Atomic and molecular data for the transitions of a number of astrophysically interesting species are summarized, in- cluding energy levels, statistical weights, Einstein A-coefficients and collisional rate coefficients. Available collisional data from quantum chemical calculations and experiments are extrapolated to higher energies (up to E/k ∼ 1000 K). These data, which are made publically available through the WWW at http://www.strw.leidenuniv.nl/∼moldata, are essential input for non-LTE line radiative transfer programs. An online version of a computer program for performing statistical equilibrium calcu- lations is also made available as part of the database. Comparisons of calculated emission lines using different sets of collisional rate coefficients are presented. This database should form an important tool in analyzing observations from current and future (sub)millimetre and infrared telescopes.

/pdf/an-atomic-and-molecular-database-for-analysis-of-4xud6qp5de.pdf

An atomic and molecular database for analysis of submillimetre line observations

Context. The discovery of amino acids in meteorites fallen to Earth and the detection of glycine, the simplest of them, in samples returned from a comet to Earth strongly suggest that the chemistry of the interstellar medium is capable of producing such complex organic molecules and that they may be widespread in our Galaxy. Aims. Our goal is to investigate the degree of chemical complexity that can be reached in the interstellar medium, in particular in dense star-forming regions. Methods. We performed an unbiased, spectral line survey toward Sgr B2(N) and (M), two regions where high-mass stars are formed, with the IRAM 30 m telescope in the 3 mm atmospheric transmission window. Partial surveys at 2 and 1.3 mm were performed in parallel. The spectra were analyzed with a simple radiative transfer model that assumes local thermodynamic equilibrium but takes optical depth effects into account. Results. About 3675 and 945 spectral lines with a peak signal-to-noise ratio higher than 4 are detected at 3 mm toward Sgr B2(N) and (M), i.e. about 102 and 26 lines per GHz, respectively. This represents an increase by about a factor of two over previous surveys of Sgr B2. About 70% and 47% of the lines detected toward Sgr B2(N) and (M) are identified and assigned to 56 and 46 distinct molecules as well as to 66 and 54 less abundant isotopologues of these molecules, respectively. In addition, we report the detection of transitions from 59 and 24 catalog entries corresponding to vibrationally or torsionally excited states of some of these molecules, respectively, up to a vibration energy of 1400 cm-1 (2000 K). Excitation temperatures and column densities were derived for each species but should be used with caution. The rotation temperatures of the detected complex molecules typically range from ~50 to 200 K. Among the detected molecules, aminoacetonitrile, n- propyl cyanide, and ethyl formate were reported for the first time in space based on this survey, as were five rare isotopologues of vinyl cyanide, cyanoacetylene, and hydrogen cyanide. We also report the detection of transitions from within twelve new vibrationally or torsionally excited states of known molecules. Absorption features produced by diffuse clouds along the line of sight are detected in transitions with low rotation quantum numbers of many simple molecules and are modeled with ~30–40 velocity components with typical linewidths of ~3–5 km s-1 . Conclusions. Although the large number of unidentified lines may still allow future identification of new molecules, we expect most of these lines to belong to vibrationally or torsionally excited states or to rare isotopologues of known molecules for which spectroscopic predictions are currently missing. Significant progress in extending the inventory of complex organic molecules in Sgr B2(N) and deriving tighter constraints on their location, origin, and abundance is expected in the near future thanks to an ongoing spectral line survey at 3 mm with ALMA in its cycles 0 and 1. The present single-dish survey will serve as a solid basis for the line identification and analysis of such an interferometric survey.

/pdf/complex-organic-molecules-in-the-interstellar-medium-iram-30-1xj4x1o7z6.pdf

Complex organic molecules in the interstellar medium: IRAM 30 m line survey of Sagittarius B2(N) and (M)

We report on the discovery of the methoxy radical (CH3O) toward the cold and dense core B1-b based on the observation, with the IRAM 30 m radio telescope, of several lines at 3 and 2 mm wavelengths. Besides this new molecular species we also report on the detection of many lines arising from methyl mercaptan (CH3SH), formic acid (HCOOH), propynal (HCCCHO), acetaldehyde (CH3CHO), dimethyl ether (CH3OCH3), methyl formate (CH3OCOH), and the formyl radical (HCO). The column density of all these species is 1012 cm–2, corresponding to abundances of 10–11. The similarity in abundances for all these species strongly suggest that they are formed on the surface of dust grains and ejected to the gas phase through non-thermal desorption processes, most likely cosmic rays or secondary photons. Nevertheless, laboratory experiments indicate that the CH3O isomer released to the gas phase is CH2OH rather than the methoxy one. Possible gas-phase formation routes to CH3O from OH and methanol are discussed.

https://iopscience.iop.org/article/10.1088/2041-8205/759/2/L43/pdf

Discovery of the Methoxy Radical, CH 3 O, toward B1: Dust Grain and Gas-phase Chemistry in Cold Dark Clouds

The class II masers of methanol are associated with the early stages of formation of high-mass stars. Modelling of these dense, dusty environments has demonstrated that pumping by infrared radiation can account for the observed masers. Collisions with other molecules in the ambient gas also play a significant role, but have not been well modelled in the past. Here we examine the effects on the maser models of newly available collision rate coefficients for methanol. The new collision data do not alter which transitions become masers in the models, but do influence their brightness and the conditions under which they switch on and off. At gas temperatures above 100 K the effects are broadly consistent with a reduction in the overall collision cross-section. This means, for example, that a slightly higher gas density than identified previously can account for most of the observed masers in W3(OH). We have also examined the effects of including more excited-state energy levels in the models, and find that these play a significant role only at dust temperatures above 300 K. An updated list of class II methanol maser candidates is presented.

/pdf/models-of-class-ii-methanol-masers-based-on-improved-2c394cgfic.pdf

Models of class II methanol masers based on improved molecular data

We present an analysis of the data from our Swedish-ESO Submillimetre Telescope molecular line survey in the 1.3 mm band of the N, M, and NW positions in the Sgr B2 molecular cloud. The line emissions from 42 molecular species, and some of their isotopomers, were analyzed assuming a single temperature and a homogeneous source. In cases where a source size much smaller than the antenna beam (23'') could be estimated, optical depth effects were also accounted for. In this way rotation temperatures, molecular column densities, and in several cases also source sizes, were determined. Observed and modeled intensities are presented in rotation diagrams. A few complex molecular species, NH2CHO, CH3CHO, C2H3CN, C2H5CN, and CH3OCHO, mainly in the N source, exhibit anomalously strong intensities in their intrinsically weak b- and c-type lines. We argue that this effect can hardly be explained by high optical depths alone, and therefore propose that the possibility of radiative pumping via the low-lying vibrational states of these molecules should be investigated as an alternative explanation. The highest rotation temperatures, up to about 500 K, were found for large molecules toward Sgr B2(N), closely followed by Sgr B2(M). In Sgr B2(NW), which samples the cloud envelope, the rotation temperatures are 15-50 K. For molecules with too few observed transitions to allow determination of rotation temperatures, the column density was calculated assuming optically thin emission, and we adopted rotation temperatures of 50, 50, and 20 K in M, N, and NW, respectively. Column density ratios of isotopomers were determined. After a critical discussion of the resolution-dependent H2 column density toward the observed positions, abundances relative to H2 were calculated. We discuss the chemical differences between the three observed cloud positions and compare with the hot core, compact ridge, and outflow in Orion A. Hot core-type molecules like CH2NH, NH2CN, CH3CN, C2H3CN, and C2H5CN, as well as H2CS, are more abundant in Sgr B2(N) by factors of 3-8 as compared to the M position. Large oxygen-containing species like CH3OH, CH3CHO, CH3OCHO, CH3OCH3, and NH2CHO, of compact ridge-type, show similar or slightly enhanced abundances in N as compared to M. The C2H5OH abundances are similar in N, M, and also in NW. The SO2 and SO abundances in the M core (4 × 10-7 and 1 × 10-7, respectively) are 13 and 5 times higher than in N and are very enhanced (103 and 102 times) as compared to NW. Such high SO2 and SO abundances are also found in the prominent Orion A outflow source. In M the SO18O and S18O data suggest a 16O/18O ratio of 120. The HOCO+ ion is detected in all three positions and appears to be 3 times more abundant in the NW position. HCNH+ is seen only in NW.

http://iopscience.iop.org/article/10.1086/313376/pdf

A Three-Position Spectral Line Survey of Sagittarius B2 between 218 and 263 GHZ. II. Data Analysis

The Millimeter- and Submillimeter-Wave Spectrum of Gauche-Ethyl Alcohol

The strong electric dipole-allowed transitions of the molecular ion CO(+) in the region 235-470 GHz have been measured in the laboratory. The laboratory spectra at 235 GHz appear to confirm the claim by Erickson et al. (1981) to have observed CO(+) in OMC-1.

Laboratory millimeter and submillimeter spectrum of CCH

Ethyl alcohol (ethanol) is known to possess a pair of closely spaced excited torsional substates (gauche+, gauche-) at an energy of approximately 57 K above the ground (trans) torsional substate. We report an extended analysis of some gauche - gauche+ Q-branch ((Delta)J = 0) transitions with a three-substate fixed frame axis method (FFAM) Hamiltonian. Our approach accounts for complex trans-gauche interactions for the first time. In addition, we are able to obtain intensities for perturbed rotational transitions, and to determine the trans to gauche+ separation to be 1185399.1 MHz. A complete ground state rotational-torsional partition function accounting for the previously neglected gauche substates is presented. Based on our analysis, a total of 14 U lines obtained towards Orion KL can now be assigned to gauche substates of ethanol. Analysis of these lines yields a rotational temperature of 223 K and a total (trans + gauche) column density of 7.0 x 10(exp 15)/sq cm. The column density is in reasonable agreement with the recent value of 2-3 x 10(exp 15)/sq cm based on observations of trans-ethanol by Ohishi et al., although there is some disparity in the rotational temperatures. Eight additional U lines in the literature are assigned to transitions of gauche ethanol.

http://iopscience.iop.org/article/10.1086/303967/pdf

Gauche ethyl alcohol: laboratory assignments and interstellar identification

Methyl formate (HCOOCH3), a threefold internal rotor, is a well-known interstellar molecule located in hot cores of giant molecular clouds. Over a decade ago, we published separate analyses of the rotational-torsional spectra of methyl formate in its A and E ground torsional substates. The analysis of the A substate was undertaken with a standard asymmetric top Hamiltonian, while that of the E substate was undertaken with a principal axis method (PAM), which was less than entirely satisfactory. In the interim, we have measured many new lines of the rotational-torsional spectrum of both the ground A and E torsional substates of this species through the rotational quantum number J = 50 using two spectrometers: our standard klystron-based system and a new fast scan spectrometer (FASSST). The newly measured lines have been combined with previous data to form a data set consisting of over 2000 lines that has been fitted to the accuracy of our experiments using a modification of the internal axis method (IAM) we have previously applied to methanol. The spectral constants obtained from the fit have allowed us to predict the frequencies of many additional lines up to 700 GHz in frequency and up to J = 50 in rotational quantum number.

/pdf/the-millimeter-and-submillimeter-wave-spectrum-of-methyl-18kig2qp3n.pdf

The Millimeter- and Submillimeter-wave Spectrum of Methyl Formate (HCOOCH3)

The carbon monosulfide species CS32 and CS34 are investigated with a millimeter wave spectrometer designed for the study of short‐lived molecules in the frequency range from 60 000 to 300 000 Mc/sec. Spectroscopic constants obtained are:           CS32          CS34B0=24 495.592±0.006 Mc/secB0=24 103.554±0.006 Mc/secBe=24 584.367±0.006 Mc/secDe=0.03979±0.0017 Mc/secαe=117.550±0.012 Mc/secHe=0.0625±0.009 kc/secDe=0.04285±0.00166 Mc/secHe=0.0653±0.0085 kc/sec The spectrometer is described. Its sensitivity has been estimated to be the order of 10—5 to 10—6 cm—1.

K. V. L. N. Sastry

Papers

The Millimeter- and Submillimeter-Wave Spectrum of Gauche-Ethyl Alcohol

Laboratory millimeter and submillimeter spectrum of CCH

Gauche ethyl alcohol: laboratory assignments and interstellar identification

The Millimeter- and Submillimeter-wave Spectrum of Methyl Formate (HCOOCH3)

Millimeter Wave Spectroscopy of Unstable Molecular Species. I. Carbon Monosulfide