JWST/NIRSpec Observations of the Planetary Mass Companion TWA 27B

Abstract

We present 1–5 μ m spectroscopy of the young planetary mass companion TWA 27B (2M1207B) performed with NIRSpec on board the James Webb Space Telescope. In these data, the fundamental band of CH _4 is absent, and the fundamental band of CO is weak. The nondetection of CH _4 reinforces a previously observed trend of weaker CH _4 with younger ages among L dwarfs, which has been attributed to enhanced nonequilibrium chemistry among young objects. The weakness of CO may reflect an additional atmospheric property that varies with age, such as the temperature gradient or cloud thickness. We are able to reproduce the broad shape of the spectrum with an ATMO cloudless model that has T _eff = 1300 K, nonequilibrium chemistry, and a temperature gradient reduction caused by fingering convection. However, the fundamental bands of CH _4 and CO are somewhat stronger in the model. In addition, the model temperature of 1300 K is higher than expected from evolutionary models given the luminosity and age of TWA 27B ( T _eff = 1200 K). Previous models of young L-type objects suggest that the inclusion of clouds could potentially resolve these issues; it remains to be seen whether cloudy models can provide a good fit to the 1–5 μ m data from NIRSpec. TWA 27B exhibits emission in Paschen transitions and the He I triplet at 1.083 μ m, which are signatures of accretion that provide the first evidence of a circumstellar disk. We have used the NIRSpec data to estimate the bolometric luminosity of TWA 27B (log L / L _⊙ = −4.466 ± 0.014), which implies a mass of 5–6 M _Jup according to evolutionary models.

Keywords

• Brown dwarfs
• L dwarfs
• Exoplanet atmospheres
• Circumstellar disks
• Protoplanetary disks
• Substellar companion stars