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

¹ Grupo de Evolución Estelar y Pulsaciones, Facultad de Ciencias Astronómicas y Geofísicas, Universidad Nacional de La Plata, Paseo del Bosque s/n, 1900 La Plata, Argentina
e-mail: mariela@fcaglp.unlp.edu.ar
² Instituto Argentino de Radioastronomía, CCT-La Plata, CONICET, C.C. Nro. 5, 1984 Villa Elisa, Argentina
³ Departamento de Física, Centro de Ciências Físicas e Matemáticas, Universidade Federal de Santa Catarina, Campus Universitário Reitor João David Ferreira Lima, Florianópolis, Brazil
⁴ Instituto de Astrofísica La Plata, CONICET-UNLP, Paseo del Bosque s/n, 1900 La Plata, Argentina
⁵ Departamento de Astronomia, Universidade Federal do Rio Grande do Sul, Av. Bento Gonçalves 9500, 91501-970 Porto Alegre, RS, Brazil
⁶ Institut für Theoretische Physik und Astrophysik, Universität Kiel, 24098 Kiel, Germany

Received: 28 September 2015
Accepted: 4 February 2016

Abstract

Context. The discovery of pulsations in low-mass stars opens an opportunity to probe their interiors and determine their evolution by employing 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 discover brightness variabilities. In order to locate these stars in the T_eff − log g diagram, we fit optical spectra (SDSS) with synthetic non-magnetic spectra 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 T_eff − log g diagram, with two known pre-white dwarfs and seven pulsating pre-ELM white dwarf stars, δ Scuti, and SX Phe stars

Results. 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 T_eff = 7972 ± 200 K, log g = 4.25 ± 0.5 and T_eff = 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 (with ~0.17 M_⊙) or more massive SX Phe stars. We identified pulsation periods of 3278.7 and 1633.9 s for SDSS J145847.02+070754.46 and 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 dwarf stars, although their identification with SX Phe stars cannot be discarded.