Scientific Rationale

9th Meeting on Hot Subdwarfs and Related Objects


Hot subdwarf B (sdB) and O (sdO) stars are associated with the Extreme Horizontal Branch (EHB) and represent, for many of them, an intermediate evolutionary stage crossed by a fraction of low-to-intermediate mass stars, where helium is consumed in the core (the sdB stars) or in a shell during the subsequent post-EHB evolution (the sdO stars). Hot subdwarfs then evolve
directly toward the white dwarf cooling sequence, which they contribute for about 2% of the population. These hot and compact objects of typically half a solar mass are found among field stars, but also in cluster populations, and they are believed to be responsible for the UV-excess observed in some galaxies.

A large fraction of hot subdwarfs cohabit with a white dwarf companion or a low-mass main sequence star in tight binary systems. It implies that these stars must have undergone interacting binary evolution, which is key to explain the formation of Extreme Horizontal Branch stars. It is widely admitted that interaction of the progenitor, as it reaches its red-giant stage, with the companion is the main driver for the stellar envelope ejection leading to the formation of a hot B subdwarf instead of a classical Horizontal Branch star. Yet, grey zones remain, especially for the numerous hot subdwarf stars that do not have a stellar companion. Between scenarios involving the merger of two helium white dwarfs or interaction of an evolving red-giant with harder-to-detect sub-stellar companions (brown dwarfs or even planets), the hot-subdwarf community strives to clarify this still shadowy path of stellar evolution.

Our knowledge of the nature, origin, and fate of hot subdwarf stars is enlightened by progress in understanding their atmospheric and internal properties. These stars are known to possess atmospheres with strong chemical abundance anomalies driven by microscopic diffusion, in particular selective gravitational settling and radiative levitation, possibly modulated by other competing processes. Radiative levitation is also a key factor responsible for the driving of radial and nonradial oscillation modes in several classes of hot subdwarf pulsators. The V361 Hya stars showing p-modes and the V1093 Her stars oscillating in g-modes constitute the two main groups of sdB pulsators, recently completed by various discoveries including sdO multiperiodic variables found in the w Cen globular cluster and among field stars, a new class of hot g-mode pulsating sdB stars possibly driven by another mechanism, and the identification of another group of Blue Large-Amplitude Pulsators. The very rich seismic activity occurring in hot subdwarfs has open up some of these stars to direct investigation of their internal properties using asteroseismology, providing in return a wealth of new information on their inner structure and dynamics, and important hints about their past history.

Hot subdwarfs also offer highly valuable laboratories to study key physical processes shaping the structure and evolution of stars, such as those involved in binary evolution mechanisms, tidal synchronisation processes, rotation, mixing and diffusion, atmosphere modeling, stellar pulsation modeling, and core helium burning evolution. Links with related objects, such as core helium burning red-giants and white dwarf stars, can be made in this context too.

This 9th edition of the "meeting on hot subdwarf and related objects" will be the occasion to revue progress in all these topics. It will take place in a renewed observational environment significantly enriched by the availability of new data provided by large surveys, such as GAIA, TESS, NGTS, and ZTF.

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