  TRAPPIST-1 is a nearby ultra-cool dwarf star transited by seven rocky
planets. We observed three transits of its outermost planet, TRAPPIST-1h, using
the G141 grism of the Wide Field Camera 3 instrument aboard the Hubble Space
Telescope to place constraints on its potentially cold atmosphere. In order to
deal with the effect of stellar contamination, we model TRAPPIST-1 active
regions as portions of a cooler and a hotter photosphere, and generate
multi-temperature models that we compare to the out-of-transit spectrum of the
star. Using the inferred spot parameters, we produce corrected transmission
spectra for planet h under five transit configurations and compare these data
to planetary atmospheric transmission models using the forward model CHIMERA.
Our analysis reveals that TRAPPIST-1h is unlikely to host an aerosol-free
H/He-dominated atmosphere. While the current data precision limits the
constraints we can put on the planetary atmosphere, we find that the likeliest
scenario is that of a flat, featureless transmission spectrum in the WFC3/G141
bandpass due to a high mean molecular weight atmosphere (>1000x solar), no
atmosphere, or an opaque aerosol layer, all in absence of stellar
contamination. This work outlines the limitations of modeling active
photospheric regions with theoretical stellar spectra, and those brought by our
lack of knowledge of the photospheric structure of ultracool dwarf stars.
Further characterization of the planetary atmosphere of TRAPPIST-1h would
require higher precision measurements over wider wavelengths, which will be
possible with the James Webb Space Telescope.
