Short-period pulsating hot subdwarf stars observed by TESS

II. Northern ecliptic hemisphere

¹ Astronomical Observatory, University of Warsaw, Al. Ujazdowskie 4, 00-478 Warszawa, Poland
e-mail: andysbaran@gmail.com
² ARDASTELLA Research Collaboration, Missouri State University, Springfield, MO 65897, USA
³ IRAP, CNRS, UPS, CNES, Université de Toulouse, 14 av. Edouard Belin, 31400 Toulouse, France
⁴ Recogito AS, Storgaten 72, 8200 Fauske, Norway
⁵ Space Sciences, Technologies and Astrophysics Research (STAR) Institute, Université de Liège, 19C Allée du 6 Août, 4000 Liège, Belgium
⁶ Nordic Optical Telescope, Rambla José Ana Fernández Pérez 7, 38711 Breña Baja, Spain
⁷ Department of Physics and Astronomy, Aarhus University, NyMunkegade 120, 8000 Aarhus C, Denmark
⁸ Astronomical Institute of the Czech Academy of Sciences, Fričova 298, 251 65 Ondřejov, Czech Republic
⁹ Astroserver.org, Fő tér 1, 8533 Malomsok, Hungary

Received: 12 November 2023
Accepted: 23 February 2024

Abstract

We present the results of an extension of our Transiting Exoplanet Survey Satellite (TESS) search for short-period pulsations in compact stellar objects observed during the second and fourth years of the TESS mission, which targeted the northern ecliptic hemisphere. For many of the targets, we exploited unpublished spectroscopic data to confirm or re-evaluate the object’s spectral classification. From the TESS photometry, we identified 50 short-period hot-subdwarf pulsators, including 35 sdB and 15 sdOB stars. The sample contains 26 pulsators that were unknown prior to the TESS mission. Nine stars show signals at both low and high frequencies and have been categorized as “hybrid” pulsators. For each pulsator, we report the list of prewhitened frequencies, along with and their amplitude spectra calculated from the TESS data. We attempt to identify possible multiplets caused by stellar rotation and we report five candidates with rotation periods between 11 and 46 d. With the search for p-mode pulsating hot subdwarfs in TESS Sectors 1–60 complete, we discuss the completeness of the study, as well as the instability strip and the evolutionary status of the stars we found. We also compare the distribution of pulsation periods as a function of effective temperature and surface gravity with theoretical predictions. We find that the percentage of undetected pulsators in the TESS mission increases with decreasing brightness measurements of stars, reaching 25% near the 15th magnitude. When comparing the distribution of hot subdwarfs in the log g − T_eff plane with stellar models, we underline the importance of a proper treatment of the hydrogen-rich envelope composition (strongly affected by microscopic diffusion processes). We also emphasize that the stellar mass is a significant factor in understanding the instability strip. The p-mode instability strip is confirmed to be narrower than predicted by prior non-adiabatic calculations based on models incorporating equilibrium between gravitational settling and radiative levitation for iron. This implies that competing mixing processes ignored in these models must play a role in reducing the amount of levitating iron in the stellar envelope. Interestingly, we find that the coolest p-mode pulsators with T_eff ≲ 30 000 K (including the hybrid ones) tend to cluster around the terminal age of the extreme horizontal branch of canonical mass (TAEHB at ∼0.47 M_⊙). This trend is expected from the non-adiabatic pulsation calculations. Otherwise, the overall pulsation period distributions tend to reproduce the predicted trends in T_eff and log g.