2005MNRAS.360..901D

We present ISOPHOT observations at 120 and 200 µm of a 31x57 arcmin² region, with optical extinction A_Vranging 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_ν(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 (τ₂₀₀) were determined. The ratios I_ν(200)/A_Vand τ₂₀₀/A_Vof 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 τ₂₀₀of 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 ¹³CO (J= 1-0) total intensity. Very good correlations between C¹⁸O (J= 1-0) total intensity and especially τ₂₀₀are 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 τ₂₀₀is a powerful tracer of dense cores with n(H₂) ~ 10⁴/cm3, and that the change in the properties of dust grains in the observed region takes place at densities of n(H₂) ~ 10³/cm3.