We have performed 12CO(1-0), 12CO(2-1), 13CO(1-0), 13CO(2-1), C18O(1-0), C18O(2-1), HCO+(1-0) and H13CO+(1-0) observations towards two selected areas in the star forming complex NGC 6357 with angular resolutions from 21" to 55".In particular, we have mapped the molecular gas around the two HII regions G353.1+0.6 and G353.2+0.9 in the 12CO(1-0) and 13CO(1-0) transitions with a resolution of about 43". This improves on the coarser 12CO(1-0) observations previously carried out by others. We have also studied the physical properties of gas along strips through the molecular cloud/HII region interfaces.

For G353.1+0.6, the observations confirm that an ionization front is eroding a warm, dense molecular cloud located to the north of the optical nebula. The molecular gas appears fragmented on a scale size >0.5 pc and not all the components are related to the HII region. There is evidence of a density increase near the front and indications of temperature gradients toward the exciting stars. This is further suggested by the presence of 12CO(1-0) self-absorption produced by a cooler external layer. The isotopic ratio X(13CO)/X(C18O) decreases from the inner part of the clouds towards the front, contrary to what is theoretically predicted and observed in many other regions. This may be due to excitation or optical depths effects. An evolutionary scenario is proposed where the exciting stars form at the edge of a molecular cloud.

The morphology of G353.2+0.9 is rather different from what previously believed. Only a weak ``bar'' of molecular material was found to the south of the sharp ionization front observed optically and in the radio-continuum, and most of the molecular emission arises from regions behind or to the north of the HII region. This indicates that we are viewing a late stage ``blister'' configuration face-on. This region is fragmented on a scale size >0.5 pc, and a warm, dense and compact molecular fragment coincides with the elephant trunk visible in H(alpha) images. Other clouds with high (ca. 40 K) 12CO(1-0) brightness temperatures surround the nebula to the north. Around their peaks, the 12CO(2-1)/12CO(1-0) main beam temperatures ratio is significantly <1, contrary to what is observed in G353.1+0.6. In both G353.1+0.6 and G353.2+0.9 a large range of radial velocities is observed close to the ionization fronts, and the molecular structures interacting with the ionized gas have virial masses greater than masses calculated under the assumption of LTE.

These observations have allowed us to better understand the morphology of the two regions and to sketch the physical properties of molecular clouds exposed to UV radiation. Dynamical interactions between ionized and molecular gas are used in order to estimate the age (< a few 10^5 yrs) of both nebulae. Also the effects of UV radiation in determining the morphology of molecular gas are considered. The heating sources of the molecular clouds are the early type stars of the H{\sc ii} regions, at the edges of the clouds. The 12CO(1-0) opacity, tau, seems to affect 12CO(1-0) main beam temperatures and an empirical relation between tau and the visual extinction Av has been determined.

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Monitoring of the H2O maser in OH39.7+1.5 as well as spatial interferometry show clear evidence that the maser switches occasionally its beaming direction between radial and tangential paths in step with the stellar pulsation cycle. Outside the minimum, several radially beamed maser features are present, which are absent (=<20 mJy) during the minimum. Competition between gain paths causes tangentially beamed maser emission only to occur, when the temperature in the maser shell is too low to excite radially beamed masers.
The radius of the H2O maser shell extends between 6x10^4 cm (40 AU) and at least 3x10^15 cm (200 AU), with the inner radius probably set by quenching of the maser due to high gas density close to the star. Within the H2O maser shell, the circumstellar outflow accelerates from v=9 km/s up to the terminal expansion velocity of 18 km/s. The expansion velocity of the circumstellar shell is constant between the outer radius of the water maser shell and the location of the 1612 MHz OH maser shell at 3x10^16 cm, showing the absence of strong velocity gradients at large radial distances.

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We have investigated the 12C/13C abundance ratio in the far-outer Galaxy. We have used the IRAM30-m telescope to obtain the 12CO and 13CO(1-0) and (2-1) distributions towards five IRAS sources at about 16-17 kpc from the galactic center. C18O(1-0) and (2-1) were observed towards the 13CO peak positions in those clouds. The source with the strongest C18O emission, WB89-437, was subsequently observed in 13C18O(1-0) and (2-1) and in H2CS(3[1,2]-2[1,1]) and (6[1,5]-5[1,4]). To be able to compare our results with published data, we observed the same transitions towards the inner Galaxy source W33, and towards W3OH. The ratio of the 13CO and C18O column densities is about 14, slightly larger than what was found in local GMCs. This ratio is dominated by excitation and beam filling effects, and is therefore not indicative of the abundance ratios. The ratio C18O(1-0)/13C18O(1-0) directly yields the 12C/13C abundance ratio however, for which towards WB89-437 we find a 3sigma lower limit of 201 +/- 15, which means that the 12C/13C gradient found in the inner Galaxy continues further out. Our results for W33 and W3OH are consistent with earlier observations and give abundance ratios of 43.0 +/- 4.3 and 85 +/- 15, respectively. These J=1-0 measurements are however in contrast to results obtained from the corresponding J=2-1 transitions: we obtain abundance ratios of 104 +/- 60 (WB89-437), 31 +/- 2 (W33), and 24 +/- 2 (W3OH). These differences may be due to the emission of the two transitions originating in different parts of the cloud with different excitation conditions.
The 12CO emission towards WB89-437 shows strong outflow emission, and that of WB89-380 is dominated by self-absorption. The sizes of the 13CO clumps are 1-2 pc and they have peak positions located within 10" (0.5 pc) from the IRAS position. Their (virial) masses are typically several 1000 Mo.

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The properties of star-forming clouds at the edge of the galactic molecular disk (far-outer Galaxy: FOG) are analyzed. By adding data from the literature, we compare the inner- and outer Galaxy (separated at R=Ro=8.5 kpc) cloud populations.

Applying a radiation transfer model to multi-line CO observations of two FOG clouds, Tkin's between about 6 K and 15 K are derived, comparable to those of clouds at smaller R. A statistical analysis of TA* both of clouds without embedded heating sources, and of clouds associated with IRAS sources, confirms there is no evidence for a gradient in cloud temperatures across the Galaxy for R>8.5 kpc.

The column density of H2, as derived from N(H2)=X x Wco (=\int TR*dv), and the H2 LTE column density show a reasonable agreement for X=2.3E+20 cm^{-2}(Kkm/s)^{-1} (derived for the inner Galaxy), if the LTE analysis takes into account a galactic gradient of (H2/13CO). The average and median values of X=Nlte/(\int TR*dv), 3.3 +/- 1.7 [1sigma] and 2.9E+20 respectively, then are within 30-45% of the inner Galaxy value. Due to saturation effects the average X increases from 3E+20 (Wco =< 20) to 6E+20 cm^{-2}(Kkm/s)^{-1} (Wco > 40 Kkm/s).

Including data from the literature we have a sample of 204 molecular clouds with 3< R <20 kpc. Assuming X=2.3E+20 for all inner- and outer Galaxy clouds we find separate, nearly parallel relations in diagrams of logMwco versus logr and logdv versus logr. Although part of the offset of a factor of 2 in r may be due to the difficulties related to the identification of inner Galaxy clouds, it may be mainly explained by inner Galaxy clouds having a higher density, or by a variation of X with R (or with mass). We find Mwco proportional to r^{2.0 +/- 0.1} for all clouds, and dv proportional to r^{0.48 +/- 0.04} (inner) and to r^{0.53 +/- 0.03} (outer).

Inner- and outer Galaxy clouds define a single relation in a diagram of logLco versus logdv: Lco proportional to dv^{3.91 +/- 0.12}. This is in contrast to published results where an offset is found between clouds inside and outside the solar circle. We conclude that a diagram of CO luminosity versus line width is not a good instrument to determine whether there is a dependence of X on R.

The mass spectrum for all outer Galaxy molecular clouds in the extended sample with Mwco >= 3.75 10^4 is dN/dMwco proportional to Mwco^{-1.62 +/- 0.04}. For all 204 clouds, the slope is -1.79 +/- 0.03.

The average ratio of virial- to Wco-mass of all clouds is 1.4 +/- 1.3 (1sigma), with a possible dependency on R and/or mass: there are indications that the ratio increases for clouds at larger R, or for clouds of smaller mass, but the cloud-cloud variations are large. Most clouds in the sample could be in equilibrium through confinement by external pressure. All clouds can be made to be in virial equilibrium, by allowing X to change by a factor of 4 between R approx. 4 and 20 kpc. This would require the galactic abundance gradient to be steeper than what is expected from an extrapolation of the presently available data.

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We present observations of NH3(1,1) and (2,2) lines in two flux-limited samples of IRAS sources selected according to colour criteria which should result in a high fraction of Young Stellar Objects. The first sample contains sources (named 'Low') whose evolutionary status is essentially unknown, while the second sample contains sources (named 'High') possibly associated with ultracompact HII regions, the distinction being based on the IRAS [25-12] colour. Indications from a previous study of H2O maser emission suggest that the sources in the first group may be in an evolutionary phase prior to the appearance of an HII region, thus being among the youngest known high-mass forming objects.

Low sources were detected in ammonia with a lower rate than High sources (45% and 80% respectively); the only difference between the two groups is in the linewidths: the (1,1) lines are generally narrower than the (2,2) lines in High sources (FWHMs median values are 1.81 km/s and 2.00 km/s respectively), while the opposite is true in Low sources (FWHMs median values are 1.72 km/s and 1.33 km.s for the (1,1) and (2,2) lines respectively). We propose that the Low group consists of two distinct populations of evolutionary different objects, based on the (non-) association with ammonia emission. The Low sources showing ammonia emission are characterized by more quiescent envelopes than those surrounding High sources, and the relationships between relevant physical quantities derived from our observations and the IRAS colours suggest that High sources, contrary to Low, may harbour objects which dominate the physical and dynamical properties of the clump, thus possibly implying the relative youth of this subgroup of the Low sources.

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We present the velocity field of the outer Galaxy over a range in galactic longitude from gl=90 degs to gl=270 degs, out to a galactocentric radius R of about 17 kpc (2R0). The field also covers a region of the inner Galaxy, within 2-3 kpc from the Sun. The data set consists of a sample of HII regions/reflection nebulae, for which we have (spectro-) photometric distances, and their associated molecular clouds (for which we have radial velocities). The velocity field includes non-circular motions, and consequently can be used to derive kinematic distances of objects for which only the position and radial velocity (Vlsr) are available.

Adding HI tangent point data to the sample of HII regions/reflection nebulae, the data set covers a range in R from R=0.2R0 to R=2R0. For R0=8.5 kpc and Theta0=220 km/s, the best fit of a rotation curve of the form Theta/Theta0 = a1(R/R0)^{a2} + a3 is obtained with a1=1.00767, a2=0.0394, and a3=0.00712. The curve is approximately flat, but the outermost points indicate a slight rise in rotational velocity. We find that Oort's constant A=12.6 km/s/kpc. The mass of the Galaxy within R=2R0 is found to be 4.1E+11 M0.

The velocity residuals (observed Vlsr minus Vlsr expected from circular rotation) show a pattern which suggest deviations from circular rotation that are consistent with spiral density wave streaming. No clear evidence of spiral structure is seen in the spatial distribution of the early type stars in our sample. Streaming motions (i.e. the systematic component of the velocity residuals) are found, with a mean value of about 12 km/s, implying 2-D values of about 17 km/s. We find that the molecular gas is streaming past the LSR, from gl=180 degs to gl=0 degs, at about 3.8 km/s.

The rotation curves fitted Northern and Southern Hemisphere data separately, are found to differ only slightly (the difference between the rotational velocities at 20 kpc being about 5%); this implies that, to first order, the galactic velocity field is axisymmetric.

From a sample of local molecular clouds (between 0.7 kpc and 2 kpc from the Sun), we derive the height of the Sun above the plane to be 13 pc +/- 7 pc. The scale height of these clouds is about 65 pc.

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The ditribution of molecular clouds in the outer Galaxy is derived and compared with that of the HI gas layer. Candidate clouds were selected (as described in paper I -- 1989 A&AS 80, 149) from the IRAS Point Source Catalogue on the basis of location on the sky in the second and third galactic quadrants and on the basis of infrared radiation characteristics consistent with those known to be typical of molecular clouds with embedded heat sources. Of the 1302 selected IRAS sources, 1077 revealed CO J=1-0 emission and subsequently provided a kinematic distance. Both the number of outer Galaxy clouds and the range of distances are larger in the sample investigated here than in what has previously been available. The completeness of the sample is discussed, as is the nature of possible errors in the distance determinations, which are based on the almost flat rotation curve Theta=Theta0*(R/R0)^{0.0382} derived by Brand and collaborators evaluated for the galactic constants R0=8.5 kpc and Theta0=220 km/s. The molecular clouds in the sample can be traced to about 20 kpc from the galactic center. Because the clouds detected all have embedded IRAS sources, the results here suggest that star formation is occurring in the far-outer Galaxy. The ensemble of clouds show the same warped shape and flaring thickness as shown by the outer-Galaxy HI layer. The warp can be followed in the IRAS-selected molecular clouds from its onset near R=11 kpc to the edge of the ensemble near 20 kpc. Over this range, the z-thickness of the ensemble increases smoothly, approximately doubling between 13 and 17 kpc. Although the HI layer is substantially thicker than the molecular one in the inner Galaxy, the two tracers approach the same thickness in the outer Galaxy. Comparison of the radial scalelengths of the HI and molecular clouds determined at R>14 kpc shows that the HI gas layer terminates less abruptly than the cloud ensemble.

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