TESS first look at evolved compact pulsators Discovery and asteroseismic probing of the g-mode hot B subdwarf pulsator EC 21494-7018

Heber U, Charpinet S, Brassard P, Fontaine G, Van Grootel V, Zong W, Giammichele N, Heber U, Bognar Z, Geier S, Green EM, Hermes JJ, Kilkenny D, Ostensen RH, Pelisoli , Silvotti R, Telting JH, Vuckovic M, Worters HL, Baran AS, Bell KJ, Bradley PA, Debes JH, Kawaler SD, Kolaczek-Szymanski P, Murphy SJ, Pigulski A, Sodor A, Uzundag M, Handberg R, Kjeldsen H, Ricker GR, Vanderspek RK (2019)

Publication Status: Published

Publication Type: Journal article

Publication year: 2019

Journal

Publisher: EDP SCIENCES S A

Book Volume: 632

Article Number: ARTN A90

DOI: 10.1051/0004-6361/201935395

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 +/- 0.009x2006;M-circle dot) 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 (greater than or similar to 2;M-circle dot) 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-circle dot), radius (0.1694 +/- 0.0081;R-circle dot), and luminosity (8.2 +/- 1.1;L-circle dot). 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 similar to 43% (in mass) of its central helium and possesses a relatively large mixed core (M-core;=;0.198 +/- 0.010;M-circle dot), 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(-0.05)(+0.13)X(O)core=0.16-0.05+0.13$ X(\mathrm{O})_{\mathrm{core}}=0.16_{-0.05}<^>{+0.13} $) 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 C-12(alpha,;gamma)O-16 nuclear reaction rate.

Authors with CRIS profile

Involved external institutions

Nordic Optical Telescope Spain (ES) Universität Potsdam Germany (DE) Uniwersytet Pedagogiczny im. Komisji Edukacji Narodowej w Krakowie Poland (PL) Los Alamos National Laboratory United States (USA) (US) University of Washington United States (USA) (US) Université de Montréal Canada (CA) National Institute for Astrophysics / Istituto Nazionale Astrofisica (INAF) Italy (IT) Université Fédérale de Toulouse Midi-Pyrénées France (FR) Massachusetts Institute of Technology (MIT) United States (USA) (US) Beijing Normal University China (CN) Aarhus University Denmark (DK) University of Valparaíso (UV) / Universidad de Valparaiso Chile (CL) University of Sydney (USYD) Australia (AU) Hungarian Academy of Sciences / Magyar Tudományos Akadémia (MTA) Hungary (HU) University of Wroclaw / Uniwersytet Wrocławski Poland (PL) Missouri State University United States (USA) (US) Boston University United States (USA) (US) University of Arizona United States (USA) (US) Iowa State University of Science and Technology (ISU) United States (USA) (US) University of the Western Cape (UWC) South Africa (ZA) Space Telescope Science Institute (STScI) United States (USA) (US) South African Astronomical Observatory (SAAO) South Africa (ZA) University of Liège (ULg) / Université de Liège Belgium (BE)

How to cite

APA:

Heber, U., Charpinet, S., Brassard, P., Fontaine, G., Van Grootel, V., Zong, W.,... Vanderspek, R.K. (2019). TESS first look at evolved compact pulsators Discovery and asteroseismic probing of the g-mode hot B subdwarf pulsator EC 21494-7018. Astronomy & Astrophysics, 632. https://dx.doi.org/10.1051/0004-6361/201935395

MLA:

Heber, Ulrich, et al. "TESS first look at evolved compact pulsators Discovery and asteroseismic probing of the g-mode hot B subdwarf pulsator EC 21494-7018." Astronomy & Astrophysics 632 (2019).

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