HST/WFC3 Complete Phase-resolved Spectroscopy of White-dwarf-brown-dwarf Binaries WD 0137 and EPIC 2122

• Published in: The Astronomical journal 2022-01, Vol.163 (1), p.17
• Main Authors: Zhou, Yifan, Apai, Dániel, Tan, Xianyu, Lothringer, Joshua D., Lew, Ben W. P., Casewell, Sarah L., Parmentier, Vivien, Marley, Mark S., Xu, Siyi, Mayorga, L. C.
• Format: Article
• Language: English
• Subjects: Astronomy; Atmospheric circulation; Atmospheric circulation models; Atmospheric models; Brightness temperature; Brown dwarf stars; Brown dwarfs; Exoplanet atmospheres; Field cameras; Fourier series; Heat transfer; Hubble Space Telescope; Irradiation; Night; Radiative transfer; Space telescopes; Spectra; Spectroscopy; Stellar atmospheres; Time series analysis; White dwarf stars
• ISSN: 0004-6256; 1538-3881
• DOI: 10.3847/1538-3881/ac3095

Brown dwarfs in close-in orbits around white dwarfs offer an excellent opportunity to investigate properties of fast-rotating, tidally locked, and highly irradiated atmospheres. We present Hubble Space Telescope Wide Field Camera 3 G141 phase-resolved observations of two brown-dwarf-white-dwarf binaries: WD 0137-349 and EPIC 212235321. Their 1.1–1.7 μ m phase curves demonstrate rotational modulations with semi-amplitudes of 5.27% ± 0.02% and 29.1% ± 0.1%; both can be fit well by multi-order Fourier series models. The high-order Fourier components have the same phase as the first-order and are likely caused by hot spots located at the substellar points, suggesting inefficient day/night heat transfer. Both brown dwarfs’ phase-resolved spectra can be accurately represented by linear combinations of their respective day- and nightside spectra. Fitting the irradiated brown dwarf model grids to the dayside spectra require a filling factor of ∼50%, further supporting a hot spot dominating the dayside emission. The nightside spectrum of WD 0137-349B is fit reasonably well by non-irradiated substellar models, and the one of EPIC 21223521B can be approximated by a Planck function. We find strong spectral variations in the brown dwarfs’ day/night flux and brightness temperature contrasts, highlighting the limitations of band-integrated measurements in probing heat transfer in irradiated objects. On the color–magnitude diagram, WD 0137-349B evolves along a cloudless model track connecting the early-L and mid-T spectral types, suggesting that clouds and disequilibrium chemistry have a negligible effect on this object. A full interpretation of these high-quality phase-resolved spectra calls for new models that couple atmospheric circulation and radiative transfer under high-irradiation conditions.