Luca Bizzocchi

Detection of (15)NNH+ in L1544: non-LTE modelling of dyazenilium hyperfine line emission and accurate (14)N/(15)N values

Samples of pristine Solar System material found in meteorites and interplanetary dust particles are highly enriched in (15)N. Conspicuous nitrogen isotopic anomalies have also been measured in comets, and the (14)N/(15)N abundance ratio of the Earth is itself larger than the recognised pre-solar value by almost a factor of two. Ion--molecules, low-temperature chemical reactions in the proto-solar nebula have been repeatedly indicated as responsible for these (15)N-enhancements. We have searched for (15)N variants of the N2H+ ion in L1544, a prototypical starless cloud core which is one of the best candidate sources for detection owing to its low central core temperature and high CO depletion. The goal is the evaluation of accurate and reliable (14)N/(15)N ratio values for this species in the interstellar gas. A deep integration of the (15)NNH+ (1-0) line at 90.4 GHz has been obtained with the IRAM 30 m telescope. Non-LTE radiative transfer modelling has been performed on the J=1-0 emissions of the parent and (15)N-containing dyazenilium ions, using a Bonnor--Ebert sphere as a model for the source. A high-quality fit of the N2H+ (1--0) hyperfine spectrum has allowed us to derive a revised value of the N2H+ column density in L1544. Analysis of the observed N(15)NH+ and (15)NNH+ spectra yielded an abundance ratio N[N(15)NH+]/N[(15)NNH+] = 1.1 +/- 0.3. The obtained (14)N/(15)N ratio is ~ 1000 +/- 200, suggestive of a sizeable (15)N depletion in this molecular ion. Such a result is not consistent with the prediction of present nitrogen chemical models. As chemical models predict large (15)N fractionation of N2H+, we suggest that (15)N(14)N, or (15)N in some other molecular form, is preferentially depleted onto dust grains.

Rotational spectroscopy of the isotopic species of silicon monosulfide, SiS.

Pure rotational transitions of silicon monosulfide ((28)Si(32)S) and its rare isotopic species have been observed in their ground as well as vibrationally excited states by employing Fourier transform microwave (FTMW) spectroscopy of a supersonic molecular beam at centimetre wavelengths (13-37 GHz) and by using long-path absorption spectroscopy at millimetre and submillimetre wavelengths (127-925 GHz). The latter measurements include 91 transition frequencies for (28)Si(32)S, (28)Si(33)S, (28)Si(34)S, (29)Si(32)S and (30)Si(32)S in upsilon = 0, as well as 5 lines for (28)Si(32)S in upsilon = 1, with rotational quantum numbers J< or = 52. The centimetre-wave measurements include more than 300 newly recorded lines. Together with previous data they result in almost 600 transitions (J = 0 and 1) from all twelve possible isotopic species, including (29)Si(36)S and (30)Si(36)S, which have fractional abundances of about 7 x 10(-6) and 4.5 x 10(-6), respectively. Rotational transitions were observed from upsilon = 0 for the least abundant isotopic species to as high as upsilon = 51 for the main species. Owing to the high spectral resolution of the FTMW spectrometer, hyperfine structure from the nuclear electric quadrupole moment of (33)S was resolved for species containing this isotope, as was much smaller nuclear spin-rotation splitting for isotopic species involving (29)Si. By combining the measurements here with previously published microwave and infrared data in one global fit, an improved set of spectroscopic parameters for SiS has been derived which include several terms describing the breakdown of the Born-Oppenheimer approximation. With this parameter set, highly accurate rotational frequencies for this important astronomical molecule can now be predicted well into the terahertz region.

MILLIMETER-WAVE SPECTROSCOPY AND COUPLED CLUSTER CALCULATIONS FOR NCCP

The 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 13C and 15N containing species were studied, and lines in the v2, v3, v4, and v5 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 αr vibration–rotation coupling constants and the ground-state rotational constants of the less abundant isotopic species. r0 and rs 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 co...

Millimeter-wave spectroscopy of HC3P isotopomers and coupled-cluster calculations: the molecular structure of phosphabutadiyne

The rotational spectrum of the unstable HC3P molecule has been investigated in the millimeter-wave region. Five different isotopomers were studied, including also the three 13C-containing species which were detected in natural abundance. The obtained ground state rotational constants allowed us to evaluate the r0 and rs 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.

Improved Rest Frequencies of HCO+ at 1 THz

The ground-state rotational spectrum of HCO+, produced in a negative-glow discharge cell, has been recorded up to 892 GHz. The sources of systematic error have been carefully accounted for, thus obtaining accurate rotational and centrifugal distortion constants. The present measurements have allowed us to determine a value of the sextic distortion constant, HJ, which, unlike the recent determination of Lattanzi et al., is consistent with the values determined for DCO+ and for the isoelectronic molecule HCN. The new set of spectroscopic constants allows us to predict the rotational spectrum of the formyl ion with an accuracy of 1 part in 108 near 1 THz. Such an accuracy is important for kinematic studies of dense cores in molecular clouds and for future far-infrared observations.

Pyrolysis of sulfur tetrafluoride over boron: Excited-state rotational spectra and equilibrium structure of fluorothioborine (FBS)

The unstable FBS molecule has been produced in the gas phase by a high-temperature reaction between crystalline boron and sulfur tetrafluoride. Its rotational spectrum has been observed in the millimeter-wave region, from 75 to 460 GHz, for different isotopic species and vibrational states. All the excited states which approximately lie below 1700 cm−1, that are 1000 (F–B stretch), 0110 (FBS bend), 0001 (B=S stretch), 2000, 0200, 0220, 0310, 0330, 0400, 0420, 0440, 1110, 1200, and 1220, have been investigated for the most abundant isotopomer F11B32S. The analysis of the spectra has been performed taking simultaneously into account the Fermi interaction which couples the states ν1,ν2,ν3 with ν1−1, ν2+2,ν3, and l-type resonances between different sublevels of a given vibrational bending state. This procedure allowed us to calculate directly deperturbed parameters and, in addition, yielded reliable estimates of the vibrational energy difference between the interacting levels and of the normal coordinate cubi...

Detection of N

Context. Excess levels of 15 N isotopes which have been detected in primitive solar system materials are explained as a remnant of interstellar chemistry which took place in regions of the protosolar nebula. Aims. Chemical models of nitrogen fractionation in cold clouds predict an enhancement in the gas-phase abundance of 15 N-bearing molecules, thus we have searched for 15 N variants of the N2H + ion in L1544, which is one of the best candidate sources for detection owing to its low central core temperature and high CO depletion. Methods. With the IRAM 30 m telescope we have obtained deep integrations of the N 15 NH + (1−0) line at 91.2 GHz. Results. The N 15 NH + (1−0) line has been detected toward the dust emission peak of L1544. The 14 N/ 15 N abundance ratio in N 15 NH + resulted 446 ± 71, very close to the protosolar value of ∼ 450, higher than the terrestrial ratio of ∼270, and significantly lower than the lower limit in L1544 found by Gerin et al. (2009, ApJ, 570, L101) in the same object using ammonia isotopologues.

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The 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 v11=1 excited state of both normal and deuterated species. Accurate values of rotational, centrifugal distortion and q11 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.

NH+ in L1544

The pure rotational spectra of 27 isotopic species of SnSe and SnTe have been measured in the frequency range of 5-24 GHz using a Fabry-Perot-type resonator pulsed-jet Fourier-transform microwave spectrometer. Gaseous samples of both chalcogenides were prepared by laser ablation of suitable target rods and were stabilized in supersonic jets of Ar. Global multi-isotopolog analyses of all available high-resolution data produced spectroscopic Dunham parameters Y01, Y11, Y21, Y31, Y02, and Y12 for both species, as well as Born-Oppenheimer breakdown (BOB) coefficients delta01 for Sn, Se, and Te. A direct fit of the same data sets to an appropriate radial Hamiltonian yielded analytic potential energy functions and BOB radial functions for the X 1Sigma+ electronic state of both SnSe and SnTe. Additionally, the magnetic hyperfine interaction produced by the dipolar nuclei 119Sn, 117Sn, 77Se, and 125Te was observed, yielding first determinations of the corresponding spin-rotation coupling constants.

Millimeter-wave spectroscopy and coupled cluster calculations for a new phosphorus–carbon chain: HC5P

Pure rotational transitions of silicon monoxide, involving the main ((28)Si(16)O) as well as several rare isotopic species, were observed in their ground vibrational states by employing long-path absorption spectroscopy between 86 and 825 GHz (1 ≤ J ≤ 18). Fourier transform microwave spectroscopy was used to study the J = 0 transition frequencies in the ground and several vibrationally excited states. The vibrational excitation of the newly studied isotopologues extend to between υ = 9 and 29 for (28)Si(17)O and (30)Si(16)O, respectively. Data were extended for some previously investigated species up to υ = 51 for the main isotopologue. The high spectral resolution allowed us to resolve the hyperfine structure in (28)Si(17)O caused by the nuclear electric quadrupole and magnetic dipole moments of (17)O for the first time, and to resolve the much smaller nuclear spin-rotation splitting for isotopic species containing (29)Si. These data were combined with previous rotational and rovibrational (infrared) data to determine an improved set of spectroscopic parameters of SiO in one global fit which takes the breakdown of the Born-Oppenheimer approximation into account. Highly accurate rotational transition frequencies for this important astronomical molecule can now be predicted well into the terahertz region with this parameter set. In addition, a more complete comparison among physical properties of group 14/16 diatomics is possible.