Context.
Understanding the details of the formation process of massive (i.e.
M
≳ 8–10
M
⊙
) stars is a long-standing problem in astrophysics. They form and evolve very quickly, and almost their ...entire formation process takes place deeply embedded in their parental clumps. Together with the fact that these objects are rare and at a relatively large distance, this makes observing them very challenging.
Aims.
We present a method for deriving accurate timescales of the evolutionary phases of the high-mass star formation process.
Methods.
We modelled a representative number of massive clumps of the ATLASGAL-TOP100 sample that cover all the evolutionary stages. The models describe an isothermal collapse and the subsequent warm-up phase, for which we followed the chemical evolution. The timescale of each phase was derived by comparing the results of the models with the properties of the sources of the ATLASGAL-TOP100 sample, taking into account the mass and luminosity of the clumps, and the column densities of methyl acetylene (CH
3
CCH), acetonitrile (CH
3
CN), formaldehyde (H
2
CO), and methanol (CH
3
OH).
Results.
We find that the molecular tracers we chose are affected by the thermal evolution of the clumps, showing steep ice evaporation gradients from 10
3
to 10
5
AU during the warm-up phase. We succeed in reproducing the observed column densities of CH
3
CCH and CH
3
CN, but H
2
CO and CH
3
OH agree less with the observed values. The total (massive) star formation time is found to be ~5.2 × 10
5
yr, which is defined by the timescales of the individual evolutionary phases of the ATLASGAL-TOP100 sample: ~5 × 10
4
yr for 70-μm weak, ~1.2 × 10
5
yr for mid-IR weak, ~2.4 × 10
5
yr for mid-IR bright, and ~1.1 × 10
5
yr for HII-region phases.
Conclusions.
With an appropriate selection of molecular tracers that can act as chemical clocks, our model allows obtaining robust estimates of the duration of the individual phases of the high-mass star formation process. It also has the advantage of being capable of including additional tracers aimed at increasing the accuracy of the estimated timescales.
Disentangling the different stages of the star formation process, in particular in the high-mass regime, is a challenge in astrophysics. Chemical clocks could help alleviate this problem, but their ...evolution strongly depends on many parameters, leading to degeneracy in the interpretation of the observational data. One of these uncertainties is the degree of CO depletion. We present here the first self-consistent magneto-hydrodynamic simulations of high-mass, star-forming regions at different scales, fully coupled with a nonequilibrium chemical network, which includes C-N-O bearing molecules. Depletion and desorption processes are treated time dependently. The results show that full CO depletion (i.e., all gas-phase CO frozen-out on the surface of dust grains) can be reached very quickly, in one-third or even smaller fractions of the freefall time, whether the collapse proceeds on slow or fast timescales. This leads to a high level of deuteration in a short time, both for typical tracers like N2H+, as well as for the main ion H 3 + , the latter being in general larger and more extended. N2 depletion is slightly less efficient, and no direct effects on N-bearing molecules and deuterium fractionation are observed. We show that CO depletion is not the only driver of deuteration, and that there is a strong impact on Dfrac when changing the grain size. We finally apply a two-dimensional Gaussian point-spread function to our results to mimic observations with single-dish and interferometers. Our findings suggest that the low-values observed in high-mass star-forming clumps are in reality masking a full-depletion stage in the inner 0.1 pc region.
ABSTRACT
An estimate of the degree of CO-depletion (fD) provides information on the physical conditions occurring in the innermost and densest regions of molecular clouds. A key parameter in these ...studies is the size of the depletion radius, i.e. the radius within which the C-bearing species, and in particular CO, are largely frozen on to dust grains. A strong depletion state (i.e. fD > 10, as assumed in our models) is highly favoured in the innermost regions of dark clouds, where the temperature is <20 K and the number density of molecular hydrogen exceeds a few × 104 cm−3. In this work, we estimate the size of the depleted region by studying the Infrared Dark Cloud (IRDC) G351.77−0.51. Continuum observations performed with the Herschel Space Observatory and the LArge APEX BOlometer CAmera, together with APEX C18O and C17O J = 2→1 line observations, allowed us to recover the large-scale beam- and line-of-sight-averaged depletion map of the cloud. We built a simple model to investigate the depletion in the inner regions of the clumps in the filament and the filament itself. The model suggests that the depletion radius ranges from 0.02 to 0.15 pc, comparable with the typical filament width (i.e. ∼0.1 pc). At these radii, the number density of H2 reaches values between 0.2 and 5.5 × 105 cm−3. These results provide information on the approximate spatial scales on which different chemical processes operate in high-mass star-forming regions and also suggest caution when using CO for kinematical studies in IRDCs.
Context . Cosmic rays are a key component of the interstellar medium because they regulate the dynamics and chemical processes in the densest and coldest regions of molecular clouds. Still, the ...cosmic-ray ionisation rate of H 2 ( ζ H 2 ion ) is one of the most debated parameters characterising molecular clouds because of the uncertainties in the adopted chemical networks and analysis techniques. Aims . This work aims to homogeneously estimate the ζ H 2 ion at parsec scales towards the Orion Molecular Clouds OMC-2 and OMC-3. We explore the change in ζ H 2 ion across a whole star-forming region by probing a range of column densities that has never been explored before. The significant increase in statistics obtained by studying an entire region allows us to place stronger constraints on the range of ζ H 2 ion values and exploit its connection with the physical properties of the interstellar medium. Methods . The most recent ζ H 2 ion estimates are based on o-H 2 D + , which is a direct product of the interaction between cosmic rays and H 2 in cold clouds. Since observations of o-H 2 D + are challenging, we proxy its abundance through CO depletion by employing C 18 O (2–1) observations towards OMC-2 and OMC-3, taking advantage of the existing correlation between the two parameters. Using additional observations of HCO + (1–0) and DCO + (3–2), we determine the deuteration fraction, and we finally derive the map of ζ H 2 ion in these two regions. Results . The C 18 O depletion correlates with both the total column density of H 2 and the N 2 H + emission across OMC-2 and OMC-3. The obtained depletion factors and deuteration fractions are consistent with previous values obtained in low- and high-mass star-forming regions. These two parameters additionally show a positive correlation in the coldest fields of our maps. We derive cosmic-ray ionisation rates of ζ H 2 ion ~ 5 × 10 -18 -10 -16 s -1 . These values agree well with previous estimates based on o-H 2 D + observations. The ζ H 2 ion also shows a functional dependence on the column density of H 2 across a full order of magnitude (~10 22 –10 23 cm −2 ). The estimated values of ζ H 2 ion decrease overall for increasing N (H 2 ), as predicted by theoretical models. Conclusions . The results delivered by our approach are comparable with theoretical predictions and previous independent studies. This confirms the robustness of the analytical framework and promotes CO depletion as a viable proxy of o-H 2 D + . We also explore the main limitations of the method by varying the physical size of the gas crossed by the cosmic rays (i.e. the path length). By employing a path length obtained from low-resolution observations, we recover values of the ζ H 2 ion that are well below any existing theoretical and observational prediction. This discrepancy highlights the need for interferometric observations in order to reliably constrain the ζ H 2 ion at parsec scales as well.
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.
Context . Cosmic rays (CRs) heavily impact the chemistry and physics of cold and dense star-forming regions. However, the characterisation of their ionisation rate continues to pose a challenge from ...the observational point of view. Aims . In the past, a few analytical formulas have been proposed to infer the cosmic-ray ionisation rate, ζ 2 , from molecular line observations. These have been derived from the chemical kinetics of the involved species, but they have not yet been validated using synthetic data processed with a standard observative pipeline. In this work, we aim to bridge this gap. Methods . We performed a radiative transfer on a set of three-dimensional magneto-hydrodynamical simulations of prestellar cores, exploring different initial ζ 2 , evolutionary stages, types of radiative transfer (for instance assuming local-thermodynamic-equilibrium conditions), and telescope responses. We then computed the column densities of the involved tracers to determine ζ 2 , employing a recently proposed method based on the detection of H 2 D + . We compared this approach with a previous method, based on more common tracers. Both approaches are commonly used. Results . Our results confirm that the equation based on the detection of H 2 D + accurately retrieves the actual ζ 2 within a factor of two to three in the physical conditions explored in our tests. Since we have also explored a non-local thermodynamic equilibrium (non-LTE) radiative transfer, this work indirectly offers insights into the excitation temperatures of common transitions at moderate volume densities (n ≈ 10 5 cm −3 ). We also performed a few tests using a previous methodology that is independent of H 2 D + , which overestimates the actual ζ 2 by at least two orders of magnitude. We considered a new derivation of this method, however, we found that it still leads to high over-estimations. Conclusions . The method based on H 2 D + is further validated in this work and demonstrates a reliable method for estimating ζ 2 in cold and dense gas. On the contrary, the former analytical equation, as already pointed out by its authors, has no global domain of application. Thus, we find that it ought to be employed with caution.
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.
SUMMARY
Objective
This study aimed to compare the tooth color matching of two dental colorimetric methods: the spectrophotometric analysis (SPM) and the standardized digital photocolorimetric ...analysis (DPC).
Methods
The color of 60 maxillary central incisors of 30 volunteers (22.5 ± 7.6 years) was analyzed. In the DPC method, tooth photographs were standardized with the eLABor_aid protocol, processed with Adobe Photoshop Lightroom software, and the values of L*, a*, and b* were obtained with a Digital Color Meter software. For the SPM, L*, a*, and b* were measured directly with a handheld spectrophotometer. Data were submitted to paired t-test and Pearson correlation test (α=0.05). Mean color difference between the two methods was calculated with CIELAB formula.
Results
All color coordinates revealed different values when comparing DPC to SPM in the same tooth (p<0.0001). Mean color difference (ΔEab) between SPM and DPC was 11.5 ±3.1. A positive correlation was observed for L* (R2=0.73,p<0.0001), a* (R2=0.31, p=0.017), and b* (R2=0.83, p<0.0001).
Conclusions
Even though the color coordinate values were different in both methods, they were correlated, revealing that the DPC is a viable alternative to determine the tooth color matching.
The development of new acaricides is a long and very expensive process. Worryingly, there is increasing resistance to available acaricides worldwide leading to the real possibility that our dwindling ...supply of effective acaricides will be exhausted unless action is taken to increase the number of new acaricidal products and reduce the rate of resistance development. In 1995, eight major animal health pharmaceutical companies formed the Veterinary Parasite Resistance Group (VPRG) to act as an expert consultative group to guide the FAO in resistance management and collaborate in the prudent use of acaricides. In this paper, members of the VPRG discuss the problems and processes in acaricide development, resistance in the field to commonly used acaricides and the different considerations when targeting the cattle and pet market, and give their view of the future for tick control from the perspective of the animal health industry.
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