New insights into the dust properties of the Taurus molecular cloud TMC-2 and its surroundings

Abstract

We present ISOPHOT observations at 120 and 200 μm of a 31 x 57 arcmin 2 region, with optical extinction Av ranging between 0.5 and 11 mag, that encloses the Taurus molecular cloud TMC-2. The far-infrared emission is separated into a warm and a cold component using the ISOPHOT data and IRAS measurements at 60 and 100 μm. This separation is based on the very different morphologies of the 60 and 200 μm emission maps. The 60 μm emission is used as a spatial template for the warm component, and the 200 μm emission [I v (200)] as a template for the cold component. The warm component presents an average colour temperature of nearly 20 K. The colour temperature map of the cold component is nearly uniform, with a mean temperature of 12.5 K. The optical depths at 200 μm of the warm and cold components (τ 200 ) were determined. The ratios I v (200)/A V and τ 200 /A V of the cold component indicate changes in the optical properties of the dust grains, which present a far-infrared emissivity that is a few times larger than that of the big grains in the diffuse interstellar medium. Comparisons of the emissions and τ 200 of the cold component with carbon monoxide measurements, which trace the molecular gas, were performed. The cold component emission, in particular at 100 μm, correlates very well with 13 CO (J = 1-0) total intensity. Very good correlations between C 18 O(J = 1-0) total intensity and especially τ 200 are found for two distinct regions, one that encloses the core TMC-2 and a northern region that also contains molecular condensations. These results confirm that τ 200 is a powerful tracer of dense cores with n(H 2 ) 10 4 cm -3 , and that the change in the properties of dust grains in the observed region takes place at densities of n(H 2 ) 10 3 cm -3 .