J. G. A. Wouterloot

Photon dominated regions in the spiral arms of M 83 and M 51

We present [C ] 3 P1– 3 P0 spectra at four spiral arm positions and the nuclei of the nearby galaxies M 83 and M 51 obtained at the JCMT. The spiral arm positions lie at galacto-centric distances of between 2 kpc and 6 kpc. This data is complemented with maps of CO 1–0, 2–1, and 3–2, and ISO/LWS far-infrared data of [C ] (158 µm), [O  ]( 63µm), and [N ] (122 µm) allowing for the investigation of a complete set of all major gas cooling lines. From the intensity of the [N ] line, we estimate that between 15% and 30% of the observed [C ] emission originates from the dense ionized phase of the ISM. The analysis indicates that emission from the diffuse ionized medium is negligible. In combination with the FIR dust continuum, we find gas heating efficiencies below ∼0.21% in the nuclei, and between 0.25 and 0.36% at the outer positions. Comparison with models of photon-dominated regions (PDRs) with the standard ratios [O ](63)/[C ]PDR and ([O ](63)+[C ]PDR )v s. TIR, the total infrared intensity, yields two solutions. The physically most plausible solution exhibits slightly lower densities and higher FUV fields than found when using a full set of line ratios, [C ]PDR/[C ](1–0), [C ](1–0)/CO(3–2), CO(3–2)/CO(1–0), [C ]/CO(3–2), and, [O ](63)/[C ]PDR. The best fits to the latter ratios yield densities of 10 4 cm −3 and FUV fields of ∼G0 = 20–30 times the average interstellar field without much variation. At the outer positions, the observed total infrared intensities are in agreement with the derived best fitting FUV intensities. The ratio of the two intensities lies at 4–5 at the nuclei, indicating the presence of other mechanisms heating the dust. The [C ] area filling factors lie below 2% at all positions, consistent with low volume filling factors of the emitting gas. The fit of the model to the line ratios improves significantly if we assume that [C ] stems from a larger region than CO 2–1. Improved modelling would need to address the filling factors of the various submm and FIR tracers, taking into consideration the presence of density gradients of the emitting gas by including cloud mass and size distributions within the beam.

Molecular abundances in the high-latitude molecular clouds

This paper presents CO, H/sub 2/CO, OH, and extinction data for several high-latitude molecular clouds (HLMCs) in an attempt to determine the molecular abundances and gas properties of a typical HLMC. The CO abundance with respect to H/sub 2/ is comparable to what is found for dark clouds. The H/sub 2/CO abundance is within a few factors of the values found in dark cloud surveys. The OH abundance in the HLMCs is an order of magnitude greater than the value determined for dark clouds. The data indicate that the HLMCs show abundance and column densities more similar to those of the dark clouds than to those of the diffuse clouds. 61 references.

Formaldehyde in the far outer galaxy: constraining the outer boundary of the galactic habitable zone.

We present results from an initial survey of the 2(12)-1(11) transition of formaldehyde (H2CO) at 140.8 GHz in giant molecular clouds in the far outer Galaxy (RG >or= 16 kpc). Formaldehyde is a key prebiotic molecule that likely plays an important role in the development of amino acids. Determining the outermost extent of the H2CO distribution can constrain the outer limit of the Galactic Habitable Zone, the region where conditions for the formation of life are thought to be most favorable. We surveyed 69 molecular clouds in the outer Galaxy, ranging from 12 to 23.5 kpc in galactocentric radius. Formaldehyde emission at 140.8 GHz was detected in 65% of the clouds. The H2CO spectral line was detected in 26 of the clouds with RG > 16 kpc (detection rate of 59%), including 6 clouds with RG > 20 kpc (detection rate of 55%). Formaldehyde is readily found in the far outer Galaxy-even beyond the edge of the old stellar disk. Determining the relatively widespread distribution of H2CO in the far outer Galaxy is a first step in establishing how favorable an environment this vast region of the Galaxy may be toward the formation of life.

SCUBA mapping of outer Galaxy protostellar candidates

Aims. We aim to study dust properties of massive star forming regions in the outer Galaxy, in a direction opposite to the Galactic center. Methods. We present observations of six outer Galaxy point sources IRAS 01045+6505, 01420+6401, 05271+3059, 05345+3556, 20222+3541 and 20406+4555, taken with the Submillimeter Common-User Bolometer Array (SCUBA) on the James Clerk Maxwell Telescope (JCMT) at 450 and 850 µm. Single temperature greybody models are fitted to the Spectral Energy Distribution of the detected sub-mm cores to derive dust temperature, dust emissivity index and optical depth at 250 µm. The observed radial intensity profiles of the sub-mm cores were fitted with power laws to derive the indices describing the density distribution. Results. At a resolution of 15 �� all six IRAS point sources show multiple emission peaks. Only four out of fourteen detected sub-mm cores show associated mid-infrared emission. For the sub-mm cores we derive dust temperatures of 32 ± 5 K and dust emissivity indices between 0.9 and 2.5. The density profiles of the sub-mm cores can be fitted by a single power law distribution with indices −1.5 ± 0.3, with most cores showing an index of −1.5. This is consistent with most observations of massive star forming regions and supports predictions of models of star formation which consider non-thermal support against gravitational collapse.

The velocity field of the outer galaxy

Results are presented of a study of the outer Galaxy velocity field. We compiled a catalogue of HII regions and reflection nebulae with 2300⩽l⩽305° and |b|⩽150. For these objects heliocentric distances and radial velocities have been determined. By combining these data with similar data from the literature, for l<230°, we construct the velocity field of the outer Galaxy in the second and third quadrants. This field includes non-circular motions and is therefore very useful for the determination of kinematic distances.

Molecular gas and stars in the translucent cloud MBM 18 (LDN 1569)

Context. We investigate star formation in translucent, high-latitu de clouds. Aims. Our aim is to understand the star-formation history and rate in the solar neighbourhood. Methods. We used spectroscopic observations of newly found candidate Hα emission-line stars to establish their pre-main-sequence nature. The environment was studied through molecular line observations of the cloud (MBM 18/LDN 1569) in which the stars are presumably embedded. Results. Ten candidate Hα emission-line stars were found in an objective grism survey of a∼ 1 square degree region in MBM 18, of which seven have been observed spectroscopically in this study. Four of these have weak (| W(Hα)|< 5 A) Hα emission, and six out of seven have spectral types M1−M4 V. One star is of type F7-G1 V, and has Hα in absorption. The spectra of three of the M-stars may show an absorption line of LiI, although none of these is an unambiguous detection. The M-stars lie at distances between∼60 pc and 250 pc, while most distance determinations of MBM 18 found in the literature agree on 120− 150 pc. For the six M-stars a good fit is obtained with pre-main-sequence isochrones indi cating ages between 7.5 and 15 Myr. The mass of the molecular material, derived from the integrated 12 CO(1‐0) emission, is∼ 160 M⊙ (for a distance of 120 pc). This is much smaller than the viria l mass (∼ 10 3 M⊙), and the cloud is not gravitationally bound. Using a clump-finding routine, we identify 12 clumps from the CO-data, with masses between 2.2 and 22 M⊙. All clumps have a virial mass at least six times higher than t heir CO-mass, and thus none are in gravitational equilibrium. A similar situation is found fr om higher-resolution CO-observations of the northern part of the cloud. Conclusions. Considering the relative weakness or absence of the Hα emission, the absence of other emission lines, and the lack of clear LiI absorption, the targets are not T Tauri stars. Wi th ages between 7.5 and 15 Myr they are old enough to explain the lack of lithium in their spectra. Based on the derived distances, some of the stars may lie inside the molecular cloud. From the fact that the cloud as a whole, as well as the individual clumps, are not gravitationally bound, in combination with the ages of the stars we conclude that it is not likely that (these) stars were formed in MBM 18.