Hubble Space Telescope Imaging and Spectral Analysis of Two Brown Dwarf Binaries at the L Dwarf/T Dwarf Transition

TL;DR

This study uses Hubble imaging and spectral analysis to characterize two brown dwarf binaries near the L/T transition, revealing unusual properties and potential multiplicity, advancing understanding of substellar atmospheres and evolution.

Contribution

It provides detailed spectral and photometric analysis of two brown dwarf binaries, identifying their spectral types and suggesting possible explanations for their unusual properties.

Findings

Component spectral types are L7+L6 and L5+L6.5.

One primary appears later-type than its secondary despite being brighter.

Thick condensate clouds and unresolved binarity may explain observed anomalies.

Abstract

We present a detailed examination of the brown dwarf multiples 2MASS J08503593+1057156 and 2MASS J17281150+3948593, both suspected of harboring components that straddle the L dwarf/T dwarf transition. Resolved photometry from Hubble Space Telescope/NICMOS show opposite trends in the relative colors of the components, with the secondary of 2MASS J0850+1057 being redder than its primary, while that of 2MASS J1728+3948 is bluer. We determine near-infrared component types by matching combined-light, near-infrared spectral data to binary templates, with component spectra scaled to resolved NICMOS and Kp photometry. Combinations of L7 + L6 for 2MASS J0850+1057 and L5 + L6.5 for 2MASS J1728+3948 are inferred. Remarkably, the primary of 2MASS J0850+1057 appears to have a later-type classification compared to its secondary, despite being 0.8-1.2 mag brighter in the near-infrared, while the primary of 2MASS J1728+3948 is unusually early for its combined-light optical classification. Comparison to absolute magnitude/spectral type trends also distinguishes these components, with 2MASS J0850+1057A being ~1 mag brighter and 2MASS J1728+3948A ~0.5 mag fainter than equivalently-classified field counterparts. We deduce that thick condensate clouds are likely responsible for the unusual properties of 2MASS J1728+3948A, while 2MASS J0850+1057A is either an inflated young brown dwarf or a tight unresolved binary, making it potentially part of a wide, low-mass, hierarchical quintuple system.