Searching for gas emission lines inSpitzerInfrared Spectrograph (IRS) spectra of young stars in Taurus

Abstract

Context. Our knowledge of circumstellar disks has traditionally been based on studies of dust. However, gas dominates the disk mass and its study is key to our understanding of accretion, outflows, and ultimately planet formation. The Spitzer Space Telescope provides access to gas emission lines in the mid-infrared, providing crucial new diagnostics of the physical conditions in accretion disks and outflows. Aims. We seek to identify gas emission lines in mid-infrared spectra of 64 pre-main-sequence stars in Taurus. Using line luminosities and other known star-disk-outflow parameters, we aim to identify correlations that will help to constrain gas heating, excitation mechanisms, and the line formation. Methods. We have based our study on Spitzer observations using the Infrared Spectrograph (IRS), mainly with the high-resolution modules. Line luminosities (or 3σ upper limits) have been obtained by fitting Gaussian profiles to the lines. We have further searched for correlations between the line luminosities and different parameters related to the star-disk system. Results. We have detected H 2 (17.03, 28.22 μm) emission in 6 objects, [Ne II] (12.81 μm) emission in 18 objects, and [Fe II] (17.93, 25.99 μm) emission in 7 objects. [Ne II] detections are found primarily in Class II objects. The luminosity of the [Ne II] line (L NeII ) is in general higher for objects known to drive jets than for those without known jets, but the two groups are not statistically distinguishable. L NeII is correlated with X-ray luminosity, but for Class II objects only. L NeII is also correlated with disk mass and accretion rate when the sample is divided into high and low accretors. Furthermore, we find correlations of L NeII with mid-IR continuum luminosity and with luminosity of the [O I] (6300 Å) line, the latter being an outflow tracer. L [FeII] correlates withṀ acc . No correlations were found between L H 2 and several tested parameters. Conclusions. Our study reveals a general trend toward accretion-related phenomena as the origin of the gas emission lines. Shocks in jets and outflowing material are more likely to play a significant role than shocks in infalling material. The role of X-ray irradiation is less prominent but still present for [Ne II], in particular for Class II sources, while the lack of correlation between [Fe II] and [Ne II] points toward different emitting mechanisms.

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

“…They concluded that [Ne II] emission originates mainly in shocks for Class I protostars. This supports the statistical studies of Güdel et al (2010) and Baldovin-Saavedra et al (2011) that jets, if present, tend to dominate the [Ne II] emission. Sacco et al (2012) also argued that the emission line stems from the inner disk (≤20−40 AU) for stars with transition and pre-transition disks.…”

“…We recently studied the gas emission with Spitzer-IRS in a large sample of young stars (Güdel et al 2010;Baldovin-Saavedra et al 2011) obtaining a large number of detections of [Ne II]. For our dedicated VLT-VISIR high-spectral resolution study, we selected objects accessible by the VLT for which the line fluxes obtained with Spitzer are higher than 10 −15 erg cm −2 s −1 , enough for follow-up ground based observations.…”

“…The Spitzer Space Telescope (Werner et al 2004) made a large contribution to the studies of protoplanetary disks in the mid-infrared (mid-IR) by revealing the presence of many emission lines from organic molecules (Carr & Najita 2008), water (Carr & Najita 2008;Salyk et al 2008;Pontoppidan et al 2010), and atomic species Lahuis et al 2007;Flaccomio et al 2009;Güdel et al 2010;Baldovin-Saavedra et al 2011). Among the many emission lines detected by Spitzer, [Ne II] 12.81 μm gained particular interest.…”

Ionization and heating by X-rays and cosmic rays

“…Analysis of Ne ii lines is particularly important since it relates directly to the uppermost material in a disk that has been ionized by the same X-ray radiation. Blueshifts of order a few km s −1 are seen in some T Tauri stars (Herczeg et al 2007;Najita et al 2009;Pascucci et al 2007;Pascucci & Sterzik 2009) but caution is in order because a mixture of photoevaporative winds, contributions from fast jets, and emission from steady disk surfaces may be present (Baldovin-Saavedra et al 2011). …”

The Gas Disk: Evolution and Chemistry

“…The mid-infrared forbidden transition of [Ne II] (12.81 µm) was first detected with the Spitzer Infrared Spectrograph (IRS, , Lahuis et al 2007. Although the Spitzer spectrograph lacks the spectral resolution needed to confirm or rule out a disk, wind, or jet origin of the line, interesting conclusions were extracted from studies based on statistically significant samples (see for example Lahuis et al 2007, Flaccomio et al 2009, Güdel et al 2010, Baldovin-Saavedra et al 2011. For example, the line luminosities of sources known to drive outflows and jets are brighter by 1-2 orders of magnitude than those of sources without outflows/jets.…”