UNDERSTANDING THE ORIGIN OF THE [O I] LOW-VELOCITY COMPONENT FROM T TAURI STARS

The formation time, masses, and location of planets are strongly impacted by the physical mechanisms that disperse protoplanetary disks and the timescale over which protoplanetary material is cleared out. Accretion of matter onto the central star, protostellar winds/jets, magnetic disk winds, and photoevaporative winds operate concurrently. Hence, disentangling their relative contribution to disk dispersal requires identifying diagnostics that trace different star-disk environments. Here, we analyze the low-velocity component (LVC) of the oxygen optical forbidden lines, which is found to be blueshifted by a few km s(-1) with respect to the stellar velocity. We find that the [O I] LVC profiles are different from those of [Ne II] at 12.81 mu m and CO at 4.7 mu m lines pointing to different origins for these gas lines. We report a correlation between the luminosity of the [O I] LVC and the accretion luminosity L-acc. We do not find any correlation with the X-ray luminosity, while we find that the higher is the stellar far-UV (FUV) luminosity, the higher is the luminosity of the [O I] LVC. In addition, we show that the [O I] lambda 6300/lambda 5577 ratio is low (ranging between 1 and 8). These findings favor an origin of the [O I] LVC in a region where OH is photodissociated by stellar FUV photons and argue against thermal emission from an X-ray-heated layer. Detailed modeling of two spectra with the highest S/N and resolution shows that there are two components within the LVC: a broad, centrally peaked component that can be attributed to gas arising in a warm disk surface in Keplerian rotation (with FWHM between similar to 40 and similar to 60 km s(-1)), and a narrow component (with FWHM similar to 10 km s(-1) and small blueshifts of similar to 2 km s(-1)) that may arise in a cool (<= 1000 K) molecular wind.