Very cold cores in the Taurus Molecular Ring as seen by ISO

Abstract

Three prominent cold objects of the Taurus Molecular Ring (TMR) were revealed by our ISOPHOT 200 µ mm ap of the south-eastern part of the Heiles Cloud 2 (HCL 2) cloud complex. One corresponds to the cyanopolyyne peak region of the TMC-1 ridge, (TMC-1 CP), one is the HCL2-E cloud, and one which we call HCL2-ES lies south of TMC-1. The 200/100 µm colour temperatures and column densities of the three ISOPHOT cold clouds are ≈12 K, and 1.2 ± 0.7 × 10 22 cm −2 respectively, as calculated from ISO/IRAS surface brightnesses. As Nagoya-4 m C 18 O (1-0) spectra show, these are dense molecular clouds with N(H) > 10 21 cm −2 column density peaks. The ISOPHOT 200 µm surface brightness is well correlated with the C 18 O line intensity (corr. coef . ≈ 70%). The large dust particle emissivity is found to be increased in the prototypical very dense core TMC-1 CP. As the low linewidths (∆v = 0.8 ± 0. 2k m s −1 ) indicate, the level of turbulent energy density is 50% lower in these three clouds than in other clouds of HCL2. Dense cores were identified inside the C 18 O clouds by NH3 measurements with the Effelsberg-100 m telescope and Nobeyama-45 m H 13 CO + data. The density of the dense cores is n ≥ 1.1 × 10 5 cm −3 ,a nd their kinetic temperatures are <10 K, in good agreement with the FIR results. The total molecular gas mass in the gravitationally bound cloud cores of TMC-1 CP and HCL2-E is about 21 Mand 8 Mrespectively. The cores, TMC-1 CPb and HCL2-Eab are associated with 3 low mass YSO candidate 2MASS point sources, while 35 other low mass YSO candidates are seen elsewhere in TMR south, which we consider as evidence for ongoing low mass star formation.