EDP Sciences
Free access
Volume 372, Number 1, June II 2001
Page(s) 302 - 316
Section Diffuse matter in space
DOI http://dx.doi.org/10.1051/0004-6361:20010486

A&A 372, 302-316 (2001)
DOI: 10.1051/0004-6361:20010486

Dense gas and cold dust in the dark core B217

S. Hotzel1, J. Harju2, D. Lemke1, K. Mattila2 and C. M. Walmsley3

1  Max-Planck-Institut für Astronomie, Königstuhl 17, 69117 Heidelberg, Germany
2  Observatory, PO Box 14, 00014 University of Helsinki, Finland
3  Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, 50125 Firenze, Italy

(Received 11 October 2000 / Accepted 28 March 2001 )

The Barnard object B217 was observed in the infrared and radio region. The 170 $\mu$m continuum was detected with ISOPHOT, the ammonia 1.3 cm radio lines with the Effelsberg 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 $\mu$m with IRAS/HIRES data in order to study the dust characteristics in nearby star forming regions on small spatial scales. The (170 $\mu$m/100 $\mu$m) 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 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 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- $5\times 10^{-8}$, 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.

Key words: ISM: clouds -- ISM: individual objects: Barnard 217 -- ISM: molecules -- infrared: ISM -- radio lines: ISM -- surveys

Offprint request: S. Hotzel, hotzel@mpia-hd.mpg.de

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