We report the results of a 100 deg 2 survey of the Taurus molecular cloud region in 12 CO and 13 CO J ¼ 1 ! 0. The image of the cloud in each velocity channel includes '3 ; 10 6 Nyquist-sampled pixels on a 20 00 grid. The high sensitivity and large spatial dynamic range of the maps reveal a very complex, highly structured cloud morphology, including filaments, cavities, and rings. The axes of the striations seen in the 12 CO emission from relatively diffuse gas are aligned with the direction of the magnetic field. We have developed a statistical method for analyzing the pixels in which 12 CO but not 13 CO is detected, which allows us to determine the CO column in the diffuse portion of the cloud, as well as in the denser regions in which we detect both isotopologues. Using a column-density-dependent model for the CO fractional abundance, we derive the mass of the region mapped to be 2:4 ; 10 4 M , more than twice as large as would be obtained using a canonical fixed fractional abundance of 13 CO, and a factor of 3 greater than would be obtained considering only the high column density regions. We determine that half the mass of the cloud is in regions having column density below 2:1 ; 10 21 cm À2. The distribution of young stars in the region covered is highly nonuniform, with the probability of finding a star in a pixel with a specified column density rising sharply for N (H 2 ) ¼ 6 ; 10 21 cm À2 . We determine a relatively low star formation efficiency (mass of young stars/mass of molecular gas), between 0.3% and 1.2%, and an average star formation rate during the past 3 Myr of 8 ; 10 À5 stars yr À1.

A summary of global properties and an evaluation of the equilibrium state of molecular regions in the outer Galaxy are presented from the decomposition of the FCRAO Outer Galaxy Survey and targeted 12CO and 13CO observations of four giant molecular cloud complexes. The ensemble of identified objects includes both small, isolated clouds and clumps within larger cloud complexes. 12CO velocity dispersions show little variation with cloud sizes for radii less than 10 pc. It is demonstrated that the internal motions of regions with molecular masses greater than 10**4 msuns are bound by self gravity, yet, the constituent clumps of cloud complexes and isolated molecular clouds with M < 10**3 msuns are not in self gravitational equilibrium. The required external pressures to maintain the equilibrium of this population are (1-2)x10**4 cm-3-K.Comment: To appear in ApJ, 32 pages, 13 figures, 2 table

We report a study of the relation between dust and gas over a 100 deg 2 area in the Taurus molecular cloud. We compare the H 2 column density derived from dust extinction with the CO column density derived from the 12 CO and 13 CO 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 A V and N (CO) is linear between A V ≃ 3 and 10 mag in the region associated with the B213-L1495 filament. In other regions the linear relation is flattened for A V 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 CO 2 ices and A V , 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 × 10 4 cm −3 and a CO depletion age of about 4.2 × 10 5 years. 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 H 2 mass of Taurus to be about 1.5 × 10 4 M ⊙ , independently derived from the A V and N (CO) maps. We derive a CO integrated intensity to H 2 conversion factor of about 2.1×10 20 cm −2 (K km s −1 ) −1 , which applies even in the region where the [CO]/[H 2 ] 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.

We present a comprehensive study of the physical and chemical conditions along the TMC-1 ridge. Temperatures were estimated from observations of CH3CCH, NH3, and CO. Densities were obtained from a multitransition study of HC3N. The values of the density and temperature allow column densities for 13 molecular species to be estimated from statistical equilibrium calculations, using observations of rarer isotopomers where possible, to minimize opacity effects. The most striking abundance variations relative to HCO+ along the ridge were seen for HC3N, CH3CCH, and SO, while smaller variations were seen in CS, C2H, and HCN. On the other hand, the NH3, HNC, and N2H+ abundances relative to HCO+ were determined to be constant, indicating that the so-called NH3 peak in TMC-1 is probably a peak in the ammonia column density rather than a relative abundance peak. In contrast, the well-studied cyanopolyyne peak is most likely due to an enhancement in the abundance of long-chain carbon species. Comparisons of the derived abundances to the results of time-dependent chemical models show good overall agreement for chemical timescales around 10(5) yr. We find that the observed abundance gradients can be explained either by a small variation in the chemical timescale from 1.2 x 10(5) to 1.8 x 10(5) yr or by a factor of 2 change in the density along the ridge. Alternatively, a variation in the C/O ratio from 0.4 to 0.5 along the ridge produces an abundance gradient similar to that observed.