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

Maxted

Abstract

The star 1SWASP J162842.31+101416.7 (WASP 1628+10) is one of several EL CVn-type stars recently identified using the WASP database, i.e., an eclipsing binary star in which an A-type dwarf star (WASP 1628+10A) eclipses the remnant of a disrupted red giant star (WASP1628+10B). 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+10B is a pre-He-WD. WASP 1628+10A appears to be a normal A2V star with a mass of $1.36 \pm 0.05 M_{\odot}$. By fitting models to the spectrum of this star around the H$\gamma$ line we find that it has an effective temperature T$_{\rm eff,A} = 7500 \pm 200$K and a metallicity [Fe/H]$ = -0.3 \pm 0.3$. The mass of WASP 1628+10B is only $0.135 \pm 0.02M_{\odot}$. The effective temperature of this pre-He-WD is approximately 9200K. The Ultracam photometry of WASP 1628+10 shows variability at several frequencies around 40 cycles per day, which is typical for $\delta$ Sct-type pulsations often observed in early A-type stars like WASP 1628+10A. We also observe frequencies near 114 cycles/day and 129 cycles/day, much higher than the frequencies normally seen in $\delta$ Sct stars. Additional photometry through the primary eclipse will be required to confirm that these higher frequencies are due to pulsations in WASP 1628+10B. If confirmed, this would be only the second known example of a pre-He-WD showing high-frequency pulsations.

Citation

Maxted. (2014). WASP 1628+10 - an EL CVn-type binary with a very-low-mass stripped-red-giant star and multi-periodic pulsations. Monthly Notices of the Royal Astronomical Society, 208-216. https://doi.org/10.1093/mnras/stu1465

• Acceptance Date: Jul 21, 2014
• Publication Date: Oct 11, 2014
• Journal: Monthly Notices of the Royal Astronomical Society
• Print ISSN: 0035-8711
• Publisher: Oxford University Press
• Pages: 208-216
• DOI: https://doi.org/10.1093/mnras/stu1465
• Keywords: binaries, close, eclipsing, spectroscopic, stars, individual, 1SWASP J162842.31+101416.7
• Publisher URL: http://dx.doi.org/10.1093/mnras/stu1465