  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
understand the star and planet formation process. Spitzer can access gas
emission lines in the mid-infrared, providing new diagnostics of the physical
conditions in accretion disks and outflows. We have studied the spectra of 64
pre-main-sequence stars in Taurus using Spitzer/IRS observations. We have
detected 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 [Ne
II] line, is in general higher for objects known to drive jets than for those
without known jets, but the two groups are not statistically distinguishable.
We have searched for correlations between the line luminosities and different
parameters related to the star-disk system. The [Ne II] luminosity is
correlated with X-ray luminosity for Class II objects. The [NeII] luminosity is
correlated with disk mass and accretion rate when the sample is divided into
high and low accretors. We also find correlations between [NeII] luminosity and
mid-IR continuum luminosity and with luminosity of the [O I] (6300 \AA) line,
the latter being an outflow tracer. [Fe II] luminosity correlates with mass
accretion rate. No correlations were found between H2 luminosity and several
tested parameters. 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, the lack of
correlation between [Fe II] and [Ne II] points toward different emitting
mechanisms.
