EL CVn-type binaries - discovery of 17 helium white dwarf precursors in bright eclipsing binary star systems

Abstract

The star 1SWASP J024743.37−251549.2 was recently discovered to be a binary star in which an A-type dwarf star eclipses the remnant of a disrupted red giant star (WASP 0247−25 B). The remnant is 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., it is a pre-He-WD. We have used the WASP photometric database to find 17 eclipsing binary stars with orbital periods P=0.7 -2.2 d with similar lightcurves to 1SWASP J024743.37−251549.2. The only star in this group previously identified as a variable star is the brightest one, EL CVn, which we adopt as the prototype for this class of eclipsing binary star. The characteristic lightcurves of EL CVn-type stars show a total eclipse by an A-type dwarf star of a smaller, hotter star and a secondary eclipse of comparable depth to the primary eclipse. We have used new spectroscopic observations for 6 of these systems to confirm that the companions to the A-type stars in these binaries have very low masses (≈ 0.2M ⊙ ). This includes the companion to EL CVn which was not previously known to be a pre-He-WD. EL CVn-type binary star systems will enable us to study the formation of very low-mass white dwarfs in great detail, particularly in those cases where the pre-He-WD star shows non-radial pulsations similar to those recently discovered in WASP0247−25 B.

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

“…Other free parameters in the least-squares fit were: a normalisation constant, the surface brightness ratio J = SB/SA, where SA is the surface brightness of WASP 1628+10 A 3 and similarly for SB; the sum of the radii relative to semi-major axis, s = (RA + RB)/a; the ratio of the radii, k = RB/RA; the orbital inclination, i; the phase of primary eclipse, ∆. The orbital phase of the observations was calculated using the ephemeris for the time of primary eclipse (min I) derived from a similar lightcurve fit to the WASP photometry of WASP 1628+10 by Maxted et al (2014), which we quote here for convenience:…”

“…Instead, we simply quote the values derived from each lightcurve plus the values derived by Maxted et al (2014) for the same parameters using the WASP photometry and an r' lightcurve obtained with the PIRATE telescope. We then adopt the mean values of the parameters that do not depend on wavelength from these 5 lightcurve fits and use the standard error on the mean to estimate the standard errors for these estimates.…”

“…N is the number of points used in the least-squares fit and RMS is the standard deviation of the residuals. Results for the fits to the PIRATE r' and WASP lightcurves are from Maxted et al (2014) given in Table 4 and the estimated standard errors reflect the range of different values derived from different methods for measuring the radial velocity. To measure the radial velocity of WASP 1628+10 B, we first subtracted a scaled and shifted version of a best-fit synthetic spectrum of WASP 1628+10 A from all the spectra observed between orbital phases 0.1 and 0.9.…”

“…We assumed surface gravity values of log gA = 4.25 for WASP 1628+10 A and log gB = 4.5 for WASP 1628+10 B. 2011) 9 is E(B−V)=0.056 and our estimated standard error on this value is 0.034 magnitudes (Maxted et al 2014). Additional constraints included in the fit are the observed values of the luminosity ratio, ℓ r ′ , and and surface brightness ratio, J r ′ , from the fits to the r'-band lightcurves given in Table 3.…”

“…Maxted et al (2014) showed that this star is an eclipsing binary star with an orbital period of 0.72 days that contains an A-type dwarf star (WASP 1628+10 A) and the precursor of a helium white dwarf (pre-He-WD) with a mass 0.3M ⊙ (WASP 1628+10 B).…”