1,721,095 research outputs found
First sample of N2H+nitrogen isotopic ratio measurements in low-mass protostars
Full text linkContext 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+â 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
Vibrationally excited states of HC5P: millimetre-wave spectroscopy and coupled cluster calculations
No full textA more efficient method for the production of the unstable HC5P molecule has been found, based on copyrolysis of phosphorus trichloride and cyclopentene. This allowed us to extend the study of the rotational spectrum of HC5P to a large number of vibrationally excited states which approximately lie between 150 and 650 cm(-1), namely (nu(6)nu(7)nu(8)nu(9)nu(10)nu(11)) = (000002) (000003), (000010), (000020), (000100), (001000), (010000), (100000), (000011), (000101), and (001001). The anharmonic resonances which couple the (100000) stretching state with the (000020) and (000101) bending states, and the l-type resonances which occur between the different sublevels of a given bending state have been taken into account in the analysis of the spectra, which yielded determinations of the alpha(6), alpha(7), alpha(8), alpha(9), and alpha(10) vibration-rotation coupling constants, and of the X-L(11,X-11), X-L(10,X-10), X-L(10,X-11), X-L(9,X-11), and x(L(8,11)) anharmonicity constants. The experimental work was assisted by coupled-cluster single double triple [CCSD(T)] calculations, performed using the cc-pVQZ basis, which provided accurate predictions for a variety of spectroscopic constants including harmonic vibrational wavenumbers, vibration-rotation coupling constants and l-type doubling constants
Millimeter-wave spectroscopy of HC3P isotopomers and coupled-cluster calculations: the molecular structure of phosphabutadiyne
No full textThe rotational spectrum of the unstable HC3P molecule has been investigated in the millimeter-wave region. Five different isotopomers were studied, including also the three C-13-containing species which were detected in natural abundance. The obtained ground state rotational constants allowed us to evaluate the r(0) and r(s) molecular structures of phosphabutadiyne. CCSD(T) calculations were performed using the cc-pVQZ basis, and theoretically computed vibration-rotation coupling constants were combined with experimental ground state rotational constants to determine an accurate equilibrium structure. (C) 2000 Published by Elsevier Science B.V. All rights reserved
Collisional excitation of NH(3ς-) by Ar: A new ab initio 3D potential energy surface and scattering calculations
No full textCollisional excitation of light hydrides is important to fully understand the complex chemical and physical processes of atmospheric and astrophysical environments. Here, we focus on the NH(X3ς-)-Ar van der Waals system. First, we have calculated a new three-dimensional Potential Energy Surface (PES), which explicitly includes the NH bond vibration. We have carried out the ab initio calculations of the PES employing the open-shell single- and double-excitation couple cluster method with noniterative perturbational treatment of the triple excitations. To achieve a better accuracy, we have first obtained the energies using the augmented correlation-consistent aug-cc-pVXZ (X = T, Q, 5) basis sets and then we have extrapolated the final values to the complete basis set limit. We have also studied the collisional excitation of NH(X3ς-)-Ar at the close-coupling level, employing our new PES. We calculated collisional excitation cross sections of the fine-structure levels of NH by Ar for energies up to 3000 cm-1. After thermal average of the cross sections, we have then obtained the rate coefficients for temperatures up to 350 K. The propensity rules between the fine-structure levels are in good agreement with those of similar collisional systems, even though they are not as strong and pronounced as for lighter systems, such as NH-He. The final theoretical values are also compared with the few available experimental data
Millimeter-Wave and Diode Laser Spectroscopy of I 13CN: Analysis of the v3 Band System
No full textThe v3, fundamental band (C-N stretch) and four associated hot bands of I 13CN have been recorded in the 2130 cm-1 region using a diode laser spectrometer. The analysis of the five infrared bands has been supported by the measurements of new millimeter-wave rotational lines for the 0000, 01 10, 1000, 0200, and 0220 states. Accurate values of the band origins and of the lower and upper state rotational constants have been determined. © 1997 Academic Press
Why does ammonia not freeze out in the centre of pre-stellar cores?
No full textWe carried out a parameter-space exploration of the ammonia abundance in the pre-stellar core L1544, where it has been observed to increase toward the centre of the core with no signs of freeze-out onto grain surfaces. We considered static and dynamical physical models coupled with elaborate chemical and radiative transfer calculations, and explored the effects of varying model parameters on the (ortho + para) ammonia abundance profile. None of our models are able to reproduce the inward-increasing tendency in the observed profile; ammonia depletion always occurs in the centre of the core. In particular, our study shows that including the chemical desorption process, where exothermic association reactions on the grain surface can result in the immediate desorption of the product molecule, leads to ammonia abundances that are over an order of magnitude above the observed level in the innermost 15 000 au of the core - at least when one employs a constant efficiency for the chemical desorption process, irrespective of the ice composition. Our results seemingly constrain the chemical desorption efficiency of ammonia on water ice to below 1 per cent. It is increasingly evident that timedependent effects must be considered so that the results of chemical models can be reconciled with observations
Millimeter-wave spectroscopy and coupled cluster calculations for NCCP
No full textThe rotational spectrum of the unstable NCCP molecule has been investigated in the millimeter-wave region. The ground-state spectra of the most abundant isotopomer and of the C-13 and N-15 containing species were studied, and lines in the upsilon(2), upsilon(3), upsilon(4), and upsilon(5) vibrationally excited states were detected for the normal isotopomer. Electric quadrupole and magnetic spin-rotation coupling constants of the nitrogen nucleus were also determined. The experimental work was assisted by high level coupled-cluster single double triple [CCSD(T)] calculations, performed using the cc-pVQZ basis, which provided accurate predictions for the alpha(r) vibration-rotation coupling constants and the ground-state rotational constants of the less abundant isotopic species. r(0) and r(s) molecular structures of NCCP were derived directly from the experimental ground-state rotational constants of four different isotopomers, and an accurate equilibrium structure could be evaluated by combining theoretically computed vibration-rotation coupling constants with experimental ground-state rotational constants. (C) 2000 American Institute of Physics. [S0021-9606(00)00928-4]
Millimeter-wave spectroscopy and coupled cluster calculations for a new phosphorus–carbon chain: HC5P
No full textThe linear, unstable HC5P molecule has been detected for the first time in the pyrolysis products of phosphorus trichloride and toluene mixtures. Its rotational spectrum has been investigated in the millimeter-wave region (78-195 GHz) for the ground and v(11) = 1 excited state of both normal and deuterated species. Accurate values of rotational, centrifugal distortion and q(11) l-type doubling constants have been obtained. The experimental work was assisted by coupled-cluster single double triple [CCSD(T)] calculations, which provided accurate predictions for the equilibrium structure and the dipole moment of this new carbon chain, phosphorus bearing molecule. (C) 2003 American Institute of Physics
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