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

Charpinet, S. and Brassard, P. and Fontaine, G. and Van Grootel, V. and Zong, W. and Giammichele, N. and Heber, U. and Bognar, Zs. and Geier, S. and Green, E. M. and Hermes, J. J. and Kilkenny, D. and Ostensen, R. H. and Pelisoli, I. and Silvotti, R. and Telting, J. H. and Vuckovic, M. 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 Kolaczek-Szymanski, P. and Murphy, S. J. and Pigulski, A. and Sodor, À and Uzundag, M. and Handberg, R. and Kjeldsen, H. and Ricker, G. R. and 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 and Astrophysics, 632:A90. pp. 1-23. ISSN 0004-6361

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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.


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Item Type: Article (Commonwealth Reporting Category C)
Refereed: Yes
Item Status: Live Archive
Faculty/School / Institute/Centre: No Faculty
Faculty/School / Institute/Centre: No Faculty
Date Deposited: 14 Jul 2022 23:42
Last Modified: 26 Aug 2022 00:09
Uncontrolled Keywords: asteroseismology; stars: interiors; stars: oscillations; stars: horizontal-branch; stars: individual: TIC 278659026; subdwarfs; Astrophysics - Solar and Stellar Astrophysics
Fields of Research (2020): 51 PHYSICAL SCIENCES > 5101 Astronomical sciences > 510109 Stellar astronomy and planetary systems
Socio-Economic Objectives (2020): 28 EXPANDING KNOWLEDGE > 2801 Expanding knowledge > 280120 Expanding knowledge in the physical sciences
Identification Number or DOI: https://doi.org/10.1051/0004-6361/201935395
URI: http://eprints.usq.edu.au/id/eprint/47870

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