Single peaked CO emission line profiles from the inner regions of protoplanetary disks

Abstract

Context. Protoplanetary disks generally exhibit strong line emission from the CO fundamental v = 1-0 ro-vibrational band around 4.7 μm. The lines are usually interpreted as being formed in the Keplerian disk, as opposed to other kinematic components of the young stellar system. Aims. This paper investigates a set of disks that show CO emission line profiles characterized by a single, narrow peak and a broad base extending to >50 km s-1, not readily explained by just Keplerian motions of gas in the inner disk. Methods. High resolution (R = 105) M-band spectroscopy has been obtained using CRIRES at the Very Large Telescope in order to fully resolve fundamental ro-vibrational CO emission line profiles around 4.7 μm. Results. Line profiles with a narrow peak and broad wings are found for 8 disks among a sample of ~50 disks around T Tauri stars with CO emission. The lines are very symmetric, have high line/continuum ratios and have central velocity shifts of < 5 km s-1 relative to the stellar radial velocity. The disks in this subsample are accreting onto their central stars at high rates relative to the parent sample. All 8 disks show CO emission lines from the v = 2 vibrational state and 4/8 disks show emission up to v = 4. Excitation analyses of the integrated line fluxes reveal a significant difference between typical rotational (∼300-800 K) and vibrational (∼1700 K) temperatures, suggesting that the lines are excited, at least in part, by UV-fluorescence. For at least one source, the narrow and broad components show different excitation temperatures, but generally the two component fits have similar central velocities and temperature. Analysis of their spatial distribution shows that the lines are formed within a few AU of the central star. Conclusions. It is concluded that these broad centrally peaked line profiles are inconsistent with the double peaked profiles expected from just an inclined disk in Keplerian rotation. Models in which the low velocity emission arises from large disk radii are excluded based on the small spatial distribution. Alternative non-Keplerian line formation mechanisms are discussed, including thermally and magnetically launched winds and funnel flows. The most likely interpretation is that the broad-based centrally peaked line profiles originate from a combination of emission from the inner part (