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Arxiv: Analyzing JWST/NIRSpec Hydrogen Line Detections at TWA 27B: Constraining Accretion Properties and Geometry Published: 7/22/2024 9:00:01 PM Updated: 7/24/2024 11:43:27 AM

Paper abstract: Hydrogen lines from forming planets are crucial for understanding planetformation. However, the number of planetary hydrogen line detections is stilllimited. Recent JWST/NIRSpec observations have detected Paschen and Bracketthydrogen lines at TWA 27 B (2M1207b). TWA 27 B is classified as aplanetary-mass companison (PMC) rather than a planet due to its large massratio to the central object (~ 5 M_\mathrm{J} compared to 25M_\mathrm{J}). Nevertheless, TWA 27 B's hydrogen line emission is expected tobe same as for planets, given its small mass. We aim to constrain the accretionproperties and accretion geometry of TWA 27 B, contributing to ourunderstanding of hydrogen line emission mechanism common to both PMCs andplanets. We conduct spectral fitting of four bright hydrogen lines(Pa-\alpha, Pa-\beta, Pa-\gamma, Pa-\delta) with an accretion-shockemission model tailored for forming planets. We estimate the mass accretionrate at \dot{M} ~ 3 \times 10^{-9}\, M_\mathrm{J}\,\mathrm{yr}^{-1}with our fiducial parameters, though this is subject to an uncertainty of up tofactor of ten. Our analysis also indicates a dense accretion flow, n\gtrsim10^{13}\,\mathrm{cm^{-3}} just before the shock, implying a smallaccretion-shock filling factor f_\mathrm{f} on the planetary surface(f_\mathrm{f} <~ 5\times10^{-4}). This finding suggests thatmagnetospheric accretion is occurring at TWA 27 B. Additionally, we carry out acomparative analysis of hydrogen-line emission color to identify the emissionmechanism, but the associated uncertainties proved too large for definitiveconclusions. This underscores the need for further high-precision observationalstudies to elucidate these emission mechanisms fully.