Analyzing JWST/NIRSpec Hydrogen Line Detections at TWA 27B: Constraining Accretion Properties and Geometry

Abstract

Hydrogen lines from forming planets are crucial for understanding planet formation. However, the number of planetary hydrogen line detections is still limited. Recent JWST/NIRSpec observations have detected Paschen and Brackett hydrogen lines at TWA 27 B (2M1207b). Although classified as a planetary- mass companison (PMC) rather than a planet due to its large mass ratio to the central star, TWA 27 B's hydrogen line emissions are expected to be same as the planetary one, given its small mass (≈5M _J). We aim to constrain the accretion properties and accretion geometry of TWA 27 B, contributing to our understanding of hydrogen-line emission mechanisms common to both PMCs and planets. We conduct spectral fitting of four bright hydrogen lines (Pa-α, Pa-β, Pa-γ, Pa-δ) with an accretion-shock emission model tailored for forming planets. We estimate the mass accretion rate at Ṁ≈3×10‑9MJyr‑1 with our fiducial parameters, though this is subject to an uncertainty of up to factor of ten. Our analysis also indicates a dense accretion flow, n ≳ 10¹³ cm^‑3 just before the shock, implying a small accretion-shock filling factor f _f on the planetary surface (f _f ≲ 5 × 10^‑4). This finding suggests that magnetospheric accretion is occurring at TWA 27 B. Additionally, we carry out a comparative analysis of hydrogen-line emission color to identify the emission mechanism, but the associated uncertainties proved too large for definitive conclusions. This underscores the need for further high-precision observational studies to elucidate these emission mechanisms fully.