TESS first look at evolved compact pulsators

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.

TESS first look at evolved compact pulsators

“…See Winget & Kepler (2008), Fontaine & Brassard (2008), and Althaus et al (2010) for reviews of the field of white dwarf asteroseismology, and Córsico et al (2019) for coverage of the most recent decade of discovery in the era of extensive space-based photometry from Kepler and K2. As part of the initial activities of the TASC WG8.2, we present analyses of examples of each type of pulsating white dwarf observed at two-minute cadence in the first TESS Sectors in a series of first-light papers. These follow the TASC WG8.3 first-light analysis of a pulsating hot subdwarf in TESS data from Charpinet et al (2019). In this paper, we study the DBV pulsator WD 0158−160 (also EC 01585−1600, G 272-B2A), which was observed by TESS as target TIC 257459955 in Sector 3.…”

Asteroseismic Observations of Hot Subdwarfs

“…Hot subdwarfs not accessible to MOST, CoRoT, Kepler and K2 may now in principle be photometrically monitored from space for an extended period. Charpinet et al (2019) discuss the prospects for asteroseismic observations of sdB stars in more detail.…”

“…Prins et al obtain ground-based time-series photometry for most of their p-mode candidates and discover J19384 + 5824 to be a V361 Hya star in the TESS 189 days viewing zone; it has a period of 172 s and an amplitude of 0.0091 magnitudes. Charpinet et al (2019) present the discovery and detailed asteroseismic analysis of a new g-mode hot subdwarf pulsator, EC 21494-7018 (TIC 278659026), monitored in the TESS first sector using a 120-second cadence. The light curve analysis reveals EC 21494-7018 to be a sdB pulsator having up to twenty independent g-mode frequencies.…”

Space observations of AA Doradus provide consistent mass determinations. New HW-Vir systems observed with TESS

“…Stellar oscillations predicted and discovered in sdB stars by Charpinet et al (1997) and Kilkenny et al (1997), respectively, are potentially useful for sdB mass estimations. Asteroseismology uses pulsation properties to describe stellar interiors, with a recent example of a mass estimation in EC 21494-7018 reported by Charpinet et al (2019). Surprisingly, the estimated mass is 0.39M , which is significantly below the canonical mass.…”