Precursor to a Low-Mass Helium White Dwarf

The Wide Angle Search for Planets (WASP) program has led to the discovery of several dozen transiting exoplanets. In addition to detecting transiting exoplanets, the unique capabilities of the WASP program also allows it to observe various other astrophysical phenomena such as eclipsing binary star systems where two stars in a binary star system periodically eclipse one another. An eclipsing binary star system known as WASP 1628+10 is one such example that was identified from the WASP database. WASP 1628+10 consists of an A-type star (WASP 1628+10A) and the remnant of a disrupted red giant star (WASP 1628+10B). Both components circle around each other in a tight orbit with an orbital period of 0.72 days.

Figure 1: Artist’s impression of a white dwarf. Image credit: ESO/L. Calcada.

Figure 2: Radial velocities of WASP 1628+10A induced by the gravitational “tugging” from WASP 1628+10B. The radial velocity half-amplitude is 23 km/s. Pierre F. L. Maxted et al. (2014).

A paper by Pierre F. L. Maxted et al. (2014) presents new spectroscopic observations of WASP 1628+10. These observations allowed the physical parameters of WASP 1628+10A and WASP 1628+10B to be measured. WASP 1628+10A has 1.36 ± 0.05 times the Sun’s mass, 1.57 ± 0.02 times the Sun’s radius and ~7 times the Sun’s luminosity; while WASP 1628+10B has 0.135 ± 0.02 times the Sun’s mass, 0.348 ± 0.008 times the Sun’s radius and is roughly as luminous as the Sun. Additionally, the effective surface temperatures of WASP 1628+10A and WASP 1628+10B are 7500 ± 200 K and 9200 ± 600 K, respectively.

The new observations confirm WASP 1628+10B is indeed the precursor of a helium white dwarf (pre-He-WD). Low-mass white dwarfs with less than 0.35 solar-masses are believed to form from the evolution of binary star systems. The process generally involves the transfer of mass from a puffed-up red giant star onto a companion star. Eventually, what remains of the red giant star is a degenerate helium core. Since it no longer has sufficient mass to fuse helium into heaver elements, it settles down as an anomalously low mass white dwarf composed almost entirely of helium. Such a star is known as a helium white dwarf (He-WD).

Models of the pre-He-WD in WASP 1628+10 indicate that it has an envelope of hydrogen amounting to no more than ~0.005 solar masses. Episodes of unstable hydrogen fusion (i.e. shell flashes) are expected to cause WASP 1628+10B to shed its hydrogen envelope as it transitions from a pre-He-WD to a He-WD. High frequency pulsations have also been observed for both components of WASP 1628+10. The pulsations are believed to be sensitive to internal process within the stars and can allow detailed studies to be made of their interiors. In particular, it permits new observational opportunities for the study of the interior structure of a low-mass white dwarf.

References:

- Pierre F. L. Maxted et al. (2014), “WASP 1628+10 - an EL CVn-type binary with a very-low-mass stripped-red-giant star and multi-periodic pulsations”, arXiv:1407.5415 [astro-ph.SR]

- Pierre F. L. Maxted et al., “Multi-periodic pulsations of a stripped red-giant star in an eclipsing binary system”, Nature 498, 463–465 (27 June 2013)