Optically visible post-AGB stars, post-RGB stars and young stellar objects in the Large Magellanic Cloud

MNRAS 454, 1468–1502 (2015) doi:10.1093/mnras/stv1202 Optically visible post-AGB stars, post-RGB stars and young stellar objects in the Large Magellanic Cloud D. Kamath,1‹ P. R. Wood2 and H. Van Winckel1 1 Instituut voor Sterrenkunde, K.U.Leuven, Celestijnenlaan 200D bus 2401, B-3001 Leuven, Belgium School of Astronomy and Astrophysics, Mount Stromlo Observatory, Weston Creek, ACT 2611, Australia 2 Research Accepted 2015 May 27. Received 2015 May 25; in original form 2015 February 21 We have carried out a search for optically visible post-asymptotic giant branch (post-AGB) stars in the Large Magellanic Cloud (LMC). First, we selected candidates with a mid-IR excess and then obtained their optical spectra. We disentangled contaminants with unique spectra such as M stars, C stars, planetary nebulae, quasi-stellar objects and background galaxies. Subsequently, we performed a detailed spectroscopic analysis of the remaining candidates to estimate their stellar parameters such as effective temperature, surface gravity (log g), metallicity ([Fe/H]), reddening and their luminosities. This resulted in a sample of 35 likely post-AGB candidates with late-G to late-A spectral types, low log g, and [Fe/H] < −0.5. Furthermore, our study confirmed the existence of the dusty post-red giant branch (postRGB) stars, discovered previously in our Small Magellanic Cloud survey, by revealing 119 such objects in the LMC. These objects have mid-IR excesses and stellar parameters (Teff , log g, [Fe/H]) similar to those of post-AGB stars except that their luminosities (< 2500 L ), and hence masses and radii, are lower. These post-RGB stars are likely to be products of binary interaction on the RGB. The post-AGB and post-RGB objects show spectral energy distribution properties similar to the Galactic post-AGB stars, where some have a surrounding circumstellar shell, while some others have a surrounding stable disc similar to the Galactic post-AGB binaries. This study also resulted in a new sample of 162 young stellar objects, identified based on a robust log g criterion. Other interesting outcomes include objects with an UV continuum and an emission line spectrum; luminous supergiants; hot main-sequence stars; and 15 B[e] star candidates, 12 of which are newly discovered in this study. Key words: techniques: photometric – techniques: spectroscopic – stars: AGB and postAGB – stars: fundamental parameters – planetary nebulae: general – Magellanic Clouds. 1 I N T RO D U C T I O N Post-asymptotic giant branch (post-AGB) stars are low- to intermediate-mass stars that have evolved off the asymptotic giant branch (AGB) because a strong dusty mass-loss has removed almost the whole stellar envelope. For single stars, this mass-loss occurs during a phase of very high mass-loss rates called the superwind phase (e.g. Vassiliadis & Wood 1993). During the post-AGB phase, which lasts for around ∼102 –105 yr (depending on the core mass), the radius of the star decreases and the star evolves to higher temperatures (from 3 × 103 K to >2 × 104 K) with a constant luminosity (e.g. Vassiliadis & Wood 1993, 1994). When the hot central star ionizes the surrounding mass-loss shell, the object is referred to as a  E-mail: planetary nebula (PN; Balick & Frank 2002). During the post-AGB phase, the warm stellar photosphere makes it possible to quantify the chemical abundances in post-AGB stars for a very wide range of elements from CNO up to the most heavy s-process elements, well beyond the Ba peak (Reyniers & Van Winckel 2003), that are brought to the stellar surface during the AGB phase (Karakas & Lattanzio 2007, 2014). For low- to intermediate-mass AGB stars in binary systems, massloss can be induced via binary interaction (e.g. Waters, Waelkens & Van Winckel 1997; Chen, Han & Tout 2011; Nie, Wood & Nicholls 2012; Abate et al. 2013; Soker 2014). A significant fraction of the ejected matter may end up in a circumbinary disc of dust and gas, and inside the disc is a binary system containing the postAGB star (e.g. de Ruyter et al. 2006). Binary interaction alters the intrinsic properties (such as: chemical composition, pulsation, mass-loss, dust-formation, circumstellar envelope morphology etc.)  C 2015 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society ABSTRACT Post-AGBs, post-RGBs and YSOs in the LMC Until recently, in the Small Magellanic Cloud (SMC), only five post-AGB candidates had been identified (Whitelock et al. 1989; Kučinskas et al. 2000; Kraemer et al. 2006; Volk et al. 2011; De Smedt et al. 2012). Similarly, in the Large Magellanic Cloud (LMC), Wood & Cohen (2001) had identified around 25 post-AGB stars based on a near-IR photometric and optical spectroscopic study of point sources from the Mid-course Space Experiment IR survey (Egan, Van Dyk & Price 2001). Subsequently, based on mid-IR and optical photometry, van Aarle et al. (2011) constructed a catalogue of 1337 optically visible post-AGB candidates in the LMC. They carried out a classification based on visual analysis of optical spectra of 105 of these candidates which resulted in 70 post-AGB candidates in the LMC. Recently, Matsuura et al. (2014) confirmed the evolutionary nature of seven post-AGB stars that were previously identified by Wood & Cohen (2001) and van Aarle et al. (2011). Based on polycyclic aromatic hydrocarbon (PAH) features that were found in Spitzer Space Telescope (SST) IRS spectra of these objects, they characterized the circumstellar dust which resulted in five objects being classified as C-rich post-AGB stars and two being classified as O-rich post-AGB stars. To fully understand the post-AGB star population in the MCs and to provide constraints for stellar evolutionary and nucleosynthesis models, a more complete sample of these objects is definitely required. Therefore, in our recent studies, we exploited the release of the mid-IR SST surveys SAGE for the LMC (Blum et al. 2006; Meixner et al. 2006) and S3 MC (Bolatto et al. 2007) and SAGESMC (Gordon et al. 2011) for the SMC to identify post-AGB stars in the MCs with mid-IR excesses indicative of a past history of heavy dusty mass-loss. We also performed an extensive low-resolution optical spectroscopic survey to systematically characterize the optically bright post-AGB stars in the SMC and LMC. The results of our SMC study are presented in Kamath, Wood & Van Winckel (2014, hereafter referred to as Paper I). That study provided the first extensive spectroscopically verified catalogue of optically visible post-AGB candidates in the SMC. We were able to find a sample of 21 likely post-AGB candidates. These objects showed SED properties similar to the Galactic post-AGB stars mentioned above. The SMC study also resulted in the discovery of a sample of 42 new, low-luminosity, dusty objects that are likely to be post-red giant branch (post-RGB) candidates. These objects have mid-IR excesses, stellar parameters and SEDs similar (...truncated)