New Magellanic Cloud R Coronae Borealis and DY Persei type stars from the EROS-2 database: the connection between RCBs, DYPers, and ordinary carbon stars

Abstract

Context. R Coronae Borealis stars (RCB) are a rare type of evolved carbon-rich supergiant stars that are increasingly thought to result from the merger of two white dwarfs, called the Double degenerate scenario. This scenario is also studied as a source, at higher mass, of type Ia Supernovae (SnIa) explosions. Therefore a better understanding of RCBs composition would help to constrain simulations of such events. Aims. We searched for and studied RCB stars in the EROS Magellanic Clouds database. We also extended our research to DY Per type stars (DYPers) that are expected to be cooler RCBs (T ∼ 3500 K) and much more numerous than their hotter counterparts. With the aim of studying possible evolutionary connections between RCBs and DYPers, and also ordinary carbon stars, we compared their publically available broad band photometry in the optical, near, and mid-infrared. Methods. The light curves of ∼70 millions stars, monitored for 6.7 years (from July 1996 to February 2003), have been analysed to search for the main signature of RCBs and DYPers: a large (up to 9 mag) drop in luminosity. Carbon stars with fading episodes were also found by inspecting numerous light curves of objects that presented an infrared excess in the 2MASS and Spitzer-SAGE and S 3 MC databases. Follow-up optical spectroscopy was used to confirm each photometric candidate found. Results. We have discovered and confirmed 6 new Magellanic Cloud RCB stars and 7 new DYPers, but also listed new candidates: 3 RCBs and 14 DYPers. Optical and infrared colour magnitude diagrams that give new insights into these two sets of stars are discussed. We estimated a range of Magellanic RCB shell temperatures between 360 and 600 K. Conclusions. We confirm the wide range of absolute luminosity known for RCB stars, M V ∼ −5.2 to -2.6. Our study further shows that mid-infrared surveys are ideal to search for RCB stars, since they have thinner and cooler circumstellar shells than classical post-AGB stars. In addition, by increasing the number of known DYPers by ∼400%, we have been able to shed light on the similarities in the spectral energy distribution between DYPers and ordinary carbon stars. We also observed that DYPer circumstellar shells are fainter and hotter than those of RCBs. This suggests that DYPers may simply be ordinary carbon stars with ejection events, but more abundance analysis is necessary to give a status on a possible evolutionnary connexion between RCBs and DYPers.Key words. stars: AGB and post-AGB -stars: carbon -supergiants -Magellanic Clouds Based on observations made with the CNRS/INSU MARLY telescope at the European Southern Observatory, La Silla, Chile.Figures 7-13 are only available in electronic form at

Long period variables in the Large Magellanic Cloud from the EROS-2 survey

“…We identified 32 variables of R Coronae Borealis (RCB) and DY Per types already known in EROS-2 and listed in Tisserand et al (2009). We show them in Fig.…”

“…In the light curves provided, this artifact was corrected to some extent (cf. Tisserand 2004) and explains the smaller number of stars with a period of around one year, as indicated by the vertical dashed line in Fig. 15.…”

A new search for R Coronae Borealis stars in the SMC

“…11]), identified using OGLE data ( [7]), strongly reminiscent of an RCB star, and which we accordingly classify as a "transitional" DY Per/RCB star (see [7] for more details). EROS2 project ( [11]) were also analyzed. Their names, coordinates and light curves can be found in [7].…”

HERSCHELOBSERVATIONS OF A NEWLY DISCOVERED UX Ori STAR IN THE LARGE MAGELLANIC CLOUD

“…The EROS-2 star, lm056-0K-21075 (also MACHO-9.4391.25), was found to be variable with irregular declines reminiscent of an R Coronae Borealis (RCB) star (Tisserand et al 2009). However, its visible spectrum is rich in hydrogen lines so this star is not a hydrogendeficient RCB star.…”

“…A finding chart is shown in Figure 1. Figure 2 shows the visible spectrum of SAGE050756-703453, obtained on JD 2454510.0 with the Dual-Beam Spectrograph (DBS) (Rodgers et al 1988) attached to the ANU 2.3m telescope at Siding Spring Observatory (Tisserand et al 2009). The visible wavelength band is split by a dichroic at around 6000Å and feeds two spectrographs, with red and blue optimized detectors.…”