Kavli Affiliate: Avi Shporer
| First 5 Authors: Michael Zhang, Jacob L. Bean, David Wilson, Girish Duvvuri, Christian Schneider
| Summary:
TOI-836 is a $sim2-3$ Gyr K dwarf with an inner super Earth
($R=1.7,R_oplus$, $P=3.8,d$) and an outer mini Neptune ($R=2.6,R_oplus$,
$P=8.6,d$). Recent JWST/NIRSpec 2.8–5.2 $mu$m observations have revealed
flat transmission spectra for both planets. We present Keck/NIRSPEC
observations of escaping helium from this system. While planet b shows no
absorption in the 1083 nm line to deep limits ($<0.2$%), 836c shows strong
(0.7%) absorption in both visits. These results demonstrate that the inner
super-Earth has lost its primordial atmosphere while the outer mini-Neptune has
not. Self-consistent 1D radiative-hydrodynamic models of c using pyTPCI, an
updated version of The PLUTO-CLOUDY Interface, reveal that the helium 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% of the
model prediction for 100x metallicity, suggesting that c may have an
atmospheric metallicity close to 100x solar. This is 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.
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