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Abstract. We have conducted a survey of SiO emission towards galactic H 2 O and OH masers and ultracompact HII regions using the 15-m SEST and the 20-m Onsala telescope. With the SEST the transitions (v = 0, J = 2 − 1) and (v = 0, J = 3−2) of SiO at 3 and 2 mm were measured simultaneously. With Onsala only the (v = 0, J = 2 − 1) line was accessible. Altogether 369 objects were observed and SiO was detected towards 137 of them. The detection rate is highest towards the most intense H 2 O masers, which probably require powerful shocks to be excited. The SiO detection rate correlates also with the integrated farinfrared flux density and the FIR luminosity of the associated IRAS point source, indicating that the occurrence of shocks is related to the amount of radiation from the central stellar source(s). For flux and luminosity limited samples the SiO detection rate is higher in the inner 7 kpc from the galactic centre than elsewhere. This suggests that dense cores belonging to the so called "molecular ring" provide particularly favourable conditions for the production of gaseous SiO.The full widths above 2σ of the SiO(J = 2 − 1) lines, which are likely to be related to the associated shock velocities, range from 2 to 60 km s −1 except for the line in Ori KL which has a full width of about 100 km s −1 . The median of our sample is 19 km s −1 . The SiO lines are singlepeaked and the peak velocities are always close to the ambient cloud velocity as determined from published CS observations. These line characteristics are compared with the predictions of kinematical bow-shock models. The SiO line shapes correspond with the model of Raga & Cabrit (1993) where the emission arises from turbulent wakes behind bow-shocks. However, the number of symmetric, relSend offprint requests to: J. HarjuThe entire Tables B.1 and B.2 and the spectra of all the detected SiO sources are only available in electronic form at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsweb.u-strasbg.fr/Abstract.html atively narrow profiles indicates that at least in some of the observed sources SiO emission arises also from the quiescent gas component.We suggest that this is due to evaporation of silicon compounds from grain mantles and their reprocessing to SiO in dense quiescent gas according to the model of McKay (1995McKay ( , 1996. These reactions may be initiated and sustained by ionizing radiation from shocks, in the same way as the enhancement of HCO + near Herbig-Haro objects has been explained in the model of Wolfire & Königl (1993).The excitation temperatures of SiO(J = 2 − 1) and (J = 3 − 2) transitions were determined towards three strong sources using measurements in isotopically substituted SiO. In all three sources the transitions are clearly subthermally excited, implying moderate densities (< 3 10 6 cm −3 ) in the SiO emission regions.

Context. We present the first results from the project Galactic cold cores, where the cold interstellar clouds detected by the Planck satellite are studied with Herschel photometric observations. The final Planck catalogue is expected to contain several thousand sources. The Herschel observations during the science demonstration phase provided the first glimpse into the nature of these sources. Aims. The main goal of the project is to derive the physical properties of the cold core population revealed by Planck. We examine three fields and confirm the Planck detections with Herschel data, which we also use to establish the evolutionary stage of the identified cores. Methods. We study the morphology and spectral energy distribution of the sources using the combined wavelength coverage of Planck and Herschel. The dust colour temperatures and emissivity indices are determined. The masses of the cores are determined with distance estimates which are taken from the literature and are confirmed by kinematic and extinction information. Results. The observations reveal extended regions of cold dust with dust colour temperatures down to T dust ∼ 11 K. The fields represent different evolutionary stages ranging from a quiescent, cold filament in Musca to regions of active star formation in Cepheus. Conclusions. The Herschel observations confirm that the all-sky survey of Planck is capable of making a large number of new cold core detections. Our results suggest that many of the sources may already have left the pre-stellar phase or are at least closely associated with active star formation. High-resolution Herschel observations are needed to establish the true nature of the Planck detections.

Context. Cold cores in interstellar molecular clouds represent the very first phase in star formation. The physical conditions of these objects are studied in order to understand how molecular clouds evolve and how stellar masses are determined. Aims. The purpose of this study is to probe conditions in the dense, starless clump Ophichus D (Oph D). Methods. The ground-state (1 10 → 1 11 ) rotational transition of ortho-H 2 D + was observed with APEX towards the density peak of Oph D.Results. The width of the H 2 D + line indicates that the kinetic temperature in the core is about 6 K. So far, this is the most direct evidence of such cold gas in molecular clouds. The observed H 2 D + spectrum can be reproduced with a hydrostatic model with the temperature increasing from about 6 K in the centre to almost 10 K at the surface. The model is unstable against any increase in the external pressure, and the core is likely to form a low-mass star. Conclusions. The results suggest that an equilibrium configuration is a feasible intermediate stage of star formation even if the larger scale structure of the cloud is thought to be determined by turbulent fragmentation. In comparison with the isothermal case, the inward decrease in the temperature makes smaller, i.e. less massive, cores susceptible to externally triggered collapse.

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