Herschel Key Program, "Dust, Ice, and Gas In Time" (DIGIT): the origin
  of molecular and atomic emission in low-mass protostars in Taurus
  Six low-mass embedded sources (L1489, L1551-IRS5, TMR1, TMC1-A, L1527, and
TMC1) in Taurus have been observed with Herschel-PACS to cover the full
spectrum from 50 to 210 $\mu$m as part of the Herschel key program, "Dust, Ice,
and Gas In Time (DIGIT)". The relatively low intensity of the interstellar
radiation field surrounding Taurus minimizes contamination of the [C II]
emission associated with the sources by diffuse emission from the cloud
surface, allowing study of the [C II] emission from the source. In several
sources, the [C II] emission is distributed along the outflow, as is the [O I]
emission. The atomic line luminosities correlate well with each other, as do
the molecular lines, but the atomic and molecular lines correlate poorly. The
relative contribution of CO to the total gas cooling is constant at $\sim$30 %,
while the cooling fraction by H$_2$O varies from source to source, suggesting
different shock properties resulting in different photodissociation levels of
H$_2$O. The gas with a power-law temperature distribution with a moderately
high density can reproduce the observed CO fluxes, indicative of CO close to
LTE. However, H$_2$O is mostly subthermally excited. L1551-IRS5 is the most
luminous source ($L_{\rm bol}=24.5$ L$_\odot$) and the [O I] 63.1 $\mu$m line
accounts for more than 70 % of its FIR line luminosity, suggesting complete
photodissociation of H$_2$O by a J-shock. In L1551-IRS5, the central velocity
shifts of the [O I] line, which exceed the wavelength calibration uncertainty
($\sim$70 km s$^{-1}$) of PACS, are consistent with the known red- and
blue-shifted outflow direction.
