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

• Published in: The Astronomical journal Vol. 168; no. 4; pp. 155 - 164
• Main Authors: Aoyama, Yuhiko, Marleau, Gabriel-Dominique, Hashimoto, Jun
• Format: Journal Article
• Language: English
• Published: Madison The American Astronomical Society 01.10.2024; IOP Publishing
• Subjects: Accretion; Emission analysis; Exoplanet formation; Extrasolar gaseous giant planets; H I line emission; Hydrogen; James Webb Space Telescope; Line spectra; Magnetospheres; Observational studies; Parameter uncertainty; Planet formation; Planetary surfaces; Planets; Spectral energy distribution; Uncertainty analysis
• ISSN: 0004-6256; 1538-3881
• DOI: 10.3847/1538-3881/ad67df

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 (≈5 M 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 M ̇ ≈ 3 × 10 − 9 M J yr − 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 ≳ 10 13 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.