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

Yifan Zhou, Dániel Apai, Xianyu Tan, Joshua D. Lothringer, Ben W.P. Lew, Sarah L. Casewell, Vivien Parmentier, Mark S. Marley, Siyi Xu, L. C. Mayorga

Research output: Contribution to journalArticlepeer-review

Abstract

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 μm 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.

Original languageEnglish (US)
Article number17
JournalAstronomical Journal
Volume163
Issue number1
DOIs
StatePublished - Jan 1 2022

ASJC Scopus subject areas

  • Astronomy and Astrophysics
  • Space and Planetary Science

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