Dense gas and cold dust in the dark core B217

Abstract

The Barnard object B217 was observed in the infrared and radio region. The 170 m continuum was detected with ISOPHOT, the ammonia 1.3 cm radio lines with the Eelsberg 100 m-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 170 mw ith IRAS/HIRES data in order to study the dust characteristics in nearby star forming regions on small spatial scales. The (170m/100m) dust colour temperature is 11 K{12 K in the dense cores and 12 K{14 K in the other regions of B217. The low dust temperatures cannot be explained by attenuation of the interstellar radiation eld alone and may reflect a change in the optical properties of the dust as compared to diuse clouds. In B217SW, molecular depletion through freeze-out onto grains is suggested by the comparison of our FIR and NH3 data with previous C 18 O observations. On the basis of our ammonia data investigation, we nd in B217SW dense gas with kinetic temperatures between 9 K and 12 K, increasing outwards. Using near-infrared extinction and NH3 collisional excitation calculations, the fractional ammonia abundance (N(NH3)=N (H2)) is found to be 3{5 10 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.