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Universität zu Köln
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Mathematisch-Naturwissenschaftliche Fakultät
Fachgruppe Physik

I. Physikalisches Institut

Detailed studies of Galactic photon dominated regions

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Introduction

In detailed observations of selected massive star formation regions, we study the influence of UV radiation on the clouds. Below, we present also the preparatory work for observations with HIFI and SOFIA.

We were able to deepen the understanding of observed line emission originating from Photon Dominated Regions (PDRs), where chemical reactions are driven by heating UV photons from nearby stars. The theory predicts a transition between ionized carbon at the cloud surface to CO deeper inside with enhanced atomic carbon abundance in the transition zone (Hollenbach & Tielens 1997). The heating through UV is balanced by cooling through the FIR dust continuum and line coolants such as [OI], [CII], mid- and high-J CO, and [CI]. Therefore, line emission is an ideal diagnostic probe for the clouds' global energy balance. Selected ratios of those lines are sensitive to certain kinetic temperatures, e.g., [CI] 2-1/[CI] 1-0 for temperatures up to 100K, and CO 7-6/[CI] 2-1 for number densities above 105cm-3 in the warm phase. All three lines are simultaneously observable with SMART (SubMillimeter Array Receiver for Two Frequencies) at KOSMA, making it a very effective instrument for PDR studies. 13CO 8-7, as well in the SMART tuning range, is very important to determine upper limits of density and temperature. Besides SMART, other instruments (such as KAO, ISO/LWS, and the JCMT) were used to obtain complementary data. Especially the ISO/LWS archive provides a wealth of line coolants. Only a combined analysis of those lines will give sufficient input to compare with PDR models.

Our sample of massive star forming regions includes sources such as DR21 W3, S106, Orion A, and ON-1. We do not aim at very large scale structure analysis of Galactic molecular clouds here, but rather study the conditions in the vicinity to the UV sources, i.e., the active spots. We present some of the recent results below.

 

Orion A molecular cloud

Located at a distance of only 450 pc, the Orion molecular cloud is the nearest region of high-mass star formation (Mookerjea et al. 2003). A map of the [CII] integrated intensity and continuum at 158 µm has been obtained for the Orion A region with the 1m-TIFR balloon-borne FIR telescope. The map includes the well studied objects Theta 1 Ori C, BN/KL, and M43. Combined with the velocity-integrated intensity of 13CO 1-0, [CI] 1-0, and CO 3-2 lines, the emission occurs in a sequenceof [CII]/CO/[CI] (wrt. to the Av, away from Theta 1 Ori C). This is in contradiction to the established PDR scenario (e.g. Kaufman et al. 1999). However, the PDR models are able to explain the observed line intensity ratios satisfactorily for positions away from the energizing source. At the [CII] peak, the position of the ionizing O6-type star, models fail to provide a solution, because of the observed excess in [CII] emission. The conclusion of the analysis is that plane-parallel PDR models predict line emission reasonably well for regions with diffuse UV radiation.

Collaboration: The Orion A observations form part of the CII observations conducted with the balloon-borne Indian/Japanese FIR telescope.

Resulting publication: Mookerjea et al. 2003, A&A, 404, 569

S106 molecular cloud

The HII/molecular cloud region around the single ionizing O8-type star S106 IR (e.g., Schneider et al. 2002, 2003) is located at a distance of 600 pc in the Cygnus region. The PDR shows enhancements of the major cooling lines [OI], [CII], mid-J and high-J CO. Neutral atomic carbon emission is less prominent at the position of the star since the UV radiation probably already ionized a significant fraction. The [CI] abundance peak is thus shifted toward a more distant NH3 peak. Ionized carbon contributes ~1% to the cooling, while the 63 micron [OI] line turns out to be the dominant cooling line. PDR models reflect that the UV field, derived from line ratio fitting, decreases with distance from the star. The found enhancement by a factor of 105 above the mean interstellar radiation field is typical for conditions close to massive stars. Discrepancies between models and observations, mainly in the density distribution, may arise from the immanent simplifications of plane-parallel PDR models, such as smaller surface in a non clumped medium and the presence of several PDR slabs along the line of sight.

Resulting publications: Schneider et al. 2003, A&A, 406, 915; 2005 (in prep.)

DR21 molecular cloud

The DR21 HII-region/molecular cloud at a distance of 1.7 kpc is part of the Cygnus X complex. The central massive protostellar cluster is still enshrouded in a dense molecular ridge but developed a very elongated ouflow lobe of shocked gas. Recent Spitzer observations (Marston et al. 2004) show that H2 emission originating in the outflow is directly tracing the shock. The PDR tracers CO 4-3, CO 7-6, and both [CI] lines were mapped with SMART and 13CO 8-7 was detected toward the peak (Jakob et al. 2005a). Other cooling lines were observed towards a few positions with ISO/LWS. One ISO position is directly covering the extended outflow and thus allows to better understand the influence of PDRs in the presence of shocks. The region also harbours cores at different evolutionary stages along the ridge and presumably sequentially triggered and initiated through the proximity to one of the richest stellar clusters Cyg OB2.

Collaboration:The DR21 observations are part of the larger Cygnus-X project conducted together with S.Bontemps and N.Schneider as described above.

Resulting publications: Jakob et al. 2005a (in prep.)

W3Main molecular cloud

The interaction of an O/B association with molecular gas is even more direct toward the W3 complex. W3 Main (Kramer et al. 2004) at a distance of 1.8 kpc harbours a cluster of roughly 20 OB stars that heats the whole cloud through very intense UV radiation. This is directly evident from the very extended [CII] emission (Howe et al. 1991). It thus makes sense to model the radiation field in order to constrain the UV parameter of the PDR models. The high excitation of mid-J and high-J CO lines (provided by SMART and ISO/LWS observations) is best explained by a dense (n=105 cm-3) and warm (T=140K) phase (Kramer et al. 2004). However, the largest fraction of the molecular gas, visible in low-J CO lines only, remains in a phase of moderate density and temperature. The line cooling is predominantly via [OI](63 micron), but the CO lines (J>=7) also contribute significantly, in total the line emission contributes about 0.1% to the cooling. PDR models overestimate the gas and dust cooling by one order of magnitude, suggesting a low beam filling factor. [CI] and CO line ratios further indicate high densities in the framework of the PDR models. These findings may be explained by spherical clumps of varying size (described by a power-law distribution).

Resulting publications: Kramer et al. 2004, A&A, 424, 887

ON1 cloudlet

Sources coinciding with very small HII regions and maser signposts are believed to be in an early evolutionary stage. ON-1 for instance is an isolated object (Israel & Wootten, 1983) with a bolometric luminosity that corresponds to a B0.5-type star. It was studied in collaboration with F. Israel in a successful short term project of our PhD student H. Jakob visiting Leiden Observatory in 2004. The rich data set comprising a SCUBA 850 micron map together with maps of CO,13CO, C18O, HCO+, H2CO, and [CI]) allows for a detailed analysis (Jakob et al. 2005b). Radiative transfer modelling of line profiles (e.g., Hogerheijde & van der Tak 2000, Ossenkopf et al. 2001) has become a usesful analysis method. For a spherical power-law density and temperature structure derived from the dust continuum, the line fluxes are fitted well by a model of 270 solar masses. The observed line profiles are consistent with an expanding shell surrounded by a layer of much colder and clumped quiescent gas. The position-velocity maps may either be explained by combined keplerian-solid-body rotation or an outflow. Once a reasonable source model is established, abundances derived from chemical models will be tested.

Collaborations: The observations of ON1 are conducted in close collaboration with F.Israel (Leiden).

Resulting publications: Jakob et al. 2005b (in prep.)