Measurement of interstellar extinction for classical T Tauri stars using far-UV H2 line fluxes

TL;DR

This study measures interstellar extinction in classical T Tauri stars using far-UV H2 line fluxes from Hubble data, confirming previous optical estimates and exploring the assumptions behind the method.

Contribution

It introduces a novel approach to determine extinction using far-UV H2 lines and assesses the validity of the optical thinness assumption in these measurements.

Findings

Results largely agree with optical extinction values.

Confirmed degeneracy between $A_{V}$ and $R_{V}$ and discussed ways to resolve it.

Evaluated the assumption of optical thinness of lines.

Abstract

Understanding the interstellar and potentially circumstellar extinction in the sight lines of classical T Tauri stars is an important ingredient for constructing reliable spectral energy distributions, which catalyze protoplanetary disk chemistry, for example. Therefore, some attempts of measuring $A_{V}$ toward individual stars have been made using partly different wavelength regimes and different underlying assumptions. We used strong lines of Ly{\alpha} fluorescent H2 and derived the extinction based on the assumption of optically thin transitions. We investigated a sample of 72 classical T Tauri stars observed with the Hubble Space Telescope in the framework of the ULLYSES program. We computed $A_{V}$ and $R_{V}$ values for the 34 objects with sufficient data quality and an additionally $A_{V}$ value for the canonical $R_{V}$ = 3.1 value. Our results agree largely with values obtained from optical data. Moreover, we confirm the degeneracy between $A_{V}$ and $R_{V}$ and present possibilities to break this. Finally, we discuss whether the assumption of optical thin lines is valid.