Context. The increased sensitivity and high spectral resolution of millimeter telescopes allow the detection of an increasing number of isotopically substituted molecules in the interstellar medium. ...The 14N/15N ratio is difficult to measure directly for molecules containing carbon. Aims. Using a time-dependent gas-phase chemical model, we check the underlying hypothesis that the 13C/12C ratio of nitriles and isonitriles is equal to the elemental value. Methods. We built a chemical network that contains D, 13C, and 15N molecular species after a careful check of the possible fractionation reactions at work in the gas phase. Results. Model results obtained for two different physical conditions that correspond to a moderately dense cloud in an early evolutionary stage and a dense, depleted prestellar core tend to show that ammonia and its singly deuterated form are somewhat enriched in 15N, which agrees with observations. The 14N/15N ratio in N2H+ is found to be close to the elemental value, in contrast to previous models that obtain a significant enrichment, because we found that the fractionation reaction between 15N and N2H+ has a barrier in the entrance channel. The high values of the N2H+/15NNH+ and N2H+/N15NH+ ratios derived in L1544 cannot be reproduced in our model. Finally, we find that nitriles and isonitriles are in fact significantly depleted in 13C, thereby challenging previous interpretations of observed C15N, HC15N, and H15NC abundances from 13C containing isotopologues.
Context.
Carbon fractionation has been studied from a theoretical point of view with different models of time-dependent chemistry, including both isotope-selective photodissociation and ...low-temperature isotopic exchange reactions.
Aims.
Recent chemical models predict that isotopic exchange reactions may lead to a depletion of
13
C in nitrile-bearing species, with
12
C/
13
C ratios two times higher than the elemental abundance ratio of 68 in the local interstellar medium. Since the carbon isotopic ratio is commonly used to evaluate the
14
N/
15
N ratios with the double-isotope method, it is important to study carbon fractionation in detail to avoid incorrect assumptions.
Methods.
In this work, we implemented a gas-grain chemical model with new isotopic exchange reactions and investigated their introduction in the context of dense and cold molecular gas. In particular, we investigated the
12
C/
13
C ratios of HNC, HCN, and CN using a grid of models, with temperatures and densities ranging from 10 to 50 K and 2 × 10
3
to 2 × 10
7
cm
−3
, respectively.
Results.
We suggest a possible
13
C exchange through the
13
C + C
3
→
12
C +
13
CC
2
reaction, which does not result in dilution, but rather in
13
C enhancement, for molecules that are formed starting from atomic carbon. This effect is efficient in a range of time between the formation of CO and its freeze-out on grains. Furthermore, the parameter-space exploration shows, on average, that the
12
C/
13
C ratios of nitriles are predicted to be a factor 0.8–1.9 different from the local
12
C/
13
C of 68 for high-mass star-forming regions. This result also affects the
14
N/
15
N ratio: a value of 330 obtained with the double-isotope method is predicted to vary in the range 260–630, up to 1150, depending on the physical conditions. Finally, we studied the
12
C/
13
C ratios of nitriles by varying the cosmic-ray ionisation rate,
ζ
: the
12
C/
13
C ratios increase with
ζ
because of secondary photons and cosmic-ray reactions.
The quasi bound spectrum of H2 Roueff, E. M.; Abgrall, H.
Molecular physics,
09/2023, Letnik:
121, Številka:
17-18
Journal Article
Recenzirano
We compute the radiative ro-vibrational emission spectrum of H
involving quasi-bound states via a simple numerical method of resolution of the Schrödinger equation by introducing a modified effective ...molecular potential. The comparison of the eigenvalues obtained with our approximation and other theoretical methods based on scattering resonance properties is excellent. Electric quadrupole and magnetic dipole contributions are calculated and we confirm the previous computations of Forrey of the electric quadrupole transition Einstein coefficients. The astrophysical relevance of such quasi-bound levels is emphasized.
An exhaustive chemical characterization of dense cores is mandatory to our understanding of chemical composition changes from a starless to a protostellar stage. However, only a few sources have had ...their molecular composition characterized in detail. Here we present a
3 mm line survey of L483, a dense core around a Class 0 protostar, which was observed with the IRAM 30m telescope in the 80-116 GHz frequency range. We detected 71 molecules (140 including different isotopologs), most of which are present in the cold and quiescent ambient cloud according to their narrow lines (FWHM ~0.5 km s
) and low rotational temperatures (≲10 K). Of particular interest among the detected molecules are the
isomer of HCOOH, the complex organic molecules HCOOCH
, CH
OCH
, and C
H
OH, a wide variety of carbon chains, nitrogen oxides like N
O, and saturated molecules like CH
SH, in addition to eight new interstellar molecules (HCCO, HCS, HSC, NCCNH
, CNCN, NCO, H
NCO
, and NS
) whose detection has already been reported. In general, fractional molecular abundances in L483 are systematically lower than in TMC-1 (especially for carbon chains), tend to be higher than in L1544 and B1-b, and are similar to those in L1527. Apart from the overabundance of carbon chains in TMC-1, we find that L483 does not have a marked chemical differentiation with respect to starless/prestellar cores like TMC-1 and L1544, although it does chemically differentiate from Class 0 hot corino sources like IRAS 16293-2422. This fact suggests that the chemical composition of the ambient cloud of some Class 0 sources could be largely inherited from the dark cloud starless/prestellar phase. We explore the use of potential chemical evolutionary indicators, such as the HNCO/C
S, SO
/C
S, and CH
SH/C
S ratios, to trace the prestellar/protostellar transition. We also derived isotopic ratios for a variety of molecules, many of which show isotopic ratios close to the values for the local interstellar medium (remarkably all those involving
S and
S), while there are also several isotopic anomalies like an extreme depletion in
C for one of the two isotopologs of
-C
H
, a drastic enrichment in
O for SO and HNCO (SO being also largely enriched in
O), and different abundances for the two
C substituted species of C
H and the two
N substituted species of N
H
. We report the first detection in space of some minor isotopologs like
-C
D. The exhaustive chemical characterization of L483 presented here, together with similar studies of other prestellar and protostellar sources, should allow us to identify the main factors that regulate the chemical composition of cores along the process of formation of low-mass protostars.
A NEW REFERENCE CHEMICAL COMPOSITION FOR TMC-1 Gratier, P.; Majumdar, L.; Ohishi, M. ...
The Astrophysical journal. Supplement series,
08/2016, Letnik:
225, Številka:
2
Journal Article
Recenzirano
Odprti dostop
ABSTRACT Recent detections of complex organic molecules in dark clouds have rekindled interest in the astrochemical modeling of these environments. Because of its relative closeness and rich ...molecular complexity, TMC-1 has been extensively observed to study the chemical processes taking place in dark clouds. We use local thermodynamical equilibrium radiative transfer modeling coupled with a Bayesian statistical method which takes into account outliers to analyze the data from the Nobeyama spectral survey of TMC-1 between 8 and 50 GHz. We compute the abundance relative to molecular hydrogen of 57 molecules, including 19 isotopologues in TMC-1 along with their associated uncertainty. The new results are in general agreement with previous abundance determination from Ohishi & Kaifu and the values reported in the review from Agúndez & Wakelam. However, in some cases, large opacity and low signal to noise effects allow only upper or lower limits to be derived, respectively.
Our observations of TMC-1 with the Yebes 40 m radio telescope in the 31.0–50.3 GHz range allowed us to detect a group of unidentified lines, showing a complex line pattern indicative of an open-shell ...species. The observed frequencies of these lines and the similarity of the spectral pattern with that of the 2
0, 2
–1
0, 1
rotational transition of H
2
CCN indicate that the lines arise from the deuterated cyanomethyl radical, HDCCN. Using Fourier transform microwave spectroscopy experiments combined with electric discharges, we succeeded in producing the radical HDCCN in the laboratory and observed its 1
0, 1
–0
0, 0
and 2
0, 2
–1
0, 1
rotational transitions. From our observations and assuming a rotational temperature of 5 K, we derive an abundance ratio H
2
CCN/HDCCN = 20 ± 4. The high abundance of the deuterated form of H
2
CCN is well accounted for by a standard gas-phase model, in which deuteration is driven by deuteron transfer from the H
2
D
+
molecular ion.
Context. The high spectral resolution R ∼ 45 000 provided by IGRINS (Immersion Grating INfrared Spectrometer) at MacDonald Observatory and R ∼ 100 000 achieved by CRIRES (CRyogenic high-resolution ...InfraRed Echelle Spectrograph) at VLT (Very Large Telescope) challenges the present knowledge of infrared spectra. Aims. We aim to predict the full infrared spectrum of molecular hydrogen at a comparable accuracy. Methods. We take advantage of the recent theoretical ab initio studies on molecular hydrogen to compute both the electric quadrupole and magnetic dipole transitions taking place within the ground electronic molecular state of hydrogen. Results. We computed the full infrared spectrum of molecular hydrogen at an unprecedented accuracy and derive for the first time the emission probabilities including both electric quadrupole (ΔJ = 0, ±2) and magnetic dipole transitions (ΔJ = 0) as well as the total radiative lifetime of each rovibrational state. Inclusion of magnetic dipole transitions increases the emission probabilities by factors of a few for highly excited rotational levels, which occur in the 3–20 μ range.
Context. Barnard B1b has been revealed as one of the most interesting globules from the chemical and dynamical point of view. It presents a rich molecular chemistry characterized by large abundances ...of deuterated and complex molecules. Furthermore, this globule hosts an extremely young Class 0 object and one candidate for the first hydrostatic core (FHSC) proving the youth of this star-forming region. Aims. Our aim is to determine the cosmic ray ionization rate, ζH2, and the depletion factors in this extremely young star-forming region. These parameters determine the dynamical evolution of the core. Methods. We carried out a spectral survey toward Barnard 1b as part of the IRAM large program “IRAM Chemical survey of sun-like star-forming regions” (ASAI) using the IRAM 30-m telescope at Pico Veleta (Spain). This provided a very complete inventory of neutral and ionic C-, N-, and S- bearing species with, from our knowledge, the first secure detections of the deuterated ions DCS+ and DOCO+. We use a state-of-the-art pseudo-time-dependent gas-phase chemical model that includes the ortho and para forms of H2, H2+, D2+, H3+, H2D+, D2H+, D2, and D3+ to determine the local value of the cosmic ray ionization rate and the depletion factors. Results. Our model assumes n(H2) = 105 cm-3 and Tk = 12 K, as derived from our previous works. The observational data are well fitted with ζH2 between 3 × 10-17 s-1 and 10-16 s-1 and the elemental abundances O/H = 3 × 10-5, N/H = 6.4−8 × 10-5, C/H = 1.7 × 10-5, and S/H between 6.0 × 10-7 and 1.0 × 10-6. The large number of neutral/protonated species detected allows us to derive the elemental abundances and cosmic ray ionization rate simultaneously. Elemental depletions are estimated to be ~10 for C and O, ~1 for N, and ~25 for S. Conclusions. Barnard B1b presents similar depletions of C and O as those measured in prestellar cores. The depletion of sulfur is higher than that of C and O, but not as extreme as in cold cores. In fact, it is similar to the values found in some bipolar outflows, hot cores, and photon-dominated regions. Several scenarios are discussed to account for these peculiar abundances. We propose that it is the consequence of the initial conditions (important outflows and enhanced UV fields in the surroundings) and a rapid collapse (~0.1 Myr) that allows most S- and N-bearing species to remain in the gas phase to great optical depths. The interaction of the compact outflow associated with B1b-S with the surrounding material could enhance the abundances of S-bearing molecules, as well.
We report the detection in space of a new molecular species that has been characterized spectroscopically and fully identified from astrophysical data. The observations were carried out with the IRAM ...30 m telescope. The molecule is ubiquitous as its transition has been found in cold molecular clouds, prestellar cores, and shocks. However, it is not found in the hot cores of Orion-KL and in the carbon-rich evolved star IRC+10216. Three rotational transitions in perfect harmonic relation have been identified in the prestellar core B1b. The molecule has a 1 electronic ground state and its transition presents the hyperfine structure characteristic of a molecule containing a nucleus with spin 1. A careful analysis of possible carriers shows that the best candidate is NS+. The derived rotational constant agrees within 0.3%-0.7% with ab initio calculations. NS+ was also produced in the laboratory to unambiguously validate the astrophysical assignment. The observed rotational frequencies and determined molecular constants confirm the discovery of the nitrogen sulfide cation in space. The chemistry of NS+ and related nitrogen-bearing species has been analyzed by means of a time-dependent gas-phase model. The model reproduces well the observed NS/NS+ abundance ratio, in the range 30-50, and indicates that NS+ is formed by reactions of the neutral atoms N and S with the cations SH+ and NH+, respectively.
Aims. We aim to investigate fractionation reactions involved in the 12C/13C, 16O/18O, and 17O balance. Methods. Full-dimensional rovibrational calculations were used to compute numerically exact ...rovibrational energies and thermal equilibrium conditions to derive the reaction rate coefficients. A nonlinear least-squares method was employed to represent the rate coefficients by analytic functions. Results. New exothermicities are derived for 30 isotopic exchange reactions of HCO+ with CO. For each of the reactions, we provide the analytic three-parameter Arrhenius-Kooij formula for both the forward reaction and backward reaction rate coefficients, that can further be used in astrochemical kinetic models. Rotational constants derived here for the 17O containing forms of HCO+ may assist detection of these cations in outer space.
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