Shell helium-burning hot subdwarf B stars as candidates for blue large-amplitude pulsators

¹ Yunnan Observatories, Chinese Academy of Sciences, 396 Yangfangwang, Guandu District, Kunming 650216, PR China
² Research School of Astronomy and Astrophysics, The Australian National University, Canberra, ACT 2611, Australia
e-mail: heran.xiong@anu.edu.au
³ Key Laboratory for the Structure and Evolution of Celestial Objects, Chinese Academy of Sciences, 396 Yangfangwang, Guandu District, Kunming 650216, PR China
⁴ Center for Astronomical Mega-Science, Chinese Academy of Sciences, 20A Datun Road, Chaoyang District, Beijing 100012, PR China
⁵ Astronomical Institute of the Czech Academy of Sciences, Fričova 298, 25165 Ondřejov, Czech Republic
⁶ Department of Astronomy, Beijing Normal University, X958+PX3, Muduo Rd, Beitaipingzhuang, Haidian District, Beijing 100088, PR China
⁷ School of Astronomy & Space Science, University of the Chinese Academy of Sciences, 20A Datun Road, Chaoyang District, Beijing 100012, PR China
⁸ The Key Laboratory of Optical Astronomy, National Astronomical Observatories, The Chinese Academy of Sciences, Datun Road, Beijing 100012, PR China
⁹ University of Chinese of Academy of Science, Yuquan Road 19, Shijingshan Block, Beijing 100049, PR China
¹⁰ Anton Pannekoek Institute for Astronomy, University of Amsterdam, Science Park 904, 1098XH Amsterdam, The Netherlands
¹¹ Max Planck Institute for Astrophysics, Karl-Schwarzschild-Str. 1, 85748 Garching, Germany
¹² Institute of Theoretical Physics, Shanxi University, Shanxi Road, Taiyuan 030006, PR China
¹³ Key Laboratory of Space Astronomy and Technology, National Astronomical Observatories, Chinese Academy of Sciences, Beijing 100101, PR China

Received: 22 July 2022
Accepted: 12 September 2022

Abstract

Blue large-amplitude pulsators (BLAPs) are a newly discovered type of variable star. Their typical pulsation periods are on the order of a few tens of minutes, with relatively large amplitudes of 0.2–0.4 mag in optical bands, and their rates of period changes are on the order of 10⁻⁷ yr⁻¹ (both positive and negative). They are extremely rare objects and attempts to explain their origins and internal structures have attracted a great deal of attention. Previous studies have proposed that BLAPs may be pre-white dwarfs, with masses around 0.3 M_⊙, or core-helium-burning stars in the range of ∼0.7 − 1.1 M_⊙. In this work, we use a number of MESA models to compute and explore whether BLAPs could be explained as shell helium-burning subdwarfs type B (SHeB sdBs). The models that best match existing observational constraints have helium core masses in the range of ∼0.45 − 0.5 M_⊙. Our model predicts that the positive rate of period change may evolve to negative. The formation channels for SHeB sdBs involve binary evolution and although the vast majority of BLAPs do not appear to be binaries (with the exception of HD 133729), the observational constraints are still very poor. Motivated by these findings, we explored the Roche lobe overflow channel. Of the 304 binary evolution models we computed, about half of them are able to produce SHeB sdBs in long-period binaries that evade detection from the limited observations that are currently available.