[Paper Review] The population of hot subdwarf stars studied with Gaia II. The Gaia DR2 catalogue of hot subluminous stars

S. Geier, R. Raddi+2 more|arXiv (Cornell University)|Oct 22, 2018

Stellar, planetary, and galactic studies66 references4 citations

TL;DR

This paper presents a Gaia DR2-based all-sky catalogue of 39,800 hot subluminous star candidates, selected via colour, absolute magnitude, and reduced proper motion cuts. The catalogue is magnitude-limited to G < 19 mag, nearly complete within 1.5 kpc outside the Galactic plane, and serves as a key target list for upcoming photometric and spectroscopic surveys to advance understanding of hot subdwarf evolution and binary interactions.

ABSTRACT

Based on data from the ESA Gaia Data Release 2 (DR2) and several ground-based, multi-band photometry surveys we 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 $G&lt;19\,{ m mag}$ 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 $1.5\,{ m kpc}$. 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.

Motivation & Objective

• To compile a comprehensive, all-sky catalogue of hot subluminous star candidates using Gaia DR2 astrometry and multi-band photometry.
• To enable statistically significant spectroscopic follow-up of hot subdwarfs to study their formation and evolution, particularly in binary systems.
• To improve the completeness and reliability of hot subdwarf samples beyond previous surveys, especially in the Galactic halo and outer disk.
• To support future large-scale surveys by providing a well-defined, magnitude-limited target list with low contamination from cooler stars (~10%).
• To facilitate asteroseismic and atmospheric studies by identifying pulsating and chemically peculiar subdwarf stars.

Proposed method

• Applied colour, absolute magnitude, and reduced proper motion cuts to Gaia DR2 data to select hot subluminous star candidates.
• Combined Gaia DR2 astrometry with multi-band photometry from ground-based surveys (e.g., SDSS, IPHAS, VPHAS+, GALEX, 2MASS) to enhance selection fidelity.
• Used a magnitude limit of G < 19 mag to ensure completeness within ~1.5 kpc for most of the sky, excluding the Galactic plane and LMC/SMC.
• Applied statistical corrections to estimate completeness and contamination levels, finding ~10% contamination by cooler stars.
• Classified candidates primarily as B/O-type hot subdwarfs, with secondary populations of blue horizontal branch stars, hot post-AGB stars, and central stars of planetary nebulae.
• Validated the catalogue’s reliability using existing spectroscopic samples and cross-matching with known hot subdwarf databases.

Experimental results

Research questions

• RQ1 What is the completeness and contamination level of a Gaia DR2-based catalogue of hot subluminous stars across the sky?
• RQ2 How can multi-band photometry and astrometry from Gaia DR2 be combined to optimally select hot subdwarf candidates?
• RQ3 To what extent is the catalogue complete within 1.5 kpc outside the Galactic plane and in the halo?
• RQ4 What is the expected distribution of spectral types (e.g., sdO/B, HBB, post-AGB) in the final sample?
• RQ5 How can this catalogue serve as a foundation for future large-scale spectroscopic and photometric surveys of hot subdwarfs?

Key findings

• The catalogue contains 39,800 hot subluminous star candidates, with a magnitude limit of G < 19 mag, covering the entire sky.
• The catalogue is expected to be nearly complete up to ~1.5 kpc in the Galactic halo and outer disk, excluding the Galactic plane and LMC/SMC.
• Contamination by cooler stars is estimated at ~10%, indicating high purity for hot subdwarf selection.
• The majority of candidates are expected to be B/O-type hot subdwarfs, followed by late B-type blue horizontal branch stars and post-AGB stars.
• The sample includes a significant number of potential pulsating subdwarfs and candidates for gravitational wave progenitors.
• The catalogue is designed as a foundational target list for upcoming surveys such as WEAVE, 4MOST, and PLATO, enabling statistically robust studies of hot subdwarf evolution and binarity.