Evolved Massive Stars at Low-metallicity I. A Source Catalog for the Small Magellanic Cloud

Abstract

We present a clean, magnitude-limited (IRAC1 or WISE1 ≤ 15.0 mag) multiwavelength source catalog for the Small Magellanic Cloud (SMC) with 45,466 targets in total, with the purpose of building an anchor for future studies, especially for the massive star populations at low-metallicity. The catalog contains data in 50 different bands including 21 optical and 29 infrared bands, retrieved from SEIP, VMC, IRSF, AKARI, HERITAGE, Gaia, SkyMapper, NSC, Massey et al. (2002), and GALEX, ranging from the ultraviolet to the far-infrared. Additionally, radial velocities and spectral classifications were collected from the literature, as well as infrared and optical variability statistics were retrieved from WISE, SAGE-Var, VMC, IRSF, Gaia, NSC, and OGLE. The catalog was essentially built upon a 1 crossmatching and a 3 deblending between the Spitzer Enhanced Imaging Products (SEIP) source list and Gaia Data Release 2 (DR2) photometric data. Further constraints on the proper motions and parallaxes from Gaia DR2 allowed us to remove the foreground contamination. We estimated that about 99.5% of the targets in our catalog were most likely genuine members of the SMC. By using the evolutionary tracks and synthetic photometry from MESA Isochrones & Stellar Tracks and the theoretical J − K S color cuts, we identified 1,405 red supergiant, 217 yellow supergiant and 1,369 blue supergiant candidates in the SMC in five different color-magnitude diagrams (CMDs), where attention should also be paid to the incompleteness of our sample. We ranked the candidates based on the intersection of different CMDs. A comparison between the models and observational data shows that the lower limit of initial mass for the RSGs population may be as low as 7 or even 6 M and the RSG is well separated from the AGB population even at faint magnitude, making RSGs a unique population connecting the evolved massive and intermediate stars, since stars with initial mass around 6 to 8 M are thought to go through a second dredge-up to become asymptotic giant branch stars. We encourage the interested reader to further exploit the potential of our catalog.

A dearth of young and bright massive stars in the Small Magellanic Cloud

“…For this, we used two criteria. First, we removed stars from the catalog that did not match the proper motion criteria of Yang et al (2019). These authors found that the SMC proper motion distributions fit Gaussians with µ ra = 0.695 ± 0.240 in the right ascension direction and µ dec = −1.206 ± 0.140 in the declination direction well.…”

“…9, that is, those with evolutionary masses in excess of 18 M and ages younger than 10 Myr. For each of these sources, we calculated their proper motion relative to the SMC by subtracting an SMC bulk motion of µ ra = 0.695 mas/yr and µ ra = −1.206 mas/yr (Yang et al 2019) from their GAIA proper motions. Then, we used their inferred age (Sect.…”

A census of OBe stars in nearby metal-poor dwarf galaxies reveals a high fraction of extreme rotators

“…For the SMC, the method is similar to the one from Schootemeijer et al (2021): sources with a GAIA parallax over parallax error value larger than five are removed, as are sources that do not fit the proper motion criteria. More specifically, for the SMC we remove sources more than 2σ away from its bulk proper motion (µ), for which we adopt the values from Yang et al (2019): µ ra = 0.695 ± 0.240 mas/yr (Right Ascension (R.A.)) and µ dec = −1.206 ± 0.140 mas/yr (Declination (Dec.)). For the LMC, we take the same approach but for its bulk motion we adopt µ ra = 1.750 ± 0.375 mas/yr and µ dec = 0.250 ± 0.375 mas/yr, based on Fig.…”