SDSS 1557


SDSS 1557 is a binary system composed of a white dwarf and a brown dwarf. The system is surrounded by a circumbinary debris disk. The debris disk was formed when a minor planet was tidally disrupted around the white dwarf in the past.

The brown dwarf companion

In 2011 it was found that the system did show Y- and J-band excess, which hinted at a companion. Follow-up observations with instruments on the Gemini Observatory and the Very Large Telescope revealed the secondary, the brown dwarf SDSS 1557B, and a circumbinary disk around the binary. The researchers measured the radial velocity changes with the help of the Magnesium absorption line at 4482 Å and found that a 66 brown dwarf orbits the white dwarf at around 0.7, with the orbital period being around 2.27 hours. The irradiated brown dwarf also shows a hydrogen-alpha emission line.
Additional follow-up came with Hubble Wide [Field Camera 3|WFC3], using time-resolved spectrophotometry. SDSS 1557B is similar to ultra-short period planets and is likely tidally locked. White dwarfs give off more radiation in the ultra-violet than it is the case for main-sequence stars. This leads to a higher UV-exposure for SDSS 1557B when compared to a regular hot Jupiter. The fact that SDSS 1557B is tidally locked creates vast temperature changes in the dayside and nightside of the brown dwarf. The researchers found that the brown dwarf is inefficient at redistributing the heat from the dayside to the nightside. They also find that the nightside is likely dominated by clouds and the dayside is likely dominated by opaque H− and likely has a temperature inversion.

The circumbinary disk

The system was first suspected to be a white dwarf with a circumstellar disk in 2011 from K-band excess. The system also displayed high metal abundances, showing that the white dwarf was polluted with planetary debris.
The disk ring lies at around 3.3, exterior to the Roche lobe. The dust grains of the disk have a temperature of 1,100 Kelvin. The material from the disk crosses the gap between disk and white dwarf in streams. A process that is well known for binaries and seen in other binaries, such as CoRoT 223992193.

Past evolution of the system

The system formed at least 1.5 Gyr ago as a low-mass-ratio binary of a star and a companion with a semi-major axis of less than one astronomical unit in the past. The minor planet on the other hand had an orbit that was larger than a few AU. The brown dwarf was engulfed when the star became a giant, an evolutionary stage known as common envelope. Around 33 Myrs ago the common envelope was ejected, forming a low-mass Helium core white dwarf. This formed the present binary, called SDSS 1557. A minor planet, likely an asteroid larger than 4 km, with a mass of at least 1014 kg survived the giant phase of the star. It was scattered towards the binary and tidally disrupted by the white dwarf when it crossed the Roche radius. The resulting debris cloud became the disk we see today.