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

Abstract

Context. The discovery of pulsations in low-mass stars opens an opportunity for probing their interiors and to determine their evolution, by employing the tools of asteroseismology. Aims. We aim to analyze high-speed photometry of SDSSJ145847.02$+$070754.46 and SDSSJ173001.94$+$070600.25 and discover brightness variabilities. In order to locate these stars in the $T_{\mathrm{eff}}-\log g$ 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 obtained by us for these stars with the 2.15m telescope at CASLEO, Argentina. We analyzed their light curves and we apply the Discrete Fourier Transform to determine the pulsation frequencies. Finally, we compare both stars in the $T_{\mathrm{eff}}-\log g$ diagram, with known two pre-white dwarfs, seven pulsating pre-ELM white dwarf stars, $\delta$ Scuti and SX Phe stars. Results. We report the discovery of pulsations in SDSSJ145847.02$+$070754.46 and SDSSJ173001.94$+$070600.25. We determine their effective temperature and surface gravity to be $T_{\mathrm{eff}}$ = 7 972 $\pm$ 200 K, $\log g$ = 4.25 $\pm$ 0.5 and $T_{\mathrm{eff}}$ = 7 925 $\pm$ 200 K, $\log g$ = 4.25 $\pm$ 0.5, respectively. With these parameters these new pulsating low-mass stars can be identified with either ELM white dwarfs (with ~ 0.17 Mo) or more massive SX Phe stars. We identified pulsation periods of 3 278.7 and 1 633.9 s for SDSSJ145847.02$+$070754.46 and a pulsation period of 3 367.1 s for SDSSJ173001.94$+$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.

5 pages, 5 figures, 1 table, accepted for publication in A&AL

References in corpus (8)

• New evolutionary sequences for extremely low mass white dwarfs: Homogeneous mass and age determinations, and asteroseismic prospects
• The ELM Survey. I. A Complete Sample of Extremely Low Mass White Dwarfs
• Multi-periodic pulsations of a stripped red giant star in an eclipsing binary
• The ELM Survey. V. Merging Massive White Dwarf Binaries
• A new class of pulsating white dwarf of extremely low mass: the fourth and fifth members
• Results of a Search for gamma Dor and delta Sct Stars with the Kepler Spacecraft
• WASP 1628+10 - an EL CVn-type binary with a very-low-mass stripped-red-giant star and multi-periodic pulsations
• PSR J1738+0333: The First Millisecond Pulsar + Pulsating White Dwarf Binary

Cited by in corpus (11)

• Pulsating white dwarfs: new insights
• Models of low-mass helium white dwarfs including gravitational settling, thermal and chemical diffusion, and rotational mixing
• Pruning The ELM Survey: Characterizing Candidate Low-Mass White Dwarfs Through Photometric Variability
• The Physical Nature of Subdwarf A Stars: White Dwarf Impostors
• Asteroseismic test of rotational mixing in low-mass white dwarfs
• Pulsating low-mass white dwarfs in the frame of new evolutionary sequences: IV. The secular rate of period change
• Pulsating low-mass white dwarfs in the frame of new evolutionary sequences: V. Asteroseismology of ELMV white dwarf stars
• A Model of the Pulsating Extremely Low-Mass White Dwarf Precursor WASP 0247-25B
• The McDonald Observatory search for pulsating sdA stars: asteroseismic support for multiple populations
• A long-period pre-ELM system discovered from LAMOST medium-resolution survey
• Comparing the asteroseismic properties of pulsating pre-extremely low mass white dwarf and delta Scuti stars