Context. Deuteration is a powerful tracer of the history of the cold prestellar phase in star-forming regions. Apart from methanol, little is known about deuterium fractionation of complex organic molecules in the interstellar medium, especially in regions forming high-mass stars.Aims. Our goal is to detect deuterated complex organic molecules toward the high mass star-forming region Sagittarius B2 (Sgr B2) and derive the level of deuteration for these molecules.Methods. We use a complete 3-mm spectral line survey performed with the Atacama Large Millimeter/submillimeter Array (ALMA) to search for deuterated complex organic molecules toward the hot molecular core Sgr B2(N2). We constructed population diagrams and integrated intensity maps to fit rotational temperatures and emission sizes for each molecule. Column densities are derived by modeling the full spectrum under the assumption of local thermodynamic equilibrium. We compare the results to predictions of two astrochemical models that treat the deuteration process.Results. We report the detection of CH2 DCN toward Sgr B2(N2) with a deuteration level of 0.4%, and tentative detections of CH2 DOH, CH2 DCH2 CN, the chiral molecule CH3 CHDCN, and DC3 N with levels in the range 0.05%–0.12%. A stringent deuteration upper limit is obtained for CH3 OD ( DOCHO. Ethyl cyanide is less deuterated than methyl cyanide by at least a factor five. The [CH2 DOH]/[CH3 OD] abundance ratio is higher than 1.8. It may still be consistent with the value obtained in Orion KL. Except for methyl cyanide, the measured deuteration levels lie at least a factor four below the predictions of current astrochemical models. The deuteration levels in Sgr B2(N2) are also lower than in Orion KL by a factor of a few up to a factor ten.Conclusions. The discrepancy between the deuteration levels of Sgr B2(N2) and the predictions of chemical models, and the difference between Sgr B2(N2) and Orion KL may both be due to the higher kinetic temperatures that characterize the Galactic center region compared to nearby clouds. Alternatively, they may result from a lower overall abundance of deuterium itself in the Galactic center region by up to a factor ten.

/pdf/exploring-molecular-complexity-with-alma-emoca-deuterated-5ai7xvbqu7.pdf

Exploring molecular complexity with ALMA (EMoCA): Deuterated complex organic molecules in Sagittarius B2(N2)

This paper presents a systematic survey of the deuterated and 13C isotopologues of a variety of oxygen-bearing complex organic molecules on Solar System scales toward the protostar IRAS 16293-2422B. We use the data from an unbiased molecular line survey between 329 and 363 GHz from the Atacama Large Millimeter/submillimeter Array (ALMA). The observations probe scales of 60 AU where most of the organic molecules have sublimated off dust grains and are present in the gas-phase. The complex organic molecules can be divided into two groups with one group, the simpler species, showing a D/H ratio of approximately 2% and the other, the more complex species, D/H ratios of 4-8%. This division may reflect the formation time of each species in the ices before or during warm-up/infall of material through the protostellar envelope. No significant differences are seen in the deuteration of different functional groups for individual species, possibly a result of the short time-scale for infall through the innermost warm regions where exchange reactions between different species may be taking place. The species show differences in excitation temperatures between 125 K and 300 K. This likely reflects the binding energies/sublimation temperatures of the individual species, in good agreement to what has previously been found for high-mass sources. For dimethyl ether, the 12C/13C ratio is found to be lower by up to a factor of 2 compared to typical ISM values similar to what has previously been inferred for glycolaldehyde. The results point to the importance of ice surface chemistry for the formation of these complex organic molecules at different stages in the evolution of embedded protostars and demonstrate the use of accurate isotope measurements for understanding the history of individual species.

/pdf/the-alma-pils-survey-isotopic-composition-of-oxygen-342htoqnfa.pdf

The ALMA-PILS survey: isotopic composition of oxygen-containing complex organic molecules toward IRAS 16293-2422B

In this paper, we couple a numerical kinetic-energy operator approach to the direct-vibrational self-consistent field (VSCF)/vibrational configuration interaction (VCI) method for the calculation of vibrational anharmonic frequencies. By combining this with fast-VSCF, an efficient direct evaluation of the ab initio potential-energy surface (PES), we introduce a general formalism for the computation of vibrational bound states of molecular systems exhibiting large-amplitude motion such as methyl-group torsion. We validate our approach on an analytical two-dimensional model and apply it to the methanol molecule. We show that curvilinear coordinates lead to a significant improvement in the VSCF/VCI description of the torsional frequency in methanol, even for a simple two-mode coupling expansion of the PES. Moreover, we demonstrate that a curvilinear formulation of the fast-VSCF/VCI scheme improves its speed by a factor of two and its accuracy by a factor of 3.

Fast vibrational configuration interaction using generalized curvilinear coordinates and self-consistent basis.

Aims Deuterated molecules have been detected and studied toward Orion BN/KL in the past decades, mostly with single-dish telescopes However, high angular resolution data are critical not only for interpreting the spatial distribution of the deuteration ratio but also for understanding this complex region in terms of cloud evolution involving star-forming activities and stellar feedbacks Therefore, it is important to investigate the deuterated ratio of methanol, one of the most abundant grain-surface species, on a scale of a few arcseconds to better understand the physical conditions related to deuteration in Orion BN/KL
Methods Orion BN/KL was extensively observed with the IRAM Plateau de Bure Interferometer from 1999 to 2007 in the 1 to 3 mm range The angular resolution varies from 1″8×0″8 to 3″6×2″3 and the spectral resolution varies from 04 to 19 km s-1 Deuterated methanol CH2DOH and CH3OD and CH3OH lines were searched for within our 3 mm and 13 mm data sets
Results We present here the first high angular resolution (1″8×0″8) images of deuterated methanol CH2DOH in Orion BN/KL Six CH2DOH lines were detected around 1058, 2235, and 2259 GHz In addition, three E-type methanol lines around 101–102 GHz were detected and were used to derive the corresponding CH3OH rotational temperatures and column densities toward different regions across Orion BN/KL The strongest CH2DOH and CH3OH emissions come from the Hot Core southwest region with a velocity that is typical of the Compact Ridge (VLSR ≈ 8 km s-1) We derive [CH2DOH]/[CH3OH] abundance ratios of 08–13 × 10-3 toward three CH2DOH emission peaks A new transition of CH3OD was detected at 2262 GHz for the first time in the interstellar medium Its distribution is similar to that of CH2DOH Besides, we find that the [CH2DOH]/[CH3OD] abundance ratios are lower than unity in the central part of BN/KL Furthermore, the HDO 31,2−22,1 line at 2259 GHz was detected and its emission distribution shows a shift of a few arcseconds with respect to the deuterated methanol emission that likely results from different excitation effects The deuteration ratios derived along Orion BN/KL are not markedly different from one clump to another However, various processes such as slow heating due to ongoing star formation, heating by luminous infrared sources, or heating by shocks could be competing to explain some local differences observed for these ratios

/pdf/deuterated-methanol-in-orion-bn-kl-3ec2mpjxx8.pdf

M. Smirnov

Papers

New assignments in the torsional spectrum of CH2DOH