Herschel-HIFI observations of H2O, NH3 and N2H+ toward high-mass starless and proto-stellar clumps identified by the Hi-GAL survey

Our present understanding of high-mass star formation still remains very schematic. In particular, it is not yet clear how much of the difference between low-mass and high-mass star formation occurs during the earliest star formation phases. The chemical characteristics of massive cold clumps, and the comparison with those of their low-mass counterparts, could provide crucial clues about the exact role that chemistry plays in differentiating the early phases of low-mass and high-mass star formation. Water, in particular, is a unique probe of physical and chemical conditions in star-forming regions. Using the HIFI instrument of Herschel we have observed the ortho-NH3 (1_0-0_0) (572GHz), ortho-H2O (1_10-1_01) (557GHz) and N2H+ (6-5) (559GHz) lines toward a sample of high-mass starless and proto-stellar clumps selected from the "Herschel} Infrared Galactic Plane Survey" (Hi-GAL). We compare our results to previous studies of low-mass and high-mass proto-stellar objects. At least one of the three molecular lines was detected in 4 (out of 35) and 7 (out of 17) objects in the l=59deg and l=30deg galactic regions, respectively. All detected sources are proto-stellar. The water spectra are complex and consist of several kinematic components, identified through a Gaussian decomposition, and in a few sources inverse and regular P-Cygni profiles have been detected. All water line profiles of the l=59deg region are dominated by a broad Gaussian emission feature, indicating that the bulk of the water emission arises in outflows. No such broad emission is detected toward the l=30deg objects. The ammonia line in some cases also shows line wings and an inverse P-Cygni profile, thus confirming that NH3 rotational transitions can be used to probe the dynamics of high-mass star forming regions. Both bolometric and water line luminosity increase with the continuum temperature.Comment: This paper includes 7 main figures and 6 tables, in addition to the figures with the spectra of the individual sources which are presented as on-line material. Accepted for publication on Astronomy and Astrophysic

First evidence for molecular interfaces between outflows and ambient clouds in high-mass star-forming regions?

We present new observations of the Cep A East region of massive star formation and describe an extended and dynamically distinct feature not previously recognized. This feature is present in emission from H2CS, OCS, CH3OH, and HDO at −5.5 km s−1 but is not traced by the conventional tracers of star-forming regions, H2S, SO2, SO, and CS. The feature is extended up to at least 0.1 pc. We show that the feature is neither a hot core nor a shocked outflow. However, the chemistry of the feature is consistent with predictions from a model of an eroding interface between a fast wind and a dense core; mixing between the two media occurs in the interface on a timescale of 10–50 yr. If these observations are confirmed by detailed maps and by detections in species also predicted to be abundant (e.g., HCO+, H2CO, and NH3), this feature would be the first detection of such an interface in regions of massive star formation. An important implication of the model is that a significant reservoir of sulfur in grain mantles is required to be in the form of OCS

A theoretical investigation of the reaction between the amidogen, NH, and the ethyl, C2H5, radicals: a possible gas-phase formation route of interstellar and planetary ethanimine

The reaction between the amidogen, NH, radical and the ethyl, C2H5, radical has been investigated by performing electronic structure calculations of the underlying doublet potential energy surface. Rate coefficients and product branching ratios have also been estimated by combining capture and RRKM calculations. According to our results, the reaction is very fast, close to the gas-kinetics limit. However, the main product channel, with a yield of ca. 86-88% in the range of temperatures investigated, is the one leading to methanimine and the methyl radical. The channels leading to the two E-, Z- stereoisomers of ethanimine account only for ca. 5-7% each. The resulting ratio [E-CH3CHNH]/[Z-CH3CHNH] is ca. 1.2, that is a value rather lower than that determined in the Green Bank Telescope PRIMOS radio astronomy survey spectra of Sagittarius B2 North (ca. 3). Considering that ice chemistry would produce essentially only the most stable isomer, a possible conclusion is that the observed [E-CH3CHNH]/[Z-CH3CHNH] ratio is compatible with a combination of gas-phase and grain chemistry. More observational and laboratory data are needed to definitely address this issue

Wide field CO J = 3->2 mapping of the Serpens Cloud Core

Context. Outflows provide indirect means to get an insight on diverse star formation associated phenomena. On scales of individual protostellar cores, outflows combined with intrinsic core properties can be used to study the mass accretion/ejection process of heavily embedded protostellar sources. Methods. An area comprising 460"x230" of the Serpens cloud core has been mapped in 12 CO J = 3\to 2 with the HARP-B heterodyne array at the James Clerk Maxwell Telescope; J = 3\to 2 observations are more sensitive tracers of hot outflow gas than lower J CO transitions; combined with the high sensitivity of the HARP-B receptors outflows are sharply outlined, enabling their association with individual protostellar cores. Results. Most of ~20 observed outflows are found to be associated with known protostellar sources in bipolar or unipolar configurations. All but two outflow/core pairs in our sample tend to have a projected orientation spanning roughly NW-SE. The overall momentum driven by outflows in Serpens lies between 3.2 and 5.1 x 10^(-1) M\odot km s^(-1), the kinetic energy from 4.3 to 6.7 x 10^(43) erg and momentum flux is between 2.8 and 4.4 x 10^(-4) M\odot km s^(-1) yr^(-1). Bolometric luminosities of protostellar cores based on Spitzer photometry are found up to an order of magnitude lower than previous estimations derived with IRAS/ISO data. Conclusions. We confirm the validity of the existing correlations between the momentum flux and bolometric luminosity of Class I sources for the homogenous sample of Serpens, though we suggest that they should be revised by a shift to lower luminosities. All protostars classified as Class 0 sources stand well above the known Class I correlations, indicating a decline in momentum flux between the two classes.Comment: 15 pages, 10 figures, accepted for publication in A&

High SiO abundance in the HH212 protostellar jet

Previous SiO maps of the innermost regions of HH212 set strong constraints on the structure and origin of this jet. They rule out a fast wide-angle wind, and tentatively favor a magneto-centrifugal disk wind launched out to 0.6 AU. We aim to assess the SiO content at the base of the HH212 jet to set an independent constraint on the location of the jet launch zone with respect to the dust sublimation radius. We present the first sub-arcsecond (0"44x0"96) CO map of the HH212 jet base, obtained with the IRAM Plateau de Bure Interferometer. Combining this with previous SiO(5-4) data, we infer the CO(2-1) opacity and mass-flux in the high-velocity jet and arrive at a much tighter lower limit to the SiO abundance than possible from the (optically thick) SiO emission alone. Gas-phase SiO at high velocity contains at least 10% of the elemental silicon if the jet is dusty, and at least 40% if the jet is dust-free, if CO and SiO have similar excitation temperatures. Such a high SiO content is challenging for current chemical models of both dust-free winds and dusty interstellar shocks. Updated chemical models (equatorial dust-free winds, highly magnetized dusty shocks) and observations of higher J CO lines are required to elucidate the dust content and launch radius of the HH212 high-velocity jet.Comment: 4 pages, 2 figure

The CHESS survey of the L1157-B1 shock: the dissociative jet shock as revealed by Herschel--PACS

Outflows generated by protostars heavily affect the kinematics and chemistry of the hosting molecular cloud through strong shocks that enhance the abundance of some molecules. L1157 is the prototype of chemically active outflows, and a strong shock, called B1, is taking place in its blue lobe between the precessing jet and the hosting cloud. We present the Herschel-PACS 55--210 micron spectra of the L1157-B1 shock, showing emission lines from CO, H2O, OH, and [OI]. The spatial resolution of the PACS spectrometer allows us to map the warm gas traced by far-infrared (FIR) lines with unprecedented detail. The rotational diagram of the high-Jup CO lines indicates high-excitation conditions (Tex ~ 210 +/- 10 K). We used a radiative transfer code to model the hot CO gas emission observed with PACS and in the CO (13-12) and (10-9) lines measured by Herschel-HIFI. We derive 20010^5 cm-3. The CO emission comes from a region of about 7 arcsec located at the rear of the bow shock where the [OI] and OH emission also originate. Comparison with shock models shows that the bright [OI] and OH emissions trace a dissociative J-type shock, which is also supported by a previous detection of [FeII] at the same position. The inferred mass-flux is consistent with the "reverse" shock where the jet is impacting on the L1157-B1 bow shock. The same shock may contribute significantly to the high-Jup CO emission.Comment: 7 pages, 9 figures, accepted for publication in Astronomy and Astrophysic

SiO excitation from dense shocks in the earliest stages of massive star formation

Molecular outflows are a direct consequence of accretion, and therefore they represent one of the best tracers of accretion processes in the still poorly understood early phases of high-mass star formation. Previous studies suggested that the SiO abundance decreases with the evolution of a massive young stellar object probably because of a decay of jet activity, as witnessed in low-mass star-forming regions. We investigate the SiO excitation conditions and its abundance in outflows from a sample of massive young stellar objects through observations of the SiO(8-7) and CO(4-3) lines with the APEX telescope. Through a non-LTE analysis, we find that the excitation conditions of SiO increase with the velocity of the emitting gas. We also compute the SiO abundance through the SiO and CO integrated intensities at high velocities. For the sources in our sample we find no significant variation of the SiO abundance with evolution for a bolometric luminosity-to-mass ratio of between 4 and 50 $L_\odot/M_\odot$. We also find a weak increase of the SiO(8-7) luminosity with the bolometric luminosity-to-mass ratio. We speculate that this might be explained with an increase of density in the gas traced by SiO. We find that the densities constrained by the SiO observations require the use of shock models that include grain-grain processing. For the first time, such models are compared and found to be compatible with SiO observations. A pre-shock density of $10^5\,$cm$^{-3}$ is globally inferred from these comparisons. Shocks with a velocity higher than 25 km s$^{-1}$ are invoked for the objects in our sample where the SiO is observed with a corresponding velocity dispersion. Our comparison of shock models with observations suggests that sputtering of silicon-bearing material (corresponding to less than 10% of the total silicon abundance) from the grain mantles is occurring.Comment: Accepted for publication by A&

The L1157-B1 astrochemical laboratory: testing the origin of DCN

L1157-B1 is the brightest shocked region of the large-scale molecular outflow, considered the prototype of chemically rich outflows, being the ideal laboratory to study how shocks affect the molecular gas. Several deuterated molecules have been previously detected with the IRAM 30m, most of them formed on grain mantles and then released into the gas phase due to the shock. We aim to observationally investigate the role of the different chemical processes at work that lead to formation the of DCN and test the predictions of the chemical models for its formation. We performed high-angular resolution observations with NOEMA of the DCN(2-1) and H13CN(2-1) lines to compute the deuterated fraction, Dfrac(HCN). We detected emission of DCN(2-1) and H13CN(2-1) arising from L1157-B1 shock. Dfrac(HCN) is ~4x10$^{-3}$ and given the uncertainties, we did not find significant variations across the bow-shock. Contrary to HDCO, whose emission delineates the region of impact between the jet and the ambient material, DCN is more widespread and not limited to the impact region. This is consistent with the idea that gas-phase chemistry is playing a major role in the deuteration of HCN in the head of the bow-shock, where HDCO is undetected as it is a product of grain-surface chemistry. The spectra of DCN and H13CN match the spectral signature of the outflow cavity walls, suggesting that their emission result from shocked gas. The analysis of the time dependent gas-grain chemical model UCL-CHEM coupled with a C-type shock model shows that the observed Dfrac(HCN) is reached during the post-shock phase, matching the dynamical timescale of the shock. Our results indicate that the presence of DCN in L1157-B1 is a combination of gas-phase chemistry that produces the widespread DCN emission, dominating in the head of the bow-shock, and sputtering from grain mantles toward the jet impact region.Comment: Accepted for publication in A&A. 7 pages, 5 Figures, 1 Tabl