Lyman alpha dominance of the Classical T Tauri FUV Radiation Field

TL;DR

This study uses Hubble Space Telescope spectra to demonstrate that Lyman alpha radiation overwhelmingly dominates the FUV emission in Classical T Tauri Stars, significantly influencing disk chemistry.

Contribution

It provides the first direct and indirect measurements confirming Lyman alpha as the primary FUV component in CTTSs, using H2 fluorescence to reconstruct Lyman alpha profiles.

Findings

Lyman alpha accounts for 70-90% of FUV flux in surveyed CTTSs.

Lyman alpha flux correlates with overall FUV flux, indicating an accretion-related origin.

Reconstructed Lyman alpha profiles reveal its dominance in the FUV radiation field.

Abstract

Far-ultraviolet (FUV) radiation plays an important role in determining chemical abundances in protoplanetary disks. HI Lyman alpha is suspected to be the dominant component of the FUV emission from Classical T Tauri Stars (CTTSs), but is difficult to measure directly due to circumstellar and interstellar HI absorption. To better characterize the intrinsic Lyman alpha radiation, we present FUV spectra of 14 CTTSs taken with the Hubble Space Telescope COS and STIS instruments. H2 fluorescence, commonly seen in the spectra of CTTSs, is excited by Lyman alpha photons, providing an indirect measure of the Lyman alpha flux incident upon the warm disk surface. We use observed H2 progression fluxes to reconstruct the CTTS Lyman alpha profiles. The Lyman alpha flux correlates with total measured FUV flux, in agreement with an accretion-related source of FUV emission. With a geometry-independent analysis, we confirm that in accreting T Tauri systems Lyman alpha radiation dominates the FUV flux (~1150 - 1700 Angstroms). In the systems surveyed this one line comprises 70 - 90 % of the total FUV flux.