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Far infrared spectroscopy of low Go PDRs in low-mass star forming clouds

René Liseau 1, Glenn White 2, & Bengt Larsson 1

1 Stockholm Observatory, SE-133 36, Saltsjöbaden, Sweden

2 Queen Mary & Westfield College, Dept. of Physics, University of London, Mile End Road, GB-London E1-4NS, UK




Spatial maps and strip scans of low-mass star forming clouds in the solar neighbourhood have been obtained. These observations have been performed with the ISO- LWS (beam size $\sim$80 $^{\prime\prime}$) and cover the spectral range of $\sim$45 to 200$\mu$m at the resolution of a few hundred. The data are absolutely calibrated to an accuracy of 30% and thus reasonably well suited for comparison with predictions from theoretical model calculations.

In this paper, we address primarily the spatial distribution of the emission in [C II] 158 $\mu$m $^{2}{\rm P}_{3/2}$ - $^{2}{\rm P}_{1/2}$ ([C II]158$\mu$m), O0 $^{3}{\rm P}_{1}$ - $^{3}{\rm P}_{2}$ ([O I]63$\mu$m) and O0 $^{3}{\rm P}_{0}$ - $^{3}{\rm P}_{1}$ ([O I]145$\mu$m). The [C II] 158 $\mu$m line emission originates in the outer, externally illuminated layers of the cloud ( PDR) and its spatial distribution holds information about the cloud surface geometry and about the efficiency at which the cloud material converts far UV radiation into [C II]158$\mu$m line photons. This model provides also estimates of the strength of the incident UV field, commonly expressed in units of G0 (the Habing Field).

Combining this informtion with the observed distribution of the [O I]63$\mu$m emission leads to estimates of the PDR density distribution from detailed model calculations. These standard PDR models fit the LWS observations of the [C II]158$\mu$m and [O I]63$\mu$m very well and predict, e.g., [C I]609$\mu$m line intensities which are in good agreement with independent observations, but leave the observed excess flux in the [O I]145$\mu$m line unexplained. These PDR models include recent updates of physical processes and their rates (low temperature collisions; self-shielding of C0, H2 and CO; chemistry; grain and PAH populations). The theory of line excitation and transfer is examined and possible reasons for the mismatch between observed and calculated [O I]145$\mu$m strengths are discussed.

next up previous contents index
Next: Morphology and excitation of Up: Poster session C Interstellar Previous: A superbubble surrounding the
"The Universe as seen by ISO", 20 - 23 October 1998, Paris: Abstract Book