2023AJ....166..216S

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 × 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.