Dense gas and cold dust in the dark core B217 *
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
Corresponding author: S. Hotzel, hotzel@mpia-hd.mpg.de
Received:
11
October
2000
Accepted:
28
March
2001
The Barnard object B217 was observed in the infrared and radio region.
The 170 μm continuum was detected with ISOPHOT, the ammonia 1.3 cm
radio lines
with the Effelsberg 100 m-telescope.
Mapping B217SW in
and 
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 μm with IRAS/HIRES data in order to study
the dust characteristics in nearby star forming regions on small
spatial scales.
The (170 μm/100 μm)
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-
, 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
Based on observations with ISO, an ESA project with instruments funded by ESA Member States (especially the PI countries: France, Germany, The Netherlands and the UK) and with the participation of ISAS and NASA. Members of the Consortium on the ISOPHOT Serendipity Survey (CISS) are MPIA Heidelberg, ESA ISO SOC Villafranca, AIP Potsdam, IPAC Pasadena, Imperial College London.
© ESO, 2001