UV Fluorescence Traces Gas and LyA Evolution in Protoplanetary Disks

Kavli Affiliate: Gregory J. Herczeg

| First 5 Authors: Nicole Arulanantham, Kevin France, Keri Hoadley, P. C. Schneider, Catherine C. Espaillat

| Summary:

Ultraviolet spectra of protoplanetary disks trace distributions of warm gas
at radii where rocky planets form. We combine HST-COS observations of H2 and CO
emission from 12 classical T Tauri stars to more extensively map inner disk
surface layers, where gas temperature distributions allow radially stratified
fluorescence from the two species. We calculate empirical emitting radii for
each species under the assumption that the line widths are entirely set by
Keplerian broadening, demonstrating that the CO fluorescence originates further
from the stars (r ~ 20 AU) than the H2 (r ~ 0.8 AU). This is supported by 2-D
radiative transfer models, which show that the peak and outer radii of the CO
flux distributions generally extend further into the outer disk than the H2.
These results also indicate that additional sources of LyA photons remain
unaccounted for, requiring more complex models to fully reproduce the molecular
gas emission. As a first step, we confirm that the morphologies of the UV-CO
bands and LyA radiation fields are significantly correlated and discover that
both trace the degree of dust disk evolution. The UV tracers appear to follow
the same sequence of disk evolution as forbidden line emission from jets and
winds, as the observed LyA profiles transition between dominant red wing and
dominant blue wing shapes when the high-velocity optical emission disappears.
Our results suggest a scenario where UV radiation fields, disk winds and jets,
and molecular gas evolve in harmony with the dust disks throughout their

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