Characterizing CO Fourth Positive Emission in Young Circumstellar Disks

TL;DR

This study analyzes ultraviolet CO emission in young star disks, revealing how it relates to disk properties and providing insights into the gas conditions in protoplanetary environments.

Contribution

It introduces a method to model UV CO emission in T Tauri stars, linking emission characteristics to disk accretion and age, and distinguishes UV-emitting CO from IR-emitting CO.

Findings

UV CO emission correlates with accretion rate and age.

Measured CO excitation temperatures are below 600 K.

UV CO emission originates from a different CO population than IR emission.

Abstract

Carbon Monoxide is a commonly used IR/sub-mm tracer of gas in protoplanetary disks. We present an analysis of ultraviolet CO emission in {HST}-COS spectra for 12 Classical T Tauri stars. Several ro-vibrational bands of the CO A^1\Pi - X^1\Sigma^+ (Fourth Positive) electronic transition system are spectrally resolved from emission of other atoms and H_2. The CO A^1\Pi v'=14 state is populated by absorption of Ly\alpha photons, created at the accretion column on the stellar surface. For targets with strong CO emission, we model the Ly\alpha radiation field as an input for a simple fluorescence model to estimate CO rotational excitation temperatures and column densities. Typical column densities range from N_{CO} = 10^{18} - 10^{19} cm^{-2}. Our measured excitation temperatures are mostly below T_{CO} = 600 K, cooler than typical M-band CO emission. These temperatures and the emission line widths imply that the UV emission originates in a different population of CO than that which is IR-emitting. We also find a significant correlation between CO emission and the disk accretion rate M_{acc} and age. Our analysis shows that ultraviolet CO emission can be a useful diagnostic of CTTS disk gas.