GEMS JWST: Transmission spectroscopy of TOI-5205b reveals significant stellar contamination and a metal-poor atmosphere

Abstract

Recent discoveries of transiting giant exoplanets ($R_p\gtrsim 8{\mathrm{R}}_{\oplus }$) around M dwarfs (GEMS) present an opportunity to investigate their atmospheric compositions and explore how such massive planets form around low-mass stars contrary to the prediction from formation models. We present the first transmission spectra of TOI-5205b, a short-period ($P=1.63\mathrm{days}$) Jupiter-like planet ($M_p=1.08{\mathrm{M}}_{\mathrm{J}}$ and $R_p=0.94{\mathrm{R}}_{\mathrm{J}}$) orbiting an M4 dwarf ($M_{\star }=0.392{\mathrm{M}}_{\odot }$, $R_{\star }=0.394{\mathrm{R}}_{\odot }$). We obtained three transits using the PRISM mode of the JWST Near Infrared Spectrograph (NIRSpec) spanning $0.6-5.3$ um. The data reveal significant stellar contamination that is evident in the light curves as spot-crossing events and in the transmission spectra as a larger transit depth at bluer wavelengths. Atmospheric retrievals demonstrate that stellar contamination from unocculted starspots and faculae is the dominant component of the transmission spectrum at wavelengths $\lambda \lesssim 3.0$ um, reducing the sensitivity to the presence of clouds or hazes in our models and preventing detection of ${\mathrm{H}}_2\mathrm{O}$. The wavelength coverage enabled a robust detection of $\mathrm{C}{\mathrm{H}}_4$ and ${\mathrm{H}}_2\mathrm{S}$, which have detectable molecular features between $3.0-5.0$ um. For both clear or cloudy atmospheres, Bayesian retrievals consistently favored an atmosphere with sub-solar metallicity ($3\sigma$ upper limit of $\log [M/H]\lesssim -1.24$) and super-solar C/O ratio ($3\sigma$ lower limit of $\log [C/O]\gtrsim 0.09$), although this may partly be driven by the non-detection of water due to stellar contamination. Planetary interior models predict a bulk metallicity of 10--20\%, which is larger than the atmospheric metallicity and suggests that the interior of TOI-5205b is decoupled from its atmosphere.

29 pages + appendix, 23 figures including 6 figure sets, 13 tables, data behind the figures will appear in journal, accepted for publication in AJ. Updates from first version: improvements to the spot fitting and associated retrievals. Updates from second version: abstract revised to satisfy journal word limit

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