Suchergebnisse

Context. An astronomical survey of interstellar molecular clouds needs a previous analysis of the spectra in the microwave and sub-mm energy range of organic molecules to be able to identify them. We obtained very accurate spectroscopic constants in a comprehensive laboratory analysis of rotational spectra. These constants can be used to predict the transitions frequencies very precisely that were not measured in the laboratory. Aims. We present the experimental study and its theoretical analysis for two 13C-methyl formate isotopologues to detect these two isotopologues for the first time in their excited torsional states, which lie at 130 cm-1 (200 K) in Orion-KL. Methods. New spectra of HCOO13CH3 (13C2) methyl formate were recorded with the mm- and submm-wave spectrometer in Lille from 50 to 940 GHz. A global fit for vt = 0 and 1 was accomplished with the BELGI program to reproduce the experimental spectra with greater accuracy. Results. We analysed 5728 and 2881 new lines for vt = 0 and 1 for HCOO13CH3. These new lines were globally fitted with 846 previously published lines for vt = 0. In consequence, 52 parameters of the RAM Hamiltonian were accurately determined and the value of the barrier height (V3 = 369.93168(395) cm-1) was improved. We report the detection of the first excited torsional states (vt = 1) in Orion-KL for the 13C2 and 13C1 methyl formate based on the present analysis and previously published data. We provide column densities, isotopic abundances, and vibrational temperatures for these species. Conclusions. Following this work, accurate prediction can be provided. This permits detecting 135 features of the first excited torsional states of 13C-methyl formate isotopologues in Orion-KL in the 80−280 GHz frequency range, without missing lines. ; M.C. acknowledges the financial support the Spanish Government through grant FIS2011-28738-C02-02, and by a joint project within the framework of a CNRS (France) and CSIC (Spain) agreement (Code No.: 2011 FR0018). J.C., A.L. and B.T. thank the Spanish MINECO for funding support from grants CSD2009-00038, AYA2009-07304, and AYA2012-32032. This work was supported by the Centre National d'Etudes Spatiales (CNES) and the Action sur Projets de l'INSU, "Physique et Chimie du Milieu Interstellaire". This work was also performed under the ANR-08-BLAN-0054.

Context. Astronomical survey of interstellar molecular clouds needs a previous analysis of the spectra in the microwave and sub-mm energy range to be able to identify them. We obtained very accurate spectroscopic constants in a comprehensive laboratory analysis of rotational spectra. These constants can be used to predict transition frequencies that were not measured in the laboratory very precisely. Aims. We present an experimental study and a theoretical analysis of two 18O-methyl formate isotopologues, which were subsequently detected for the first time in Orion KL. Methods. The experimental spectra of both methyl formate isotopologues recorded in the microwave and sub-mm range from 1 to 660 GHz. Both spectra were analysed by using the rho-axis method (RAM) which takes into account the CH3 internal rotation. Results. We obtained spectroscopic constants of both 18O- methyl formate with high accuracy. Thousands of transitions were assigned and others predicted, which allowed us to detect both species in the IRAM 30 m line survey of Orion KL. ; M.C. acknowledges the financial support provided by the Andalusian Government (Spain) project number P07-FQM-03014. He also thanks Universite Paris 12 for inviting him. J.C. and B.T. thank Spanish MICINN for support under grants AYA2006-14786, AYA2009-07304 and the CONSOLIDER program "ASTROMOL" CSD2009-00038. This work was supported by the Centre National d'Etudes Spatiales (CNES) and the Action sur Projets de l'INSU, "Physique et Chimie du Milieu Interstellaire". This work was also done under the ANR-08-BLAN-0054.

We have surveyed a sample of massive star-forming regions located over a range of distances from the Galactic centre for methyl formate, HCOOCH3, and its isotopologues H13COOCH3 and HCOO13CH3. The observations were carried out with the APEX telescope in the frequency range 283.4-287.4 GHz. Based on the APEX observations, we report tentative detections of the 13C-methyl formate isotopologue HCOO13CH3 towards the following four massive star-forming regions: Sgr B2(N-LMH), NGC 6334 IRS 1, W51 e2 and G19.61-0.23. In addition, we have used the 1 mm ALMA science verification observations of Orion-KL and confirm the detection of the 13C-methyl formate species in Orion-KL and image its spatial distribution. Our analysis shows that the 12C/13C isotope ratio in methyl formate toward Orion-KL Compact Ridge and Hot Core-SW components (68.4±10.1 and 71.4±7.8, respectively) are, for both the 13C-methyl formate isotopologues, commensurate with the average 12C/13C ratio of CO derived toward Orion-KL. Likewise, regarding the other sources, our results are consistent with the 12C/13C in CO. We also report the spectroscopic characterization, which includes a complete partition function, of the complex H13COOCH3 and HCOO13CH3 species. New spectroscopic data for both isotopomers H13COOCH3 and HCOO13CH3, presented in this study, has made it possible to measure this fundamentally important isotope ratio in a large organic molecule for the first time. ; This work was supported by the National Science Foundation under grant 1008800. We are grateful to the Ministerio de Economia y Competitividad of Spain for the financial support through grant No. FIS2011-28738-C02-02 and to the French Government through grant No. ANR-08-BLAN-0054 and the French PCMI (Programme National de Physique Chimie du Milieu Interstellaire). This paper makes use of the following ALMA data: ADS/JAO. ALMA#2011.0.00009.SV.ALMAis a partnership of ESO (representing its member states), NSF (USA), and NINS (Japan), together with NRC (Canada) and NSC and ASIAA (Taiwan), in cooperation with the Republic of Chile. The Joint ALMA Observatory is operated by ESO, AUI/NRAO, and NAOJ. C.F. thanks Dahbia Talbi, Eric Herbst, and Anthony Remijan for enlightening discussions. Finally, we thank the anonymous referee for helpful comments.