Two new pulsating low-mass pre-white dwarfs or SX Phoenicis stars?

CÓRSICO, ALEJANDRO H.; CORTI, MARIELA A.; KOESTER, D.; KEPLER, S.O.; SÁNCHEZ ARIAS, JULIETA P.; KANAAN, ANTONIO ; ALTHAUS, LEANDRO G.

ASTRONOMY AND ASTROPHYSICS

EDP SCIENCES S A

Lugar: Paris; Año: 2016 vol. 58 p. 5 - 8

0004-6361

Context. The discovery of pulsations in low-mass stars opens an opportunity to probe their interiors and determine their evolution byemploying the tools of asteroseismology.Aims. We aim to analyse high-speed photometry of SDSS J145847.02+070754.46 and SDSS J173001.94+070600.25 and discoverbrightness variabilities. In order to locate these stars in the Teff-log g diagram, we fit optical spectra (SDSS) with synthetic nonmagneticspectra derived from model atmospheres.Methods. To carry out this study, we used the photometric data we obtained for these stars with the 2.15 m telescope at CASLEO,Argentina. We analysed their light curves and applied the discrete Fourier transform (FT) to determine the pulsation frequencies.Finally, we compare both stars in the Teff- log g diagram, with two known pre-white dwarfs and seven pulsating pre-ELM whitedwarf stars, Scuti, and SX Phe starsResults. We report the discovery of pulsations in SDSS J145847.02+070754.46 and SDSS J173001.94+070600.25. We determine their effective temperature and surface gravity to be Te = 7972 +- 200 K, log g = 4.25 +- 0.5 and Te = 7925 +- 200 K, log g = 4.25 +-0.5, respectively. With these parameters, these new pulsating low-mass stars can be identified with either ELM white dwarfs (with0:17 Msun) or more massive SX Phe stars. We identified pulsation periods of 3278.7 and 1633.9 s for SDSS J145847.02+070754.46and a pulsation period of 3367.1 s for SDSS J173001.94+070600.25. These two new objects, together with those of Maxted et al.(2013, 2014), indicate the possible existence of a new instability domain towards the late stages of evolution of low-mass white dwarfstars, although their identification with SX Phe stars cannot be discarded.