Emission from Water Vapor and Absorption from Other Gases at 5-7.5 Microns in Spitzer-IRS Spectra of Protoplanetary Disks

TL;DR

This study analyzes Spitzer-IRS spectra of 13 T Tauri stars, revealing water vapor emission and absorption features from other gases in their protoplanetary disks, indicating diverse molecular compositions within the inner disk regions.

Contribution

First detailed analysis of 5-7.5 micron spectra showing water vapor emission and absorption in T Tauri star disks, identifying molecules like H2O, H2CO, and HCOOH.

Findings

Seven stars show water vapor emission at 6.6 microns.

Six stars exhibit absorption features possibly from H2CO or HCOOH.

Gas molecules are located within the inner few AU of the disks.

Abstract

We present spectra of 13 T Tauri stars in the Taurus-Auriga star-forming region showing emission in Spitzer Space Telescope Infrared Spectrograph (IRS) 5-7.5 micron spectra from water vapor and absorption from other gases in these stars' protoplanetary disks. Seven stars' spectra show an emission feature at 6.6 microns due to the nu_2 = 1-0 bending mode of water vapor, with the shape of the spectrum suggesting water vapor temperatures > 500 K, though some of these spectra also show indications of an absorption band, likely from another molecule. This water vapor emission contrasts with the absorption from warm water vapor seen in the spectrum of the FU Orionis star V1057 Cyg. The other six of the thirteen stars have spectra showing a strong absorption band, peaking in strength at 5.6-5.7 microns, which for some is consistent with gaseous formaldehyde (H2CO) and for others is consistent with gaseous formic acid (HCOOH). There are indications that some of these six stars may also have weak water vapor emission. Modeling of these stars' spectra suggests these gases are present in the inner few AU of their host disks, consistent with recent studies of infrared spectra showing gas in protoplanetary disks.