A. Gusdorf

The W43-MM1 mini-starburst ridge, a test for star formation efficiency models

Context. Star formation e ciency (SFE) theories are currently based on statistical distributions of turbulent cloud structures and a simple model of star formation from cores. They remain poorly tested, especially at the highest densities. Aims. We investigate the e ects of gas density on the SFE through measurements of the core formation e ciency (CFE). With a total mass of 2 10 4 M , the W43-MM1 ridge is one of the most convincing candidate precursors of Galactic starburst clusters and thus one of the best places to investigate star formation. Methods. We used high-angular resolution maps obtained at 3 mm and 1 mm within the W43-MM1 ridge with the IRAM Plateau de Bure Interferometer to reveal a cluster of 11 massive dense cores, and, one of the most massive protostellar cores known. A Herschel column density image provided the mass distribution of the cloud gas. We then measured the “instantaneous” CFE and estimated the SFE and the star formation rate (SFR) within subregions of the W43-MM1 ridge. Results. The high SFE found in the ridge ( 6% enclosed in 8 pc 3 ) confirms its ability to form a starburst cluster. There is, however, a clear lack of dense cores in the eastern part of the ridge, which may be currently assembling. The CFE and the SFE are observed to increase with volume gas density, while the SFR per free fall time steeply decreases with the virial parameter, vir. Statistical models of the SFR may describe the outskirts of the W43-MM1 ridge well, but struggle to reproduce its inner part, which corresponds to measurements at low vir. It may be that ridges do not follow the log-normal density distribution, Larson relations, and stationary conditions forced in the statistical SFR models.

The CHESS survey of the L1157-B1 bow-shock: high and low excitation water vapor

Context. Molecular outflows powered by young protostars strongly affect the kinematics and chemistry of the natal molecular cloud through strong shocks. This results in substantial modifications of the abundance of several species. In particular, water is a powerful tracer of shocked material because of its sensitivity to both physical conditions and chemical processes. Aims. As part of the Chemical HErschel Surveys of Star-forming regions (CHESS) guaranteed time key program, we aim at investigating the physical and chemical conditions of H2O in the brightest shock region B1 of the L1157 molecular outflow. Methods. We observed several ortho- and para-H2O transitions using the HIFI and PACS instruments on board Herschel toward L1157-B1, providing a detailed picture of the kinematics and spatial distribution of the gas. We performed a large velocity gradient (LVG) analysis to derive the physical conditions of H2O shocked material, and ultimately obtain its abundance. Results. We detected 13 H2O lines with both instruments probing a wide range of excitation conditions. This is the largest data set of water lines observed in a protostellar shock and it provides both the kinematics and the spatial information of the emitting gas. The PACS maps reveal that H2O traces weak and extended emission associated with the outflow identified also with HIFI in the o-H2O line at 556.9 GHz, and a compact (∼10 �� ) bright, higher excitation region. The LVG analysis of H2O lines in the bowshock show the presence of two gas components with different excitation conditions: a warm (Tkin � 200‐300 K) and dense (n(H2) � (1‐3) × 10 6 cm −3 ) component with an assumed extent of 10 �� , and a compact (∼2 �� ‐5 �� ) and hot, tenuous (Tkin � 900‐1400 K, n(H2) � 10 3−4 cm −3 ) gas component that is needed to account for the line fluxes of high Eu transitions. The fractional abundance of the warm and hot H2O gas components is estimated to be (0.7‐2) × 10 −6 and (1‐3) × 10 −4 , respectively. Finally, we identified an additional component in absorption in the HIFI spectra of H2O lines that connect with the ground state level. This absorption probably arises from the photodesorption of icy mantles of a water-enriched layer at the edges of the cloud, driven by the external UV illumination of the interstellar radiation field.

Shockingly low water abundances in Herschel/PACS observations of low-mass protostars in Perseus

Context. Protostars interact with their surroundings through jets a nd winds impinging on the envelope and creating shocks, but the nature of these shocks is still poorly understood. Aims. Our aim is to survey far-infrared molecular line emission from a uniform and significant sample of deeply-embedded low-m ass young stellar objects (YSOs) in order to characterize shocks and the possible role of ultraviolet radiation in the immed iate protostellar environment. Methods. Herschel/PACS spectral maps of 22 objects in the Perseus molecular cloud were obtained as part of the William Herschel Line Legacy (WILL) survey. Line emission from H2O, CO, and OH is tested against shock models from the literature. Results. Observed line ratios are remarkably similar and do not show variations with physical parameters of the sources (luminosity, envelope mass). Most ratios are also comparable to those found at off-source outflow positions. Observations show good agreemen t with the shock models when line ratios of the same species are compared. Ratios of various H2O lines provide a particularly good diagnostic of pre-shock gas densities, nH ∼ 10 5 cm −3 , in agreement with typical densities obtained from observations of the postshock gas when a compression factor on the order of 10 is applied (for non-dissociative shocks). The corresponding shock velocities, obtained from comparison with CO line ratios, are above 20 km s −1 . However, the observations consistently show H2O-to-CO and H2O-to-OH line ratios that are one to two orders of magnitude lower than predicted by the existing shock models. Conclusions. The overestimated model H2O fluxes are most likely caused by an overabundance of H 2O in the models since the excitation is well-reproduced. Illumination of the shocke d material by ultraviolet photons produced either in the sta r-disk system or, more locally, in the shock, would decrease the H2O abundances and reconcile the models with observations. Detections of hot H2O and strong OH lines support this scenario.

SiO emission from low- and high-velocity shocks in Cygnus-X massive dense clumps

We used PdBI observations of SiO (2-1) to investigate the morphology and profile of the SiO emission within several massive dense clumps (MDCs) in Cygnus-X. We find that most molecular outflows are detected in both SiO and CO, although there are some cases of CO outflows with no SiO counterpart. We find a significant amount of narrow line SiO emission that appears to be unrelated to outflows. The fraction of the total SiO luminosity that is not associated with outflows is highly variable in the different MDCs (from 10% to 90%); this might be a problem when extrapolating outflow properties from SiO luminosities without resolving individual outflows. The extent of the narrow SiO emission varies from rather compact (~ 0.03 pc) to widespread (~0.2 pc), and its kinematics often differs from those found by other high-density tracers such as H13CO+. We find that the least centrally concentrated clumps with the least massive protostellar cores have the most widespread narrow SiO emission. In line with previous evidence of SiO emission associated with low-velocity shocks, we propose an evolutionary picture to explain the existence and distribution of narrow SiO line profiles. In this scenario, the least centrally condensed MDCs are at an early stage where the SiO emission traces shocks from the large-scale collapse of material onto the MDC (e.g. CygX-N40). As the MDC collapses, the SiO emission becomes more confined to the close surroundings of cores, tracing the post-shock material from the infalling MDC against the dense cores (e.g. CygX-N3, N12, and N48). At later stages, when single massive protostars are formed, the SiO luminosity is largely dominated by powerful outflows, and the weaker narrow component shows perhaps the last remnants of the initial collapse (e.g. CygX-N53 and N63).

Probing magnetohydrodynamic shocks with high-J CO observations: W28F

Context. Observing supernova remnants (SNRs) and modelling the shocks they are associated with is the best way to quantify the energy SNRs re-distribute back into the interstellar medium (ISM). Aims. We present comparisons of shock models with CO observations in the F knot of the W28 supernova remnant. These comparisons constitute a valuable tool to constrain both the shock characteristics and pre-shock conditions. Methods. New CO observations from the shocked regions with the APEX and SOFIA telescopes are presented and combined. The integrated intensities are compared to the outputs of a grid of models, which were combined from an MHD shock code that calculates the dynamical and chemical structure of these regions and a radiative transfer module based on the large velocity gradient (LVG) approximation. Results. We base our modelling method on the higher J CO transitions, which unambiguously trace the passage of a shock wave. We provide fits for the blue- and red-lobe components of the observed shocks. We find that only stationary, C-type shock models can reproduce the observed levels of CO emission. Our best models are found for a pre-shock density of 10 4 cm −3 , with the magnetic field strength varying between 45 and 100 μG, and a slightly higher shock velocity for the so-called blue-shock (∼25 km s −1 ) than for the red one (∼20 km s −1 ). Our models also satisfactorily account for the pure rotational H2 emission that is observed with Spitzer.

Evidence of a SiO collimated outflow from a massive YSO in IRAS 17233–3606

Studies of molecular outflows in high-mass young stellar objects reveal important information about the formation process of massive stars. We therefore selected the close-by IRAS 17233–3606 massive star-forming region to perform SiO observations with the SMA interferometer in the (5−4) line and with the APEX single-dish telescope in the (5−4) and (8–7) transitions. In this paper, we present a study of one of the outflows in the region, OF1, which shows several properties similar to jets driven by low-mass protostars, such as HH211 and HH212. It is compact and collimated, and associated with extremely high velocity CO emission, and SiO emission at high velocities. We used a state-of-the-art shock model to constrain the pre-shock density and shock velocity of OF1. The model also allowed us to self-consistently estimate the mass of the OF1 outflow. The shock parameters inferred by the SiO modelling are comparable with those found for low-mass protostars, only with higher pre-shock density values, yielding an outflow mass in agreement with those obtained for molecular outflows driven by early B-type young stellar objects. Our study shows that it is possible to model the SiO emission in high-mass star-forming regions in the same way as for shocks from low-mass young stellar objects.

Water and acetaldehyde in HH212: The first hot corino in Orion

Aims. Using the unprecedented combination of high resolution and sensitivity offered by ALMA, we aim to investigate whether and how hot corinos, circumstellar disks, and ejected gas are related in young solar-mass protostars. Methods. We observed CH3CHO and deuterated water (HDO) high-excitation (Eu up to 335 K) lines towards the Sun-like protostar HH212−MM1. Results. For the first time, we have obtained images of CH3CHO and HDO emission in the inner � 100 AU of HH212. The multifrequency line analysis allows us to contrain the density (≥10 7 cm −3 ), temperature (� 100 K), and CH3CHO abundance (� 0.2−2 × 10 −9 ) of the emitting region. The HDO profile is asymmetric at low velocities (≤ 2k ms −1 from Vsys). If the HDO line is optically thick, this points to an extremely small (∼20−40 AU) and dense (≥10 9 cm −3 ) emitting region. Conclusions. We report thefirst detection of a hot corino in Orion. The HDO asymmetric profile indicates a contribution of outflowing gas from the compact central region, possibly associated with a dense disk wind.

APEX observations of supernova remnants - I. Non-stationary magnetohydrodynamic shocks in W44

Aims. The interaction of supernova remnants (SNRs) with molecular clouds gives rise to strong molecular emission in the far-IR and sub-mm wavelength regimes. The application of MHD shock models in the interpretation of this line emission can yield valuable information on the energetic and chemical impact of supernova remnants. Methods. New mapping observations with the APEX telescope in CO (3-2), (4-3), (6-5), (7-6) and 13CO (3-2) towards two regions in the supernova remnant W44 are presented. Integrated intensities are extracted on five different positions, corresponding to local maxima of CO emission. The integrated intensities are compared to the outputs of a grid of models, which combine an MHD shock code with a radiative transfer module based on the large velocity gradient approximation. Results. All extracted spectra show ambient and line-of-sight components as well as blue- and red-shifted wings indicating the presence of shocked gas. Basing the shock model fits only on the highest-lying transitions that unambiguously trace the shock-heated gas, we find that the observed CO line emission is compatible with non-stationary shocks and a pre-shock density of 10^4 cm-3. The ages of the modelled shocks scatter between values of \sim1000 and \sim3000 years. The shock velocities in W44F are found to lie between 20 and 25 km/s, while in W44E fast shocks (30-35 km/s) as well as slower shocks (\sim20 km/s) are compatible with the observed spectral line energy diagrams. The pre-shock magnetic field strength components perpendicular to the line of sight in both regions have values between 100 and 200 \muG. Our best-fitting models allow us to predict the full ladder of CO transitions, the shocked gas mass in one beam as well as the momentum- and energy injection.

Water emission from the high-mass star-forming region IRAS 17233-3606 - High water abundances at high velocities

We investigate the physical and chemical processes at work during the formation of a massive protostar based on the observation of water in an outflow from a very young object previously detect ed in H2 and SiO in the IRAS 17233‐3606 region. We estimated the abundance of water to understand its chemistry, and to constrain the mass of the emitting outflow. We present new observat ions of shocked water obtained with the HIFI receiver onboard Herschel. We detected water at high velocities in a range similar to SiO. We self-consistently fitted these observations along with pre vious SiO data through a state-of-the-art, one-dimensional, stationary C-shock model. We found that a single model can explain the SiO and H2O emission in the red and blue wings of the spectra. Remarkably, one common area, similar to that found for H2 emission, fits both the SiO and H 2O emission regions. This shock model subsequently allowed us to assess the shocked water column density, NH2O = 1.2 10 18 cm −2 , mass, MH2O = 12.5 M⊕, and its maximum fractional abundance with respect to the total density, xH2O = 1.4 10 −4 . The corresponding water abundance in fractional column density units ranges between 2.5 10 −5 and 1.2 10 −5 , in agreement with recent results obtained in outflows from l ow- and high-mass young stellar

CO Spectral Line Energy Distributions in Galactic Sources: Empirical Interpretation of Extragalactic Observations

The relative populations in rotational transitions of CO can be useful for inferring gas conditions and excitation mechanisms at work in the interstellar medium. We present CO emission lines from rotational transitions observed with Herschel/HIFI in the star-forming cores Orion S, Orion KL, Sgr B2(M), and W49N. Integrated line fluxes from these observations are combined with those from Herschel/PACS observations of the same sources to construct CO spectral line energy distributions (SLEDs) from 5 ≤ J_u ≤ 48. These CO SLEDs are compared to those reported in other galaxies, with the intention of empirically determining which mechanisms dominate excitation in such systems. We find that CO SLEDs in Galactic star-forming cores cannot be used to reproduce those observed in other galaxies, although the discrepancies arise primarily as a result of beam filling factors. The much larger regions sampled by the Herschel beams at distances of several megaparsecs contain significant amounts of cooler gas, which dominate the extragalactic CO SLEDs, in contrast to observations of Galactic star-forming regions, which are focused specifically on cores containing primarily hot molecular gas.