WASP 1628+10 – an EL CVn-type binary with a very low mass stripped red giant star and multiperiodic pulsations

Abstract

The star 1SWASP J162842.31+101416.7 (WASP 1628+10) is one of several EL CVntype stars recently identified using the WASP database, i.e., an eclipsing binary star in which an A-type dwarf star (WASP 1628+10 A) eclipses the remnant of a disrupted red giant star (WASP 1628+10 B). We have measured the masses, radii and luminosities of the stars in WASP 1628+10 using photometry obtained in three bands (u', g', r') with the Ultracam instrument and medium-resolution spectroscopy. The properties of the remnant are well-matched by models for stars in a rarely-observed state evolving to higher effective temperatures at nearly constant luminosity prior to becoming a very low-mass white dwarf composed almost entirely of helium, i.e., we confirm that WASP 1628+10 B is a pre-He-WD. WASP 1628+10 A appears to be a normal A2 V star with a mass of 1.36 ± 0.05M ⊙ . By fitting models to the spectrum of this star around the Hγ line we find that it has an effective temperature T eff,A = 7500 ± 200 K and a metallicity [Fe/H] = −0.3±0.3. The mass of WASP 1628+10 B is only 0.135±0.02M ⊙ . The effective temperature of this pre-He-WD is approximately 9200 K. The Ultracam photometry of WASP 1628+10 shows variability at several frequencies around 40 cycles per day, which is typical for δ Sct-type pulsations often observed in early A-type stars like WASP 1628+10 A. We also observe frequencies near 114 cycles/day and 129 cycles/day, much higher than the frequencies normally seen in δ Sct stars. Additional photometry through the primary eclipse will be required to confirm that these higher frequencies are due to pulsations in WASP 1628+10 B. If confirmed, this would be only the second known example of a pre-He-WD showing high-frequency pulsations.

Pulsating white dwarfs

“…[23]- [27] found pulsations in eight of these ELMs, similar to the pulsations seen in DAVs (ZZ Ceti stars), as described in [29]. [30] found 17 pre-ELMs, i.e., helium-core white dwarf precursors, and [31,32] report pulsations in six of them. Pulsations are an important tool to study the stellar interior, and [33] - [37] report on theoretical models and pulsations of ELMs.…”

DISCOVERY OF TWO NEW THERMALLY BLOATED LOW-MASS WHITE DWARFS AMONG THEKEPLERBINARIES

“…The filled red circles are the approximate current locations of the six known thermally bloated hot white dwarfs found among the Kepler binaries. The open blue circles are eight lowmass white dwarfs found in double-degenerate systems (Hermes et al 2014), while the open green circles are for systems similar to those reported here and that were found using the WASP photometric database (Maxted et al 2013(Maxted et al , 2014; these have been added for comparison.…”

“…After this work was completed we became aware of an impressive collection of 18 similar bloated hot WD systems found in the WASP photometric database (Maxted et al 2013(Maxted et al , 2014, and references therein). Two of these have accurate measurements of the mass, radius, and T for the bloated hot WD and show multiperiodic pulsations.…”

The Formation of EL CVn-type Binaries

“…In this paper, we define a binary as an EL CVn-type binary if it has an A-or Ftype dwarf star (with mass of 1.05 − 2.9M⊙) and a proto-He WD being in the constant-L phase. Multi-periodic pulsations were first discovered in the proto-He WD companion of ⋆ E-mail: cxf@ynao.ac.cn 1SWASP J024743.37-251549.2 (J0247-25 hereinafter) where the proto-He WD has a mass of ∼ 0.2M⊙ (Maxted et al 2011(Maxted et al , 2013, then in 1SWASP J162842.31+101416.7 (WASP 1628+10) where the mass of the proto-He WD companion is only about 0.135 ± 0.02M⊙ (Maxted et al 2014b). This provides an opportunity to study the structure and formation of such low-mass WDs in details through the application of asteroseismology to this new class of pulsating variable stars.…”

“…For example, the reconstructions of the evolutionary history for WASP J0247-25 and WASP 1628+10 (Maxted et al 2013(Maxted et al , 2014b showed that both the objects are probably from stable mass transfer where the mass donor starts to lose mass before it leaves the MS, i.e. the central H abundance is larger than zero.…”