Characterization of the Coriolis Coupled Far-Infrared Bands of syn-Vinyl Alcohol.

Context. The diffuse and translucent molecular clouds traced in absorption along the line of sight to strong background sources have so far been investigated mainly in the spectral domain because of limited angular resolution or small sizes of the background sources. Aims. We aim to resolve and investigate the spatial structure of molecular clouds traced by several molecules detected in absorption along the line of sight to Sgr B2(N). Methods. We have used spectral line data from the EMoCA survey performed with the Atacama Large Millimeter/submillimeter Array (ALMA), taking advantage of its high sensitivity and angular resolution. The velocity structure across the field of view is investigated by automatically fitting synthetic spectra to the detected absorption features, which allows us to decompose them into individual clouds located in the Galactic centre (GC) region and in spiral arms along the line of sight. We compute opacity maps for all detected molecules. We investigated the spatial and kinematical structure of the individual clouds with statistical methods and perform a principal component analysis to search for correlations between the detected molecules. To investigate the nature of the molecular clouds along the line of sight to Sgr B2, we also used archival Mopra data. Results. We identify, on the basis of c-C3H2, 15 main velocity components along the line of sight to Sgr B2(N) and several components associated with the envelope of Sgr B2 itself. The c-C3H2 column densities reveal two categories of clouds. Clouds in Category I (3 kpc arm, 4 kpc arm, and some GC clouds) have smaller c-C3H2 column densities, smaller linewidths, and smaller widths of their column density PDFs than clouds in Category II (Scutum arm, Sgr arm, and other GC clouds). We derive opacity maps for the following molecules: c-C3H2, H13CO+, 13CO, HNC and its isotopologue HN13C, HC15N, CS and its isotopologues C34S and 13CS, SiO, SO, and CH3OH. These maps reveal that most molecules trace relatively homogeneous structures that are more extended than the field of view defined by the background continuum emission (about 15′′, that is 0.08–0.6 pc depending on the distance). SO and SiO show more complex structures with smaller clumps of size ~5–8′′. Our analysis suggests that the driving of the turbulence is mainly solenoidal in the investigated clouds. Conclusions. On the basis of HCO+, we conclude that most line-of-sight clouds towards Sgr B2 are translucent, including all clouds where complex organic molecules were recently detected. We also conclude that CCH and CH are good probes of H2 in both diffuse and translucent clouds, while HCO+ and c-C3H2 in translucent clouds depart from the correlations with H2 found in diffuse clouds.

We report the discovery of widespread maser emission in non-metastable inversion transitions of NH3 toward various parts of the Sagittarius B2 molecular cloud and star-forming region complex. We detect masers in the J, K = (6, 3), (7,4), (8,5), (9,6), and (10,7) transitions toward Sgr B2(M) and Sgr B2(N), an NH3 (6,3) maser in Sgr B2(NS), and NH3 (7,4), (9,6), and (10,7) masers in Sgr B2(S). With the high angular resolution data of the Karl G. Jansky Very Large Array (JVLA) in the A-configuration, we identify 18 maser spots. Nine maser spots arise from Sgr B2(N), one from Sgr B2(NS), five from Sgr B2(M), and three in Sgr B2(S). Compared to our Effelsberg single-dish data, the JVLA data indicate no missing flux. The detected maser spots are not resolved by our JVLA observations. Lower limits to the brightness temperature are > 3000 K and reach up to several 105 K, manifesting the lines’ maser nature. In view of the masers’ velocity differences with respect to adjacent hot molecular cores and/or UCH II regions, it is argued that all the measured ammonia maser lines may be associated with shocks caused either by outflows or by the expansion of UCH II regions. Overall, Sgr B2 is unique in that it allows us to measure many NH3 masers simultaneously, which may be essential in order to elucidate their thus far poorly understood origin and excitation.

Context. The interstellar detections of isocyanic acid (HNCO), methyl isocyanate (CH3NCO), and very recently also ethyl isocyanate (C2H5NCO) invite the question of whether or not vinyl isocyanate (C2H3NCO) can be detected in the interstellar medium. There are only low-frequency spectroscopic data (<40 GHz) available for this species in the literature, which makes predictions at higher frequencies rather uncertain, which in turn hampers searches for this molecule in space using millimeter (mm) wave astronomy. Aims. The aim of the present study is on one hand to extend the laboratory rotational spectrum of vinyl isocyanate to the mm wave region and on the other to search, for the first time, for its presence in the high-mass star-forming region Sgr B2, where other isocyanates and a plethora of complex organic molecules are observed. Methods. We recorded the pure rotational spectrum of vinyl isocyanate in the frequency regions 127.5–218 and 285–330 GHz using the Prague mm wave spectrometer. The spectral analysis was supported by high-level quantum-chemical calculations. On the astronomy side, we assumed local thermodynamic equilibrium to compute synthetic spectra of vinyl isocyanate and to search for it in the ReMoCA survey performed with the Atacama Large Millimeter/submillimeter Array (ALMA) toward the high-mass star-forming protocluster Sgr B2(N). Additionally, we searched for the related molecule ethyl isocyanate in the same source. Results. Accurate values for the rotational and centrifugal distortion constants are reported for the ground vibrational states of trans and cis vinyl isocyanate from the analysis of more than 1000 transitions. We report nondetections of vinyl and ethyl isocyanate toward the main hot core of Sgr B2(N). We find that vinyl and ethyl isocyanate are at least 11 and 3 times less abundant than methyl isocyanate in this source, respectively. Conclusions. Although the precise formation mechanism of interstellar methyl isocyanate itself remains uncertain, we infer from existing astrochemical models that our observational upper limit for the CH3NCO:C2H5NCO ratio in Sgr B2(N) is consistent with ethyl isocyanate being formed on dust grains via the abstraction or photodissociation of an H atom from methyl isocyanate, followed by the addition of a methyl radical. The dominance of such a process for ethyl isocyanate production, combined with the absence of an analogous mechanism for vinyl isocyanate, would indicate that the ratio C2H3NCO:C2H5NCO should be less than unity. Even though vinyl isocyanate was not detected toward Sgr B2(N), the results of this work represent a significant improvement on previous low-frequency studies and will help the astronomical community to continue searching for this species in the Universe.

During the last quarter century, chemists have responded magnificently to the challenges raised by astronomers in their attempts to understand the variety of molecules detected in interstellar clouds. Observations have shown the chemistry of these regions to be surprisingly complex, and now more than 100 molecular species have been identified in interstellar and circumstellar regions of the Galaxy. The chemistry of interstellar clouds that gives rise to these molecules is now believed to be reasonably well understood (see section 2) in terms of a network of some thousands of binary reactions between several hundred species.1,2 Astronomers are now applying the techniques of astrochemistry to interpret observations of star-forming regions.3 These regions are much more complex in physical terms than quiescent interstellar clouds. In starforming regions, interstellar gas is being compressed, the force of gravity overcoming the resistance provided by gas pressure, magnetohydro-dynamic (MHD) turbulence, magnetic pressure, and rotation. The chemistry is not in steady state during this collapse, and can therefore be used as a tracer of the evolution of the collapse. In addition, the chemistry modifies and controls the collapse through the provision of molecular coolants of the gas, and by determining the fractional ionization in the gas. It is this ionization that affects the level of magnetic and turbulent support available to the cloud. Molecular rotational emissions at millimeter and submillimeter wavelengths are both the main cooling processes and the most effective probes of these regions. Interstellar gas in the Galaxy is observed to be distributed in an irregular fashion, in clouds of a range of sizes. Much of the mass is encompassed in so-called giant molecular clouds (GMCs) which range in mass from about 104 to about 106 solar masses (the solar mass is about 2 × 1030 kg), and have linear extents of several hundred light years (a light year, ly, is about 1 × 1016 m). The gas in GMCs is largely H2, but because that molecule has no dipole moment, the material is most effectively traced in the 1-0 rotational emission of CO, the next most abundant molecule (CO/H2 = 10-4 by number). Isotopomers of CO are also used. The CO emission identifies cold gas of number density ∼103 H2 molecules cm-3. A detailed study4 of one particular GMC, the Rosette molecular cloud (RMC), shows that it contains almost 2 × 105 solar masses of gas, extending over 100 ly. The gas in the RMC is fragmented into about 70 clumps with masses ranging from a few tens to a few thousands of solar masses. The clumps are embedded in a more tenuous medium, typically contain 102-103 H2 molecules cm-3, and are cool (j30 K). Observations show that clumps with larger column densities of CO (J1016 CO molecules cm-2) are more likely to contain embedded stars. Therefore, clumps satisfying this criterion are likely to be the sites of star formation in the RMC. Collapse of a clump leads to fragmentation and the formation of a cluster of dense cores. Carbon monoxide (12C16O) is not an effective tracer of gas in dense cores because the CO lines are optically thick and CO level populations are thermalized at lower densities. However, species of lower abundance than 12C16O can trace the dense gas in cores (J104 H2 molecules cm-3), and they include NH3, CN, H2CO, and CS. A typical core cluster5 is illustrated in Figure 1. It is a contour map in intensity of 1-0 rotational emission from the minor isotopic species 12C18O. This core cluster contains cores which may evolve to form new stars. Several stars have already formed and are detected as infrared sources (IRS 1-4). A primary goal of astrophysics is the detailed study of the collapse of a dense core to form a star. It is also important to gain an understanding of how gravity overcomes the various resistances to collapse, and how a young star interacts with its environment through the stellar winds and jets that develop at the earliest stages of the star’s existence. The answers to these questions are certainly contained in the emissions from the molecules and dust present in the collapsing core. Identifying a collapsing core is observationally difficult. The indicators should be molecular lines that are broadened by the infalling velocities. In this Account, we describe how the search for the infall signature has led to a recognition that the interaction of gas and dust in the infalling gas produces profound changes to the chemistry and physics of star-forming regions. This interaction is poorly understood, and the nature of the star-forming process will

The cyclic evolution of a multimodal interstellar grain population and its chemical, morphological and optical properties are described in terms of dust in molecular and diffuse clouds. The temperatures characteristic of the “large” tenth micron core-mantle particles, which dominate the mass of the dust and the extinction in the visual, are calculated. For typical situations the tenth micron particle temperatures range from 6 K to 15 K in cool molecular clouds and are about 16 K in diffuse clouds. The small (hundredth micron) hump particles and the very small FUV particles (large molecules) are not only warmer but, because of temperature fluctuations, emit radiation at much shorter wavelengths than the tenth micron particles. The model predicts that the relative proportion of small to large particles is larger in diffuse clouds than in molecular clouds so that the mid infrared emission in diffuse clouds is far higher than in molecular clouds. The core-mantle particles are significantly cooler than the graphite-silicate mixture and the emission peak is 70μm higher.KeywordsMolecular CloudDust EmissionInterstellar DustExtinction CurveDust ModelThese keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.

We have used the Odin submillimetre-wave satellite telescope to observe the ground state transitions of ortho-ammonia and ortho-water, including their 15N, 18O, and 17O isotopologues, towards Sgr B2. The extensive simultaneous velocity coverage of the observations, >500 km/s, ensures that we can probe the conditions of both the warm, dense gas of the molecular cloud Sgr B2 near the Galactic centre, and the more diffuse gas in the Galactic disk clouds along the line-of-sight. We present ground-state NH3 absorption in seven distinct velocity features along the line-of-sight towards Sgr B2. We find a nearly linear correlation between the column densities of NH3 and CS, and a square-root relation to N2H+. The ammonia abundance in these diffuse Galactic disk clouds is estimated to be about (0.5-1)e-8, similar to that observed for diffuse clouds in the outer Galaxy. On the basis of the detection of H218O absorption in the 3 kpc arm, and the absence of such a feature in the H217O spectrum, we conclude that the water abundance is around 1e-7, compared to ~1e-8 for NH3. The Sgr B2 molecular cloud itself is seen in absorption in NH3, 15NH3, H2O, H218O, and H217O, with emission superimposed on the absorption in the main isotopologues. The non-LTE excitation of NH3 in the environment of Sgr B2 can be explained without invoking an unusually hot (500 K) molecular layer. A hot layer is similarly not required to explain the line profiles of the 1_{1,0}-1_{0,1} transition from H2O and its isotopologues. The relatively weak 15NH3 absorption in the Sgr B2 molecular cloud indicates a high [14N/15N] isotopic ratio >600. The abundance ratio of H218O and H217O is found to be relatively low, 2.5--3. These results together indicate that the dominant nucleosynthesis process in the Galactic centre is CNO hydrogen burning.

Context.Thioformamide NH2CHS is a sulfur-bearing analog of formamide NH2CHO. The latter was detected in the interstellar medium back in the 1970s. Most of the sulfur-containing molecules detected in the interstellar medium are analogs of corresponding oxygen-containing compounds. Therefore, thioformamide is an interesting candidate for a search in the interstellar medium.Aims.A previous study of the rotational spectrum of thioformamide was limited to frequencies below 70 GHz and to transitions withJ ≤ 3. The aim of this study is to provide accurate spectroscopic parameters and rotational transition frequencies for thioformamide to enable astronomical searches for this molecule using radio telescope arrays at millimeter wavelengths.Methods.The rotational spectrum of thioformamide was measured and analyzed in the frequency range 150−660 GHz using the Lille spectrometer. We searched for thioformamide toward the high-mass star-forming region Sagittarius (Sgr) B2(N) using the ReMoCA spectral line survey carried out with the Atacama Large Millimeter/submillimeter Array.Results.Accurate rigid rotor and centrifugal distortion constants were obtained from the analysis of the ground state of parent,34S,13C, and15N singly substituted isotopic species of thioformamide. In addition, for the parent isotopolog, the lowest two excited vibrational states,v12 = 1 andv9 = 1, were analyzed using a model that takes Coriolis coupling into account. Thioformamide was not detected toward the hot cores Sgr B2(N1S) and Sgr B2(N2). The sensitive upper limits indicate that thioformamide is nearly three orders of magnitude at least less abundant than formamide. This is markedly different from methanethiol, which is only about two orders of magnitude less abundant than methanol in both sources.Conclusions.The different behavior shown by methanethiol versus thioformamide may be caused by the preferential formation of the latter (on grains) at late times and low temperatures, when CS abundances are depressed. This reduces the thioformamide-to-formamide ratio, because the HCS radical is not as readily available under these conditions.

A review is given of low-energy cosmic rays (1 MeV-10 GeV), which play an important role in the physics and chemistry of interstellar medium of our Galaxy. According to the generally accepted theory of star formation, cosmic rays penetrate into molecular clouds and ionize the dense gaseous medium of star formation centers besides due to a process of ambipolar diffusion they establish a star formation time scale of about 100-1000 thousand years. The source of cosmic rays in the Galaxy are supernovae remnants where diffusion acceleration at the shock front accelerates particles up to energies of 1015 eV. Being the main source of ionization in the inner regions of molecular clouds, cosmic rays play a fundamental role in the global chemistry of clouds, triggering the entire chain of ion-molecular reactions that make it possible to obtain basic molecules. The review also noted the importance of cosmic rays in atmospheric chemistry: playing a significant role in the formation of nitric oxide, especially with an increase in the flux, they cause a decrease in the concentration of ozone in the atmosphere with all climatic consequences.

The abundances of polyatomic molecules that may be formed by CH3(+) radiative association reactions in dense interstellar molecular clouds are reevaluated. The formation of a number of complex interstellar molecules via radiative association reactions involving ionic precursors other than CH3(+) is also investigated; these additional precursors include CH3O(+), CH3CO(+), CH5(+), HCO(+), NO(+), H2CN(+), C2H2(+), and NH3(+). The results indicate that the postulated gas-phase ion-molecule radiative association reactions could potentially explain the synthesis of most of the more complex species observed in dense molecular clouds such as Sgr B2. It is concluded, however, that in order to be conclusive, laboratory data are needed to show whether or not these reactions proceed at the required rates at low temperatures.

Microwave spectrum, molecular structure, and force field of HBO

Twenty-seven rotational lines of C3H2 have been identified in the laboratory or in astronomical sources, and the rotational and centrifugal distortion constants of this previously unobserved carbene ring determined to high accuracy. The assigned astronomical transitions include the strong, ubiquitous interstellar lines at 85,338 MHz and 18,343 MHz, which are the lowest lying transitions of ortho C3H2:2(12) to 1(01) and 1(10) to 1(01), respectively. Interstellar C3H2 can be rapidly formed by dissociative recombination of the very stable ion C3H3(+), which in turn can be produced from acetylene in only two stars. In standard molecular sources such as Ori A and Sgr B2, C3H2 is only moderately abundant, but in diffuse molecular clouds it may be one of the most abundant molecules. There is some radio spectroscopic evidence for two related molecules in Sgr B2 or TMC-1: ethynylmethylene HCCCH, a hypothetical carbon chain isomer, and cyclopropene, C3H4, a known, stable three-membered ring.

The ground state rotational spectra of 80Se and 78Se species of the hexadeutero dimethyl selenide have been measured in the region from 5 to 40 GHz. In both the cases, rotational and centrifugal distortion constants have been determined by a least square fit to about thirty transition frequencies. For the (CD3)2 80Se molecule, fourteen rotational transitions in the excited torsional states ṽ = l1, and ṽ = l2 were also recorded, out of which nine appeared as well resolved triplets. The potential barrier parameter V3 and the angle a between one of the ‘top axes’ and the ‘b axis’ have been determined by a least square fit of the mean value of the observed splittings in the ṽ = l1 and l2 states. The methyl top moment of inertia Iα was kept fixed at 6.35 amu Å2 , which is half of the observed inertia defect in the molecule.

Microwave spectrum of trans 3-fluorophenol in excited torsional states

We have investigated the gaseous and solid state molecular composition of dense interstellar material that periodically experiences processing in the shock waves associated with ongoing star formation. Our motivation is to confront these models with the stringent abundance constraints on CO2, H2O and O2, in both gas and solid phases, that have been set by ISO and SWAS. We also compare our results with the chemical composition of dark molecular clouds as determined by ground-based telescopes. Beginning with the simplest possible model needed to study molecular cloud gas-grain chemistry, we only include additional processes where they are clearly required to satisfy one or more of the ISO-SWAS constraints. When CO, N2 and atoms of N, C and S are efficiently desorbed from grains, a chemical quasi-steady-state develops after about one million years. We find that accretion of CO2 and H2O cannot explain the ISO observations; as with previous models, accretion and reaction of oxygen atoms are necessary although a high O atom abundance can still be derived from the CO that remains in the gas. The observational constraints on solid and gaseous molecular oxygen are both met in this model. However, we find that we cannot explain the lowest abundances seen by SWAS or the highest atomic carbon abundances found in molecular clouds; additional chemical processes are required and possible candidates are given. One prediction of models of this type is that there should be some regions of molecular clouds which contain high gas phase abundances of H2O, O2 and NO. A further consequence, we find, is that interstellar grain mantles could be rich in NH2OH and NO2. The search for these regions, as well as NH2OH and NO2 in ices and in hot cores, is an important further test of this scenario. The model can give good agreement with observations of simple molecules in dark molecular clouds such as TMC-1 and L134N. Despite the fact that S atoms are assumed to be continously desorbed from grain surfaces, we find that the sulphur chemistry independently experiences an "accretion catastrophe" . The S-bearing molecular abundances cease to lie within the observed range after about years and this indicates that there may be at least two efficient surface desorption mechanisms operating in dark clouds -one quasi-continous and the other operating more sporadically on this time-scale. We suggest that mantle removal on short time-scales is mediated by clump dynamics, and by the effects of star formation on longer time-scales. The applicability of this type of dynamical-chemical model for molecular cloud evolution is discussed and comparison is made with other models of dark cloud chemistry.

Relatively high abundances of methyl isocyanate (CH3NCO), a methyl derivative of isocyanic acid (HNCO), found in the Orion KL and Sgr B2 molecular clouds suggest that its ethyl derivative, ethyl isocyanate (CH3CH2NCO), may also be present. The aim of this work is to provide accurate experimental frequencies of ethyl isocyanate in its ground and excited vibrational states in the millimeter wave region to support searches for it in the interstellar medium. The rotational spectrum of ethyl isocyanate was recorded at room temperature from 80 to 340 GHz using the millimeter wave spectrometer in Valladolid. Assigned rotational transitions were analyzed using the S -reduced semirigid-rotor Hamiltonian. More than 1100 distinct frequency lines were analyzed for the ground vibrational state of the cis conformer as well as for three vibrational satellites corresponding to successive excitation of the lowest-energy C-N torsional mode. Newly determined rotational and centrifugal distortion constants were used for searches of spectral features of ethyl isocyanate in Orion KL and Sgr B2 clouds. Upper limits to CH3CH2NCO in these high-mass star-forming regions were obtained.