Optically visible post-AGB/RGB stars and young stellar objects in the Small Magellanic Cloud: candidate selection, spectral energy distributions and spectroscopic examination

Abstract

We have carried out a search for optically visible post-AGB candidates in the Small Magellanic Cloud (SMC). Firstly, we used mid-IR observations from the Spitzer Space Telescope to select optically visible candidates with excess mid-IR flux and then we obtained low-resolution optical spectra for 801 of the candidates. After removing poor quality spectra and contaminants such as M-stars, C-stars, planetary nebulae, quasistellar objects and background galaxies, we ended up with a final sample of 63 high probability post-AGB/RGB candidates of A − F spectral type. From the spectral observations, we estimated the stellar parameters: effective temperature (T eff ), surface gravity (log g), and metallicity ([Fe/H]). We also estimated the reddening and deduced the luminosity using the stellar parameters combined with photometry. For the post-AGB/RGB candidates, we found that the metallicity distribution peaks at [Fe/H] ≈ -1.00 dex. Based on a luminosity criterion, 42 of these 63 sources were classified as post-RGB candidates and the remaining 21 as post-AGB candidates. From the spectral energy distributions we were able to infer that 6 of the 63 post-AGB/RGB candidates have a surrounding circumstellar shell suggesting that they are single stars, while 27 of the post-AGB/RGB candidates have a surrounding disc, suggesting that they lie in binary systems. For the remaining 30 post-AGB/RGB candidates the nature of the circumstellar environment was unclear. Variability is displayed by 38 of the 63 post-AGB/RGB candidates with the most common variability types being the Population II Cepheids (including RV-Tauri stars) and semi-regular variables. This study has also revealed a new RV Tauri star in the SMC, J005107.19-734133.3, which shows signs of s-process enrichment. From the numbers of post-AGB/RGB stars in the SMC, we were able to estimate evolutionary rates. We find that the number of post-AGB and post-RGB candidates that we have identified are in good agreement with the stellar evolution models with some mass loss in the post-AGB phase and a small amount of re-accretion in the lower luminosity post-RGB phase. This study also resulted in a new sample of 40 young stellar objects (YSOs) of A − F spectral type. The 40 YSO candidates for which we could estimate stellar parameters are luminous and of high mass (∼ 3 − 10 M ). They lie on the cool side of the usually adopted birthline in the HR-diagram. This line separates visually obscured protostars from optically visible pre-main sequence stars, meaning that our YSO candidates have become optically visible in the region of the HR-diagram usually reserved for obscured protostars. Additionally, we also identified a group of 63 objects whose spectra are dominated by emission lines and in some cases, a significant UV continuum. These objects are very likely to be either hot post-AGB/RGB candidates or luminous YSOs.

Chemical abundance study of two stronglys-process enriched post-AGB stars in the LMC: J051213.81-693537.1 and J051848.86-700246.9

“…van Aarle et al (2011) and Kamath et al (2015), provide catalogues of spectroscopically verified optically visible post-AGB candidates in the Large Magellanic Cloud (LMC) and Kamath et al (2014) provides a catalogue of spectroscopically verified optically visible post-AGB candidates in the Small Magellanic Cloud (SMC).…”

Properties of Anomalous and Type II Cepheids in the Small and Large Magellanic Clouds

“…What was surprising is the detection of IR excess among the WVir stars below luminosities predicted by single star evolutionary tracks. This IR excess was independently found by [7][8][9] to be dusty post-RGB stars and possibly related to binary evolution.…”

An N Band Interferometric Survey of The Disks Around Post-AGB Binary Stars

“…One of the simplest arguments for the disk interpretation of the infrared excess around a large fraction of the known post-AGB stars (Kamath et al, 2014) is that they are too hot and too evolved to still produce and expell dust (van Winckel, 2003). The shape of the infrared excess is a critical quantity to distinguish between a post-AGB star that is surrounded by an expanding shell or by a Keplerian disk: the latter have a characteristic near-IR excess because some of the dust is orbiting close to the central object.…”