2025/06/13 by David Berardo, Berardo, David, Julien de Wit +31
Physics and Astronomy · #Adaptive optics and wavefront sensing #Astronomy and Astrophysical Research #History and Developments in Astronomy
paper · pdf · doi:10.48550/arxiv.2506.12140
Ly-α 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~EOH/Gyr for planet b (1~EOH is the Earth-ocean-equivalent hydrogen content), in agreement with recent claims that planet b should be airless. These upper limits are ∼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-α variability from frequent ``microflares.'' While JWST observations of TRAPPIST-1 in the near infrared have shown that ∼1030-erg flares occur every ∼6 hours, we report here ∼1029-erg flares on sub-hour timescales in the HST/STIS and also Very Large Telescope (VLT) g' observations. The FUV and optical amplitudes (∼400% vs ∼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 ± 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.