CO Fundamental Emission from V836 Tau

We present high resolution 4.7 micron CO fundamental spectroscopy of V836 Tau, a young star with properties that are between those of classical and weak T Tauri stars and which may be dissipating its circumstellar disk. We find that the CO line profiles of V836 Tau are unusual in that they are markedly double-peaked, even after correcting for stellar photospheric absorption in the spectrum. This suggests that the CO emission arises from a restricted range of disk radii (< 0.5 AU), in contrast to the situation for most classical T Tauri stars where the CO emission extends out to much larger radii (~ 1-2 AU). We discuss whether the outer radius of the emission in V836 Tau results from the physical truncation of the disk or an excitation effect. We also explore how either of these hypotheses may bear on our understanding of disk dissipation in this system.