Context. The thermal structure of a starless core is crucial for our understanding of the physics in these objects and hence for our understanding of star formation. Theory predicts a gas temperature drop in the inner ∼5000 AU of the pre-stellar core L 1544, but there has been no observational proof of this. Aims. We performed VLA observations of the NH 3 (1, 1) and (2, 2) transitions towards L 1544 in order to measure the temperature gradient between the high density core nucleus and the surrounding core envelope. Our VLA observation for the first time provide measurements of gas temperature in a core with a resolution smaller than 1000 AU. We have also obtained high resolution Plateau de Bure observations of the 110 GHz 1 11 − 1 01 para-NH 2 D line in order to further constrain the physical parameters of the high density nucleus. Methods. We combine our interferometric NH 3 and NH 2 D observations with available single dish measurements in order to estimate the effects of flux loss from extended components upon our data. We have estimated the temperature gradient using a model of the source to fit our data in the u, v plane. As the NH 3 (1, 1) line is extremely optically thick, this also involved fitting a gradient in the NH 3 abundance. In this way, we also measure the [NH 2 D]/[NH 3 ] abundance ratio in the inner nucleus. Results. We find that indeed the temperature decreases toward the core nucleus from 12 K down to 5.5 K resulting in an increase of a factor of 50% in the estimated density of the core from the dust continuum if compared with the estimates done with constant temperature of 8.75 K. Current models of the thermal equilibrium can describe consistently the observed temperature and density in this object, simultaneously fitting our temperature profile and the continuum emission. We also found a remarkably high abundance of deuterated ammonia with respect to the ammonia abundance (50% ± 20%), which proves the persistence of nitrogen bearing molecules at very high densities (2 × 10 6 cm −3 ) and shows that high-resolution observations yield higher deuteration values than single-dish observations. The NH 2 D observed transition, free of the optical depth problems that affect the NH 3 lines in the core center, is a much better probe of the high-density nucleus and, in fact, its map peak at the dust continuum peak. Our analysis of the NH 3 and NH 2 D kinematic fields shows a decrease of specific angular momentum from the large scales to the small scales.

Context. The formation processes and the evolutionary stages of high-mass stars are poorly understood compared to low-mass stars. Large-scale surveys are needed to provide an unbiased census of high column density sites that can potentially host precursors to high-mass stars. Aims. The ATLASGAL survey covers 420 sq. degree of the Galactic plane, between −80 • < < +60 • at 870 μm. Here we identify the population of embedded sources throughout the inner Galaxy. With this catalog we first investigate the general statistical properties of dust condensations in terms of their observed parameters, such as flux density and angular size. Then using mid-infrared surveys we aim to investigate their star formation activity and the Galactic distribution of star-forming and quiescent clumps. Our ultimate goal is to determine the statistical properties of quiescent and star-forming clumps within the Galaxy and to constrain the star formation processes. Methods. We optimized the source extraction method, referred to as MRE-GCL, for the ATLASGAL maps in order to generate a catalog of compact sources. This technique is based on multiscale filtering to remove extended emission from clouds to better determine the parameters corresponding to the embedded compact sources. In a second step we extracted the sources by fitting 2D Gaussians with the Gaussclumps algorithm. Results. We have identified in total 10861 compact submillimeter sources with fluxes above 5σ. Completeness tests show that this catalog is 97% complete above 5σ and >99% complete above 7σ. Correlating this sample of clumps with mid-infrared point source catalogs (MSX at 21.3 μm and WISE at 22 μm), we have determined a lower limit of 33% that is associated with embedded protostellar objects. We note that the proportion of clumps associated with mid-infrared sources increases with increasing flux density, achieving a rather constant fraction of ∼75% of all clumps with fluxes over 5 Jy/beam being associated with star formation. Examining the source counts as a function of Galactic longitude, we are able to identify the most prominent star-forming regions in the Galaxy. Conclusions. We present here the compact source catalog of the full ATLASGAL survey and investigate their characteristic properties. From the fraction of the likely massive quiescent clumps (∼25%), we estimate a formation time scale of ∼7.5 ± 2.5 × 10 4 yr for the deeply embedded phase before the emergence of luminous young stellar objects. Such a short duration for the formation of high-mass stars in massive clumps clearly proves that the earliest phases have to be dynamic with supersonic motions.

Abstract.We have considered the thermal equilibrium in pre-protostellar cores in the approximation where the dust temperature is independent of interactions with the gas and where the gas is heated both by collisions with dust grains and ionization by cosmic rays. We have then used these results to study the stability of cores in hydrostatic equilibrium in the limit where thermal pressure dominates over magnetic field and turbulence. We compare the density distribution derived in this manner with results obtained in the isothermal case. We find that for cores with characteristics similar to those observed in nearby molecular clouds, the gas and dust temperatures are coupled in the core interior with densities above ∼3×10 4 cm −3 . As a consequence, one expects that the gas temperature like the dust temperature decreases towards the center of these objects. However, the regime where gas and dust temperatures are coupled coincides approximately with that in which CO and many other molecular species deplete onto dust grain surfaces. At larger radii and lower densities, the gas and dust temperatures decouple and the gas temperature tends to the value expected for cosmic ray heating alone. The density structure which one computes taking into account such deviations from isothermality are not greatly different from that expected for an isothermal Bonnor-Ebert sphere. It is impossible in the framework of these models to have a stable equilibrium core with mass above ∼5 M and column density compatible with observed values (N H > 2 × 10 22 cm −2 or A V > 10 mag). We conclude from this that observed high mass cores are either supported by magnetic field or turbulence or are already in a state of collapse. Lower mass cores on the other hand have stable states where thermal pressure alone provides support against gravitation and we conclude that the much studied object B68 may be in a state of stable equilibrium if the internal gas temperature is computed in self-consistent fashion. Finally we note that in molecular clouds such as Ophiuchus and Orion with high radiation fields and pressures, gas and dust temperatures are expected to be well coupled and hence in the absence of an internal heat source, one expects temperatures to decrease towards core centers and to be relatively high as compared to low pressure clouds like Taurus.

Context. The processes leading to the birth of high-mass stars are poorly understood. The key first step to reveal their formation processes is characterising the clumps and cores from which they form. Aims. We define a representative sample of massive clumps in different evolutionary stages selected from the APEX Telescope Large Area Survey of the Galaxy (ATLASGAL), from which we aim to establish a census of molecular tracers of their evolution. As a first step, we study the shock tracer, SiO, mainly associated with shocks from jets probing accretion processes. In low-mass young stellar objects (YSOs), outflow and jet activity decreases with time during the star formation processes. Recently, a similar scenario was suggested for massive clumps based on SiO observations. Here we analyse observations of the SiO (2−1) and (5−4) lines in a statistically significant sample to constrain the change of SiO abundance and the excitation conditions as a function of evolutionary stage of massive star-forming clumps. Methods. We performed an unbiased spectral line survey covering the 3-mm atmospheric window between 84−117 GHz with the IRAM 30 m telescope of a sample of 430 sources of the ATLASGAL survey, covering various evolutionary stages of massive clumps. A smaller sample of 128 clumps has been observed in the SiO (5−4) transition with the APEX telescope to complement the (2−1) line and probe the excitation conditions of the emitting gas. We derived detection rates to assess the star formation activity of the sample, and we estimated the column density and abundance using both an LTE approximation and non-LTE calculations for a smaller subsample, where both transitions have been observed. Results. We characterise the physical properties of the selected sources, which greatly supersedes the largest samples studied so far, and show that they are representative of different evolutionary stages. We report a high detection rate of >75% of the SiO (2−1) line and a >90% detection rate from the dedicated follow-ups in the (5−4) transition. Up to 25% of the infrared-quiet clumps exhibit high-velocity line wings, suggesting that molecular tracers are more efficient tools to determine the level of star formation activity than infrared colour criteria. We also find infrared-quiet clumps that exhibit only a low-velocity component (FWHM ∼ 5−6 km s −1 ) SiO emission in the (2−1) line. In the current picture, where this is attributed to low-velocity shocks from cloud-cloud collisions, this can be used to pinpoint the youngest, thus, likely prestellar massive structures. Using the optically thin isotopologue ( 29 SiO), we estimate that the (2−1) line is optically thin towards most of the sample. Furthermore, based on the line ratio of the (5−4) to the (2−1) line, our study reveals a trend of changing excitation conditions that lead to brighter emission in the (5−4) line towards more evolved sources. Our models show that a proper treatment of non-LTE effects and beam dilution is necessary to constrain trends in the SiO column den...