Kavli Affiliate: Sara Seager
| First 5 Authors: David Berardo, Julien de Wit, Michael Gillon, Ward S. Howard, Vincent Bourrier
| Summary:
Ly-$alpha$ observations provide a powerful probe of stellar activity and
atmospheric escape in exoplanetary systems. We present here an analysis of 104
HST/STIS orbits monitoring the TRAPPIST-1 system between 2017 and 2022,
covering 3–5 transits for each of its seven planets. We rule out transit
depths $gtrsim20%$, which translates into an upper limit on the escape rate
of $1064~EO_H$/Gyr for planet b ($1~EO_H$ is the Earth-ocean-equivalent
hydrogen content), in agreement with recent claims that planet b should be
airless. These upper limits are $sim$3 times larger than expected from the
photon noise due to a large baseline scatter, which we ultimately link to
TRAPPIST-1’s intrinsic Ly-$alpha$ variability from frequent “microflares.”
While JWST observations of TRAPPIST-1 in the near infrared have shown that
$sim10^{30}$-erg flares occur every $sim$6 hours, we report here
$sim10^{29}$-erg flares on sub-hour timescales in the HST/STIS and also Very
Large Telescope (VLT) $g^{‘}$ observations. The FUV and optical amplitudes
($sim$400$%$ vs $sim$3$%$, respectively) for flares with similar
waiting-times indicate flare temperatures of 11000$^{+4200}_{-3100}$~K over
0.011$^{+0.03}_{-0.01}$% of the stellar disk. Finally, our multi-year baseline
reveals a variability with $P = 3.27 pm 0.04$ days, providing further
validation of the previously reported 3.295-day rotation period for TRAPPIST-1.
These results highlight the importance of accounting for stellar
microvariability when searching for exospheres around active M dwarfs.
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