APEX observations of ortho-H 2 D + towards dense cores in the Orion B9 filament

et al. 2020

A&A

Context. Deuteration has been suggested to be a reliable chemical clock of star-forming regions due to its strong dependence on density and temperature changes during cloud contraction. In particular, the H3+ isotopologues (e.g. ortho-H2D+) seem to act as good proxies of the evolutionary stages of the star formation process. While this has been widely explored in low-mass star-forming regions, in the high-mass counterparts only a few studies have been pursued, and the reliability of deuteration as a chemical clock remains inconclusive. Aims. We present a large sample of o-H2D+ observations in high-mass star-forming regions and discuss possible empirical correlations with relevant physical quantities to assess its role as a chronometer of star-forming regions through different evolutionary stages. Methods. APEX observations of the ground-state transition of o-H2D+ were analysed in a large sample of high-mass clumps selected from the ATLASGAL survey at different evolutionary stages. Column densities and beam-averaged abundances of o-H2D+ with respect to H2, X(o-H2D+), were obtained by modelling the spectra under the assumption of local thermodynamic equilibrium. Results. We detect 16 sources in o-H2D+ and find clear correlations between X(o-H2D+) and the clump bolometric luminosity and the dust temperature, while only a mild correlation is found with the CO-depletion factor. In addition, we see a clear correlation with the luminosity-to-mass ratio, which is known to trace the evolution of the star formation process. This would indicate that the deuterated forms of H3+ are more abundant in the very early stages of the star formation process and that deuteration is influenced by the time evolution of the clumps. In this respect, our findings would suggest that the X(o-H2D+) abundance is mainly affected by the thermal changes rather than density changes in the gas. We have employed these findings together with observations of H13CO+, DCO+, and C17O to provide an estimate of the cosmic-ray ionisation rate in a sub-sample of eight clumps based on recent analytical work. Conclusions. Our study presents the largest sample of o-H2D+ in star-forming regions to date. The results confirm that the deuteration process is strongly affected by temperature and suggests that o-H2D+ can be considered a reliable chemical clock during the star formation processes, as proved by its strong temporal dependence.

Section: Discussionsupporting

confidence: 89%

Section: Atlasgal-idmentioning

confidence: 58%

Section: Discussionmentioning

confidence: 96%

Section: O-h 2 D + Detectionsmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 98%

See 3 more Smart Citations

Survey of ortho-H₂D⁺ in high-mass star-forming regions

Sabatini

et al. 2020

A&A

Synthetic observations using POLARIS: an application to simulations of massive prestellar cores

Zamponi

et al. 2022

Astrophys Space Sci

Young massive stars are usually found embedded in dense and massive molecular clumps which are known for being highly obscured and distant. During their formation process, the degree of deuteration can be used as a potential indicator of the very early formation stages. This is particularly effective when employing the abundance of H2D+. However, its low abundances and large distances make detections in massive sources hard to achieve. We present an application of the radiative transfer code POLARIS, with the goal to test the observability of the ortho-H2D+ transition $1_{10}$ 1 10 -$1_{11}$ 1 11 (∼372.42 GHz) using simulations of high-mass collapsing cores that include deuteration chemistry. We analyzed an early and a late stage of the collapse of a 60 M⊙ core, testing different source distances. For all cases, we generated synthetic single-dish and interferometric observations and studied the differences in both techniques. The column densities we derive are comparable to values reported for similar sources. These estimates depend on the extent over which they are averaged, and sources with compact emission they can be highly affected by beam dilution. Combined ALMA-ACA observations improve in signal-to-noise ratio and lead to better column density estimates as compared to ALMA alone. We confirm the feasibility to study ortho-H2D+ emission up to distances of ∼7 kpc. We provide a proof-of-concept of our framework for synthetic observations and highlight its importance when comparing numerical simulations with real observations. This work also proves how relevant it is to combine single-dish and interferometric measurements to derive appropriate source column densities.

Identification of pre-stellar cores in high-mass star forming clumps via H₂D⁺ observations with ALMA

Redaelli

et al. 2021

A&A

Context. The different theoretical models concerning the formation of high-mass stars make distinct predictions regarding their progenitors, which are the high-mass pre-stellar cores. However, no conclusive observation of such objects has been made to date. Aims. We aim to study the very early stages of high-mass star formation in two infrared-dark massive clumps. Our goal is to identify the core population that they harbour and to investigate their physical and chemical properties at high spatial resolution. Methods. We obtained Atacama Large Millimeter/submillimeter Array (ALMA) Cycle 6 observations of continuum emission at 0.8 mm and of the ortho-H2D+ transition at 372 GHz towards the two clumps. We used the SCIMES algorithm to identify substructures (i.e. cores) in the position-position-velocity space, finding 16 cores. We modelled their observed spectra using a Bayesian fitting approach in the approximation of local thermodynamic equilibrium. We derived the centroid velocity, the line width, and the molecular column density maps. We also studied the correlation between the continuum and molecular data, which in general do not present the same structure. Results. We report, for the first time, the detection of ortho-H2D+ in high-mass star-forming regions performed with an interferometer. The molecular emission shows narrow and subsonic lines, suggesting that locally, the temperature of the gas is below 10 K. From the continuum emission, we estimated the cores’ total masses and compare them with the respective virial masses. We also computed the volume density values, which are found to be higher than 106 cm−3. Conclusions. Our data confirm that ortho-H2D+ is an ideal tracer of cold and dense gas. Interestingly, almost all the H2D+-identified cores are less massive than ≈13 M⊙, with the exception of one core in AG354, which could be as massive as 39 M⊙ under the assumption of low dust temperature (5 K). Furthermore, most of them are sub-virial and larger than their Jeans masses. These results are hence difficult to explain in the context of the turbulent accretion models, which predict massive and virialised pre-stellar cores. However, we cannot exclude that the cores are still in the process of accreting mass and that magnetic fields are providing enough support for the virialisation. ALMA could also be seeing only the innermost parts of the cores, and hence the cores’ total masses could be higher than inferred in this work. Furthermore, we note that the total masses of the investigated clumps are below the average for typical high-mass clumps, and thus studies of more massive sources are needed.