JWST detection of methane emission from a young brown dwarf

Abstract

In recent studies, we have used low-resolution spectroscopy (R~100) with the James Webb Space Telescope (JWST) to uncover low-mass brown dwarfs (2-10 MJup) in the young cluster IC 348 that exhibit the so-called 3.4um absorption band from aliphatic hydrocarbons. Atmospheric models have not predicted the presence of that band in brown dwarfs at any age. Instead, methane absorption was expected in the coolest newborn brown dwarfs, but it was not detected. We have performed JWST spectroscopy at higher resolution (R~1000) on one of the brown dwarfs in IC 348, LRL 11001 (~6 MJup). The new measurement of the 3.4um band is fit well with four Gaussians that represent the CH stretching modes of CH2 and CH3 groups, indicating that the carrier is a large alkane (CnH2n+2). In addition, the spectrum contains emission lines at 3.324 and 3.333um, which we attribute to Q branches of CH4 hot bands between the octad and dyad regions (primarily nu3+nu4->nu4) and the tetradecad and pentad regions. This is the first detection of 3.33um CH4 emission from a young brown dwarf. The emission probably arises from the photosphere rather than a circumstellar disk given that no disks around stars or brown dwarfs are known to exhibit emission in these transitions. Modeling of the emission is needed to constrain the excitation mechanism, which could be either thermal (e.g., an accretion shock) or pumping by external radiation (e.g., neighboring cluster members). We speculate that vertical mixing has suppressed CH4 and enhanced large alkanes in the photosphere, but CH4 survives in the upper atmosphere, where it is available to experience the observed emission.