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

We present ISOPHOT observations at 120 and 200 mu m of a 31 x 57 arcmin(2) region, with optical extinction AV ranging between approximate to 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 mu m. This separation is based on the very different morphologies of the 60 and 200 mu m emission maps. The 60 mu m emission is used as a spatial template for the warm component, and the 200 mu m 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 mu m of the warm and cold components (tau(200)) were determined. The ratios I-v(200)/A(V) and tau(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 tau(200) of the cold component with carbon monoxide measurements, which trace the molecular gas, were performed. The cold component emission, in particular at 100 mu m, correlates very well with (CO)-C-13 (J = 1-0) total intensity. Very good correlations between (CO)-O-18 (J = 1-0) total intensity and especially tau(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 tau(200) is a powerful tracer of dense cores with n(H-2) approximate to 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) approximate to 103 cm(-3).