A search for helium in the atmospheres of three sub-Neptunes and a super-Earth around M-dwarfs

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Abstract Thousands of sub-Neptunes have been discovered mainly through space-based surveys such as the Kepler and Transiting Exoplanet Survey Satellite (TESS). Their bulk compositions and internal structures are thought to reflect their formation and evolutionary pathways, and atmospheric observations provide constraints on these processes. The near-infrared helium triplet is a potential tracer of extended, escaping ${\rm H/He}$ atmospheres. Recent models that include geometric effects suggest that planets orbiting nearby late M-dwarfs may offer favorable conditions for detecting this signal. Nevertheless, helium has been reported for only three planets around M-dwarfs to date, in contrast to the larger number of detections around especially K-dwarfs, motivating further surveys of M-dwarf systems. We conducted high-resolution transmission spectroscopy of three sub-Neptunes (TOI-2136b, TOI-654b, and LP 791-18c) and a super-Earth (TOI-1634b) orbiting M-dwarfs with the InfraRed Doppler (IRD) spectrograph on the Subaru Telescope. We find no statistically significant helium absorption in any target; accordingly, we derive 95% confidence upper limits on the helium line depth of 1.36%, 0.60%, 2.07%, and 3.00%, and on the equivalent width of 7.3, 2.1, 7.4, and 9.1 mÅ, for TOI-2136b, TOI-1634b, TOI-654b, and LP 791-18c, respectively. We further explored constraints on the upper atmospheric temperature and mass-loss rate by comparing these results with isothermal Parker-wind models. While we have compared with self-consistent ATES models of primordial ${\rm H/He}$ atmospheres spanning a range of assumed X-ray luminosities, changes in the assumed XUV flux do not appear to account for the non-detections. The results suggest that these planets have metal-enriched ${\rm H/He}$ primary atmospheres or non-primordial atmospheres, such as water-rich envelopes. Future observations of other absorption lines, such as Lyman-$\alpha$, H-$\alpha$, and H$_2$O, may provide further constraints on these atmospheres.

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