Understanding the Origin of the [O I] Low-Velocity Component From T Tauri Stars

Abstract

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 μm and CO at 4.7 μm 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] λ6300/λ5577 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 ∼40 and ∼60 km s −1 ), and a narrow component (with FWHM ∼ 10 km s −1 and small blueshifts of ∼2 km s −1 ) that may arise in a cool ( 1000 K) molecular wind.

Disk Dispersal: Theoretical Understanding and Observational Constraints

“…For EUV-heated, fully ionized winds, data is consistent with mass loss rates of ∼ 10 −10 M ⊙ yr −1 , while partly neutral X-ray heated gas implies higher rates ∼ 10 −9 to 10 −8 M ⊙ yr −1 . Other low velocity wind tracers such as [OI] forbidden line emission and CO rovibrational emission are more difficult to interpret with contributions from multiple components (Rigliaco et al 2013). For a more in-depth discussion, see Alexander et al (2014).…”

X-shooter spectroscopy of young stellar objects

“…It could be emitted at the base of a magnetically driven disk wind, as suggested by Hartigan et al (1995) for the O  lines, but it can also be a tracer of a photoevaporative disk wind, as shown by, e.g., Pascucci & Sterzik (2009) for the LVC of the Ne  mid-IR emission lines. Lately, Rigliaco et al (2013) have shown that the LVC O  lines could also have multiple components, with one component tracing gas in Keplerian rotation and another component tracing a photoevaporative wind. Acke et al (2005) find that in Herbig Ae/Be stars, the [O ] 630.03 nm emission could come from the disk surface layers.…”

X-Shooter study of accretion inρ-Ophiucus: very low-mass stars and brown dwarfs

“…Star-disk interaction processes can be studied observationally through the strong signatures they introduce in the spectra of YSOs. Accretion shocks give rise to continuum excess emission in the UV (e.g., Valenti et al 1993;Gullbring et al 1998Gullbring et al , 2000Calvet et al 2000) and the prominent emission of permitted lines across the whole spectrum (e.g., Muzerolle et al 1998aMuzerolle et al ,b,c, 2003Natta et al 2004), while winds are traced by various forbidden emission lines (e.g., Hartigan et al 1995;Rigliaco et al 2013;Natta et al 2014). In recent years, new instruments have provided the possibility of studying these processes simultaneously in large samples of objects.…”