Probing the Inner Regions of Protoplanetary Disks with CO Absorption Line Spectroscopy

TL;DR

This study uses ultraviolet CO absorption spectroscopy from Hubble to probe the inner regions of protoplanetary disks, revealing the physical conditions and structure of the disk atmosphere or slow disk wind.

Contribution

It provides new measurements of CO absorption lines in protoplanetary disks, constraining gas densities, temperatures, and velocities, and offers insights into disk atmospheres and winds.

Findings

CO column densities range from 10^{16} to 10^{18} cm^{-2}

Rotational temperatures are between 300 and 700 K

Gas velocities suggest a disk atmosphere or slow wind

Abstract

Carbon monoxide (CO) is the most commonly used tracer of molecular gas in the inner regions of protoplanetary disks. CO can be used to constrain the excitation and structure of the circumstellar environment. Absorption line spectroscopy provides an accurate assessment of a single line-of-sight through the protoplanetary disk system, giving more straightforward estimates of column densities and temperatures than CO and molecular hydrogen emission line studies. We analyze new observations of ultraviolet CO absorption from the Hubble Space Telescope along the sightlines to six classical T Tauri stars. Gas velocities consistent with the stellar velocities, combined with the moderate-to-high disk inclinations, argue against the absorbing CO gas originating in a fast-moving disk wind. We conclude that the far-ultraviolet observations provide a direct measure of the disk atmosphere or possibly a slow disk wind. The CO absorption lines are reproduced by model spectra with column densities in the range N(^{12}CO) ~ 10^{16} - 10^{18} cm^{-2} and N(^{13}CO) ~ 10^{15} - 10^{17} cm^{-2}, rotational temperatures T_{rot}(CO) ~ 300 - 700 K, and Doppler b-values, b ~ 0.5 - 1.5 km s^{-1}. We use these results to constrain the line-of-sight density of the warm molecular gas (n_{CO} ~ 70 - 4000 cm^{-3}) and put these observations in context with protoplanetary disk models.