Fig 5 - uploaded by Romane Le Gal
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Impact of the k p→o rate coefficient of the H + NH 2 Hexchange reaction on the NH 2 OPR as a function of temperature computed with Model 1 for the physical conditions given in the super title at steady state and an earlier time. The details of Model 1 are described in Sect. 2.3.
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Based on recent $Herschel$ results, the ortho-to-para ratio (OPR) of NH$_2$ has been measured towards the following high-mass star-forming regions: W31C (G10.6-0.4), W49N (G43.2-0.1), W51 (G49.5-0.4), and G34.3+0.1. The OPR at thermal equilibrium ranges from the statistical limit of three at high temperatures to infinity as the temperature tends to...
Citations
... As described in Le Gal et al. (2017), for a molecule containing two identical hydrogen nuclei, such as CH 2 CN, we expect a statistical ortho/para ratio of 3:1, and therefore we infer a total column density, N tot , of (8.4 ± 0.7) × 10 12 cm −2 . Indeed, due to the X 2 B1 symmetry of the ground electronic state of CH 2 CN, the ortho-to-para ratio of CH 2 CN decreases toward the statistical 3:1 value with increasing temperature as NH 2 (Le Gal et al. 2016). CH 2 CN is a heavier molecule than NH 2 , therefore, its ortho-to-para ratio will reach the statistical ratio for lower temperatures than NH 2 . ...
... CH 2 CN is a heavier molecule than NH 2 , therefore, its ortho-to-para ratio will reach the statistical ratio for lower temperatures than NH 2 . Thus, according to Figure 1 of Le Gal et al. (2016), the relatively high rotational temperature we derived confirms that it is reasonable to consider a 3:1 statistical ortho-para ratio for CH 2 CN. Given this ratio, we can calculate the expected para column density and using this value and the expected integrated intensity of the para lines that we do not detect (located at 241.353 and 241.381). ...
We report the first detection of the molecule cyanomethyl, CH 2 CN, in a protoplanetary disk. Until now, CH 2 CN had only been observed at earlier evolutionary stages, in the molecular clouds TMC-1, Sgr2, and L483, in the prestellar core L1544, and toward the protostar L1527. We detect six transitions of ortho-CH 2 CN toward the disk around nearby T Tauri star TW Hya. An excitation analysis reveals that the disk-averaged column density, N , for ortho-CH 2 CN is (6.3 ± 0.5) × 10 ¹² cm ⁻² , which is rescaled to reflect a 3:1 ortho-para ratio, resulting in a total column density, N tot , of (8.4 ± 0.7) × 10 ¹² cm ⁻² . We calculate a disk-average rotational temperature, T rot = 40 ± 5 K, while a radially resolved analysis shows that T rot remains relatively constant across the radius of the disk. This high rotation temperature suggests that in a static disk and if vertical mixing can be neglected, CH 2 CN is largely formed through gas-phase reactions in the upper layers of the disk, rather than solid-state reactions on the surface of grains in the disk midplane. The integrated intensity radial profiles show a ring structure consistent with molecules such as CN and DCN. We note that this is also consistent with previous lower-resolution observations of centrally peaked CH 3 CN emission toward the TW Hya disks, since the observed emission gap disappears when convolving our observations with a larger beam size. We obtain a CH 2 CN/CH 3 CN ratio ranging between 4 and 10. This high CH 2 CN/CH 3 CN is reproduced in a representative chemical model of the TW Hya disk that employs standard static disk chemistry model assumptions, i.e., without any additional tuning.
... As described in Le Gal et al. (2017), for a molecule containing two identical Hydrogen nuclei, such as CH 2 CN, we expect a statistical ortho/para ratio of 3:1, and therefore we infer a total column density, N tot , of (8.4 ± 0.7) × 10 12 cm −2 . Indeed, due to the X 2 B1 symmetry of the ground electronic state of CH 2 CN, the ortho-to-para ratio of CH 2 CN decreases toward the statistical 3:1 value with increasing temperature as NH 2 (Le Gal et al. 2016). CH 2 CN is a heavier molecule than NH 2 , therefore, its ortho-to-para ratio will reach the statistical ratio for lower temperatures than NH 2 . ...
... CH 2 CN is a heavier molecule than NH 2 , therefore, its ortho-to-para ratio will reach the statistical ratio for lower temperatures than NH 2 . Thus, according to Fig. 1 of Le Gal et al. (2016), the relatively high rotational temperature we derived confirms that it is reasonable to consider a 3:1 statistical ortho-para ratio for CH 2 CN. Given this ratio, we can calculate the expected para column density and using this value and the the expected integrated intensity of the para lines that we do not detect (located at 241.353 and 241.381). ...
We report the first detection of the molecule cyanomethyl, CH$_2$CN, in a protoplanetary disk. Until now, CH$_2$CN had only been observed at earlier evolutionary stages, in the giant molecular clouds TMC-1 and Sgr 2, and the prestellar core L1544. We detect six transitions of ortho-CH$_2$CN towards the disk around nearby T Tauri star TW Hya. An excitation analysis reveals that the disk-averaged column density, $N$, for ortho-CH$_2$CN is $(6.3\pm 0.5)\times10^{12}$ cm$^{-2}$, which is rescaled to reflect a 3:1 ortho-para ratio, resulting in a total column density, $N_{\rm tot}$, of $(8.4\pm 0.7)\times10^{12}$ cm$^{-2}$. We calculate a disk-average rotational temperature, $T_{\rm{rot}}$ = $40 \pm 5$ K, while a radially resolved analysis shows that $T_{\rm{rot}}$ remains relatively constant across the radius of the disk. This high rotation temperature suggests that in a static disk and if vertical mixing can be neglected,CH$_2$CN is largely formed through gas-phase reactions in the upper layers of the disk, rather than solid-state reactions on the surface of grains in the disk midplane. The integrated intensity radial profiles show a ring structure consistent with molecules such as CN and DCN. We note that this is also consistent with previous lower-resolution observations of centrally peaked CH$_3$CN emission towards the TW Hya disks, since the observed emission gap disappears when convolving our observations with a larger beam size. We obtain a CH$_2$CN/CH$_3$CN ratio ranging between 4 and 10. This high CH$_2$CN/CH$_3$CN is reproduced in a representative chemical model of the TW Hya disk that employs standard static disk chemistry model assumptions, i.e. without any additional tuning.
... A recent study has also emphasized the importance of the H-exchange reaction NH 2 + H in the ortho-para conversion of NH 2 . 12 However, the analysis of the NH 2 rotational spectra, especially those in emission, was hampered by the lack of collisional rate coefficients. Without these data, only approximate estimates of the molecular column density are possible assuming local thermodynamic equilibrium (LTE), which is generally not a good approximation. ...
We present quantum close-coupling calculations for the rotational excitation of the interstellar amidogen radical NH2 due to collisions with H2 molecules. The calculations are based on a recent, high-accuracy full-dimensional NH4potential energy surface adapted for rigid-rotor scattering calculations. The collisional cross section calculations are performed for all transitions among the first 15 energy levels of both ortho- and para-NH2 and for total energies up to 1500 cm⁻¹. Both para- and ortho-H2 colliding partners are considered. The cross sections for collision with para- and ortho-H2 are found to differ significantly, the magnitude of the ortho-H2 ones being dominant. No strong propensity rules are observed but transitions with Δkc=0 are slightly favored.
Context. We present a full analysis of a broadband spectral line survey of Sagittarius B2 (Main), one of the most chemically rich regions in the Galaxy located within the giant molecular cloud complex Sgr B2 in the central molecular zone.
Aims. Our goal is to derive the molecular abundances and temperatures of the high-mass star-forming region Sgr B2(M) and thus its physical and astrochemical conditions.
Methods. Sgr B2(M) was observed using the Heterodyne Instrument for the Far-Infrared (HIFI) on board the Herschel Space Observatory in a spectral line survey from 480 to 1907 GHz at a spectral resolution of 1.1 MHz, which provides one of the largest spectral coverages ever obtained toward this high-mass star-forming region in the submillimeter with high spectral resolution and includes frequencies >1 THz that are unobservable from the ground. We modeled the molecular emission from the submillimeter to the far-infrared using the XCLASS program, which assumes local thermodynamic equilibrium. For each molecule, a quantitative description was determined taking all emission and absorption features of that species across the entire spectral range into account. Because of the wide frequency coverage, our models are constrained by transitions over an unprecedented range in excitation energy. Additionally, we derived velocity resolved ortho/para ratios for those molecules for which ortho and para resolved molecular parameters are available. Finally, the temperature and velocity distributions are analyzed and the derived abundances are compared with those obtained for Sgr B2(N) from a similar HIFI survey.
Results. A total of 92 isotopologues were identified, arising from 49 different molecules, ranging from free ions to complex organic compounds and originating from a variety of environments from the cold envelope to hot and dense gas within the cores. Sulfur dioxide, methanol, and water are the dominant contributors. Vibrationally excited HCN ( v 2 = 1) and HNC ( v 2 = 1) are detected as well. For the ortho/para ratios, we find deviations from the high temperature values between 37 and 180%. In total 14% of all lines remain unidentified.
Conclusions. Compared to Sgr B2(N), we found less complex molecules such as CH 3 OCH 3 , CH 3 NH 2 , or NH 2 CHO, but more simple molecules such as CN, CCH, SO, and SO 2 . However some sulfur bearing molecules such as H 2 CS, CS, NS, and OCS are more abundant in N than in M. The derived molecular abundances can be used for comparison to other sources and for providing further constraints for astrochemical models.
Accurate estimation of the abundance of the NH2 radical in the interstellar medium requires accurate radiative and collisional rate coefficients. The calculation of hyperfine-resolved rate coefficients for the collisional (de-)excitation of NH2 by both ortho- and para-H2 is presented in this work. Hyperfine-resolved rate coefficients are calculated from pure rotational close-coupling rate coefficients using the Mj randomizing approximation. Rate coefficients for temperatures ranging from 5 to 150 K were computed for all hyperfine transitions among the first 15 rotational energy levels of both ortho- and para-NH2 in collisions with ortho- and para-H2. The new data were then employed in radiative transfer calculations to simulate the excitation of NH2 in typical star-forming regions such as W31C, where NH2 is seen in emission. We compared the excitation and brightness temperatures for different NH2 transitions obtained using the new and the previously available collisional data. It is found that the new rate coefficients increase the simulated line intensities by a factor ∼10–30. As a consequence, NH2 abundance derived from the observations will be significantly reduced by the use of the present rate coefficients.
Deuterium and lithium are light elements of high cosmological and astrophysical importance. In this work we report the first detection of deuterated molecules and a search for lithium hydride, ⁷ LiH, at redshift z = 0.89 in the spiral galaxy intercepting the line of sight to the quasar PKS 1830−211. We used ALMA to observe several submillimeter lines of ND, NH 2 D, and HDO, and their related isotopomers NH 2 , NH 3 , and H 2¹⁸ O, in absorption against the southwest image of the quasar, allowing us to derive XD/XH abundance ratios. The absorption spectra mainly consist of two distinct narrow velocity components for which we find remarkable differences. One velocity component shows XD/XH abundances that is about 10 times larger than the primordial elemental D/H ratio, and no variability of the absorption profile during the time span of our observations. In contrast, the other component shows a stronger deuterium fractionation. Compared to the first component, this second component has XD/XH abundances that are 100 times larger than the primordial D/H ratio, a deepening of the absorption by a factor of two within a few months, and a rich chemical composition, with relative enhancements of N 2 H ⁺ , CH 3 OH, SO 2 and complex organic molecules. We therefore speculate that this component is associated with the analog of a Galactic dark cloud, while the first component is likely more diffuse. Our search for the ⁷ LiH (1–0) line was unsuccessful and we derive an upper limit ⁷ LiH/H 2 = 4 × 10 ⁻¹³ (3 σ ) in the z = 0.89 absorber toward PKS 1830−211. Besides, with ALMA archival data, we could not confirm the previous tentative detections of this line in the z = 0.68 absorber toward B 0218+357; we derive an upper limit ⁷ LiH/H 2 = 5 × 10 ⁻¹¹ (3 σ ), although this is less constraining than our limit toward PKS 1830−211. We conclude that, as in the Milky Way, only a tiny fraction of lithium nuclei are possibly bound in LiH in these absorbers at intermediate redshift.
Amidogen (NH 2 ), a b-type asymmetric top molecule with electric dipole moment 1.82 ± 0.05 Debye, is detected in Sgr B2, in high-mass star-forming regions W31C (G10.6−0.4), W49N (G43.2−0.1), W51 (G49.5−0.4), G34.3+0.1, and in several comets. Because of two hydrogen atoms, each with nuclear spin 1/2, its rotational energy levels can be classified into ortho and para groups. We have not considered for fine structure splitting and hyper-fine structure splitting of rotational levels. For 15 rotational levels in the ground vibrational state, having energy up to 400 cm−1, for each specie, the energies of rotational levels, and Einstein A and B coefficients for radiative transitions between the levels are calculated, using accurate values of spectroscopic data. These radiative transition probabilities along with the collisional rate coefficients (obtained from a scaling law) are employed as input parameters for solving a set of statistical equilibrium equations coupled with the equations of radiative transfer for each group. Several emission lines produced by amidogen are found. For each species of NH 2 , we have considered some strongest emission lines along with the observed one, which may help for identification of NH 2 in the interstellar medium (ISM) and in the cometary material.
Multi-hydrogenated species with proper symmetry properties can present different spin configurations, and thus exist under different spin symmetry forms, labeled as para and ortho for two-hydrogen molecules. We investigated here the ortho-to-para ratio (OPR) of H$_2$Cl$^+$ in the light of new observations performed in the z=0.89 absorber toward the lensed quasar PKS 1830-211 with the Atacama Large Millimeter/submillimeter Array (ALMA). Two independent lines of sight were observed, to the southwest (SW) and northeast (NE) images of the quasar, with OPR values found to be $3.15 \pm 0.13$ and $3.1 \pm 0.5$ in each region, respectively, in agreement with a spin statistical weight of 3:1. An OPR of 3:1 for a molecule containing two identical hydrogen nuclei can refer to either a statistical result or a high-temperature limit depending on the reaction mechanism leading to its formation. It is thus crucial to identify rigorously how OPRs are produced in order to constrain the information that these probes can provide. To understand the production of the H$_2$Cl$^+$ OPR, we undertook a careful theoretical study of the reaction mechanisms involved with the aid of quasi-classical trajectory calculations on a new global potential energy surface fit to a large number of high-level ab initio data. Our study shows that the major formation reaction for H$_2$Cl$^+$ produces this ion via a hydrogen abstraction rather than a scrambling mechanism. Such a mechanism leads to a 3:1 OPR, which is not changed by destruction and possible thermalization reactions for H$_2$Cl$^+$ and is thus likely to be the cause of observed 3:1 OPR ratios, contrary to the normal assumption of scrambling.
