a, LiF1b, LiF2a, and LiF2b), which have to be combined together to give the final FUSE spectrum. These spectral segments do overlap and provide a way to renormalize the spectra in the SiC1, SiC2, and LiF2 channels to the flux in the LiF1 channel (which is the most reliable segment). In the present case, the two SiC channels experienced intermittent (and unexplained) drops in the count rates (which was more severe for the SiC2 channel than for the SiC1 channel). When combining the 8 spectral segments together, we took care to renormalize the SiC spectral segments to the LiF spectral segments.

The FUSE spectrum with line identifications is displayed in Fig.1. Many ISM molecular hydrogen absorption lines are indicated with vertical tick marks. Numerous helium lines dominate the shortest wavelengths. Also seen are neon emission lines. FPN is a fixed pattern noise from the FUSE detector.

The FUSE spectrum was previously analyzed by Eriksson et al. (2006) who studied the locations, origins and excitation mechanisms of the emission lines, and by Skopal et al. (2017) who used the FUSE spectrum to analyse the O vi 1032 Å line as part of a multi-wavelength analysis.

In preparation for the spectral analysis, we dereddened the FUSE spectrum assuming $E(B-V)=0.10$ (see Table 1) and using our dereddening script based on the extinction curve from Fitzpatrick & Massa (2007) with $R=3.1$.

2.2 Spectral Modeling and Analysis.

The spectral analysis is carried out by fitting the observed FUSE spectrum of AG Peg with theoretical (synthetic) model spectra of WDs and accretion disks.

We use Hubeny’s suite of codes TLUSTY and SYNSPEC (Hubeny, 1988; Hubeny & Lanz, 1995) to generate synthetic spectra for high-gravity stellar stmosphere WD models. A one-dimensional vertical stellar atmosphere structure is first generated with TLUSTY for a given surface gravity ($log(g)$), effective surface temperature ($T_{\rm eff}$), and surface composition (here we assume solar composition). The code SYNSPEC is then used to solve for the radiation field and generate a synthetic stellar spectrum over a given wavelength range between 900 Å and 7500 Å. Finally the code ROTIN is used to reproduce rotational and instrumental broadening as well as limb darkening. In this manner we generate a grid of stellar photospheric models covering a wide range of effective temperatures and surface gravities.

The accretion disk spectra are generated by dividing the disk into rings, each with a given radius, temperature, effective surface (vertical) gravity, and density obtained from the standard disk model (Pringle, 1981). For each ring the code TLUSTY is then run to generate a one-dimensional vertical structure. This is then followed by a run of SYNSPEC to create a spectrum for each individual ring. The ring spectra are then combined using DISKSYN which includes the effects of Keplerian rotational broadening, inclination and limb darkening. All our disk models used here, as well as Wade & Hubeny (1998)’s disk models, have solar abundances.

A complete description of our recently-updated accretion disk modeling is given in Godon et al. (2017); Darnley et al. (2017) and Godon et al. (2018). The main implementation in our disk models is that the inner and outer radii of the disk are chosen to fit the parameters of the system that is being modeled. In the present case, the outer radius of the disk was extended to several hundred times the radius of the accreting white dwarf, where the disk temperature drops to $\sim$ 3,500 K. In the disks of Wade & Hubeny (1998), the outer disk extends only to where the temperature reaches 10,000 K. Nevertheless, because of the large mass accretion rate (and ensuing higher disk temperature) and the short wavelength coverage of FUSE, the inclusion of a more extended (and colder) outer disk does not contribute additional flux to the FUV (our new disk spectra models also extend into the optical where the larger outer disk significantly increases the flux).

In the present work we also use model accretion disks from the optically thick, steady state disk model grid of Wade & Hubeny (1998) as explained in the next section.

Since the FUSE spectrum of AG Peg is heavily affected by ISM molecular hydrogen absoprtion lines, we decided to model the ISM lines to obtain a better fit. Since our main purpose is not to assess the hydrogen atomic and molecular column densities, but only to improve our spectral fit, we use some of the ISM models we generated in Godon et al. (2009) and refer the reader to that work for further details.

3 Results

In the following, we adopt a distance of 800 pc, an inclination of $50^{\circ}$, a WD mass of about $0.65M_{\odot}$ and we expect the WD temperature to be at least of the order of 100,000 K (see Table 1). We carry out all the disk and photosphere model fits for these values.

3.1 NLTE WD Photosphere Analysis

The input gravity of the models was varied from $log(g)=6.0$ to $log(g)=8.0$ in steps of 0.5, and the WD temperature was varied from $\sim 40,000$ K to $\sim 200,000$K (in steps of 5,000 K and 10,000 K). We find that the best fit WD photosphere models are obtained for the lowest gravity ($log(g)=6.0,6.5$) in our grid of models and for a temperature $T_{\rm wd}=100,000$ K to 180,000 K. The radius of the WD is then obtained by scaling the theoretical spectrum obtained for a distance of 800 pc to the observed spectrum. The exact mass of the WD is uknown but it is likely around $\sim 0.65M_{\odot}$ (see Table 1), and an output value of the gravity, $log(g)$, can then be obtained and compared to the input value of $log(g)$ for self-consistency.

Explicitly, for $T_{\rm wd}=100,000$ K, the WD radius scaled to a distance of 800 pc is $R_{\rm wd}=55.5\times 10^{9}$cm ($\sim 0.08R_{\odot}$), giving $log(g)=6.44$ for an assumed WD mass of $M_{\rm wd}=0.65M_{\odot}$ (see Table 2). Values of $log(g)$ are also listed for a $0.4M_{\odot}$ and $1M_{\odot}$ WD masses and are consistant with the best fits obtained for $log(g)=6.0$ and $log(g)=6.5$.

For the hottest model $T_{\rm wd}=180,000$ K, the WD radius scaled to a distance of 800 pc is $R_{\rm wd}=39.6\times 10^{9}$cm ($0.057R_{\odot}$), giving $log(g)=6.74$ for a $0.65M_{\odot}$ WD mass (Table 2). The 100,000 K model is slightly defficient in flux around 930-950 Å, while the 180,000 K model provides slightly too much flux in that region. At longer wavelengths, the models provide the same fit. Among these models, the best-fitting photosphere (namely in the 930-950 Å region) has $T_{\rm wd}=150,000$ K, and the WD radius scaled to a distance of 800 pc is $R_{\rm wd}=42.9\times 10^{9}$cm ($0.0615R_{\odot}$), giving $log(g)=6.67$ for a $0.65M_{\odot}$ WD mass. This WD photosphere model fit to the FUSE spectrum is displayed in Fig.2, and includes the addition of ISM absorption lines. We note that our derived value of $T_{\rm wd}$ agrees with the multi-wavelength analysis of Skopal et al. (2017) who took $T_{\rm wd}=160,000$ K but did not model the FUSE spectrum explicitly. The low mass (and/or low gravity) of the white dwarf is consistent with the very small mass function derived from the full orbit solution of (Fekel et al., 2000).

To further check the self-consistency of these models, we compute the luminosity (see Table 2, last column). Our best-fit model has a luminosity of 1729$L_{\odot}$, which agrees with Skopal et al. (2017)’s estimate, which is not surprizing since the temperature is of the same order as Skopal et al. (2017)’s. However, our hotest model has a luminosity about twice as large which must be ruled out.

3.2 NLTE Accretion Disk Analysis

We extended our analysis of the FUSE spectrum to optically thick, steady state accretion disk models. We first used the grid of UV disk models by Wade & Hubeny (1998), and found out that AG Peg must have a high mass transfer rate if one ascribes all of the FUV flux to accretion luminosity. We tried the highest mass accretion rate models of Wade & Hubeny (1998) with $\dot{M}=10^{-8}M_{\odot}$/yr, $M_{\rm wd}=0.55M_{\odot}$ and $M_{\rm wd}=0.80M_{\odot}$, all with an inclination $i=41^{\circ}$. However, all these disk models yielded derived distances much shorter, and did not provide enough flux in the short wavelength range of FUSE ($\lambda<960$ Å). When linearly scaling these disk models to 800 pc , we found that the mass accretion rate should be of the order of $\dot{M}\approx 10^{-7}-10^{-6}M_{\odot}$/yr. Consequently, we generated accretion disk models for larger mass accretion rates using TLUSTY and SYNSPEC (Hubeny, 1988; Hubeny & Lanz, 1995) as described in the previous section.

We chose a WD mass $M_{\rm wd}=0.7M_{\odot}$, with a radius $R_{\rm wd}=8,500$ km, and an inclination $i=50^{\circ}$. The disk extends to where its temperature $T_{\rm disk}$ drops to $\approx 3,500$ K. For a given mass accretion rate the disk model scales to a given distance.

To match a distance of 800 pc we obtained that the mass accretion rate has to be $\dot{M}=1.8\times 10^{-7}M_{\odot}$/yr. This disk model has an inner radius of 8,500 km and an outer radius of 1 million km, and is presented in Figure 3. The fit in the shorter wavelength of FUSE is not as good as for the single hot WD model fit (Figure 2), as the model is deficient in flux.

We note that even if we increase the mass accretion to $\dot{M}=1\times 10^{-6}M_{\odot}$/yr, the fit in the short wavelength range is still not as good as the single WD component. This model (shown in Figure 4) leads a distance of 1320 pc.

We also tried combined WD + accretion disk models where the WD contributes a significant fraction of the flux, and the result was intermediate between the single WD models and the single disk models. Namely, the WD+disk models were improved over the single disk models, but did not provide a fit as good as the single WD models.

4 Discussion and Conclusions

On the basis of fitting very hot NLTE white dwarf photosphere models to the FUSE spectrum of AG Peg in quiescence and comparing with the best fitting high $\dot{M}$ ($10^{-7}-10^{-6}M_{\odot}$/yr) disk models, we report evidence that a very hot white dwarf with a temperature $T_{\rm wd}$ = 150,000K dominates the entire FUSE wavelength range while an accretion disk with high $\dot{M}$ gives a poorer fit in the short wavelengths ($\lambda<$ 960 Å). This hot WD model has a gravity $\log(g)\sim 6.5$, and the distance of 800 pc gives a scaled radius $R_{\rm wd}\sim 43\times 10^{9}$ cm ($\sim 0.06R_{\odot}$). The addition of an accretion disk only degrades the hot WD fit and implies that the WD is possibly over-shining the disk.

It is interesting to compare our results on the hot component of AG Peg using FUSE spectra with a recent multi-wavelength analysis by Skopal et al. (2017). They computed the peak luminosity of the June 2015 Z And-like outburst of AG Peg to be $2-11\times 10^{37}$ergs/s (for a distance of 0.8 kpc). They estimated that the white dwarf had to be accreting at $\sim 3\times 10^{-7}M_{\odot}$/yr, which they claimed exceeds the stable burning limit by which one assumes is the steady state limit in which the accreted material is burned at the same rate that it accretes.

Our best-fitting NLTE high gravity photosphere models were fit best with $T_{\rm wd}=150,000$ K which is in agreement with the modeling of the UV/Optical continuum in quiescence from 1993 and 1996 Skopal et al. (2017), and considerably lower than the hot component of AG Peg in outburst. The luminosity of our best-fit model also agrees with the quiescent luminosity estimate of Skopal et al. (2017). During the outburst, the effective radius of the hot component, $R_{h}$, varies between $\sim 0.09R_{\odot}$ and $\sim 0.15R_{\odot}$. The accretion rate they found, on average, was a factor of 2-3 larger during the outburst than during quiescence.

Note that Mürset et al. (1991) obtained temperatures and luminosities of the hot component in symbiotic novae using the modified Zanstra Method. For AG Peg they obtained a hot component temperature between 95,000K and 100,000K during the decline phase. However, we note that Mürset et al. (1991) adopted a shorter distance (650 pc vs 800 pc) as well as a smaller E(B-V) = 0.05 (vs 0.10) which leads to a lower effective temperature for the hot component (since the UV continuum is less steep and a much lower luminosity than that derived by Kenyon et al. (1993, 2001)). Other attempts assumed black bodies to fit the SED of AG Peg encompassing the optical all the way to the IUE SWP range. These efforts obtained temperatures significantly cooler than the temperature of 150,000 K that we obtained from the best fitting NLTE model atmosphere to the FUSE spectrum.

It is interesting to note that our method of deriving the temperature of the hot component gives a temperature ($Log(T_{\rm eff})=5.17$) similar to the temperature obtained from an analysis of the optical H i and He ii lines (see Fig.5 in Kenyon et al., 2001), but much higher than the temperature derived from an analysis of the UV lines or the UV continuum. The luminosity we derive here is significantly larger than derived by these 3 methods (Kenyon et al., 2001). In the present work we dereddened the FUSE spectrum using the extinction law given by Fitzpatrick & Massa (2007), while Kenyon et al. (2001) adopted Mathis (1990)’s extinction curve (which itself is the Cardelli et al. (1989) extinction law). In the FUV region the difference between Cardelli et al. (1989) and Fitzpatrick & Massa (2007) extinction laws increases with decreasing wavelength and can gives different results (see e.g. Selvelli & Gilmozzi, 2013). It is possible that the difference in the extinction laws in the shortest wavelengths of FUSE increased the flux (in the dereddened spectrum) to better match a 150,000 K model, since the difference between the 100,000 K model and the 150,000 K model is very small. A temperature closer to 100,000 K gives a luminosity in better agreement with the quiescent luminosity.

It is also possible that the observed difference in temperature is due to a real change in the WD surface temperature. The short wavelength range of the FUSE spacecraft down to the Lyman Limit probes the radiation from the innermost disk/boundary layer/white dwarf, which likely accounts for the higher temperature that we obtain. No previous attempts to derive a temperature for the hot component in AG Peg covered a wavelength range down to the Lyman Limit.

Overall, our result suggests the possibility that the system is dominated by a very hot, low gravity white dwarf. This is similar to our analysis of the FUSE spectrum of the S-Type symbiotic star RW Hydrae (Sion et al., 2017), which indicates it also contains a very hot, bare white dwarf with no evidence of an accretion disk. An important question now is whether the Z And-type outbursts are due to bursts of accretion from the M3/4 III companion or if they are also thermonclear powered like the 1850 nova outburst.

On the other hand, the actual structure of an accretion disk surrounding a hot stellar component in symbiotic binary is poorly understood compared to catalcysmic variables. First, the scale of a symbiotic binary relative to a cataclysmic variable is vastly larger and second, the mass transfer occurs via a red giant donor which may or may not fill its Roche lobe. This points to the possibility that the disk models might be different than the standard disk model. 3D hydrodynamic model simulations of symbiotic binaries (de Val-Borro et al., 2009, 2017) reveal that a disk-like structure resembling a steady-state disk does appear to form, but the thin disk approximation may break down in symbiotics. Given the uncertainty in what a true symbiotic accretion disk would look like, and that a very hot low gravity degenerate star dominates the FUV flux, the presence of a steady-state (standard) accretion disk cannot be summarily ruled out nor denied.

However, we note that for the hot WD to contribute a significant fraction of the total flux to the FUSE spectrum, in addition to that of an accretion disk, its emitting photosphere radius has to be inflated (Section 3.1). With a radius of the order of $R_{\rm wd}\sim 0.05R_{\odot}$, rather than $R_{\rm wd}\sim 0.01R_{\odot}$, the accretion disk temperature drops significantly (Godon et al., 2017). For example, at a mass accretion rate of $\dot{M}=10^{-7}M_{\odot}$/yr the peak temperature in the disk reaches 103,000 K if $R_{\rm wd}=8,500$ km (which is the radius of a $\sim 30,000$ K WD of mass $0.7M_{\odot}$). However, for a WD radius of $R_{\rm wd}=35,000$ km $\approx 0.05R_{\odot}$, the peak temperature of such a disk drops to 36,500 K. At a mass accretion rate of $\dot{M}=10^{-6}M_{\odot}$/yr, the disk peak temperature drops from $\sim$171,000 K to $\sim 65,000$ K. In both cases ($\dot{M}=10^{-7}M_{\odot}$/yr and $\dot{M}=10^{-6}M_{\odot}$/yr) the disk will contributes little flux to the FUSE spectrum if $R_{\rm wd}=0.05R_{\odot}$. As the accretion luminosity decreases like $\propto 1/R_{\rm wd}$, so does the disk luminosity, and as long as the WD radius remains inflated it will dominate the FUV spectrum.

This work is supported by funding from the National Aeronautics and Space Administration (NASA) under grant number NNX17AF36G issued through the Office of Astrophysics and DATA Analysis Program (ADAP) to Villanova University. This study has also been supported in part by the National Science Centre, Poland, grant OPUS 2017/27/B/ST9/01940. PG is pleased to thank William (Bill) P. Blair at the Henry Augustus Rowland Department of Physics & Astronomy at the Johns Hopkins University, Baltimore, Maryland, USA, for his kind hospitality. We made use of online data from the AAVSO and we are thankful to the AAVSO and its members worldwide for their constant monitoring of CVs and for making their data public.

Patrick Godon https://orcid.org/0000-0002-4806-5319 Joanna Mikolajewska https://orcid.org/0000-0003-3457-0020