Transmission spectroscopy is currently the most powerful technique to study a
wide range of planetary atmospheres, leveraging the filtering of a stars light
by a planets atmosphere rather than its own emission. However, both a planet
and its star contribute to the information encoded in a transmission spectrum
and a particular challenge relate to disentangling their contributions. As
measurements improve, the lack of fidelity of stellar spectra models present a
bottleneck for accurate disentanglement. Considering JWST and future
high-precision spectroscopy missions, we investigate the ability to derive
empirical constraints on the emission spectra of stellar surface
heterogeneities (i.e., spots and faculae) using the same facility as used to
acquire the transmission spectra intended to characterize a given atmosphere.
Using TRAPPIST-1 as a test case, we demonstrate that it is possible to
constrain the photospheric spectrum to 0.2% and the spectra of stellar
heterogeneities to within 1-5%, which will be valuable benchmarks to inform the
new generation of theoretical stellar models. Long baseline of observations
(90% of the stellar rotation period) are necessary to ensure the photon-limited
(i.e., instrument-limited) exploration of exoplanetary atmospheres via
transmission spectroscopy.Comment: 10 pages, 3 figure