HST/COS Spectra of DF Tau and V4046 SGR: First Detection of Molecular Hydrogen Absorption Against the Ly$\alpha$ Emission Line

TL;DR

This study reports the first detection of molecular hydrogen absorption in the Lyman-alpha emission profiles of two classical T Tauri stars, revealing insights into their circumstellar environments and accretion processes.

Contribution

It presents the first observation of H$_{2}$ absorption against Lyman-alpha in CTTSs and analyzes the H I absorption needed to explain the emission ratios, enhancing understanding of star-disk interactions.

Findings

H$_{2}$ absorption detected in both stars' Lyman-alpha profiles.

H I absorption velocities suggest different origins: disk/interstellar medium for DF Tau, accretion columns for V4046 Sgr.

Corrected H$_{2}$ emission ratios close to unity after accounting for H I absorption.

Abstract

We report the first detection of molecular hydrogen (H$_{2}$) absorption in the Lyman-$\alpha$ emission line profiles of two classical T Tauri stars (CTTSs), DF Tau and V4046 Sgr, observed by \emph{HST}/COS. This absorption is the energy source for many of the Lyman-band H$_{2}$ fluorescent lines commonly seen in the far-ultraviolet spectra of CTTSs. We find that the absorbed energy in the H$_{2}$ pumping transitions from a portion of the Lyman-$\alpha$ line significantly differ from the amount of energy in the resulting fluorescent emission. By assuming additional absorption in the H I Lyman-$\alpha$ profile along our light of sight, we can correct the H$_{2}$ absorption/emission ratios so that they are close to unity. The required H I absorption for DF Tau is at a velocity close to the radial velocity of the star, consistent with H I absorption in the edge-on disk and interstellar medium. For V4046 Sgr, a nearly face-on system, the required absorption is between +100 km/s and +290 km/s, most likely resulting from H I gas in the accretion columns falling onto the star.