2001A&A...372..302H

The Barnard object B217 was observed in the infrared and radio region. The 170µm continuum was detected with ISOPHOT, the ammonia 1.3 cm radio lines with the Effelsberg 100m-telescope. Mapping B217SW in (J,K)=(1,1) and (2,2) inversion lines revealed the temperature and density distribution of the gas and made it possible to investigate the dynamical state of this dense core inside B217. The ISOPHOT Serendipity Survey (ISOSS) detected the cold dust emission of B217 in all of 3 slews crossing the region. Combining ISOSS with IRAS data, we derive the core parameters of the dust from FIR emission and compare them with the NH₃ data, which sample the densest region of the core. This study shows the power of combining ISOSS 170µm with IRAS/HIRES data in order to study the dust characteristics in nearby star forming regions on small spatial scales. The (170µm/100µm) dust colour temperature is 11K-12K in the dense cores and 12K-14K in the other regions of B217. The low dust temperatures cannot be explained by attenuation of the interstellar radiation field alone and may reflect a change in the optical properties of the dust as compared to diffuse clouds. In B217SW, molecular depletion through freeze-out onto grains is suggested by the comparison of our FIR and NH₃ data with previous C¹⁸O observations. On the basis of our ammonia data investigation, we find in B217SW dense gas with kinetic temperatures between 9K and 12K, increasing outwards. Using near-infrared extinction and NH₃ collisional excitation calculations, the fractional ammonia abundance (N(NH₃)/N(H₂)) is found to be 3-5x10^–8, and the comparison of gas and dust observations supports this range. Knowing the ammonia abundance, we calculate the thermal, turbulent and gravitational energies of the dense core, which appears to be close to hydrostatic equilibrium. Our results are compatible with B217SW being now on the verge of collapse or in an early collapse phase.