Deuterium Fractionation as an Evolutionary Probe in the Infrared Dark Cloud G28.34+0.06

We have observed the J=3-2 transition of N2H+ and N2D+ to investigate the trend of deuterium fractionation with evolutionary stage in three selected regions in the Infrared Dark Cloud (IRDC) G28.34+0.06 with the Submillimeter Telescope (SMT) and the Submillimeter Array (SMA). A comprehensible enhancement of roughly 3 orders of magnitude in deuterium fractionation over the local interstellar D/H ratio is observed in all sources. In particular, our sample of massive star-forming cores in G28.34+0.06 shows a moderate decreasing trend over a factor of 3 in the N(N2D+)/N(N2H+) ratio with evolutionary stage, a behavior resembling what previously found in low-mass protostellar cores. This suggests a possible extension for the use of the N(N2D+)/N(N2H+) ratio as an evolutionary tracer to high-mass protostellar candidates. In the most evolved core, MM1, the N2H+ (3-2) emission appears to avoid the warm region traced by dust continuum emission and emission of 13CO sublimated from grain mantles, indicating an instant release of gas-phase CO. The majority of the N2H+ and N2D+ emission is associated with extended structures larger than 8" (~ 0.2 pc).Comment: 14 pages, 3 figures, and 2 tables, accepted by the Astrophysical Journal Letter

The NH2D/NH3 ratio toward pre-protostellar cores around the UCHII region in IRAS 20293+3952

The deuterium fractionation, Dfrac, has been proposed as an evolutionary indicator in pre-protostellar and protostellar cores of low-mass star-forming regions. We investigate Dfrac, with high angular resolution, in the cluster environment surrounding the UCHII region IRAS 20293+3952. We performed high angular resolution observations with the IRAM Plateau de Bure Interferometer (PdBI) of the ortho-NH2D 1_{11}-1_{01} line at 85.926 GHz and compared them with previously reported VLA NH3 data. We detected strong NH2D emission toward the pre-protostellar cores identified in NH3 and dust emission, all located in the vicinity of the UCHII region IRAS 20293+3952. We found high values of Dfrac~0.1-0.8 in all the pre-protostellar cores and low values, Dfrac<0.1, associated with young stellar objects. The high values of Dfrac in pre-protostellar cores could be indicative of evolution, although outflow interactions and UV radiation could also play a role.Comment: 5 pages, 3 figures. Accepted for publication in Astronomy and Astrophysics Letter

Detection of N15NH+ in L1544

Excess levels of 15N isotopes which have been detected in primitive solar system materials are explained as a remnant of interstellar chemistry which took place in regions of the protosolar nebula. Chemical models of nitrogen fractionation in cold clouds predict an enhancement in the gas-phase abundance of 15N-bearing molecules, thus we have searched for 15N variants of the N2H+ ion in L1544, which is one of the best candidate sources for detection owing to its low central core temperature and high CO depletion. With the IRAM 30m telescope we have obtained deep integrations of the N2H+(1-0) line at 91.2 GHz. The N2H+(1-0) line has been detected toward the dust emission peak of L1544. The 14N/15N abundance ratio in N2H+ resulted 446+/-71, very close to the protosolar value of ~450, higher than the terrestrial ratio of ~270, and significantly lower than the lower limit in L1544 found by Gerin et al. (2009, ApJ, 570, L101) in the same object using ammonia isotopologues.Comment: Accepted for publication in Astronomy and Astrophysic

Observing the gas temperature drop in the high-density nucleus of L 1544

Abridged: 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 these objects, but there has been little observational proof of this. We performed VLA observations of the NH3 (1,1) and (2,2) transitions towards the pre-stellar core 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 111-101 para-NH2D line in order to further constrain the physical parameters of the high density nucleus. We have estimated the temperature gradient using a model of the source to fit our data in the u,v plane. 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. 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 (2e6 cm-3) and shows that high-resolution observations yield higher deuteration values than single-dish observations. Our analysis of the NH3 and NH2D kinematic fields shows a decrease of specific angular momentum from the large scales to the small scales.Comment: 12 pages, 6 figures. Accepted for publication by A&

Nitrogen chemistry and depletion in starless cores

We investigated the chemistry of nitrogen--containing species, principally isotopomers of CN, HCN, and HNC, in a sample of pre-protostellar cores. We used the IRAM 30 m telescope to measure the emission in rotational and hyperfine transitions of CN, HCN, 13CN, H13CN, HN13C, and HC15N, in L 1544, L 183, Oph D, L 1517B, L 310. The observations were made along axial cuts through the dust emission peak, at a number of regularly--spaced offset positions. The observations were reduced and analyzed to obtain the column densities, using the measurements of the less abundant isotopic variants in order to minimize the consequences of finite optical depths in the lines. The observations were compared with the predictions of a free--fall gravitational collapse model, which incorporates a non-equilibrium treatment of the relevant chemistry. We found that CN, HCN, and HNC remain present in the gas phase at densities well above that at which CO depletes on to grains. The CN:HCN and the HNC:HCN abundance ratios are larger than unity in all the objects of our sample. Furthermore, there is no observational evidence for large variations of these ratios with increasing offset from the dust emission peak and hence with density. Whilst the differential freeze--out of CN and CO can be understood in terms of the current chemistry, the behaviour of the CN:HCN ratio is more difficult to explain. Models suggest that most nitrogen is not in the gas phase but may be locked in ices. Unambiguous conclusions require measurements of the rate coefficients of the key neutral--neutral reactions at low temperatures

Detection of 6 K gas in Ophiuchus D

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. The purpose of this study is to probe conditions in the dense, starless clump Ophichus D (Oph D). The ground-state (1(10)-1(11)) rotational transition of ortho-H2D+ was observed with APEX towards the density peak of Oph D. The width of the H2D+ 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 H2D+ 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. 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.Comment: 7 pages, 5 figures, accepted for Astronomy and Astrophysic

Imaging of the CCS 22.3 GHz emission in the Taurus Molecular Cloud complex

Thioxoethenylidene (CCS) is an abundant interstellar molecule, and a good tracer of high density and evolutionary stage of dense molecular clouds. It is also a suitable candidate for Zeeman splitting observations for its high splitting factor and narrow thermal linewidths. We report here EVLA 22.3 GHz observations of three dense molecular cores TMC-1, TMC-1C and L1521B in the Taurus Molecular Cloud complex to image the CCS 2_1-1_0 transition. For all three sources, the clumpy CCS emission is most likely tracing the starless cores. However, these compact structures account for only ~ 1-13% of the integrated emission detected in single-dish observations, indicating the presence of significant large scale diffuse emission in favorable conditions for producing CCS.Comment: 5 pages, 2 figures. Accepted for publication in ApJ Letters EVLA special issue. The definitive version will be available at http://iopscience.iop.org

Highly deuterated pre-stellar cores in a high-mass star formation region

We have observed the deuterated gas in the high-mass star formation region IRAS 05345+3157 at high-angular resolution, in order to determine the morphology and the nature of such gas. We have mapped the N2H+ (1-0) line with the Plateau de Bure Interferometer, and the N2D+ (3-2) and N2H+ (3-2) lines with the Submillimeter Array. The N2D+ (3-2) integrated emission is concentrated in two condensations, with masses of 2-3 and 9 M_sun and diameters of 0.05 and 0.09 pc, respectively. The high deuterium fractionation (0.1) and the line parameters in the N2D+ condensations indicate that they are likely low- to intermediate-mass pre-stellar cores, even though other scenarios are possible.Comment: 4 pages, 2 figures, accepted for publication in Astronomy and Astrophysic

On the frequency of N2H+ and N2D+

Context : Dynamical studies of prestellar cores search for small velocity differences between different tracers. The highest radiation frequency precision is therefore required for each of these species. Aims : We want to adjust the frequency of the first three rotational transitions of N2H+ and N2D+ and extrapolate to the next three transitions. Methods : N2H+ and N2D+ are compared to NH3 the frequency of which is more accurately known and which has the advantage to be spatially coexistent with N2H+ and N2D+ in dark cloud cores. With lines among the narrowests, and N2H+ and NH3 emitting region among the largests, L183 is a good candidate to compare these species. Results : A correction of ~10 kHz for the N2H+ (J:1-0) transition has been found (~0.03 km/s) and similar corrections, from a few m/s up to ~0.05 km/s are reported for the other transitions (N2H+ J:3-2 and N2D+ J:1-0, J:2-1, and J:3-2) compared to previous astronomical determinations. Einstein spontaneous decay coefficients (Aul) are included

A (sub)millimetre study of dense cores in Orion B9

We aim to further constrain the properties and evolutionary stages of dense cores in Orion B9. The central part of Orion B9 was mapped at 350 micron with APEX/SABOCA. A sample of nine cores in the region were observed in C17O(2-1), H13CO+(4-3) (towards 3 sources), DCO+(4-3), N2H+(3-2), and N2D+(3-2) with APEX/SHFI. These data are used in conjunction with our previous APEX/LABOCA 870-micron dust continuum data. Many of the LABOCA cores show evidence of substructure in the higher-resolution SABOCA image. In particular, we report on the discovery of multiple very low-mass condensations in the prestellar core SMM 6. Based on the 350-to-870 micron flux density ratios, we determine dust temperatures of ~7.9-10.8 K, and dust emissivity indices of ~0.5-1.8. The CO depletion factors are in the range ~1.6-10.8. The degree of deuteration in N2H+ is ~0.04-0.99, where the highest value (seen towards the prestellar core SMM 1) is, to our knowledge, the most extreme level of N2H+ deuteration reported so far. The level of HCO+ deuteration is about 1-2%. We also detected D2CO towards two sources. The detection of subcondensations within SMM 6 shows that core fragmentation can already take place during the prestellar phase. The origin of this substructure is likely caused by thermal Jeans fragmentation of the elongated parent core. A low depletion factor and the presence of gas-phase D2CO in SMM 1 suggest that the core chemistry is affected by the nearby outflow. The very high N2H+ deuteration in SMM 1 is likely to be remnant of the earlier CO-depleted phase.Comment: 20 pages, 10 figures, 10 tables. Accepted for publication in Astronomy and Astrophysic