The only inflated brown dwarf in an eclipsing white dwarf-brown dwarf binary: WD1032+011B

TL;DR

This study presents detailed spectrophotometric observations of an eclipsing white dwarf-brown dwarf binary, revealing an inflated brown dwarf with a cloud-free atmosphere and significant temperature contrast driven by irradiation.

Contribution

First detailed phase-resolved spectra of an irradiated brown dwarf in a white dwarf binary, showing atmospheric properties and inflation caused by irradiation.

Findings

Brown dwarf exhibits a 210 K temperature difference between dayside and nightside.

The brown dwarf has an inflated radius likely due to white dwarf irradiation.

Spectral analysis indicates a cloud-free atmosphere with a peculiar L1 spectral type.

Abstract

Due to their short orbital periods and relatively high flux ratios, irradiated brown dwarfs in binaries with white dwarfs offer better opportunities to study irradiated atmospheres than hot Jupiters, which have lower planet-to-star flux ratios. WD1032+011 is an eclipsing, tidally locked white dwarf-brown dwarf binary with a 9950 K white dwarf orbited by a 69.7 M$_{Jup}$ brown dwarf in a 0.09 day orbit. We present time-resolved Hubble Space Telescope Wide Field Camera 3 spectrophotometric data of WD1032+011. We isolate the phase-dependent spectra of WD1032+011B, finding a 210 K difference in brightness temperature between the dayside and nightside. The spectral type of the brown dwarf is identified as L1 peculiar, with atmospheric retrievals and comparison to field brown dwarfs showing evidence for a cloud-free atmosphere. The retrieved temperature of the dayside is $1748^{+66}_{-67}$ K, with a nightside temperature of $1555^{+76}_{-62}$ K, showing an irradiation-driven temperature contrast coupled with inefficient heat redistribution from the dayside to the nightside. The brown dwarf radius is inflated, likely due to the constant irradiation from the white dwarf, making it the only known inflated brown dwarf in an eclipsing white dwarf-brown dwarf binary.