Context. The nitrogen isotopic ratio is considered an important diagnostic tool of the star formation process, and N2H+ is particularly important because it is directly linked to molecular nitrogen ...N2. However, theoretical models still do not provide an exhaustive explanation for the observed 14N/15N values. Aims. Recent theoretical works suggest that the 14N/15N behaviour is dominated by two competing reactions that destroy N2H+: dissociative recombination and reaction with CO. When CO is depleted from the gas phase, if the N2H+ recombination rate is lower with respect to that for N15NH+, the rarer isotopologue is destroyed more quickly. In prestellar cores, due to a combination of low temperatures and high densities, most CO is frozen in ices onto the dust grains, leading to high levels of depletion. On the contrary, in protostellar cores, where temperature are higher, CO ices evaporate back to the gas phase. This implies that the N2H+ isotopic ratio in protostellar cores should be lower than that in prestellar cores, and consistent with the elemental value of ≈440. We aim to test this hypothesis, producing the first sample of N2H+∕N15NH+ measurements in low-mass protostars. Methods. We observe the N2H+ and N15NH+ lowest rotational transition towards six young stellar objects in the Perseus and Taurus molecular clouds. We model the spectra with a custom python code using a constant Tex approach to fit the observations. We discuss in the Appendix the validity of this hypothesis. The derived column densities are used to compute the nitrogen isotopic ratios. Results. Our analysis yields an average of 14N/15N|pro = 420 ± 15 in the protostellar sample. This is consistent with the protosolar value of 440, and significantly lower than the average value previously obtained in a sample of prestellar objects. Conclusions. Our results are in agreement with the hypothesis that, when CO is depleted from the gas-phase, dissociative recombinations with free electrons destroy N15NH+ faster than N2H+, leading to high isotopic ratios in prestellar cores where carbon monoxide is frozen onto dust grains.
Context.
Phosphorus-bearing species are essential to the formation of life on Earth, however they have barely been detected in the interstellar medium. In particular, towards star-forming regions ...only PN and PO have been identified so far. Since only a small number of detections of P-bearing molecules are available, their chemical formation pathways are not easy to constrain and are thus highly debatable. An important factor still missing in the chemical models is the initial elemental abundance of phosphorus, that is, the depletion level of P at the start of chemical models of dense clouds.
Aims.
In order to overcome this problem, we study P-bearing species in diffuse and translucent clouds. In these objects phosphorus is expected to be mainly in the gas phase and therefore the elemental initial abundance needed in our chemical simulations corresponds to the cosmic one and is well constrained.
Methods.
For the study of P-bearing chemistry we used an advanced chemical model. We updated and significantly extended the P-chemistry network based on chemical databases and previous literature. We performed single-pointing observations with the IRAM 30 m telescope in the 3 mm range towards the line of sight to the strong continuum source B0355+508 aiming for the (2–1) transitions of PN, PO, HCP, and CP. This line of sight incorporates five diffuse and/or translucent clouds.
Results.
The (2–1) transitions of the PN, PO, HCP, and CP were not detected. We report high signal-to-noise-ratio detections of the (1–0) lines of
13
CO, HNC, and CN along with a first detection of C
34
S towards this line of sight. We have attempted to reproduce the observations of HNC, CN, CS, and CO in every cloud with our model by applying typical physical conditions for diffuse or translucent clouds. We find that towards the densest clouds with
v
LSR
= −10, − 17 km s
−1
the best-fit model is given by the parameters (
n
(H),
A
V
,
T
gas
) = (300 cm
−3
, 3 mag, 40 K).
Conclusions.
According to our best-fit model, the most abundant P-bearing species are HCP and CP (~10
−10
). The molecules PN, PO, and PH
3
also show relatively high predicted abundances of ~10
−11
. We show that the abundances of these species are sensitive to visual extinction, cosmic-ray ionization rate, and the diffusion-to-desorption energy ratio on dust grains. The production of P-bearing species is favored towards translucent rather than diffuse clouds, where the environment provides a stronger shielding from the interstellar radiation. Based on our improved model, we show that the (1–0) transitions of HCP, CP, PN, and PO are expected to be detectable with estimated intensities of up to ~200 mK.
Context. L1521E is a dense starless core in Taurus that was found to have relatively low molecular depletion by earlier studies, thus suggesting a recent formation. Aims. We aim to characterize the ...chemical structure of L1521E and compare it to the more evolved L1544 pre-stellar core. Methods. We have obtained ~2.5 × 2.5 arcminute maps toward L1521E using the IRAM-30 m telescope in transitions of various species, including C17O, CH3OH, c-C3H2, CN, SO, H2CS, and CH3CCH. We derived abundances for the observed species and compared them to those obtained toward L1544. We estimated CO depletion factors using the C17O IRAM-30 m map, an N(H2) map derived from Herschel/SPIRE data and a 1.2 mm dust continuum emission map obtained with the IRAM-30 m telescope. Results. Similarly to L1544, c-C3H2 and CH3OH peak at different positions. Most species peak toward the c-C3H2 peak including C2S, C3S, HCS+, HC3N, H2CS, CH3CCH, and C34S. C17O and SO peak close to both the c-C3H2 and the CH3OH peaks. CN and N2H+ peak close to the Herschel dust peak. We found evidence of CO depletion toward L1521E. The lower limit of the CO depletion factor derived toward the Herschel dust peak is 4.3±1.6, which is about a factor of three lower than toward L1544. We derived abundances for several species toward the dust peaks of L1521E and L1544. The abundances of most sulfur-bearing molecules such as C2S, HCS+, C34S, C33S, and HCS+ are higher toward L1521E than toward L1544 by factors of ~2–20, compared to the abundance of A-CH3OH. The abundance of methanol is very similar toward the two cores. Conclusions. The fact that the abundances of sulfur-bearing species toward L1521E are higher than toward L1544 suggests that significant sulfur depletion takes place during the dynamical evolution of dense cores, from the starless to pre-stellar stage. The CO depletion factor measured toward L1521E suggests that CO is more depleted than previously found. Similar CH3OH abundances between L1521E and L1544 hint that methanol is forming at specific physical conditions in the Taurus Molecular Cloud Complex, characterized by densities of a few ×104 cm−3 and N(H2) ≳ 1022 cm−2, when CO starts to catastrophically freeze-out, while water can still be significantly photodissociated, so that the surfaces of dust grains become rich in solid CO and CH3OH, as already found toward L1544. Methanol can thus provide selective crucial information about the transition region between dense cores and the surrounding parent cloud.
Context. Magnetic fields can significantly affect the star formation process. The theory of the magnetically driven collapse in a uniform field predicts that the contraction initially happens along ...the field lines. When the gravitational pull grows strong enough, the magnetic field lines pinch inwards, giving rise to a characteristic hourglass shape. Aims. We investigate the magnetic field structure of a young Class 0 object, IRAS 15398-3359, embedded in the Lupus I cloud. Previous observations at large scales have suggested that this source evolved in an highly magnetised environment. This object thus appears to be an ideal candidate to study the magnetically driven core collapse in the low-mass regime. Methods. We performed polarisation observations of IRAS 15398-3359 at 214 μm using the SOFIA telescope, thus tracing the linearly polarised thermal emission of cold dust. Results. Our data unveil a significant bend of the magnetic field lines from the gravitational pull. The magnetic field appears ordered and aligned with the large-scale B-field of the cloud and with the outflow direction. We estimate a magnetic field strength of B = 78 μG, which is expected to be accurate within a factor of two. The measured mass-to-flux parameter is λ = 0.95, indicating that the core is in a transcritical regime.
Context.
Cosmic rays (CRs), which are energetic particles mainly composed of protons and electrons, play an important role in the chemistry and dynamics of the interstellar medium. In dense ...environments, they represent the main ionising agent, hence driving the rich chemistry of molecular ions. Furthermore, they determine the ionisation fraction, which regulates the degree of coupling between the gas and the interstellar magnetic fields, and the heating of the gas. Estimates of the CR ionisation rate of molecular hydrogen (
ζ
2
) span several orders of magnitude, depending on the targeted sources and on the method used.
Aims.
Recent theoretical models have characterised the CR attenuation with increasing density. We aim to test these models for the attenuation of CRs in the low-mass pre-stellar core L1544.
Methods.
We used a state-of-the-art gas-grain chemical model, which accepts the CR ionisation rate profile as input, to predict the abundance profiles of four ions: N
2
H
+
, N
2
D
+
, HC
18
O
+
, and DCO
+
. Non-local thermodynamic equilibrium radiative transfer simulations were run to produce synthetic spectra based on the derived abundances. These were compared with observations obtained with the Institut de Radioastronomie Millimétrique 30 m telescope.
Results.
Our results indicate that a model with high
ζ
2
(>10
−16
s
−1
) is excluded by the observations. Also the model with the standard
ζ
2
= 1.3 × 10
−17
s
−1
produces a worse agreement with respect to the attenuation model based on Voyager observations, which is characterised by an average ⟨
ζ
2
⟩ = 3 × 10
−17
s
−1
at the column densities typical of L1544. The single-dish data, however, are not sensitive to the attenuation of the CR profile, which changes only by a factor of two in the range of column densities spanned by the core model (
N
= 2−50 × 10
21
cm
−2
). Interferometric observations at higher spatial resolution, combined with observations of transitions with lower critical density – hence tracing the low-density envelope – are needed to observe a decrease in the CR ionisation rate with density.
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 . Nitrogen fractionation is a powerful tracer of the chemical evolution during star and planet formation. It requires robust determinations of the nitrogen fractionation across different ...evolutionary stages. Aims . We aim to determine the 14 N/ 15 N and 12 C/ 13 C ratios for HCN in six starless and prestellar cores and to compare the results between the direct method using radiative transfer modeling and the indirect double isotope method, assuming a fixed 12 C/ 13 C ratio. Methods. We present IRAM observations of the HCN 1–0, HCN 3–2, HC 15 N 1–0 and H 13 CN 1–0 transitions toward six embedded cores. The 14 N/ 15 N ratio was derived using both the indirect double isotope method and directly through non-local thermodynamic equilibrium (NLTE) 1D radiative transfer modeling of the HCN emission. The latter also provides the 12 C/ 13 C ratio, which we compared to the local interstellar value. Results . The derived 14 N/ 15 N ratios using the indirect method are generally in the range of 300-550. This result could suggest an evolutionary trend in the nitrogen fractionation of HCN between starless cores and later stages of the star formation process. However, the direct method reveals lower fractionation ratios of around ~250, mainly resulting from a lower 12 C/ 13 C ratio in the range ~20–40, as compared to the local interstellar medium value of 68. Conclusions . This study reveals a significant difference between the nitrogen fractionation ratio in HCN derived using direct and indirect methods. This can influence the interpretation of the chemical evolution and reveal the pitfalls of the indirect double isotope method for fractionation studies. However, the direct method is challenging, as it requires well-constrained source models to produce accurate results. No trend in the nitrogen fractionation of HCN between earlier and later stages of the star formation process is evident when the results of the direct method are considered.
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.
Context.
Molecules containing two or more hydrogen or deuterium atoms have different nuclear spin states which behave as separate chemical species. The relative abundances of these species can give ...clues to their origin. Formation on grains is believed to yield statistical spin ratios whereas gas-phase reactions are predicted to result in clear deviations from them. This is also true for ammonia and its deuterated forms NH
2
D, NHD
2
, and ND
3
.
Aims.
Here we aim to determine the ortho/para ratios of NH
2
D and NHD
2
in dense, starless cores, where their formation is supposed to be dominated by gas-phase reactions.
Methods.
The Large APEX sub-Millimeter Array (LAsMA) multibeam receiver of the Atacama Pathfinder EXperiment (APEX) telescope was used to observe the prestellar cores H-MM1 and Oph D in Ophiuchus in the ground-state lines of ortho and para NH
2
D and NHD
2
. The fractional abundances of these molecules were derived employing three-dimensional radiative transfer modelling, using different assumptions about the abundance profiles as functions of density. We also ran gas-grain chemistry models with different scenarios concerning proton or deuteron exchanges and chemical desorption from grains to find out if one of these models can reproduce the observed spin ratios.
Results.
The observationally deduced ortho/para ratios of NH
2
D and NHD
2
are in both cores within 10% of their statistical values 3 and 2, respectively, and taking 3
σ
limits, deviations from these of about 20% are allowed. Of the chemistry models tested here, the model that assumes proton hop (as opposed to full scrambling) in reactions contributing to ammonia formation, and a constant efficiency of chemical desorption, comes nearest to the observed abundances and spin ratios.
Conclusions.
The nuclear spin ratios derived here are in contrast with spin-state chemistry models that assume full scrambling in proton donation and hydrogen abstraction reactions leading to deuterated ammonia. The efficiency of chemical desorption strongly influences the predicted abundances of NH
3
, NH
2
D, and NHD
2
, but has a lesser effect on their ortho/para ratios. For these the proton exchange scenario in the gas is decisive. We suggest that this is because of rapid re-processing of ammonia and related cations by gas-phase ion-molecule reactions.
PDF
