Bright CO ro-vibrational emission lines in the class I source GSS 30 IRS1: Probing the inner disk of a young embedded star

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