  We present a 4.5-4.85 micron R=5000 spectrum of the low mass class I young
stellar object GSS 30 IRS1 (L=25L_sun) in the rho Ophiuchus core, observed with
the infrared spectrometer (ISAAC) on the Very Large Telescope (VLT-UT1). Strong
line emission from the ro-vibrational transitions of 12CO and 13CO is detected.
In total more than 40 distinct lines are seen in the covered region. The line
emission is spatially extended and detected up to 2" = 320 AU from the central
source but is spectrally unresolved (Delta v < 30 km/s). This is the first time
strong emission in the fundamental ro-vibrational band from CO has been
observed from an embedded young stellar object. The line fluxes were modeled
using a 1-dimensional full radiative transfer code, which shows that the
emission is fully consistent with a gas in LTE at a single well constrained
temperature (T=515+/-5 K). Furthermore, the ratios between lines from the two
detected isotopic species of CO show that the 12CO lines must be optically
thick. However, this is inconsistent with the observed spatial extent of the
emission, since this implies such low CO column densities that the lines are
optically thin. A likely solution to the discrepancy is that the lines are
emitted by a smaller more dense region and then scattered in the bipolar cavity
present around the central star. This gives a rough estimate of the total
molecular gas mass of 1-100 M_earth and a physical extent of ~20-100 AU. We
propose that the most likely origin of the line emission is post-shocked gas in
a dense dissociative accretion shock from the inner 10-50 AU of a circumstellar
disk. The presence of a shock capable of dissociating molecules in the disk
will have implications for the chemical evolution of disks around young low
mass stars.
