Kavli Affiliate: Avi Shporer
| First 5 Authors: Michael Zhang, Jacob L. Bean, David Wilson, Girish Duvvuri, Christian Schneider
| Summary:
TOI-836 is a ~2-3 Gyr K dwarf with an inner super Earth ($R=1.7 R_oplus$,
$P=3.8$ d) and an outer mini Neptune ($R=2.6 R_oplus$, $P=8.6$ d).
JWST/NIRSpec 2.8–5.2 $mu$m transmission spectra are flat for both planets. We
present Keck/NIRSPEC observations of escaping helium for super-Earth b, which
shows no excess absorption in the 1083 nm triplet to deep limits (<0.2%), and
mini-Neptune c, which shows strong (0.7%) excess absorption in both visits.
These results demonstrate that planet c retains at least some primordial
atmosphere, while planet b is consistent with having lost its entire primordial
envelope. Self-consistent 1D radiative-hydrodynamic models of planet c reveal
that the helium excess absorption signal is highly sensitive to metallicity:
its equivalent width collapses by a factor of 13 as metallicity increases from
10x to 100x solar, and by a further factor of 12 as it increases to 200x solar.
The observed equivalent width is 88% the model prediction for 100x
metallicity, suggesting an atmospheric metallicity similar to K2-18b and
TOI-270d, the first two mini-Neptunes with detected absorption features in JWST
transmission spectra. We highlight the helium triplet as a potentially powerful
probe of atmospheric composition, with complementary strengths and weaknesses
to atmospheric retrievals. The main strength is its extreme sensitivity to
metallicity in the scientifically significant range of 10–200x solar, and the
main weakness is the enormous model uncertainties in outflow suppression and
confinement mechanisms, such as magnetic fields and stellar winds, which can
suppress the signal by at least a factor of ~several.
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