  Spitzer Space Telescope spectra of the low mass young stellar object (YSO)
IRS 46 (L_bol ~ 0.6 L_sun) 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 YSO's that shows these features and the first time they are
seen in the spectrum of a solar-mass YSO. Analysis of the Spitzer data combined
with Keck L- and M-band spectra gives excitation temperatures of > 350 K and
abundances of 10(-6)-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, indicating a source
diameter less than 13 AU. The (sub)millimeter continuum emission and the
absence of scattered light in near-infrared images limits the mass and
temperature of any remnant collapse envelope to less than 0.01 M_sun and 100 K,
respectively. This excludes a hot-core type region as found in high-mass YSO's.
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 2-D 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 organic chemistry,
gas temperatures and kinematics in the planet-forming zones close to a young
star.
