Abstract
Luminous hot stars (MKs ≲ 0 mag and Teff ≳ 8000 K) dominate the stellar energy input to the interstellar medium throughout cosmological time, are used as laboratories to test theories of stellar evolution and multiplicity, and serve as luminous tracers of star formation in the Milky Way and other galaxies. Massive stars occupy well-defined loci in colour–colour and colour–magnitude spaces, enabling selection based on the combination of Gaia EDR3 astrometry and photometry and 2MASS photometry, even in the presence of substantive dust extinction. In this paper we devise an all-sky sample of such luminous OBA-type stars, which was designed to be complete rather than very pure, providing targets for spectroscopic follow-up with the SDSS-V survey. To estimate the purity and completeness of our catalogue, we derive stellar parameters for the stars in common with LAMOST DR6 and we compare the sample to other O and B-type star catalogues. We estimate ‘astro-kinematic’ distances by combining parallaxes and proper motions with a model for the expected velocity and density distribution of young stars; we show that this adds useful constraints on the distances and therefore luminosities of the stars. With these distances we map the spatial distribution of a more stringently selected subsample across the Galactic disc, and find it to be highly structured, with distinct over- and under-densities. The most evident over-densities can be associated with the presumed spiral arms of the Milky Way, in particular the Sagittarius-Carina and Scutum-Centaurus arms. Yet, the spatial picture of the Milky Way’s young disc structure emerging in this study is complex, and suggests that most young stars in our Galaxy (tage < tdyn) are not neatly organised into distinct spiral arms. The combination of the comprehensive spectroscopy to come from SDSS-V (yielding velocities, ages, etc.) with future Gaia data releases will be crucial in order to reveal the dynamical nature of the spiral arms themselves.
Highlights
Luminous and hot stars are massive, and rare and short lived. They play decisive roles across different fields of astrophysics. They dominate the interaction between stars and the interstellar gas and dust in their host galaxies by heating or ionising those components; those stars interact with the ISM through powerful stellar winds and eventually supernova explosions (Mac Low & Klessen 2004; Hopkins et al 2014)
Luminous and hot stars are inevitably young, and they can serve as tracers of recent massive star formation
In the Milky Way, the term ‘spiral arm’ has been used very broadly to refer to regions encompassing HI, molecular gas, dust, and young stars, while it has been shown from external galaxies that these tracers have quite distinct morphology
Summary
Luminous and hot stars are massive, and rare and short lived They play decisive roles across different fields of astrophysics. For the current context we choose an operative definition of luminous and hot as Teff 8000 K and MK 0 mag, or – loosely speaking – OBA stars This is the set of stars that addresses the science sketched out above. We use a ‘clean’ subset of the target sample to study the structure of the Milky Way disc as traced by young, massive stars. 5 we use this filtered sample to study the 3D space distribution of the sources and we compare the distribution of OBA stars with other tracers of massive star formation and spiral arm structure.
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