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Fig. 1


Ly-α line spectrum of TRAPPIST-1, plotted as a function of Doppler velocity in the heliocentric rest frame. The black line has been averaged over the out-of-transit observations, while the blue and orange spectra were obtained at the times of TRAPPIST-1b and c transits, respectively. The dashed black line indicates the velocity of the star, and the dashed red line the velocity of the ISM/LIC cloud. The red wing of the Ly-α line cannot be observed from Earth because of ISM absorption along the line of sight, and contamination from geocoronal emission (hatched region).

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