THE RELATION BETWEEN GAS AND DUST IN THE TAURUS MOLECULAR CLOUD

We report a study of the relation between dust and gas over a 100 deg2 area in the Taurus molecular cloud. We compare the H2 column density derived from dust extinction with the CO column density derived from the 12CO and 13CO J = 1 → 0 lines. We derive the visual extinction from reddening determined from 2MASS data. The comparison is done at an angular size of 200″ corresponding to 0.14 pc at a distance of 140 pc. We find that the relation between visual extinction AV and N(CO) is linear between AV ≃ 3 and 10 mag in the region associated with the B213-L1495 filament. In other regions, the linear relation is flattened for AV≳ 4 mag. We find that the presence of temperature gradients in the molecular gas affects the determination of N(CO) by ∼30%–70% with the largest difference occurring at large column densities. Adding a correction for this effect and accounting for the observed relation between the column density of CO and CO2 ices and AV, we find a linear relationship between the column of carbon monoxide and dust for observed visual extinctions up to the maximum value in our data ≃23 mag. We have used these data to study a sample of dense cores in Taurus. Fitting an analytical column density profile to these cores we derive an average volume density of about 1.4 × 104 cm−3 and a CO depletion age of about 4.2 × 105 yr. At visual extinctions smaller than ∼3 mag, we find that the CO fractional abundance is reduced by up to two orders of magnitude. The data show a large scatter suggesting a range of physical conditions of the gas. We estimate the H2 mass of Taurus to be about 1.5 × 104 M☉, independently derived from the AV and N(CO) maps. We derive a CO integrated intensity to H2 conversion factor of about 2.1 × 1020 cm−2 (K km s−1)−1, which applies even in the region where the [CO]/[H2] ratio is reduced by up to two orders of magnitude. The distribution of column densities in our Taurus maps resembles a log-normal function but shows tails at large and low column densities. The length scale at which the high column density tail starts to be noticeable is about 0.4 pc.

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