Mass distribution in the taurus complex

Abstract

The masses of the Taurus cloud complex, Heiles Cloud 2, and TMC1 are evaluated from their dust content, from the brightness of the ¹²CO, ¹³CO and C¹⁸O lines, and from virial equilibrium. Paradoxically, the optically thick ¹²CO line, whose integrated brightness can be converted into H² mass by using Bloemen et al's (1986) conversion factor, yields a better evaluation of the mass of the Taurus complex than do the optically thin ¹³CO and C¹⁸O lines. This is not just an effect of sensitivity. The mass lies mostly in a diffuse halo, bright in the ¹²CO(J=1−0) line, but barely detectable in ¹³CO and not detectable at all in C¹⁸O. The halo density is so low that CO collisional excitation is inefficient; here, ¹³CO and C¹⁸O are hardly excited and could be photodissociated.

Inside HCL₂, the gas density is high enough that CO is thermalized by collisions; ¹³CO and C¹⁸O, rather than ₁₂CO, should be used to trace the mass. The structure of the tiniest clumps is best observed through high excitation lines, such as the 10–9 line of HC₃N and the 2₁₂–1₀₁ line of C₃H₂. In TMC1, these two lines show a chain of clumps of size 0.03–0.05 pc. These tiny clumps, just as HCL₂ itself, appear to be virialized.

for the sake of simplicity, following Elias (1978) and Cernicharo et al. (1985), we adopt an average distance of 135 pc.