Izaskun Jimenez-Serra

Parsec-scale SiO emission in an infrared dark cloud

We present high-sensitivity 2 × 4a rcmin 2 maps of the J = 2→1 rotational lines of SiO, CO, 13 CO and C 18 O, observed towards the filamentary infrared dark cloud (IRDC) G035.39−00.33. Single-pointing spectra of the SiO J = 2→1 and J = 3→2 lines towards several regions in the filament are also reported. The SiO images reveal that SiO is widespread along the IRDC (size ≥2 pc), showing two different components: one bright and compact arising from three condensations (N, E and S) and the other weak and extended along the filament. While the first component shows broad lines (linewidths of ∼4–7 km s −1 ) in both SiO J = 2→1 and SiO J = 3→2, the second one is only detected in SiO J = 2→1 and has narrow lines (∼0.8 km s −1 ). The maps of CO and its isotopologues show that low-density filaments are intersecting the IRDC and appear to merge towards the densest portion of the cloud. This resembles the molecular structures predicted by flow-driven, shock-induced and magneticallyregulated cloud formation models. As in outflows associated with low-mass star formation, the excitation temperatures and fractional abundances of SiO towards N, E and S increase with velocity from ∼ 6t o 40 Ka nd from∼10 −10 to ≥10 −8 , respectively, over a velocity range of ∼ 7k m s −1 . Since 8 μm and 24 μm sources and/or extended 4.5 μm emission are detected in N, E and S, broad SiO is likely produced in outflows associated with high-mass protostars. The excitation temperatures and fractional abundances of the narrow SiO lines, however, are very low (∼9 K and ∼10 −11 , respectively), and consistent with the processing of interstellar grains by the passage of a shock with vs ∼ 12 km s −1 . This emission could be generated (i) by a large-scale shock, perhaps remnant of the IRDC formation process, (ii) by decelerated or recently processed gas in large-scale outflows driven by 8- and 24-μm sources or (iii) by an undetected and widespread population of lower mass protostars. High-angular-resolution observations are needed to disentangle between these three scenarios.

Deuteration as an evolutionary tracer in massive-star formation

Context. Theory predicts, and observations confirm, that the column density ratio of a molecule containing D to its counterpart containing H can be used as an evolutionary tracer in the low-mass star formation process. Aims. Since it remains unclear if the high-mass star formation process is a scaled-up version of the low-mass one, we investigated whether the relation between deuteration and evolution can be applied to the high-mass regime. Methods. With the IRAM-30 m telescope, we observed rotational transitions of N 2 D + and N 2 H + and derived the deuterated fraction in 27 cores within massive star-forming regions understood to represent different evolutionary stages of the massive-star formation process. Results. The abundance of N 2 D + is higher at the pre-stellar/cluster stage, then drops during the formation of the protostellar object(s) as in the low-mass regime, remaining relatively constant during the ultra-compact HII region phase. The objects with the highest fractional abundance of N 2 D + are starless cores with properties very similar to typical pre-stellar cores of lower mass. The abundance of N 2 D + is lower in objects with higher gas temperatures as in the low-mass case but does not seem to depend on gas turbulence. Conclusions. Our results indicate that the N 2 D + -to-N 2 H + column density ratio can be used as an evolutionary indicator in both low-and high-mass star formation, and that the physical conditions influencing the abundance of deuterated species likely evolve similarly during the processes that lead to the formation of both low- and high-mass stars.

MAPPING LARGE-SCALE CO DEPLETION IN A FILAMENTARY INFRARED DARK CLOUD

Infrared Dark Clouds (IRDCs) are cold, high mass surface density and high density structures, likely to be representative of the initial conditions for massive star and star cluster formation. CO emission from IRDCs has the potential to be useful for tracing their dynamics, but may be affected by depleted gas phase abundances due to freeze out onto dust grains. Here we analyze C 18 O J = 1 → 0 and J = 2 → 1 emission line data, taken with the Instituto de Radioastronomia Milimetrica 30 m telescope, of the highly filamentary IRDC G035.39.-0033. We derive the excitation temperature as a function of position and velocity, with typical values of ∼7 K, and thus derive total mass surface densities, ΣC18O, assuming standard gas phase abundances and accounting for optical depth in the line, which can reach values of ∼1. The mass surface densities reach values of ∼0.07 g cm −2 . We compare these results to the mass surface densities derived from mid-infrared extinction mapping, ΣSMF, by Butler & Tan, which are expected to be insensitive to the dust temperatures in the cloud. With a significance of 10σ, we find ΣC18O/ΣSMF decreases by about a factor of five as Σ increases from ∼0.02 to ∼0. 2gc m −2 , which we interpret as evidence for CO depletion. Several hundred solar masses are being affected, making this one of the most massive clouds in which CO depletion has been observed directly. We present a map of the depletion factor in the filament and discuss implications for the formation of the IRDC.

The dynamical properties of dense filaments in the infrared dark cloud G035.39−00.33

Infrared Dark Clouds (IRDCs) are unique laboratories to study the initial conditions of high-mass star and star cluster formation. We present high-sensitivity and high-angular resolution IRAM PdBI observations of N2H+ (1-0) towards IRDC G035.39-00.33. It is found that G035.39-00.33 is a highly complex environment, consisting of several mildly supersonic filaments (sigma_NT/c_s ~1.5), separated in velocity by <1 km s^-1 . Where multiple spectral components are evident, moment analysis overestimates the non-thermal contribution to the line-width by a factor ~2. Large-scale velocity gradients evident in previous single-dish maps may be explained by the presence of substructure now evident in the interferometric maps. Whilst global velocity gradients are small (<0.7 km s^-1 pc^-1), there is evidence for dynamic processes on local scales (~1.5-2.5 km s^-1 pc^-1 ). Systematic trends in velocity gradient are observed towards several continuum peaks. This suggests that the kinematics are influenced by dense (and in some cases, starless) cores. These trends are interpreted as either infalling material, with accretion rates ~(7 pm 4)x10^-5 M_sun yr^-1 , or expanding shells with momentum ~24 pm 12 M_sun km s^-1 . These observations highlight the importance of high-sensitivity and high-spectral resolution data in disentangling the complex kinematic and physical structure of massive star forming regions.

L1157-B1: WATER AND AMMONIA AS DIAGNOSTICS OF SHOCK TEMPERATURE

We investigate the origin and nature of the profiles of water and ammonia observed toward the L1157-B1 clump as part of the HIFI CHESS survey using a new code coupling a gas-grain chemical model with a parametric shock model. First results from the unbiased survey reveal different molecular components at different excitation conditions coexisting in the B1 bow shock structure, with NH3, H2CO, and CH3OH emitting only at relatively low outflow velocities whereas H2O shows bright emission at high velocities. Our model suggests that these differences are purely chemical and can be explained by the presence of a C-type shock whose maximum temperature must be close to 4000 K along the B1 clump.

FRAGMENTATION OF MOLECULAR CLUMPS AND FORMATION OF A PROTOCLUSTER

Sufficiently massive clumps of molecular gas collapse under self-gravity and fragment to spawn a cluster of stars that have a range of masses. We investigate observationally the early stages of formation of a stellar cluster in a massive filamentary infrared dark cloud, G28.34+0.06 P1, in the 1.3mm continuum and spectral line emission using the ALMA. Sensitive continuum data reveal further fragmentation in five dusty cores at a resolution of several 10^3 AU. Spectral line emission from C18O, CH3OH, 13CS, H2CO and N2D+ are detected for the first time toward these dense cores. We found that three cores are chemically more evolved as compared with the other two; interestingly though, all of them are associated with collimated outflows as suggested by evidence from the CO, SiO, CH3OH, H2CO and SO emissions. The parsec-scale kinematics in NH3 exhibit velocity gradients along the filament, consistent with accretion flows toward the clumps and cores. The moderate luminosity and the chemical signatures indicate that the five cores harbor low- to intermediate-mass protostars that likely become massive ones at the end of the accretion. Despite the fact that the mass limit reached by the 1sigma dust continuum sensitivity is 30 times lower than the thermal Jeans mass, there is a lack of a distributed low-mass protostellar population in the clump. Our observations indicate that in a protocluster, low-mass stars form at a later stage after the birth of more massive protostars.

Complex, quiescent kinematics in a highly filamentary infrared dark cloud

Infrared Dark Clouds (IRDCs) host the initial conditions under which massive stars and stellar clusters form. We have obtained high sensitivity and high spectral resolution observations with the IRAM 30m antenna, which allowed us to perform detailed analysis of the kinematics within one IRDC, G035.39-00.33. We focus on the 1-0 and 3-2 transitions of N2H+, C18O (1-0), and make comparison with SiO (2-1) observations and extinction mapping. Three interacting filaments of gas are found. We report large-scale velocity coherence throughout the cloud, evidenced through small velocity gradients and relatively narrow line widths. This suggests that the merging of these filaments is somewhat gentle, possibly regulated by magnetic fields. This merging of filaments may be responsible for the weak parsec-scale SiO emission detected by Jimenez-Serra et al. 2010, via grain mantle vaporization. A systematic velocity shift between the N2H+ (1-0) and C18O (1-0) gas throughout the cloud of 0.18 +/- 0.04 kms^{-1} is also found, consistent with a scenario of collisions between filaments which is still ongoing. The N2H+ (1-0) is extended throughout the IRDC and it does not only trace dense cores, as found in nearby low-mass star-forming regions. The average H2 number density across the IRDC is ~ 5 x 10^4 cm^{-3}, at least one order of magnitude larger than in nearby molecular clouds where low-mass stars are forming. A temperature gradient perpendicular to the filament is found. From our study, we conclude that G035.39-00.33 (clearly seen in the extinction map and in N2H+) has been formed via the collision between two relatively quiescent filaments with average densities of ~ 5 x 10^3 cm^{-3}, moving with relative velocities of ~ 5 kms^{-1}. The accumulation of material at the merging points started > 1 Myr ago and it is still ongoing.

A Photoevaporating Rotating Disk in the Cepheus A HW2 Star Cluster

We present VLA and PdBI subarcsecond images (~0.15-0.6) of the radio continuum emission at 7 mm and of the SO2 J = 192, 18 → 183, 15 and J = 278, 20 → 287, 21 lines toward the Cep A HW2 region. The SO2 images reveal the presence of a hot core internally heated by an intermediate-mass protostar, and a circumstellar rotating disk around the HW2 radio jet of size 600 × 100 AU and mass ~1 M☉. Keplerian rotation for the disk velocity gradient of ~5 km s-1 requires a 9 M☉ central star, which cannot explain the total luminosity observed in the region. This may indicate that the disk does not rotate with a Keplerian law due to the extreme youth of this object. Our high-sensitivity radio continuum image at 7 mm shows, in addition to the ionized jet, an extended emission to the west (and marginally to the south) of the HW2 jet, filling the southwest cavity of the HW2 disk. From the morphology and location of this free-free continuum emission at centimeter and millimeter wavelengths (spectral index ~0.4-1.5), we propose that the disk is photoevaporating due to the UV radiation from the central star. All this indicates that the Cep A HW2 region harbors a cluster of massive stars. Disk accretion seems to be the most plausible way to form massive stars in moderate density/luminosity clusters.

DISTINCT CHEMICAL REGIONS IN THE ''PRESTELLAR'' INFRARED DARK CLOUD G028.23-00.19

We have observed the Infrared Dark Cloud (IRDC) G028.23–00.19 at 3.3 mm using the Combined Array for Research in Millimeter-wave Astronomy. In its center, the IRDC hosts one of the most massive (~1520 M_☉) quiescent, cold (12 K) clumps known (MM1). The low temperature, high NH2D abundance, narrow molecular line widths, and absence of embedded infrared sources (from 3.6 to 70 μm) indicate that the clump is likely prestellar. Strong SiO emission with broad line widths (6-9 km s^(–1)) and high abundances ((0.8-4) × 10^(–9)) is detected in the northern and southern regions of the IRDC, unassociated with MM1. We suggest that SiO is released to the gas phase from the dust grains through shocks produced by outflows from undetected intermediate-mass stars or clusters of low-mass stars deeply embedded in the IRDC. A weaker SiO component with narrow line widths (~2 km s^(–1)) and low abundances (4.3 × 10^(–11)) is detected in the center-west region, consistent with either a subcloud-subcloud collision or an unresolved population of a few low-mass stars. We report widespread CH_3OH emission throughout the whole IRDC and the first detection of extended narrow methanol emission (~2 km s^(–1)) in a cold, massive prestellar clump (MM1). We suggest that the most likely mechanism releasing methanol into the gas phase in such a cold region is the exothermicity of grain-surface reactions. HN^(13)C reveals that the IRDC is actually composed of two distinct substructures (subclouds) separated in velocity space by ~1.4 km s^(–1). The narrow SiO component arises where the subclouds overlap. The spatial distribution of C2H resembles that of NH_2D, which suggests that C_2H also traces cold gas in this IRDC.

CHEMICAL SEGREGATION TOWARD MASSIVE HOT CORES: THE AFGL2591 STAR-FORMING REGION

We present high angular resolution observations (05 × 03) carried out with the Submillimeter Array (SMA) toward the AFGL2591 high-mass star-forming region. Our SMA images reveal a clear chemical segregation within the AFGL2591 VLA 3 hot core, where different molecular species (Types I, II, and III) appear distributed in three concentric shells. This is the first time that such a chemical segregation is ever reported at linear scales ≤3000 AU within a hot core. While Type I species (H2S and 13CS) peak at the AFGL2591 VLA 3 protostar, Type II molecules (HC3N, OCS, SO, and SO2) show a double-peaked structure circumventing the continuum peak. Type III species, represented by CH3OH, form a ring-like structure surrounding the continuum emission. The excitation temperatures of SO2, HC3N, and CH3OH (185 ± 11 K, 150 ± 20 K, and 124 ± 12 K, respectively) show a temperature gradient within the AFGL2591 VLA 3 envelope, consistent with previous observations and modeling of the source. By combining the H2S, SO2, and CH3OH images, representative of the three concentric shells, we find that the global kinematics of the molecular gas follow Keplerian-like rotation around a 40 M ☉ star. The chemical segregation observed toward AFGL2591 VLA 3 is explained by the combination of molecular UV photodissociation and a high-temperature (~1000 K) gas-phase chemistry within the low extinction innermost region in the AFGL2591 VLA 3 hot core.