2005/11/29 by F. Lahuis, E. F. van Dishoeck, A. C. A. Boogert +11 · 4 citations
Chemistry · Physics and Astronomy · #Astrophysics and Star Formation Studies #Molecular Spectroscopy and Structure #Stellar, planetary, and galactic studies #astro-ph
paper · pdf · doi:10.1086/500084
published as Astrophys.J.636:L145-L148,2006 · 4 pages, 4 figures; To appear in Astrophysical Journal Letters
arxiv created 2005/11/29 · openalex publication_date 2005/12/28 · arxiv updated 2009/12/01 · openalex created_date 2025/10/10 · openalex updated_date 2026/07/28
Spitzer Space Telescope spectra of the low-mass young stellar object (YSO) IRS 46 (Lbol~0.6 Lsolar) in Ophiuchus reveal strong vibration-rotation absorption bands of gaseous C2H2, 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 J=4-3 line is barely detected with the 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 Msolar 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 edge-on 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.