On the origin of [Ne II] emission in young stars: mid-infrared and optical observations with the Very Large Telescope

Abstract

Context. The [Ne II] line 12.81 μm was proposed to be a good tracer of gas in the environments of proto-planetary disks; its origin is explained by different mechanisms: jets in outflows, photo-evaporative disk winds driven by stellar X-rays/EUV or by the X-ray irradiated proto-planetary disk atmosphere. Previous Spitzer studies gave hints toward the neon emitting mechanism by exploring correlations between the line luminosity and properties of the star-disk system. These studies concluded that the origin of the emission is likely related to accretion and outflows, with some influence from X-rays. Aims. We provide direct constraints on the origin of the [Ne II] emission using high-spatial and spectral resolution observations that allow us to study the kinematics of the emitting gas. In addition we compare the [Ne II] line with optical forbidden lines. Methods. We obtained high-resolution ground-based observations with VISIR-VLT for 15 stars and UVES-VLT for three of them. The stars were chosen for having bright neon emission lines detected with Spitzer/IRS. The velocity shifts and profiles are used to disentangle the different emitting mechanisms producing the [Ne II] line. A comparison between results from this study and previous high-resolution studies is also presented. Results. The [Ne II] line was detected in seven stars, among them the first confirmed detection of [Ne II] in a Herbig Be star, V892 Tau. In four cases, the large blueshifted lines indicate an origin in a jet. In two stars, the small shifts and asymmetric profiles indicate an origin in a photo-evaporative wind. CoKu Tau 1, seen close to edge-on, shows a spatially unresolved line centered at the stellar rest velocity, although cross-dispersion centroids move within 10 AU from one side of the star to the other as a function of wavelength. The line profile is symmetric with wings extending up to ∼±80 km s −1 . The origin of the [Ne II] line is unclear and could either be due to the bipolar jet or to the disk. For the stars with VLT-UVES observations, in several cases, the optical forbidden line profiles and shifts are very similar to the profile of the [Ne II] line, suggesting that the lines are emitted in the same region. A general trend observed with VISIR is a lower line flux when compared with the fluxes obtained with Spitzer. We found no correlation between the line full-width at half maximum and the line peak velocity. The [Ne II] line remains undetected in a large part of the sample, an indication that the emission detected with Spitzer in those stars is likely extended.

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

“…Indeed, this line also shows two distinct components like the optical forbidden lines. Similar to the [O i] line, the HVC of the [Ne ii] is clearly associated with jets/outflows (it has been also spatially resolved toward one source; see van Boekel et al 2009), while the profile and peak velocity of the LVC are consistent with a photoevaporative disk wind driven by stellar X-ray/EUV photons (Pascucci & Sterzik 2009;Sacco et al 2012;Baldovin-Saavedra et al 2012). Thus, the [Ne ii] LVC traces either the EUV-heated or the X-ray-heated disk layer we discussed above in the context of the [O i] LVC (Meijerink et al 2008;Hollenbach & Gorti 2009;.…”

The complete far-infrared and submillimeter spectrum of the Class 0 protostar Serpens SMM1 obtained withHerschel

“…Indeed, C + is not detected in IRAS 4B outflow whereas it is detected in Serpens SMM1 protostar and outflow (see Fig. 4 However, they did not detect the [Ne ii]12.8 μm line that requires fast shock velocities (Lahuis et al 2007;Baldovin-Saavedra et al 2012) or X-ray radiation (see Güdel et al 2010, for Class II disk sources).…”

The Gas Disk: Evolution and Chemistry

“…Thanks to the high-spectral resolution achievable with ground-based spectrographs in the mid-infrared (R ∼ 30000, or 10 km s −1 for VISIR at VLT), lines could be spectrally resolved, and the nature of the emission (disk, disk wind, or jet) for most of the objects could finally be unveiled. At the time of writing the neon line has been observed with ground-based instruments in ∼50 objects (see , Najita et al 2009, Pascucci and Sterzik 2009, van Boekel et al 2009, Sacco et al 2012, Baldovin-Saavedra et al 2012. About 50 % of the [Ne II] lines detected with Spitzer are confirmed with ground based observations.…”

“…considered FUV and X-rays in their input stellar spectrum and found that the [O I] lines are emitted from dissociation of OH with only a minor contribution from thermal emission. Recent observational studies have focused on the study of optical forbidden lines (Baldovin-Saavedra et al 2012, Rigliaco et al 2013, Manara et al 2014. Baldovin-Saavedra et al (2012) obtained UVES (VLT) high-resolution spectra for three objects with a [Ne II] line detected from the ground.…”

“…Recent observational studies have focused on the study of optical forbidden lines (Baldovin-Saavedra et al 2012, Rigliaco et al 2013, Manara et al 2014. Baldovin-Saavedra et al (2012) obtained UVES (VLT) high-resolution spectra for three objects with a [Ne II] line detected from the ground. In two of the objects the optical and the mid-infrared tracers display the same velocity shift and FWHM (see Fig.…”