A. Bacmann

Depletion and low gas temperature in the L183 (= L134N) prestellar core: the N2H + - N2D + tool ⋆

Context. The study of pre-stellar cores (PSCs) suffers from a lack of undepleted species to trace the gas physical properties in their very dense inner parts. Aims. We want to carry out detailed modelling of N2H+ and N2D+ cuts across the L183 main core to evaluate the depletion of these species and their usefulness as a probe of physical conditions in PSCs. Methods. We have developed a non-LTE (NLTE) Monte-Carlo code treating the 1D radiative transfer of both N2H+ and N2D+ , making use of recently published collisional coefficients with He between individual hyperfine levels. The code includes line overlap between hyperfine transitions. An extensive set of core models is calculated and compared with observations. Special attention is paid to the issue of source coupling to the antenna beam. Results. The best fitting models indicate that i) gas in the core center is very cold (7± 1 K) and thermalized with dust, ii) depletion of N2H+ does occur, starting at densities 5–7E5 cm?3 and reaching a factor of 6 (+13/?3) in abundance, iii) deuterium fractionation reaches ?70% at the core center, and iv) the density profile is proportional to r^-1 out to ?4000 AU, and to r^?2 beyond. Conclusions. Our NLTE code could be used to (re-)interpret recent and upcoming observations of N2H+ and N2D+ in many pre-stellar cores of interest, to obtain better temperature and abundance profiles.

Survey of ortho-H2D+ (1(1,0)-1(1,1)) in dense cloud cores

Aims. We present a survey of the ortho-H2D + (11,0−11,1) line toward a sample of 10 starless cores and 6 protostellar cores, carried out at the Caltech Submillimeter Observatory. The high diagnostic power of this line is revealed for the study of the chemistry, and the evolutionary and dynamical status of low-mass dense cores. Methods. The derived ortho-H2D + column densities (N(ortho-H2D + )) are compared with predictions from simple chemical models of centrally concentrated cloud cores. Results. The line is detected in 7 starless cores and in 4 protostellar cores. N(ortho-H2D + ) ranges between 2 and 40 × 10 12 cm −2 in starless cores and between 2 and 9 × 10 12 cm −2 in protostellar cores. The brightest lines are detected toward the densest and most centrally concentrated starless cores, where the CO depletion factor and the deuterium fractionation are also largest. The large scatter observed in plots of N(ortho-H2D + ) vs. the observed deuterium fractionation and vs. the CO depletion factor is likely to be due to variations in the ortho-to-para (o/p) ratio of H2D + from >0.5 for Tkin < 10 K gas in pre-stellar cores to � 0.03 (consistent with Tkin � 15 K for protostellar cores). The two Ophiuchus cores in our sample also require a relatively low o/p ratio (� 0.3). Other parameters, including the cosmic-ray ionization rate, the CO depletion factor (or, more in general, the depletion factor of neutral species), the volume density, the fraction of dust grains and PAHs also largely affect the ortho-H2D + abundance. In particular, gas temperatures above 15 K, low CO depletion factors and large abundance of negatively charged small dust grains or PAHs drastically reduce the deuterium fractionations to values inconsistent with those observed toward pre-stellar and protostellar cores. The most deuterated and H2D + -rich objects (L 429, L 1544, L 694-2 and L 183) are reproduced by chemical models of centrally concentrated (central densties � 10 6 cm −3 ) cores with chemical ages between 10 4 and 10 6 yr. Upper limits of the para-H3O + (1 − −2 + ) and para-D2H + (11,0−10,1) lines are also given. The upper limit to the para-H3O + fractional abundance is � 10 −8 and we find an upper limit to the para-D2H + /orthoH2D + column density ratio equal to 1, consistent with chemical model predictions of high density (2 × 10 6 cm −3 ) and low temperature (Tkin < 10 K) clouds. Conclusions. Our results point out the need for better determinations of temperature and density profiles in dense cores as well as for observations of para-H2D + .

Herschel spectral surveys of star-forming regions - Overview of the 555–636 GHz range

High resolution line spectra of star-forming regions are mines of information: they provide unique clues to reconstruct the chemical, dynamical, and physical structure of the observed source. We present the first results from the Herschel key project “Chemical HErschel Surveys of Star forming regions”, CHESS. We report and discuss observations towards five CHESS targets, one outflow shock spot and four protostars with luminosities bewteen 20 and 2 × 105 L_ȯ: L1157-B1, IRAS 16293-2422, OMC2-FIR4, AFGL 2591, and NGC 6334I. The observations were obtained with the heterodyne spectrometer HIFI on board Herschel, with a spectral resolution of 1 MHz. They cover the frequency range 555-636 GHz, a range largely unexplored before the launch of the Herschel satellite. A comparison of the five spectra highlights spectacular differences in the five sources, for example in the density of methanol lines, or the presence/absence of lines from S-bearing molecules or deuterated species. We discuss how these differences can be attributed to the different star-forming mass or evolutionary status. Herschel is an ESA space observatory with science instruments provided by European-led principal Investigator consortia and with important participation from NASA.Figures [see full textsee full text]-[see full textsee full text] and Tables 3, 4 (pages 6 to 8) are only available in electronic form at http://www.aanda.org

TIMASSS: The IRAS 16293-2422 Millimeter and Submillimeter Spectral Survey - I. Observations, calibration, and analysis of the line kinematics

While unbiased surveys observable from ground-based telescopes have previously been obtained towards several high mass protostars, very little exists on low mass protostars. To fill up this gap, we carried out a complete spectral survey of the bands at 3, 2, 1 and 0.8 mm towards the solar type protostar IRAS16293-2422. The observations covered about 200,GHz and were obtained with the IRAM-30m and JCMT-15m telescopes. Particular attention was devoted to the inter-calibration of the obtained spectra with previous observations. All the lines detected with more than 3 sigma and free from obvious blending effects were fitted with Gaussians to estimate their basic kinematic properties. More than 4000 lines were detected (with sigma geq 3) and identified, yielding a line density of approximatively 20 lines per GHz, comparable to previous surveys in massive hot cores. The vast majority (~2/3) of the lines are weak and due to complex organic molecules. The analysis of the profiles of more than 1000 lines belonging 70 species firmly establishes the presence of two distinct velocity components, associated with the two objects, A and B, forming the IRAS16293-2422 binary system. In the source A, the line widths of several species increase with the upper level energy of the transition, a behavior compatible with gas infalling towards a ~1 Mo object. The source B, which does not show this effect, might have a much lower central mass of ~0.1 Mo. The difference in the rest velocities of both objects is consistent with the hypothesis that the source B rotates around the source A. This spectral survey, although obtained with single-dish telescope with a low spatial resolution, allows to separate the emission from 2 different components, thanks to the large number of lines detected. The data of the survey are public and can be retrieved on the web site http://www-laog.obs.ujf-grenoble.fr/heberges/timasss.

Survey of ortho-H2D+(11,0–11,1) in dense cloud cores

Aims. We present a survey of the ortho-H2D + (11,0−11,1) line toward a sample of 10 starless cores and 6 protostellar cores, carried out at the Caltech Submillimeter Observatory. The high diagnostic power of this line is revealed for the study of the chemistry, and the evolutionary and dynamical status of low-mass dense cores. Methods. The derived ortho-H2D + column densities (N(ortho-H2D + )) are compared with predictions from simple chemical models of centrally concentrated cloud cores. Results. The line is detected in 7 starless cores and in 4 protostellar cores. N(ortho-H2D + ) ranges between 2 and 40 × 10 12 cm −2 in starless cores and between 2 and 9 × 10 12 cm −2 in protostellar cores. The brightest lines are detected toward the densest and most centrally concentrated starless cores, where the CO depletion factor and the deuterium fractionation are also largest. The large scatter observed in plots of N(ortho-H2D + ) vs. the observed deuterium fractionation and vs. the CO depletion factor is likely to be due to variations in the ortho-to-para (o/p) ratio of H2D + from >0.5 for Tkin < 10 K gas in pre-stellar cores to � 0.03 (consistent with Tkin � 15 K for protostellar cores). The two Ophiuchus cores in our sample also require a relatively low o/p ratio (� 0.3). Other parameters, including the cosmic-ray ionization rate, the CO depletion factor (or, more in general, the depletion factor of neutral species), the volume density, the fraction of dust grains and PAHs also largely affect the ortho-H2D + abundance. In particular, gas temperatures above 15 K, low CO depletion factors and large abundance of negatively charged small dust grains or PAHs drastically reduce the deuterium fractionations to values inconsistent with those observed toward pre-stellar and protostellar cores. The most deuterated and H2D + -rich objects (L 429, L 1544, L 694-2 and L 183) are reproduced by chemical models of centrally concentrated (central densties � 10 6 cm −3 ) cores with chemical ages between 10 4 and 10 6 yr. Upper limits of the para-H3O + (1 − −2 + ) and para-D2H + (11,0−10,1) lines are also given. The upper limit to the para-H3O + fractional abundance is � 10 −8 and we find an upper limit to the para-D2H + /orthoH2D + column density ratio equal to 1, consistent with chemical model predictions of high density (2 × 10 6 cm −3 ) and low temperature (Tkin < 10 K) clouds. Conclusions. Our results point out the need for better determinations of temperature and density profiles in dense cores as well as for observations of para-H2D + .

L183 (L134N) revisited. III. The gas depletion

We present a detailed study of the gas depletion in L183 (= L134N) for a set of important species, namely, CO, CS, SO, N_2H+ and NH_3. We show that all these species are depleted at some level. This level seems to depend mostly on a density threshold rather than on dust opacity. Therefore UV shielding would not be a main factor in the triggering of depletion. Our data suggest that CO, CS and SO depletion happen at densities of ˜3 × 104 cm-3, while N_2H+b and NH_3 seem to deplete at densities close to 106 cm-3. The latter result is consistent with the Bergin & Langer (cite{Bergin97}, ApJ, 486, 316) polar (H_2O) ice case but not with the more recent models of Aikawa et al. (cite{Aikawa03}, ApJ, 593, 906). CS depletion occurs much below its (J:2-1)b critical density, (7 × 105 cm-3) and therefore makes this species unsuitable to study the density structure of many dark cloud cores. Based on observations made with the CFHT, the Iram 30-m and the ARO 12-m (formerly NRAO 12-m). Appendix A is only available in electronic form at http://www.edpsciences.org

HDO abundance in the envelope of the solar-type protostar IRAS 16293-2422

We present IRAM 30m and JCMT observations of HDO lines towards the solar-type protostar IRAS 16293-2422. Five HDO transitions have been detected on-source, and two were unfruitfully searched for towards a bright spot of the outflow of IRAS 16293-2422. We interpret the data by means of the Ceccarelli, Hollenbach and Tielens (1996) model, and derive the HDO abundance in the warm inner and cold outer parts of the envelope. The emission is well explained by a jump model, with an inner abundance of 1e-7 and an outer abundance lower than 1e-9 (3 sigma). This result is in favor of HDO enhancement due to ice evaporation from the grains in theinner envelope. The deuteration ratio HDO/H2O is found to be f_in=3% and f_out < 0.2% (3 sigma) in the inner and outer envelope respectively and therefore, the fractionation also undergoes a jump in the inner part of the envelope. These results are consistent with the formation of water in the gas phase during the cold prestellar core phase and storage of the molecules on the grains, but do not explain why observations of H2O ices consistently derive a H2O ice abundance of several 1e-5 to 1e-4, some two orders of magnitude larger than the gas phase abundance of water in the hot core around IRAS 16293-2422.

Nitrogen hydrides in the cold envelope of IRAS 16293-2422

Nitrogen is the fifth most abundant element in the Universe, yet the gas-phase chemistry of N-bearing species remains poorly understood. Nitrogen hydrides are key molecules of nitrogen chemistry. Their abundance ratios place strong constraints on the production pathways and reaction rates of nitrogen-bearing molecules. We observed the class 0 protostar IRAS 16293-2422 with the heterodyne instrument HIFI, covering most of the frequency range from 0.48 to 1.78 THz at high spectral resolution. The hyperfine structure of the amidogen radical o-NH2 is resolved and seen in absorption against the continuum of the protostar. Several transitions of ammonia from 1.2 to 1.8 THz are also seen in absorption. These lines trace the low-density envelope of the protostar. Column densities and abundances are estimated for each hydride. We find that NH:NH2:NH3 ≈ 5:1:300. Dark clouds chemical models predict steady-state abundances of NH2 and NH3 in reasonable agreement with the present observations, whilst that of NH is underpredicted by more than one order of magnitude, even using updated kinetic rates. Additional modelling of the nitrogen gas-phase chemistry in dark-cloud conditions is necessary before having recourse to heterogen processes. Herschel is an ESA space observatory with science instruments provided by European-led principal Investigator consortia and with important participation from NASA.Appendices (pages 6, 7) are only available in electronic form at http://www.aanda.org

Ortho-to-para ratio of interstellar heavy water

Context. Despite the low elemental deuterium abundance in the Galaxy, enhanced molecular D/H ratios have been found in the environments of low-mass star-forming regions, and in particular the Class 0 protostar IRAS 16293-2422. Aims. The CHESS (Chemical HErschel Surveys of Star forming regions) key program aims to study the molecular complexity of the interstellar medium. The high sensitivity and spectral resolution of the Herschel/HIFI instrument provide a unique opportunity to observe the fundamental 1(1,1)-0(0,0) transition of the ortho-D2O molecule, which is inaccessible from the ground, and determine the ortho-to-para D2O ratio. Methods. We detected the fundamental transition of the ortho-D2O molecule at 607.35 GHz towards IRAS 16293-2422. The line is seen in absorption with a line opacity of 0.62 +/- 0.11 (1 sigma). From the previous ground-based observations of the fundamental 1(1,0)-1(0,1) transition of para-D2O seen in absorption at 316.80 GHz, we estimate a line opacity of 0.26 +/- 0.05 (1 sigma). Results. We show that the observed absorption is caused by the cold gas in the envelope of the protostar. Using these new observations, we estimate for the first time the ortho-to-para D2O ratio to be lower than 2.6 at a 3 sigma level of uncertainty, which should be compared with the thermal equilibrium value of 2:1.

First detection of ND in the solar-mass protostar IRAS16293-2422

Context. In the past decade, much progress has been made in characterising the processes leading to the enhanced deuterium fractionation observed in the ISM and in particular in the cold, dense parts of star forming regions such as protostellar envelopes. Very high molecular D/H ratios have been found for saturated molecules and ions. However, little is known about the deuterium fractionation in radicals, even though simple radicals often represent an intermediate stage in the formation of more complex, saturated molecules. The imidogen radical NH is such an intermediate species for the ammonia synthesis in the gas phase. Many of these light molecules however have their fundamental transitions in the submillimetre domain and their detection is hampered by the opacity of the atmosphere at these wavelengths. Herschel/HIFI represents a unique opportunity to study the deuteration and formation mechanisms of species not observable from the ground. Aims: We searched here for the deuterated radical ND in order to determine the deuterium fractionation of imidogen and constrain the deuteration mechanism of this species. Methods: We observed the solar-mass Class 0 protostar IRAS16293-2422 with the heterodyne instrument HIFI in Bands 1a (480-560 GHz), 3b (858-961 GHz), and 4a (949-1061 GHz) as part of the Herschel key programme CHESS (Chemical HErschel Survey of Star forming regions). Results: The deuterated form of the imidogen radical ND was detected and securely identified with 2 hyperfine component groups of its fundamental transition (N = 0-1) at 522.1 and 546.2 GHz, in absorption against the continuum background emitted from the nascent protostar. The 3 groups of hyperfine components of its hydrogenated counterpart NH were also detected in absorption. The absorption arises from the cold envelope, where many deuterated species have been shown to be abundant. The estimated column densities are ~2 × 1014 cm-2 for NH and ~ 1.3 × 1014 cm-2 for ND. We derive a very high deuterium fractionation with an [ND]/[NH] ratio of between 30 and 100%. Conclusions: The deuterium fractionation of imidogen is of the same order of magnitude as that in other molecules, which suggests that an efficient deuterium fractionation mechanism is at play. We discuss two possible formation pathways for ND, by means of either the reaction of N+ with HD, or deuteron/proton exchange with NH. Herschel is an ESA space observatory with science instruments provided by European-led principal Investigator consortia and with important participation from NASA.