Chemistry and kinematics of the pre-stellar core L1544: Constraints from H2D+

This paper explores the sensitivity of line profiles of H2D+, HCO+ and N2H+, observed towards the center of L1544, to various kinematic and chemical parameters. The total width of the H2D+ line can be matched by a static model and by models invoking ambipolar diffusion and gravitational collapse. The derived turbulent line width is b=0.15 km/s for the static case and <~ 0.05 km/s for the collapse case. However, line profiles of HC18O+ and N2H+ rule out the static solution. The double-peaked H2D+ line shape requires either infall speeds in the center that are much higher than predicted by ambipolar diffusion models, or a shell-type distribution of H2D+, as is the case for HCO+ and N2H+. At an offset of ~20 arcsec from the dust peak, the H2D+ abundance drops by a factor of ~5.Comment: four pages, two colour figures; to appear in The Dense Interstellar Medium in Galaxies, proceedings of the fourth Cologne-Bonn-Zermatt Symposium, Sept 22-26, 200

On the stability of nonisothermal Bonnor-Ebert spheres. III. The role of chemistry in core stabilization

Aims. We investigate the effect of chemistry on the stability of starless cores against gravitational collapse. Methods. We combine chemical and radiative transfer simulations in the context of a modified Bonnor-Ebert sphere to model the effect of chemistry on the gas temperature, and study the effect of temperature changes on core stability. Results. We find that chemistry has in general very little effect on the nondimensional radius $\xi_{\rm out}$ which parametrizes the core stability. Cores that are initially stable or unstable tend to stay near their initial states, in terms of stability (i.e., $\xi_{\rm out} \sim$ constant), as the chemistry develops. This result is independent of the initial conditions. We can however find solutions where $\xi_{\rm out}$ decreases at late times ($t \gtrsim 10^6 \, \rm yr$) which correspond to increased stabilization caused by the chemistry. Even though the core stability is unchanged by the chemistry in most of the models considered here, we cannot rule out the possibility that a core can evolve from an unstable to a stable state owing to chemical evolution. The reverse case, where an initially stable core becomes ultimately unstable, seems highly unlikely. Conclusions. Our results indicate that chemistry should be properly accounted for in studies of star-forming regions, and that further investigations of core stability especially with hydrodynamical models are warranted.Comment: 8 pages, 10 figures; accepted for publication in A&

First measurements of 15N fractionation in N2H+ toward high-mass star forming cores

We report on the first measurements of the isotopic ratio 14N/15N in N2H+ toward a statistically significant sample of high-mass star forming cores. The sources belong to the three main evolutionary categories of the high-mass star formation process: high-mass starless cores, high-mass protostellar objects, and ultracompact HII regions. Simultaneous measurements of 14N/15N in CN have been made. The 14N/15N ratios derived from N2H+ show a large spread (from ~180 up to ~1300), while those derived from CN are in between the value measured in the terrestrial atmosphere (~270) and that of the proto-Solar nebula (~440) for the large majority of the sources within the errors. However, this different spread might be due to the fact that the sources detected in the N2H+ isotopologues are more than those detected in the CN ones. The 14N/15N ratio does not change significantly with the source evolutionary stage, which indicates that time seems to be irrelevant for the fractionation of nitrogen. We also find a possible anticorrelation between the 14N/15N (as derived from N2H+) and the H/D isotopic ratios. This suggests that 15N enrichment could not be linked to the parameters that cause D enrichment, in agreement with the prediction by recent chemical models. These models, however, are not able to reproduce the observed large spread in 14N/15N, pointing out that some important routes of nitrogen fractionation could be still missing in the models.Comment: 2 Figures, accepted for publication in ApJ

Chemical tracers in proto-brown dwarfs: CO, ortho-H2_{2}CO, para-H2_{2}CO, HCO+^{+}, CS observations

We present a study of the CO isotopologues and the high-density tracers H$_{2}$CO, HCO$^{+}$, and CS in Class 0/I proto-brown dwarfs (proto-BDs). We have used the IRAM 30m telescope to observe the $^{12}$CO (2-1), $^{13}$CO (2-1), C$^{18}$O (2-1), C$^{17}$O (2-1), H$_{2}$CO (3-2), HCO$^{+}$ (3-2), and CS (5-4) lines in 7 proto-BDs. The hydrogen column density for the proto-BDs derived from the CO gas emission is $\sim$2-15 times lower than that derived from the dust continuum emission, indicating CO depletion from the gas-phase. The mean H$_{2}$CO ortho-to-para ratio is $\sim$3 for the proto-BDs and indicates gas-phase formation for H$_{2}$CO. We have investigated the correlations in the molecular abundances between the proto-BDs and protostars. Proto-BDs on average show a factor of $\sim$2 higher ortho-to-para H$_{2}$CO ratio than the protostars. Possible explanations include a difference in the H$_{2}$CO formation mechanism, spin-selective photo-dissociation, self-shielding effects, or different emitting regions for the ortho and para species. There is a tentative trend of a decline in the HCO$^{+}$ and H$_{2}$CO abundances with decreasing bolometric luminosity, while the CS and CO abundances show no particular difference between the proto-BDs and protostars. These trends reflect the scaled-down physical structures for the proto-BDs compared to protostars and differences in the peak emitting regions for these species. The C$^{17}$O isotopologue is detected in all of the proto-BDs as well as the more evolved Class Flat/Class II BDs in our sample, and can probe the quiescent gas at both early and late evolutionary stages.Comment: Accepted in MNRAS. arXiv admin note: text overlap with arXiv:1809.1016

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

Chemical tracers in proto-brown dwarfs: CN, HCN, and HNC observations

We present results from a study of nitrogen chemistry in Class 0/I proto-brown dwarfs (proto-BDs). We have used the IRAM 30 m telescope to observe the CN (2-1), HCN (3-2), and HNC (3-2) lines in 7 proto-BDs. All proto-BDs show a large CN/HCN abundance ratio of >20, and a HNC/HCN abundance ratio close to or larger than unity. The enhanced CN/HCN ratios can be explained by high UV flux originating from an active accretion zone in the proto-BDs. The larger than unity HNC/HCN ratio for the proto-BDs is likely caused by a combination of low temperature and high density. Both CN and HNC show a flat distribution with CO, indicating that these species can survive in regions where CO is depleted. We have investigated the correlations in the molecular abundances of these species for the proto-BDs with Class 0/I protostars. We find tentative trends of CN (HCN) abundances being about an order of magnitude higher (lower) in the proto-BDs compared to protostars. HNC for the proto-BDs shows a nearly constant abundance unlike the large spread of ~2 orders of magnitude seen for the protostars. Also notable is a rise in the HNC/HCN abundance ratio for the lowest luminosity objects, suggesting that this ratio is higher under low-temperature environments. None of the relatively evolved Class Flat/Class II brown dwarfs in our sample show emission in HNC. The HNC molecule can be considered as an efficient tracer to search and identify early stage sub-stellar mass objects.Comment: Accepted in MNRA

The observed chemical structure of L1544

Prior to star formation, pre-stellar cores accumulate matter towards the centre. As a consequence, their central density increases while the temperature decreases. Understanding the evolution of the chemistry and physics in this early phase is crucial to study the processes governing the formation of a star. We aim at studying the chemical differentiation of a prototypical pre-stellar core, L1544, by detailed molecular maps. In contrast with single pointing observations, we performed a deep study on the dependencies of chemistry on physical and external conditions. We present the emission maps of 39 different molecular transitions belonging to 22 different molecules in the central 6.25 arcmin$^2$ of L1544. We classified our sample in five families, depending on the location of their emission peaks within the core. Furthermore, to systematically study the correlations among different molecules, we have performed the principal component analysis (PCA) on the integrated emission maps. The PCA allows us to reduce the amount of variables in our dataset. Finally, we compare the maps of the first three principal components with the H$_2$ column density map, and the T$_{dust}$ map of the core. The results of our qualitative analysis is the classification of the molecules in our dataset in the following groups: (i) the $c$-C$_3$H$_2$ family (carbon chain molecules), (ii) the dust peak family (nitrogen-bearing species), (iii) the methanol peak family (oxygen-bearing molecules), (iv) the HNCO peak family (HNCO, propyne and its deuterated isotopologues). Only HC$^{18}$O$^+$ and $^{13}$CS do not belong to any of the above mentioned groups. The principal component maps allow us to confirm the (anti-)correlations among different families that were described in a first qualitative analysis, but also points out the correlation that could not be inferred before.Comment: 29 pages, 19 figures, 2 appendices, accepted for publication in A&A, arXiv abstract has been slightly modifie

A simple and robust event-detection algorithm for single-cell impedance cytometry

Microfluidic impedance cytometry is emerging as a powerful label-free technique for the characterization of single biological cells. In order to increase the sensitivity and the specificity of the technique, suited digital signal processing methods are required to extract meaningful information from measured impedance data. In this study, a simple and robust event-detection algorithm for impedance cytometry is presented. Since a differential measuring scheme is generally adopted, the signal recorded when a cell passes through the sensing region of the device exhibits a typical odd-symmetric pattern. This feature is exploited twice by the proposed algorithm: first, a preliminary segmentation, based on the correlation of the data stream with the simplest odd-symmetric template, is performed; then, the quality of detected events is established by evaluating their E2O index, that is, a measure of the ratio between their even and odd parts. A thorough performance analysis is reported, showing the robustness of the algorithm with respect to parameter choice and noise level. In terms of sensitivity and positive predictive value, an overall performance of 94.9% and 98.5%, respectively, was achieved on two datasets relevant to microfluidic chips with very different characteristics, considering three noise levels. The present algorithm can foster the role of impedance cytometry in single-cell analysis, which is the new frontier in "Omics.