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Our knowledge of circumstellar disks has traditionally been based on studiesof dust. However, gas dominates the disk mass and its study is key tounderstand the star and planet formation process. Spitzer can access gasemission lines in the mid-infrared, providing new diagnostics of the physicalconditions in accretion disks and outflows. We have studied the spectra of 64pre-main-sequence stars in Taurus using Spitzer/IRS observations. We havedetected H2 (17.03, 28.22 $\mu$m) emission in 6 objects, [Ne II] (12.81 $\mu$m)in 18 objects, and [Fe II] (17.93, 25.99 $\mu$m) in 7 objects. [Ne II]detections are found primarily in Class II objects. The luminosity of the [NeII] line, is in general higher for objects known to drive jets than for thosewithout known jets, but the two groups are not statistically distinguishable.We have searched for correlations between the line luminosities and differentparameters related to the star-disk system. The [Ne II] luminosity iscorrelated with X-ray luminosity for Class II objects. The [NeII] luminosity iscorrelated with disk mass and accretion rate when the sample is divided intohigh and low accretors. We also find correlations between [NeII] luminosity andmid-IR continuum luminosity and with luminosity of the [O I] (6300 \AA) line,the latter being an outflow tracer. [Fe II] luminosity correlates with massaccretion rate. No correlations were found between H2 luminosity and severaltested parameters. Our study reveals a general trend toward accretion-relatedphenomena as the origin of the gas emission lines. Shocks in jets andoutflowing material are more likely to play a significant role than shocks ininfalling material. The role of X-ray irradiation is less prominent but stillpresent for [Ne II], in particular for Class II sources, the lack ofcorrelation between [Fe II] and [Ne II] points toward different emittingmechanisms.

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