The population of hot subdwarf stars studied with Gaia

Abstract

Based on data from the ESA Gaia Data Release 2 (DR2) and several ground-based, multi-band photometry surveys we have compiled an all-sky catalogue of 39 800 hot subluminous star candidates selected in Gaia DR2 by means of colour, absolute magnitude, and reduced proper motion cuts. We expect the majority of the candidates to be hot subdwarf stars of spectral type B and O, followed by blue horizontal branch stars of late B-type (HBB), hot post-AGB stars, and central stars of planetary nebulae. The contamination by cooler stars should be about 10%. The catalogue is magnitude limited to Gaia G < 19 mag and covers the whole sky. Except within the Galactic plane and LMC/SMC regions, we expect the catalogue to be almost complete up to about 1.5 kpc. The main purpose of this catalogue is to serve as input target list for the large-scale photometric and spectroscopic surveys which are ongoing or scheduled to start in the coming years. In the long run, securing a statistically significant sample of spectroscopically confirmed hot subluminous stars is key to advance towards a more detailed understanding of the latest stages of stellar evolution for single and binary stars.

Gaia Data Release 2 catalogue of extremely low-mass white dwarf candidates

“…Same as Fig. 6, but for the colours (r − i) × (i − z).tamination by low-mass (0.30 − 0.45 M⊙) or canonical mass white dwarfs in the sample, as we further discuss below.Our catalogue has overlap with the white dwarf catalogue of Gentile Fusillo et al(2019) and with the hot subdwarf catalogue ofGeier et al (2019) (seeFig. 12).…”

A New Class of Roche Lobe–filling Hot Subdwarf Binaries

“…Such a color puts it in the regime of A-to F-type stars with T eff ≈ 10,000 K, which is significantly cooler than typical hot subdwarfs. With the 3D map extinction maps corrected magnitude, we find an absolute magnitude of M g = 4.4 ± 0.6 mag, consistent with a hot subdwarf star (Geier et al 2019).…”

“…This leads to a shift of PanSTARRS g−r colors of ≈0.25-0.3 mag per kpc toward redder color leading to a biased sample of hotter or less distant systems in color-selected samples. Typically hot subdwarfs have PanSTARRS colors of g−r≈ −0.35 mag (Geier et al 2019) leading to g−r≈0.15-0.25 mag for a 2 mag extinction at a distance of 2 kpc. These are typical colors for F-to G-type main-sequence stars and such objects will be missed in color-selected samples for blue stars.…”

“…However, particularly in dense Galactic plane regions, Gaia DR2 data often provide bad astrometry, resulting in many spurious sources. Therefore, hot subdwarfs selected based on absolute magnitudes with reddening corrections are currently done best outside the Galactic plane with no crowding (Geier et al 2019), but misses these hot subdwarfs at low Galactic latitudes that have evolved from a young stellar population and progenitor masses >2 M e . Further discoveries and extended studies using reddeningcorrected samples are needed for space density calculations and to understand whether Roche lobe-filling hot subdwarfs are more common among shell-burning hot subdwarfs rather than He core-burning hot subdwarf stars.…”

A search for variable subdwarf B stars in TESS full frame images – I. Variable objects in the southern ecliptic hemisphere

“…We chose objects that are rich in high amplitude peaks in a frequency range that is typical of g-modes in sdBVs. These stars may also be δ Scuti or β Cep stars, however Geier et al (2019) and Geier (2020) applied a color index criterium to avoid cool stars. These papers provide detailed arguments using Gaia color indices that these targets are hot subluminous stars and occupy the region −0.7 < GBP − GRP 0.7 in the Gaia colour space.…”