@article{7d4b9134a5184d1d91c9c9933c65661d,
title = "TESS first look at evolved compact pulsators: Discovery and asteroseismic probing of the g-mode hot B subdwarf pulsator EC 21494-7018",
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 (Mcore? =? 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.",
keywords = "Asteroseismology, Stars: horizontal-branch, Stars: individual: TIC 278659026, Stars: interiors, Stars: oscillations, Subdwarfs",
author = "S. Charpinet and P. Brassard and G. Fontaine and {Van Grootel}, V. and W. Zong and N. Giammichele and U. Heber and Zs Bogn{\'a}r and S. Geier and Green, {E. M.} and Hermes, {J. J.} and D. Kilkenny and {\O}stensen, {R. H.} and I. Pelisoli and R. Silvotti and Telting, {J. H.} and M. Vu{\v c}kovi{\'c} and Worters, {H. L.} and Baran, {A. S.} and Bell, {K. J.} and Bradley, {P. A.} and Debes, {J. H.} and Kawaler, {S. D.} and P. Ko{\l}aczek-Szyma{\'n}ski and Murphy, {S. J.} and A. Pigulski and S{\'o}dor and M. Uzundag and R. Handberg and H. Kjeldsen and Ricker, {G. R.} and Vanderspek, {R. K.}",
note = "Funding Information: gestions that helped improve this manuscript. St{\'e}phane Charpinet acknowledges financial support from the Centre National d{\textquoteright}{\'E}tudes Spa-tiales (CNES, France) and from the Agence Nationale de la Recherche (ANR, France) under grant ANR-17-CE31-0018, funding the INSIDE project. This work was granted access to the high-performance computing resources of the CALMIP computing center under allocation numbers 2018-p0205 and 2019-p0205. This paper includes data collected by the TESS mission. Funding for the TESS mission is provided by the NASA Explorer Program. Funding for the TESS Asteroseismic Science Operations Centre is provided by the Danish National Research Foundation (Grant agreement no.: DNRF106), ESA PRODEX (PEA 4000119301) and Stellar Astrophysics Centre (SAC) at Aarhus University. We thank the TESS team and staff and TASC/TASOC for their support of the present work. G.F. acknowledges the contribution of the Canada Research Chair Program. W.Z. acknowledges the support from the National Natural Science Foundation of China (NSFC) through the grant 11833002, the support from the China Postdoctoral Science Foundation through the grant 2018M641244 and the LAMOST fellowship as a Youth Researcher which is supported by the Special Funding for Advanced Users, budgeted and administrated by the Center for Astronomical Mega-Science, Chinese Academy of Sciences (CAMS). V.V.G. is an F.R.S.-FNRS Research Associate. ZsB and {\'A}S acknowledge the financial support of the GINOP-2.3.2-15-2016-00003, K-115709, K-113117, K-119517 and PD-123910 grants of the Hungarian National Research, Development and Innovation Office (NKFIH), and the Lend{\"u}let Program of the Hungarian Academy of Sciences, project No. LP2018-7/2018. DK acknowledges financial support form the University of the Western Cape. I. P. acknowledges funding by the Deutsche Forschungs-gemeinschaft under grant GE2506/12-1. AP and PK-S acknowledge support from the NCN grant no. 2016/21/B/ST9/01126. ASB gratefully acknowledges financial support from the Polish National Science Center under projects No. UMO-2017/26/E/ST9/00703 and UMO-2017/25/B ST9/02218. SJM is a DECRA fellow supported by the Australian Research Council (grant number DE180101104). KJB is supported by an NSF Astronomy and Astrophysics Postdoctoral Fellowship under award AST-1903828. We thank Andreas Irrgang and Simon Kreuzer for developing the SED fitting tool. This work has made use of data from the European Space Agency (ESA) mission Gaia (https://www.cosmos.esa.int/gaia), processed by the Gaia Data Processing and Analysis Consortium (DPAC, https://www.cosmos.esa.int/web/gaia/dpac/consortium). Funding for the DPAC has been provided by national institutions, in particular the institutions participating in the Gaia Multilateral Agreement. The national facility capability for SkyMapper has been funded through ARC LIEF grant LE130100104 from the Australian Research Council, awarded to the University of Sydney, the Australian National University, Swinburne University of Technology, the University of Queensland, the University of Western Australia, the University of Melbourne, Curtin University of Technology, Monash University, and the Australian Astronomical Observatory. SkyMapper is owned and operated by The Australian National University{\textquoteright}s Research School of Astronomy and Astrophysics. The survey data were processed and provided by the SkyMapper Team at ANU. The SkyMapper node of the All-Sky Virtual Observatory (ASVO) is hosted at the National Computational Infrastructure (NCI). Development and support the SkyMapper node of the ASVO has been funded in part by Astronomy Australia Limited (AAL) and the Australian Government through the Commonwealth{\textquoteright}s Education Investment Fund (EIF) and National Collaborative Research Infrastructure Strategy (NCRIS), particularly the National eResearch Collaboration Tools and Resources (NeCTAR) and the Australian National Data Service Projects (ANDS). This publication makes use of data products from the Wide-field Infrared Survey Explorer, which is a joint project of the University of California, Los Angeles, and the Jet Propulsion Laboratory/California Institute of Technology, funded by the National Aeronautics and Space Administration. This publication makes use of data products from the Two Micron All Sky Survey, which is a joint project of the University of Massachusetts and the Infrared Processing and Analysis Center/California Institute of Technology, funded by the National Aeronautics and Space Administration and the National Science Foundation. Publisher Copyright: {\textcopyright} S. Charpinet et al. 2019.",
year = "2019",
month = dec,
day = "1",
doi = "10.1051/0004-6361/201935395",
language = "English (US)",
volume = "632",
journal = "Astronomy and Astrophysics",
issn = "0004-6361",
publisher = "EDP Sciences",
}