H_2D^+ in the High-mass Star-forming Region Cygnus X

H_2D^+ is a primary ion that dominates the gas-phase chemistry of cold dense gas. Therefore, it is hailed as a unique tool in probing the earliest, prestellar phase of star formation. Observationally, its abundance and distribution is, however, just beginning to be understood in low-mass prestellar and cluster-forming cores. In high-mass star-forming regions, H_2D^+ has been detected only in two cores, and its spatial distribution remains unknown. Here, we present the first map of the ortho-H_2D^+J_(k^+,k^-) = 1_(1,0) → 1_(1,1) and N_2H^+ 4-3 transition in the DR21 filament of Cygnus X with the James Clerk Maxwell Telescope, and N_2D^+ 3-2 and dust continuum with the Submillimeter Array. We have discovered five very extended (≤34, 000 AU diameter) weak structures in H2D+ in the vicinity of, but distinctly offset from, embedded protostars. More surprisingly, the H_2D^+ peak is not associated with either a dust continuum or N_2D^+ peak. We have therefore uncovered extended massive cold dense gas that was undetected with previous molecular line and dust continuum surveys of the region. This work also shows that our picture of the structure of cores is too simplistic for cluster-forming cores and needs to be refined: neither dust continuum with existing capabilities nor emission in tracers like N_2D^+ can provide a complete census of the total prestellar gas in such regions. Sensitive H_2D^+ mapping of the entire DR21 filament is likely to discover more of such cold quiescent gas reservoirs in an otherwise active high-mass star-forming region

Deuteration as an evolutionary tracer in massive-star formation

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. 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. With the IRAM-30m telescope, we observed rotational transitions of N2D+ and N2H+ 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. Our results clearly indicate that the abundance of N2D+ 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 N2D+ are starless cores with properties very similar to typical pre-stellar cores of lower mass. The abundance of N2D+ is lower in objects with higher gas temperatures as in the low-mass case but does not seem to depend on gas turbulence. Our results indicate that the N2D+-to-N2H+ 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.Comment: Accepted by A&AL, 4 pages, 2 figures, 2 appendices (one for Tables, one for additional figures

Gravitational instabilities in a protosolar-like disc - I. Dynamics and chemistry

MGE gratefully acknowledges a studentship from the European Research Council (ERC; project PALs 320620). JDI gratefully acknowledges funding from the European Union FP7-2011 under grant agreement no. 284405. ACB's contribution was supported, in part, by The University of British Columbia and the Canada Research Chairs program. PC and TWH acknowledge the financial support of the European Research Council (ERC; project PALs 320620).To date, most simulations of the chemistry in protoplanetary discs have used 1 + 1D or 2D axisymmetric α-disc models to determine chemical compositions within young systems. This assumption is inappropriate for non-axisymmetric, gravitationally unstable discs, which may be a significant stage in early protoplanetary disc evolution. Using 3D radiative hydrodynamics, we have modelled the physical and chemical evolution of a 0.17 M⊙ self-gravitating disc over a period of 2000 yr. The 0.8 M⊙ central protostar is likely to evolve into a solar-like star, and hence this Class 0 or early Class I young stellar object may be analogous to our early Solar system. Shocks driven by gravitational instabilities enhance the desorption rates, which dominate the changes in gas-phase fractional abundances for most species. We find that at the end of the simulation, a number of species distinctly trace the spiral structure of our relatively low-mass disc, particularly CN. We compare our simulation to that of a more massive disc, and conclude that mass differences between gravitationally unstable discs may not have a strong impact on the chemical composition. We find that over the duration of our simulation, successive shock heating has a permanent effect on the abundances of HNO, CN and NH3, which may have significant implications for both simulations and observations. We also find that HCO+ may be a useful tracer of disc mass. We conclude that gravitational instabilities induced in lower mass discs can significantly, and permanently, affect the chemical evolution, and that observations with high-resolution instruments such as Atacama Large Millimeter/submillimeter Array (ALMA) offer a promising means of characterizing gravitational instabilities in protosolar discs.Publisher PDFPeer reviewe

Molecular ions in L1544. I. Kinematics

We have mapped the dense dark core L1544 in H13CO+(1-0), DCO+(2-1), DCO+(3-2), N2H+(1-0), NTH+(3-2), N2D+(2-1), N2D+(3-2), C18O(1-0), and C17O(1-0) using the IRAM 30-m telescope. We have obtained supplementary observations of HC18O+(1-0), HC17O+(1-0), and D13CO+(2-1). Many of the observed maps show a general correlation with the distribution of dust continuum emission in contrast to C18O(1-0) and C17O(1-0) which give clear evidence for depletion of CO at positions close to the continuum peak. In particular N2D+(2-1) and (3-2) and to a lesser extent N2H+(1-0) appear to be excellent tracers of the dust continuum. We find that the tracers of high density gas (in particular N2D+) show a velocity gradient along the minor axis of the L1544 core and that there is evidence for larger linewidths close to the dust emission peak. We interpret this using the model of the L1544 proposed by Ciolek & Basu (2000) and by comparing the observed velocities with those expected on the basis of their model. The results show reasonable agreement between observations and model in that the velocity gradient along the minor axis and the line broadening toward the center of L1544 are predicted by the model. This is evidence in favour of the idea that amipolar diffusion across field lines is one of the basic processes leading to gravitational collapse. However, line widths are significantly narrower than observed and are better reproduced by the Myers & Zweibel (2001) model which considers the quasistatic vertical contraction of a layer due to dissipation of its Alfvenic turbulence, indicating the importance of this process for cores in the verge of forming a star.Comment: 24 pages, 9 figures, to be published in Ap

Broad N2H+ emission towards the protostellar shock L1157-B1

We present the first detection of N2H+ towards a low-mass protostellar outflow, namely the L1157-B1 shock, at about 0.1 pc from the protostellar cocoon. The detection was obtained with the IRAM 30-m antenna. We observed emission at 93 GHz due to the J = 1-0 hyperfine lines. The analysis of the emission coupled with the HIFI CHESS multiline CO observations leads to the conclusion that the observed N2H+(1-0) line originates from the dense (> 10^5 cm-3) gas associated with the large (20-25 arcsec) cavities opened by the protostellar wind. We find a N2H+ column density of few 10^12 cm-2 corresponding to an abundance of (2-8) 10^-9. The N2H+ abundance can be matched by a model of quiescent gas evolved for more than 10^4 yr, i.e. for more than the shock kinematical age (about 2000 yr). Modelling of C-shocks confirms that the abundance of N2H+ is not increased by the passage of the shock. In summary, N2H+ is a fossil record of the pre-shock gas, formed when the density of the gas was around 10^4 cm-3, and then further compressed and accelerated by the shock.Comment: ApJ, in pres

Mid-J CO Shock Tracing Observations of Infrared Dark Clouds I

Infrared dark clouds (IRDCs) are dense, molecular structures in the interstellar medium that can harbour sites of high-mass star formation. IRDCs contain supersonic turbulence, which is expected to generate shocks that locally heat pockets of gas within the clouds. We present observations of the CO J = 8-7, 9-8, and 10-9 transitions, taken with the Herschel Space Observatory, towards four dense, starless clumps within IRDCs (C1 in G028.37+00.07, F1 and F2 in G034.43+0007, and G2 in G034.77-0.55). We detect the CO J = 8-7 and 9-8 transitions towards three of the clumps (C1, F1, and F2) at intensity levels greater than expected from photodissociation region (PDR) models. The average ratio of the 8-7 to 9-8 lines is also found to be between 1.6 and 2.6 in the three clumps with detections, significantly smaller than expected from PDR models. These low line ratios and large line intensities strongly suggest that the C1, F1, and F2 clumps contain a hot gas component not accounted for by standard PDR models. Such a hot gas component could be generated by turbulence dissipating in low velocity shocks.Comment: 14 pages, 8 figures, 5 tables, accepted by A&A, minor updates to match the final published versio

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&

Simulated Observations of Young Gravitationally Unstable Protoplanetary Discs

The formation and earliest stages of protoplanetary discs remain poorly constrained by observations. ALMA will soon revolutionise this field. Therefore, it is important to provide predictions which will be valuable for the interpretation of future high sensitivity and high angular resolution observations. Here we present simulated ALMA observations based on radiative transfer modelling of a relatively massive (0.39 M_solar) self-gravitating disc embedded in a 10 M_solar dense core, with structure similar to the pre-stellar core L1544. We focus on simple species and conclude that C17O 3-2, HCO+ 3-2, OCS 26-25 and H2CO 404-303 lines can be used to probe the disc structure and kinematics at all scales.Comment: 12 pages, 15 figures, Accepted by MNRA

Gesture and speech in maternal input to children with Down's syndrome

Background: Despite recent interest in relationships between maternal gesture and speech and communicative development in typically developing (M) children, little work has examined either speech or gesture in mothers of children with Down's syndrome (DS). Aims: To compare aspects of speech and gesture production by mothers of children with DS with that of mothers of TD children. Methods & Procedures: Participants were five mothers of children with DS (mean chronological age (CA)=47.6 months; mean mental age (MA,)=22.4 months) and five mothers of TD children. To equate for expressive language ability, children in the TD and DS groups were individually matched on the basis of: (1) gender; (2) correspondence between the TD child's chronological age and the DS child's language age; and (3) observed expressive vocabulary size. Each mother-child dyad was videotaped for approximately 30 min during free play. Data analyses focused on: (1) the number and types (speech only, gesture only, mixed) of maternal utterances; (2) the gesture types (deictic, iconic, conventional, emphatic); and (3) for mixed utterances, the structure and the temporal patterning of spoken and gestured components. Outcomes & Results: Relative to mothers of TD children, mothers of children with DS produced significantly fewer utterances overall, but the distribution of utterance types did not differ between the two groups. Relative to mothers of TD children, mothers of children with DS used proportionately more deictic gestures and made more frequent use of SHOWING. Mothers of TD children produced more POINTING gestures. Finally, mothers of children with DS produced a significantly higher proportion of utterances consisting of a single gesture and a single verbal utterance; in contrast to mothers of TD children, more complex structures (one gesture with multiple verbal utterances, one verbal utterance with multiple gestures) were never observed. Within the category of utterances consisting of a gesture and a single verbal utterance, mothers of children with DS tended to produce gestures that were held throughout the complete verbal utterance, while the gestures of mothers of TD children tended to co-occur with only a portion of the utterance. Conclusions: The findings suggest that mothers of children with DS adjust their communication to the developmental status of their child. Results are discussed in terms of the role of gesture in maternal communication and in the regulation of mother-child interaction