GK Car and GZ Nor: Two low-luminous, depleted RV Tauri stars

TL;DR

This study analyzes two RV Tauri stars, GK Car and GZ Nor, using photometric and spectroscopic data to determine their properties, circumstellar environment, and possible evolutionary status, suggesting they are likely post-RGB or post-AGB stars.

Contribution

First detailed combined photometric and spectroscopic analysis of GK Car and GZ Nor, revealing their circumstellar dust, depletion patterns, and potential binary nature, with implications for their evolutionary classification.

Findings

Both stars are surrounded by hot circumstellar dust.

Distances and luminosities are consistent between Gaia and PLC methods.

Stars show refractory element depletion and possible binary status.

Abstract

We performed a photometric and spectroscopic analysis of two RV Tauri stars GK Car and GZ Nor. Both objects are surrounded by hot circumstellar dust. Their pulsation periods, derived from ASAS photometric time series, have been used to derive their luminosities and distances via the PLC relation. In addition, for both objects, GAIA distances are available. The Gaia distances and luminosities are consistent with the values obtained from the PLC relationship. GK Car is at distance of 4.5$\pm{1.3}$ kpc and has a luminosity of 1520$\pm{840}$ L$_{\odot}$, while GZ Nor is at distance of 8.4$\pm{2.3}$ kpc and has a luminosity of 1240 $\pm{690}$ L$_{\odot}$. Our abundance analysis reveals that both stars show depletion of refractory elements with [Fe/H]=$-$1.3 and [Zn/Ti]=$+$1.2 for GK Car and [Fe/H]=$-$2.0 and [Zn/Ti]=$+$0.8 for GZ Nor. In the WISE colour-colour diagram, GK Car is located in the RV Tauri box as originally defined by \cite{evans85} and updated by \cite{gezer15}, while GZ Nor is not. Despite this, we argue that both objects are surrounded by a gravitationally bound disc. As depletion is observed in binaries, we postulate that both stars are binaries as well. RV Tauri stars are generally acknowledged to be post$-$AGB stars. Recent studies show that they might be either indeed post$-$AGB or post$-$RGB objects depending on their luminosity. For both objects, the derived luminosities are relatively low for post-AGB objects, however, the uncertainties are quite large. We conclude that they could be either post-RGB or post-AGB objects.

Tables

Table 1. Table 1: Basic parameters of GK Car and GZ Nor are from SIMBAD. WISE colours and SED types are from our previous study ( Gezer et al. , 2015 ) .

Name	R.A.
	Decl.
	l	b	[12]$-$[22]	[3.4]$-$[4.6]	SED
GK Car	11 14 01.61	$-$57 43 15	290.1955	$+$02.7218	1.301	1.448	Disc
GZ Nor	16 31 54.15	$-$55 33 07	330.6957	$-$05.0546	2.453	0.477	Uncertain

Table 2. Table 2: Photometry info. No. refers to number of observational points.

Name	mv	mI	No.	$\Delta T$ (day)
GK Car	11m.34	10m.15	530	3283
GZ Nor	13m.13	11m.64	417	3166

Table 3. Table 4: The fundamental pulsation period (P 0 ) is given in Col.2. Col. 3 is the intrinsic (V − - I) 0 values. Col. 4 is the derived E(B-V) values towards each object obtained from the reddened SED model. Calculated distances using PLC relation are shown in Cols. 5 and luminosities which are calculated using the distances in Col. 5 are given in Col.6.

Star	Ppuls	(V-I)0	E(B-V)	DistancePLC	LSED
	(days)			(kpc)	(L⊙)
GK Car	27.6, 55.2	0.624	0.41$\pm 0.1$	4.54$\pm$1.26	1520$\pm 840$
GZ Nor	36.2, 72.4	0.910	0.42$\pm 0.1$	8.38$\pm$2.32	1240$\pm 690$

Table 4. Table 5: The GAIA distance is given in Col.2, lower and upper limits in the distances are given in Col.3 and 4, respectively ( Bailer-Jones et al. , 2018 ) . Col.5, 6 and 7 are luminosities, which are calculated using the distances in Col.2, 3 and 4, respectively.

Star	Distance	Distance	Distance	LSED	LSED	LSED
		(lower)	(upper)		(lower)	(upper)
	(kpc)	(kpc)	(kpc)	(L⊙)	(L⊙)	(L⊙)
GK Car	3.782	3.376	4.293	1050	837	1352
GZ Nor	8.800	6.630	12.062	1367	776	2568

Table 5. Table 6: Abundance analysis of GK Car. The solar abundances to compute the [X/H] ratios are taken from Asplund et al. ( 2009 ) . The dust condensation temperatures, which is the temperature at which half of the particles of the element are condensed onto dust grains, under the assumption of chemical equilibrium, at constant pressure of 10 -4 bar in a solar mixture, are from Lodders et al. ( 2009 ) .

		GK Car
		Teff $=$ 5500 K
		log$g$ $=$ 1.0
		${\xi }_t$ $=$ 5.5 km/s
		[Fe/H] $=$ $-$1.32
ion	N	[X/H]	$\sigma$	Tcond
CI	5	0.06	0.16	40
NaI	4	$-$0.31	0.15	958
MgI	2	$-$0.92	0.18	1354
SiI	3	$-$0.87	0.03	1354
SI	3	$-$0.16	0.1	664
CaI	7	$-$1.46	0.20	1659
ScII	4	$-$1.72	0.07	1659
TiII	15	$-$1.63	0.14	1582
CrII	8	$-$1.15	0.17	1291
MnI	6	$-$1.17	0.09	1150
FeI	77	$-$1.32	0.11	1334
FeII	8	$-$1.3	0.09
NiI	15	$-$1.26	0.08	1353
CuI	3	$-$0.81	0.13	1037
ZnI	3	$-$0.39	0.14	726
YII	3	$-$1.56	0.16	1659
ZrII	3	$-$1.23	0.05	1764
BaII	2	$-$1.59	0.17	1162
LaII	2	$-$1.99	0.05	1578
CeII	5	$-$1.51	0.18	1487
NdII	4	$-$1.46	0.1	1602

Table 6. Table 8: Luminosities at the tip of the RGB from evolutionary tracks of Bertelli et al. ( 2009 ) for a range of initial masses and metallicities.

mass	Z = 0.017	Z = 0.004	Z = 0.002
1M⊙	3160L⊙	2880L⊙	2640L⊙
2M⊙	855L⊙	335L⊙	310L⊙
3M⊙	540L⊙	665L⊙	690L⊙

Equations

$$M_{\mathrm{v}}=-2.53(\pm 0.03)\log P-1.17(\pm 0.03)+2.55⟨(V-I{)}_0$$

Full text

††pubyear: 2019††pagerange: GK Car and GZ Nor: Two low-luminous, depleted RV Tauri stars–2

1 Introduction

RV Tauri stars are population II Cepheid variables with spectral types typically between F and K. There are 126 RV Tauri stars known in the Galaxy (see GCVS Samus et al. (2009)) and many of them have been discovered in the Small Magellanic Cloud (SMC) and Large Magellanic Cloud (LMC) (Alcock et al., 1998; Soszyński et al., 2008; Buchler et al., 2009; Soszyñski et al., 2010). They owe their name to the prototype RV Tau. A defining characteristic of RV Tauri stars is the presence of subsequent deep and shallow minima in their light curves (e.g. Pollard et al., 1997). The period between two successive deep minima is called the “formal” period and lies in the range 20-150 days. The period between a deep and shallow minimum is typically half of the formal period and is called the “fundamental” period (Soszyński et al., 2008; Soszyñski et al., 2010). There are several hypotheses for their pulsation mechanism (Tuchman et al., 1993). Non-linear, non-adiabatic hydrodynamical RV Tauri models show that the alternating minima in the light curves might be explained with double mode pulsations between the fundamental mode and the first overtone (Fokin, 1994). In addition, it can also be explained using chaotic systems (Buchler & Kovacs, 1987).

IRAS (Infrared Astronomical Satellite 1983) detected several RV Tauri stars which show large infrared excesses (IR) due to thermal emission from dust. On the basis of this finding, RV Tauri stars were classified as post-Asymptotic Giant Branch (post$-$AGB) objects by Jura (1986). As post-AGB stars, they are expected to display AGB nucleosynthesis products, which are mostly C and \it{s}$$-process elements. However, most RV Tauri stars that have been chemically studied to date show no traces of C and \it{s}$$-process enhancements, with the exceptions of HD 158616 (Van Winckel, 1997), MACHO 47.2496.8 (Reyniers et al., 2007) and SMC$-$T2CEP$-$018 (SMC J005107.19$-$734133.3) (Kamath et al., 2014). Instead, RV Tauri stars show often a chemical anomaly which is called “depletion” (e.g. Giridhar et al., 1994; Giridhar et al., 1998, 2000; Gonzalez et al., 1997b, a; Van Winckel et al., 1998; Maas et al., 2002; Maas et al., 2003, 2005; Deroo et al., 2005; Gezer et al., 2015). In a depleted photosphere, refractory elements, which have high dust condensation temperature, are underabundant, while volatiles, which has low dust condensation temperature, are more abundant (e.g. Van Winckel, 2003). It is assumed that the dust formation causes chemical fractionation in the circumstellar environment and the radiation pressure on dust grains separates the dust from the gas. The cleaned gas is re-accreted onto the stellar surface. Such a process seems to occur only in a stable circumstellar disc (Waters et al., 1992). The most characteristic chemical signatures of depleted photospheres are high [Zn/Fe], [Zn/Ti] and [S/Ti] ratios, which are used to identify a photosphere to be depleted or not (Gezer et al., 2015).

Lloyd Evans (1985) showed that IR colours of RV Tauri stars occupy a specific region in the IRAS [12]-[25], [25]-[60] diagram and he defined this region as the RV Tauri box. In our first paper (see Gezer et al. (2015), hereafter Paper I), we have expanded his study using WISE (Wide-Field Infrared Survey) data, which is similar to IRAS but deep enough to detect all Galactic RV Tauri stars. We defined a new WISE colour combination as [3.4]-[4.6] versus [12]-[22], which is a good WISE alternative of IRAS colour-colour diagram. We also performed a detailed spectral energy distribution (SED) analysis for all Galactic RV Tauri stars and showed that they display three different types of SED characteristics; disc, non-IR and uncertain types. This classification is shown in Fig. 1. A disc source displays a very distinctive SED with the IR-excess starting in the near-IR region and peaking around 10 $\mu$m. This type of SED is a clear indication of the presence of a stable compact dusty disc (de Ruyter et al., 2006; Deroo et al., 2006; Deroo et al., 2007; Gielen et al., 2011; Hillen et al., 2015). Disc-type SEDs occupy a specific region in the WISE colour-colour diagram and this region is equivalent to the RV Tauri box defined by Lloyd Evans (1985) in the IRAS colour-colour diagram. Thus, we slightly redefined the RV Tauri box as a disc box in the WISE colour-colour diagram. RV Tauri stars which do not show any infrared excess, cluster around the zero-point of the diagram, and we define this region as the non-IR box. For the remaining RV Tauri stars, the full SED is not clear, which is likely because of the large amplitude pulsations or/and a poor photometric sampling. Besides, these sources are mostly located outside of the disc and non-IR boxes in the WISE colour-colour diagram. Therefore, we classify these sources as uncertain.

Recently, Manick et al. (2018) also showed that the SMC and LMC RV Tauri stars display similar IR characteristics as their Galactic counterparts. The authors performed a systematic study of RV Tauri stars in the SMC/LMC and they yield their luminosities using the known distance to the Magellanic Clouds. They also interpret the evolutionary nature of RV Tauri stars using their luminosities. Objects which display disc-type SEDs are very likely binaries and they have a wide range of luminosities. Luminous binaries are likely post$-$AGB objects while the low luminous ones are probably post-red giant branch (post-RGB) objects. Post-RGB objects are a new class of dusty objects discovered in SMC/LMC surveys (Kamath et al., 2014, 2015). These objects are similar to post$-$AGB objects in terms of their stellar parameters and infrared excess, except that they display lower luminosities (100-2500 L*⊙*) than post$-$AGB stars. These stars very likely evolve off the RGB because of a strong binary interaction process already occurring on the RGB (Kamath et al., 2016).

In this paper, we selected two as yet poorly studied RV Tauri stars which have different SED characteristics and locations in the WISE colour-colour diagram. The one is a disc source GK Car which is located in the disc box and another one is GZ Nor which is outside of the disc box and defined as uncertain. Their positions in the WISE colour-colour diagram are shown in Fig. 1. The outline of the paper is as follows: Photometric and spectroscopic data and SEDs are presented in Section 2. Section 3 describes the photometric analysis including the pulsations and determination of luminosity and distance based on two methods; the Period-Luminosity-Distance (PLC) relationship and GAIA. Determination of atmospheric parameters and abundance analysis of the stars are presented in Section 4. In Section 5 conclusions are summarized.

2 Data

Basic parameters, WISE colours and SED types of the stars are given in Table 2.