Friday, May 27, 2016

Irradiation from an Accreting White Dwarf

J1433 is a system comprised of a brown dwarf in a very close-in orbit around a white dwarf, taking only 78.1 minutes to orbit the white dwarf. The brown dwarf was once a star that transitioned from the stellar to the sub-stellar regime as the white dwarf accreted material from it. At present, the brown dwarf is estimated to have 0.055 ± 0.008 times the mass of the Sun and the white dwarf is estimated to have 0.80 ± 0.07 times the mass of the Sun. The mass of the white dwarf in J1433 is consistent with the average white dwarf mass of known accreting white dwarf systems, but is significantly higher than the average mass of isolated white dwarfs (i.e. ~0.6 times the mass of the Sun).

Figure 1: Artist’s impression of an accreting white dwarf.

The close-in orbit of the brown dwarf subjects it to intense irradiation from the white dwarf. For the brown dwarf, its average day-side temperature is 2401 ± 10 K and its average night-side temperature is 2344 ± 7 K. The irradiation-induced average temperature difference between the day-side and night-side on the brown dwarf is ~57 K, and the maximum temperature difference between the hottest and coolest parts on the brown dwarf is ~200 K. This suggests a modest level of heat transfer between the day-side to the night-side on the brown dwarf. As for the white dwarf, its estimated temperature is 13200 ± 200 K.

Since the brown dwarf is tidally-locked, its rotation period is the same as its orbital period. Its short orbital period of only 78.1 minutes means it is rotating very rapidly. The rotation speed at the equatorial region on the brown dwarf is estimated to be at least 131 ± 46 km/s. This is expected to generate interesting atmospheric dynamics and potentially extreme atmospheric phenomena on the brown dwarf.

Figure 2: Ellipsoidal and irradiation effects associated with the brown dwarf in J1433. Hernández et al. (2016)

Hernández et al. (2016), "An irradiated brown-dwarf companion to an accreting white dwarf", arXiv:1605.07132 [astro-ph.SR]