Thermally Bloated Low-Mass White Dwarfs

White dwarfs typically pack as much mass as the Sun into a volume the size of Earth. Given that Earth’s diameter is only one percent the Sun’s, white dwarfs are very compact objects. However, white dwarfs are not always as dense and compact as they typically are. A study by Rappaport et al. (2015) reports on the discovery of two thermally bloated low-mass white dwarfs in the binary star systems KIC 9164561 and KIC 10727668. Both white dwarfs happen to transit their primary host stars and were first detected using data from NASA’s Kepler space telescope.

Figure 1: Artist’s impression of a white dwarf.

KIC 9164561 consists of a white dwarf with 0.197 ± 0.005 times the Sun’s mass and 0.277 ± 0.005 times the Sun’s diameter in a 1.267 day orbit around a 2.02 ± 0.06 solar-mass A-type star. The other binary star system, KIC 10727668, consists of a white dwarf with 0.266 ± 0.035 times the Sun’s mass and 0.151 ± 0.004 times the Sun’s diameter in a 2.306 day orbit around a 2.22 ± 0.10 solar-mass A-type star. Both white dwarfs in KIC 9164561 and KIC 10727668 are hot, low-mass helium white dwarfs with estimated temperatures of 10,410 ± 200 K and 14,110 ± 440 K, respectively.

The addition of KIC 9164561 and KIC 10727668 brings the totally number of binary systems consisting of low-mass white dwarfs in tight orbits around A-type stars in the Kepler sample to six. The other four similar binary systems are KOI 74, KOI 81, KOI 1224 and KOI 1375. Of the six binary systems, KIC 9164561 has the shortest orbital period and the largest, most thermally bloated white dwarf. Nearly a quarter the Sun’s diameter but with only 1/5th the mass, the white dwarf in KIC 9164561 has a mean density of only 12.8 ± 1.2 g/cm³, making it less dense than gold. For comparison, white dwarfs typically have mean densities at ~1 ton per cubic centimetre.

Binary systems like KIC 9164561 and KIC 10727668 can form via two possible mechanisms. The first mechanism is known as common envelope (CE) evolution where the more massive star in the binary evolves into a giant star and undergoes unstable mass transfer to its companion star. The rate of mass transfer is sufficiently rapid to cause the companion star to spiral towards the core of the giant star. Such an in-spiralling process drives the transfer of angular momentum from the companion star to the envelope of the giant star. This causes the envelope to be expelled into space, leaving behind the helium core as a low-mass white dwarf in a close-in orbit around the companion star.

The second mechanism involves stable mass transfer where the more massive star (i.e. donor star) in the binary evolves quicker and grows in size before it starts to transfer mass to its companion star. During the mass transfer process, the donor star continues to evolve as a red giant and forms a helium core. Eventually, so much mass is transferred that only the helium core of the donor star remains in the form of a low-mass white dwarf in a tight binary.

Figure 2: Transit light curve for KIC 9164561. The smaller dip corresponds to the transit of the white dwarf in front of the primary A-type star while the larger dip corresponds to the occultation of the white dwarf as it passes behind the primary. The bottom panel shows the residuals of the data minus the fitted model. Rappaport et al. (2015).

Figure 3: As in Figure 2, but for KIC 10727668. Rappaport et al. (2015).

Reference:
Rappaport et al. (2015), “Thermally Bloated Low-Mass White Dwarfs”, arXiv:1502.02303 [astro-ph.SR]