A Rotating Accretion Disk around MWC 297, a Young B1.5Ve Star

High-resolution spectra with iSHELL on IRTF in the K and M bands of the young, heavily accreting B1.5Ve star MWC 297 show numerous double-peaked CO lines. These CO lines originate in an inclined gaseous disk in Keplerian rotation. MWC 297 is the only early B star known to show a Keplerian disk in CO. Analysis of the spectra shows that 12CO 1-0 is optically thick for the low excitation lines. Even the 13CO 1-0 and 12CO 2-1 have somewhat optically thick lines at low J levels. We find that the CO emission in the disk can be fitted with CO being in a narrow ring at a radius of 12 au, with a temperature of 1500 K and a CO column density of 1.6 x 1018 cm-2. This model underestimates the line strength of high-J lines, indicating that they are excited by fluorescence. The CO overtone lines have a similar temperature. The 13CO lines are much brighter than expected from interstellar isotope ratios. The 13CO lines are wider than the 12CO ones, suggesting different excitation conditions. The same is true for 12CO 2-1. We see strong absorption in 12CO and 13CO 1-0 at low J levels, which is due to two cold foreground clouds. These clouds, one with a temperature of 8.3 K and a column density of 6.7 1017 cm-2 and the other one colder and with lower column density, can fully account for the observed extinction toward MWC 297.