Hot Subdwarf Stars (2410.11663v2)

Abstract: Hot subdwarf (SD) stars are the stripped cores of red giant stars in transition to the white dwarf sequence. The B-type subdwarfs (sdB) are powered by helium fusion in the core, more evolved ones (sdO) by shell burning. Low mass SDs may evolve through this stage without any support by nuclear fusion. Because the loss of the giants' envelopes is likely caused by mass transfer in binaries, hot SDs are test beds for close-binary evolution through stable and unstable Roche lobe overflow, common envelope formation and ejection as well as mergers. Many classes of hot SDs can be identified according to surface composition, binarity, magnetism, pulsation characteristics and population membership, including members of globular clusters. Observed binaries show a wide spread of orbital periods from 20 minutes to more than 1,000 days with white dwarf or main sequence companions. The closest systems qualify as type Ia supernova progenitors and LISA detectable gravitational wave sources. High-precision light curves from Kepler and TESS combined with radial velocity curves are used to derive masses, while asteroseismology adds information on the internal structure, slow rotation, and synchronization. Gaia's parallax measurements now allow us to place the stars in the Hertzsprung-Russell diagram and derive stellar parameters by combining them with multi-band photometry. The stellar radius can be determined to high precision this way. Newton's law can then be used to derive masses if accurate surface gravities are available. Large-scale spectroscopic surveys will provide atmospheric parameters for large samples of stars, allowing the mass distributions for the diverse subtypes to be established. These are crucial for testing binary synthesis models and constraining poorly known parameters such as the common envelope efficiency as well as the critical threshold mass-ratio for mass transfer stability.

• The paper presents a comprehensive analysis of hot subdwarf stars, illustrating their transition from red giant cores to white dwarfs through binary interactions.
• It employs large-scale survey data from SDSS, LAMOST, and Gaia to accurately determine stellar characteristics and refine evolutionary models.
• The study underscores the importance of binary evolution—including mergers and short-period systems—in shaping observable phenomena like Type Ia supernova progenitors and gravitational wave sources.

Overview of Hot Subdwarf Stars: An In-Depth Analysis

The paper on "Hot Subdwarf Stars," authored by Ulrich Heber, provides a comprehensive exploration of these stellar objects, which are quintessential for understanding stellar evolution and binary interactions. Hot subdwarf stars, particularly of the B-type (sdB) and O-type (sdO), signify critical phases in stellar evolution, marking a transition from the red giant branch (RGB) to the white dwarf cooling sequence. Characterized as helium-burning cores of red giant stars that have lost their outer envelopes, hot subdwarfs serve as crucial test beds for theoretical models concerning stellar evolution and binary star interactions.

Fundamental Characteristics and Evolution

Within the Hertzsprung-Russell diagram (HRD), hot subdwarfs are situated as crucial links transitioning from the RGB stage to the endpoint of white dwarfs. The subdwarfs are primarily powered by helium fusion, with some evolved variants engaging in shell burning around a carbon/oxygen (C/O) core. Given that the loss of the outer envelopes of RGB stars to form such subdwarfs is likely driven by binary interactions, these stars serve as valuable investigative tools for close-binary evolution, encompassing stable Roche lobe overflow (RLOF), common envelope (CE) ejection, and potential mergers.

Observational Advances and Implications

A significant aspect of understanding hot subdwarfs stems from the availability of comprehensive survey data. The paper discusses insights gained from large-scale spectroscopic surveys such as the Sloan Digital Sky Survey (SDSS) and the Large Sky Area Multi-Object Fiber Spectroscopic Telescope (LAMOST). Moreover, spatial missions like Gaia provide parallax measurements that enable accurate placements of these stars within the HRD, enhancing our ability to derive stellar parameters, including radius and mass.

Binarily-Induced Phenomena

The interaction within binary systems plays a pivotal role in shaping the evolution of hot subdwarfs. The paper elaborates on various binary evolutionary channels and the observational evidence supporting these pathways. These include systems with varied orbital periods, ranging from a mere 20 minutes to over a thousand days, comprising white dwarf or main sequence companions. The shortest period binaries are particularly notable, as they present candidates for Type Ia supernova progenitors and sources of gravitational waves detectable by future space-based observatories like LISA.

The Merger Scenario

Single hot subdwarfs may emerge from mergers of binary components, specifically white dwarfs, leading to both chemically peculiar and magnetic variants. The potential for magnetic field generation during mergers, due to dynamical interactions and rotational influences, is a subject of ongoing investigation. Newly observed magnetic iHesdOBs lend credence to these theories, positing mergers as a plausible genesis scenario for such subdwarfs.

Future Perspectives

With forthcoming advancements in observational technology and methodologies, the paper of hot subdwarfs is poised for significant progress. Precise photometric data from missions like Kepler and TESS, complemented by Gaia's astrometric precision, enable detailed asteroseismological studies and derivation of stellar parameters which are instrumental in refining evolutionary models. Future research is anticipated to further elucidate the complexities of close-binary interactions, the role of mergers, and the precise mechanisms underpinning phenomena such as gravitational wave emissions.

In summary, hot subdwarf stars are essential to understanding both single and binary stellar evolution. They exemplify the quintessential intersection of observational astronomy and theoretical astrophysics, driving a deeper understanding of the mechanisms governing stellar lifecycles and binary dynamics. As new data becomes available, the theoretical framework and observational paradigms surrounding these enigmatic stars will continue to evolve, potentially unlocking further insights into the life and death of stars and the intricate ballet of binary systems.