Revisiting the Helium and Hydrogen Accretion Indicators at TWA 27B: Weak Mass Flow at Near-Freefall Velocity

Abstract

TWA 27B (2M1207b) is the first directly-imaged planetary-mass (MP ~ 5 MJ) companion (Chauvin et al. 2004) and was observed at 0.9--5.3 micron with JWST/NIRSpec (Luhman et al. 2023). To understand the accretion properties of TWA 27B, we search for continuum-subtracted near-infrared helium and hydrogen emission lines and measure their widths and luminosities. We detect the He I triplet at 4.3 sigma and all Paschen-series lines covered by NIRSpec (Pa alpha, Pa beta, Pa gamma, Pa delta) at 4--5 sigma. The three brightest Brackett-series lines (Br alpha, Br beta, Br gamma) as well as Pf gamma and Pf delta are tentative detections at 2--3 sigma. We provide upper limits on the other hydrogen lines, including on H alpha through Hubble Space Telescope archival data. Three lines can be reliably deconvolved to reveal an intrinsic width Delta v = 67+-9 km/s, which is 60% of the surface freefall velocity. The line luminosities seem significantly too high to be due to chromospheric activity. Converting line luminosities to an accretion rate yields Mdot ~ 5e-9 MJ/yr when using scaling relationships for planetary masses, and Mdot ~ 0.1e-9 MJ/yr with extrapolated stellar scalings. Several of these lines represent first detections at an accretor of such low mass. The weak accretion rate implies that formation is likely over. This analysis shows that JWST can be used to measure low line-emitting mass accretion rates onto planetary-mass objects, motivates deeper searches for the mass reservoir feeding TWA 27B, and hints that other young directly-imaged objects might -- hitherto unbeknownst -- also be accreting.