TY  - JOUR
A1  - Baran, Andrzej S.
A1  - Telting, J. H.
A1  - Jeffery, C. Simon
A1  - Ostensen, R. H.
A1  - Vos, Joris
A1  - Reed, M. D.
A1  - Vŭcković, Maja
T1  - K2 observations of the sdBV plus dM/bd binaries PHL457 and EQPsc
JF  - Monthly notices of the Royal Astronomical Society
N2  - We present an analysis of two pulsating subdwarf B stars PHL 457 and EQ Psc observed during the K2 mission. The K2 light curves of both stars show variation consistent with irradiation of a cooler companion by the hot subdwarf. They also show higher frequency oscillations consistent with pulsation. Using new spectroscopic data, we measured the radial velocity, effective temperature, surface gravity, and helium abundance of both hot subdwarfs as a function of orbital phase. We confirm the previously published spectroscopic orbit of PHL 457, and present the first spectroscopic orbit of EQ Psc. The orbital periods are 0.313 and 0.801 d, respectively. For EQPsc, we find a strong correlation between T-eff and orbital phase, due to contribution of light from the irradiated companion. We calculated amplitude spectra, identified significant pulsation frequencies, and searched for multiplets and asymptotic period spacings. By means of multiplets and period spacing, we identified the degrees of several pulsation modes in each star. The g-mode multiplets indicate subsynchronous core rotation with periods of 4.6 d (PHL 457) and 9.4 d (EQ Psc). We made spectral energy disctribution (SED) fits of PHL 457 and EQ Psc using available broad-band photometry and Gaia data. While the SED of PHL 457 shows no evidence of a cool companion, the SED for EQPsc clearly shows an infrared (IR) access consistent with a secondary with a temperature of about 6800K and a radius of 0.23 R-circle dot. This is the first detection of an IR access in any sdB + dM binary.
KW  - binaries: general
KW  - stars: individual (PHL 457, EQ Psc)
KW  - stars: oscillations (including pulsations)
Y1  - 2019
U6  - https://doi.org/10.1093/mnras/stz2209
SN  - 0035-8711
SN  - 1365-2966
VL  - 489
IS  - 2
SP  - 1556
EP  - 1571
PB  - Oxford Univ. Press
CY  - Oxford
ER  - 
TY  - JOUR
A1  - Charpinet, Stéphane
A1  - Brassard, P.
A1  - Fontaine, G.
A1  - Van Grootel, Valerie
A1  - Zong, Weika
A1  - Giammichele, N.
A1  - Heber, Ulrich
A1  - Bognár, Zsófia
A1  - Geier, Stephan
A1  - Green, Elizabeth M.
A1  - Hermes, J. J.
A1  - Kilkenny, D.
A1  - Ostensen, R. H.
A1  - Pelisoli, Ingrid Domingos
A1  - Silvotti, R.
A1  - Telting, J. H.
A1  - Vuckovic, Maja
A1  - Worters, H. L.
A1  - Baran, Andrzej S.
A1  - Bell, Keaton J.
A1  - Bradley, Paul A.
A1  - Debes, J. H.
A1  - Kawaler, S. D.
A1  - Kolaczek-Szymanski, P.
A1  - Murphy, S. J.
A1  - Pigulski, A.
A1  - Sodor, A.
A1  - Uzundag, Murat
A1  - Handberg, R.
A1  - Kjeldsen, H.
A1  - Ricker, G. R.
A1  - Vanderspek, R. K.
T1  - TESS first look at evolved compact pulsators Discovery and asteroseismic probing of the g-mode hot B subdwarf pulsator EC 21494-7018
JF  - Astronomy and astrophysics : an international weekly journal
N2  - 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.
KW  - asteroseismology
KW  - stars
KW  - interiors
KW  - oscillations
KW  - horizontal-branch
KW  - individual
KW  - TIC 278659026
KW  - subdwarfs
Y1  - 2019
U6  - https://doi.org/10.1051/0004-6361/201935395
SN  - 0004-6361
SN  - 1432-0746
VL  - 632
PB  - EDP Sciences
CY  - Les Ulis
ER  -