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.