Abstract

Spitzer Space Telescopespectra of the low-mass young stellar object (YSO) IRS 46 ( ) in Ophiuchus L ≈ 0.6 L bol , reveal strong vibration-rotation absorption bands of gaseous C 2H2, HCN, and CO2. This is the only source out of a sample of ∼100 YSOs that shows these features, and this is the first time that they are seen in the spectrum of a solar-mass YSO. Analysis of the Spitzer data combined with Keck L- and M-band spectra reveals excitation temperatures of 350 K and abundances of 10 6 to 10 5 with respect to H2, orders of magnitude higher than those found in cold clouds. In spite of this high abundance, the HCN line is barely detected with the J p 4–3 James Clerk Maxwell Telescope (JCMT), indicating a source diameter less than 13 AU. The (sub)millimeter continuum emission and the absence of scattered light in near-infrared images limit the mass and temperature of any remnant collapsing envelope to less than 0.01 M, and 100 K, respectively. This excludes a hot-core–type region as found in high-mass YSOs. The most plausible origin of this hot gas rich in organic molecules is in the inner (!6 AU radius) region of the disk around IRS 46, either the disk itself or a disk wind. A nearly edgeon two-dimensional disk model fits the spectral energy distribution (SED) and gives a column of dense warm gas along the line of sight that is consistent with the absorption data. These data illustrate the unique potential of high-resolution infrared spectroscopy to probe the organic chemistry, gas temperatures, and gas kinematics in the planet-forming zones close to a young star. Subject headings: infrared: ISM — ISM: individual (IRS 46) — ISM: jets and outflows — ISM: molecules — planetary systems: protoplanetary disks — stars: formation

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