A Thermal Infrared Imaging Study of Very Low-Mass, Wide Separation Brown Dwarf Companions to Upper Scorpius Stars: Constraining Circumstellar Environments

TL;DR

This study uses thermal infrared imaging to analyze very low-mass brown dwarf companions to Upper Scorpius stars, revealing circumstellar disks and providing insights into their formation and environment.

Contribution

It presents new thermal IR observations of wide-separation brown dwarf companions, identifying disks and challenging previous spectral classifications, thus advancing understanding of their formation.

Findings

GSC 06214B has a circumstellar disk with IR excess.

1RXS 1609B shows no IR excess, but the system has a modest 24um excess.

HIP 78530B's spectral type is likely M3+-2, not M8.

Abstract

We present a 3-5um LBT/MMT adaptive optics imaging study of three Upper Scorpius stars with brown dwarf (BD) companions with very low-masses/mass ratios (M_BD < 25M_Jup; M_BD / M_star ~ 1-2%), and wide separations (300-700 AU): GSC 06214, 1RXS 1609, and HIP 78530. We combine these new thermal IR data with existing 1-4um and 24um photometry to constrain the properties of the BDs and identify evidence for circumprimary/secondary disks in these unusual systems. We confirm that GSC 06214B is surrounded by a disk, further showing this disk produces a broadband IR excess due to small dust near the dust sublimation radius. An unresolved 24um excess in the system may be explained by the contribution from this disk. 1RXS 1609B exhibits no 3-4um excess, nor does its primary; however, the system as a whole has a modest 24um excess, which may come from warm dust around the primary and/or BD. Neither object in the HIP 78530 system exhibits near- to mid-IR excesses. We additionally find that the 1-4um colors of HIP 78530B match a spectral type of M3+-2, inconsistent with the M8 spectral type assigned based on its near-IR spectrum, indicating it may be a low-mass star rather than a BD. We present new upper limits on additional low-mass companions in the system (<5M_Jup beyond 175AU). Finally, we examine the utility of circumsecondary disks as probes of the formation histories of wide BD companions, finding that the presence of a disk may disfavor BD formation near the primary with subsequent outward scattering.