Organic Molecules and Water in the Inner Disks of T Tauri Stars

Abstract

We report high signal-to-noise Spitzer IRS spectra of a sample of eleven classical T Tauri stars. Molecular emission from rotational transitions of H 2 O and OH and ro-vibrational bands of simple organic molecules (CO 2 , HCN, C 2 H 2 ) is common among the sources in the sample. The emission shows a range in both flux and line-to-continuum ratio for each molecule and in the flux ratios of different molecular species. The gas temperatures (200-800 K) and emitting areas we derive are consistent with the emission originating in a warm disk atmosphere in the inner planet formation region at radii < 2 AU. The H 2 O emission appears to form under a limited range of excitation conditions, as demonstrated by the similarity in relative strengths of H 2 O features from star to star and the narrow range in derived temperature and column density.Emission from highly excited rotational levels of OH is present in all stars; the OH emission flux increases with the stellar accretion rate, and the OH/H 2 O flux ratio shows a relatively small scatter. We interpret these results as evidence for OH production via FUV photo-dissociation of H 2 O in the disk surface layers. No obvious explanation is found for the observed range in the relative emission strengths of different organic molecules or in their strength with respect to water. We put forward the possibility that these variations reflect a diversity in organic abundances due to star-to-star differences in the C/O ratio of the inner disk gas. Stars with the largest HCN/H 2 O flux ratios in our sample have the largest disk masses. While larger samples are required to confirm this, we speculate that such a trend could result if higher mass disks are more efficient at planetesimal formation and sequestration of water in the outer disk, leading to enhanced C/O ratios and abundances of organic molecules in the inner disk. A comparison of our derived HCN to H 2 O column density ratio to comets, hot cores, and outer T Tauri star disks suggests that the inner disks are chemically active.

Exploring organic chemistry in planet-forming zones

“…This could imply that the C 2 H 2 /HCN abundance ratios probed through our edge-on disks presented are more similar to the hot cores and comets in Fig. 12 of Carr & Najita (2011) than to their protoplanetary disks. However, Salyk et al (2011a) and Mandell et al (2012) present C 2 H 2 /HCN abundance ratios of ∼1 for several protoplanetary disks, illustrating that different analysis methods of the emission data can lead to an order of magnitude different abundance ratios.…”

“…It is also interesting to compare the two observed C 2 H 2 /HCN abundance ratios of 0.6 and 0.8 for IRS 46 and GV Tau respectively with the one order of magnitude lower values presented in Carr & Najita (2011) for 5 protoplanetary disks in Taurus. Those ratios have been inferred from emission lines, whereas ours come from absorption data and may probe a different part of the disk (<1 AU vs up to a few AU).…”

Detection of warm water vapour in Taurus protoplanetary discs byHerschel

“…Carr & Najita (2011) detected six out of eleven stars: AA Tau, BP Tau, DK Tau, GI Tau, RW Aur, and UY Aur. Our sample contains all of their detected sources.…”

The chemistry of planet-forming regions is not interstellar

“…That the molecular emission from inner disks is indicative of an active chemistry has already been suggested, based on simple zero-dimensional calculations 7 , albeit with significant degeneracies 8 . As one step toward eliminating such retrieval degeneracies, we are developing more sophisticated models, along with additional constraints.…”

“…Many of the higher-quality spectra revealed a multitude of lines from many bulk carriers of the volatile elements carbon, oxygen and nitrogen 7,15 . The species that have so far been seen include H 2 O, CO, HCN, C 2 H 2 , CO 2 and OH.…”