PHL 5038

PHL 5038AB (or just PHL 5038) is a binary system consisting out of a white dwarf and a brown dwarf on a wide orbit. The system is 240 light years (74 parsec) distant from earth.

The white dwarf PHL 5038A was discovered in 2006 in data from the Sloan Digital Sky Survey and the brown dwarf companion was discovered in 2009 from UKIDSS infrared excess and confirmed with Gemini North to be a spacially resolved binary. It was only the fourth known brown dwarf to orbit a white dwarf at the time. The others were GD 165B, WD 0137-349B and GD 1400B.

Physical parameters
The white dwarf was first classified as a DA white dwarf, which indicates a hydrogen-dominated atmosphere without any metal pollution. A later work found weak pollution due to calcium in the atmosphere of the white dwarf thanks to XSHOOTER spectra from the Very Large Telescope. The calcium is detected as the K-line in two spectra. No infrared excess due to a disk was detected. PHL 5038A has either accreted all debris or is surrounded by a thin disk. The mass and temperature was also overestimated in the past and later works found a mass of around 0.53 to 0.57, an effective temperature of around 7500 to 7800 Kelvin and a surface gravity of around 7.9 dex. The progenitor main-sequence star had a mass of around 1.07 and it existed for around 9 billion years until it became an AGB star and around 1 billion years ago it became a white dwarf.

The brown dwarf has a spectral type of around L8-L9. Its mass was initially estimated to be 60, but this mass was likely an underestimate and more recent estimates find a mass of around 0.070 (or 73 ) and an effective temperature of 1425 K.

The same team that discovered the metal pollution of the white dwarf also re-observed the system with Gemini North to determine the orbital parameters. The semi-major axis is $66$ astronomical units and the inclination is 132 ±11°. The eccentricity is unconstrained, but likely lower than 0.615. In the past the white dwarf was more massive, making the semi-major axis half as large at 33 AU.

Evolution
Casewell et al. suggest the following evolution of the system:

The PHL 5038 system during the main-sequence had a star with a mass similar or more massive than the sun and it had a brown dwarf at an orbit of 33 AU, similar to the orbital distance of Neptune. It also likely had rocky debris in the form of planetesimals in orbit around the star, maybe similar to the asteroid belt. At the end of its lifetime the star became an AGB star with a size smaller than 2.5 AU, leaving the rocky debris mostly intact. As the star lost around half of its mass, the orbit of the brown dwarf and the planetesimals increased. The debris would be stable within 17-32 AU (circular orbit of the brown dwarf) or 5-8 AU (e=0.6 for the brown dwarf orbit). A debris belt with an increased size might be close to the edge of this stable zone and gravitational interactions with the brown dwarf would scatter planetesimals into all kinds of directions, eroding the edge of the debris belt. Some of these planetesimals will be scattered inwards and are being disrupted by the tidal forces of the white dwarf, leading to the pollution of the white dwarf atmosphere. Alternatively the disk could have been larger than the stable zone, resulting in chaotic scattering at the beginning of the white dwarf stage, until the scattering decreased.