TESS first look at evolved compact pulsators

Discovery and asteroseismic probing of the g-mode hot B subdwarf pulsator EC 21494−7018

¹ Institut de Recherche en Astrophysique et Planétologie, CNRS, Université de Toulouse, CNES, 14 Avenue Edouard Belin, 31400 Toulouse, France
e-mail: stephane.charpinet@irap.omp.eu
² Département de Physique, Université de Montréal, Québec H3C 3J7, Canada
³ Space sciences, Technologies and Astrophysics Research (STAR) Institute, Université de Liège, 19C Allée du six août, 4000 Liège, Belgium
⁴ Department of Astronomy, Beijing Normal University, Beijing 100875, PR China
⁵ Dr. Karl Remeis-Observatory & ECAP, Astronomical Institute, Friedrich-Alexander University Erlangen-Nürnberg (FAU), Sternwartstr. 7, 96049 Bamberg, Germany
⁶ Konkoly Observatory, MTA Research Centre for Astronomy and Earth Sciences, Konkoly Thege Miklós út 15-17, 1121 Budapest, Hungary
⁷ MTA CSFK Lendület Near-Field Cosmology Research Group, Hungary
⁸ Institute for Physics and Astronomy, University of Potsdam, Karl-Liebknecht-Str. 24/25, 14476 Potsdam, Germany
⁹ Steward Observatory, University of Arizona, 933 North Cherry Avenue, Tucson, AZ 85721, USA
¹⁰ Department of Astronomy, Boston University, 725 Commonwealth Ave., Boston, MA 02215, USA
¹¹ Department of Physics and Astronomy, University of the Western Cape, Private Bag X17, Bellville 7535, South Africa
¹² Department of Physics, Astronomy, and Materials Science, Missouri State University, Springfield, MO 65897, USA
¹³ INAF-Osservatorio Astrofisico di Torino, Strada dell’Osservatorio 20, 10025 Pino Torinese, Italy
¹⁴ Nordic Optical Telescope, Rambla José Ana Fernández Pérez 7, 38711 Breña Baja, Spain
¹⁵ Instituto de Física y Astronomía, Universidad de Valparaiso, Gran Bretaña 1111, Playa Ancha, Valparaíso 2360102, Chile
¹⁶ South African Astronomical Observatory, PO Box 9, Cape Town 7935, South Africa
¹⁷ Uniwersytet Pedagogiczny, Obserwatorium na Suhorze, ul. Podchorżych 2, 30-084 Kraków, Polska
¹⁸ DIRAC Institute, Department of Astronomy, University of Washington, Seattle, WA 98195-1580, USA
¹⁹ XCP-6, MS F699, Los Alamos National Laboratory, Los Alamos, NM 87545, USA
²⁰ Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA
²¹ Department of Physics and Astronomy, Iowa State University, Ames, IA 50011, USA
²² Instytut Astronomiczny, Uniwersytet Wrocławski, ul. Kopernika 11, 51-622 Wrocław, Poland
²³ Sydney Institute for Astronomy (SIfA), School of Physics, The University of Sydney, Sydney, NSW 2006, Australia
²⁴ Stellar Astrophysics Centre, Department of Physics and Astronomy, Aarhus University, Ny Munkegade 120, 8000 Aarhus C, Denmark
²⁵ Department of Physics, and Kavli Institute for Astrophysics and Space Research, Massachusetts Institute of Technology, Cambridge, MA 02139, USA

Received: 4 March 2019
Accepted: 8 October 2019

Abstract

Context. The TESS satellite was launched in 2018 to perform high-precision photometry from space over almost the whole sky in a search for exoplanets orbiting bright stars. This instrument has opened new opportunities to study variable hot subdwarfs, white dwarfs, and related compact objects. Targets of interest include white dwarf and hot subdwarf pulsators, both carrying high potential for asteroseismology.

Aims. We present the discovery and detailed asteroseismic analysis of a new g-mode hot B subdwarf (sdB) pulsator, EC 21494−7018 (TIC 278659026), monitored in TESS first sector using 120-s cadence.

Methods. The TESS light curve was analyzed with standard prewhitening techniques, followed by forward modeling using our latest generation of sdB models developed for asteroseismic investigations. By simultaneously best-matching all the observed frequencies with those computed from models, we identified the pulsation modes detected and, more importantly, we determined the global parameters and structural configuration of the star.

Results. The light curve analysis reveals that EC 21494−7018 is a sdB pulsator counting up to 20 frequencies associated with independent g-modes. The seismic analysis singles out an optimal model solution in full agreement with independent measurements provided by spectroscopy (atmospheric parameters derived from model atmospheres) and astrometry (distance evaluated from Gaia DR2 trigonometric parallax). Several key parameters of the star are derived. Its mass (0.391 ± 0.009 M_⊙) is significantly lower than the typical mass of sdB stars and suggests that its progenitor has not undergone the He-core flash; therefore this progenitor could originate from a massive (≳2 M_⊙) red giant, which is an alternative channel for the formation of sdBs. Other derived parameters include the H-rich envelope mass (0.0037 ± 0.0010 M_⊙), radius (0.1694 ± 0.0081 R_⊙), and luminosity (8.2 ± 1.1 L_⊙). The optimal model fit has a double-layered He+H composition profile, which we interpret as an incomplete but ongoing process of gravitational settling of helium at the bottom of a thick H-rich envelope. Moreover, the derived properties of the core indicate that EC 21494−7018 has burnt ∼43% (in mass) of its central helium and possesses a relatively large mixed core (M_core = 0.198 ± 0.010 M_⊙), in line with trends already uncovered from other g-mode sdB pulsators analyzed with asteroseismology. Finally, we obtain for the first time an estimate of the amount of oxygen (in mass; X(O)_core = 0.16^+0.13_−0.05) produced at this stage of evolution by an helium-burning core. This result, along with the core-size estimate, is an interesting constraint that may help to narrow down the still uncertain ¹²C(α, γ)¹⁶O nuclear reaction rate.