A general all-atom force field for atomistic simulation of common organic molecules, inorganic small molecules, and polymers was developed using state-of-the-art ab initio and empirical parametrization techniques. The valence parameters and atomic partial charges were derived by fitting to ab initio data, and the van der Waals (vdW) parameters were derived by conducting MD simulations of molecular liquids and fitting the simulated cohesive energies and equilibrium densities to experimental data. The combined parametrization procedure significantly improves the quality of a general force field. Validation studies based on large number of isolated molecules, molecular liquids and molecular crystals, representing 28 molecular classes, show that the present force field enables accurate and simultaneous prediction of structural, conformational, vibrational, and thermophysical properties for a broad range of molecules in isolation and in condensed phases. Detailed results of the parametrization and validation f...

COMPASS: An ab Initio Force-Field Optimized for Condensed-Phase ApplicationsOverview with Details on Alkane and Benzene Compounds

The Cologne Database for Molecular Spectroscopy, CDMS: a useful tool for astronomers and spectroscopists

Understanding the formation of stars in galaxies is central to much of modern astrophysics. However, a quantitative prediction of the star formation rate and the initial distribution of stellar masses remains elusive. For several decades it has been thought that the star formation process is primarily controlled by the interplay between gravity and magnetostatic support, modulated by neutral-ion drift (known as ambipolar diffusion in astrophysics). Recently, however, both observational and numerical work has begun to suggest that supersonic turbulent flows rather than static magnetic fields control star formation. To some extent, this represents a return to ideas popular before the importance of magnetic fields to the interstellar gas was fully appreciated. This review gives a historical overview of the successes and problems of both the classical dynamical theory and the standard theory of magnetostatic support, from both observational and theoretical perspectives. The outline of a new theory relying on control by driven supersonic turbulence is then presented. Numerical models demonstrate that, although supersonic turbulence can provide global support, it nevertheless produces density enhancements that allow local collapse. Inefficient, isolated star formation is a hallmark of turbulent support, while efficient, clustered star formation occurs in its absence. The consequences of this theory are then explored for both local star formation and galactic-scale star formation. It suggests that individual star-forming cores are likely not quasistatic objects, but dynamically collapsing. Accretion onto these objects varies depending on the properties of the surrounding turbulent flow; numerical models agree with observations showing decreasing rates. The initial mass distribution of stars may also be determined by the turbulent flow. Molecular clouds appear to be transient objects forming and dissolving in the larger-scale turbulent flow, or else quickly collapsing into regions of violent star formation. Global star formation in galaxies appears to be controlled by the same balance between gravity and turbulence as small-scale star formation, although modulated by cooling and differential rotation. The dominant driving mechanism in star-forming regions of galaxies appears to be supernovae, while elsewhere coupling of rotation to the gas through magnetic fields or gravity may be important.

Control of star formation by supersonic turbulence

Of the over 150 different molecular species detected in the interstellar and circumstellar media, approximately 50 contain 6 or more atoms. These molecules, labeled complex by astronomers if not by chemists, all contain the element carbon and so can be called organic. In the interstellar medium, complex molecules are detected in the denser sources only. Although, with one exception, complex molecules have only been detected in the gas phase, there is strong evidence that they can be formed in ice mantles on interstellar grains. The nature of the gaseous complex species depends dramatically on the source where they are found: in cold, dense regions they tend to be unsaturated (hydrogen-poor) and exotic, whereas in young stellar objects, they tend to be quite saturated (hydrogen-rich) and terrestrial in nature. Based on both their spectra and chemistry, complex molecules are excellent probes of the physical conditions and history of the sources where they reside. Because they are detected in young stellar objects, complex molecules are expected to be common ingredients for new planetary systems. In this review, we discuss both the observation and chemistry of complex molecules in assorted interstellar regions in the Milky Way.

/pdf/complex-organic-interstellar-molecules-sz3r2nq94z.pdf

Complex Organic Interstellar Molecules

/pdf/control-of-star-formation-by-supersonic-turbulence-7wf7lt35b2.pdf

Control of Star Formation by Supersonic Turbulence

Survey observations of CCS (J N = 4 3 -3 2 , J N = 2 1 -1 0 ), HC 3 N (J = 5-4), HC 5 N (J = 9-8, J = 17-16), and NH 3 (J, K = 1, 1) were carried out toward 49 dark cloud cores to examine the existence of a systematic relation between the chemical evolution and the physical evolution of dark clouds. The J N = 3 3 -2 2 and J N = 3 4 -2 3 lines of CCS and the J = 4-3 line of C 3 S were also observed in several cores

A survey of CCS, HC3N, HC5N, and NH3 toward dark cloud cores and their production chemistry

We have determined the abundances of HCN and HNC toward 19 nearby dark cloud cores by observations of optically thin H13CN (J = 1-0) and HN13C (J = 1-0) lines. The column density of HCN is found to be correlated with that of HNC. The abundance ratio of [HNC]/[HCN] is determined to be 0.54-4.5 in the observed dark cloud cores. These results are consistent with the idea that HCN and HNC are produced mainly by a recombination reaction of HCNH+ with electrons in dark cloud cores. Furthermore, the [HNC]/[HCN] ratio does not show any significant differences between star-forming cores and starless cores. The HCN and HNC abundances are compared with those for the OMC-1 cores previously reported. Although the abundances of HCN in the OMC-1 cores are comparable to those in the dark cloud cores, the abundances of HNC in OMC-1 are 1-2 orders of magnitude less than those in dark cloud cores. It is suggested that HNC is destroyed by neutral-neutral reactions in high kinetic temperature regions.

https://iopscience.iop.org/article/10.1086/306032/pdf

Abundances of HCN and HNC in dark cloud cores

ASTE (Atacama Submillimeter Telescope Experiment) is a project to install and operate a 10-m submillimeter telescope in the high altitude site (4,800 m) in Atacama desert, northern Chile. The project is aimed to explore the southern sky with submillimeter waves as well as to develop and evaluate various instruments and observing techniques. The telescope was shipped and re-assembled in Chilean site in early 2002, including the establishment of the on site infrastructure. Following evaluation of the telescope and receivers, scientific observations such as supernova remnants, galaxies, star forming regions and proto-planetary nebulae, have been carried out since early 2003. The high-precision 10-m antenna was measured to have the surface accuracy of 18.9 mm and the relative pointing accuracy was 1.2" r.m.s. for both azimuth and elevation. The subreflector is equipped with wobbling capability. Several types of receivers have been on board the telescope; the heterodyne-receivers operating at 100, 230, 345, 500 and 800 GHz bands including cartridge-type receivers, as well as a bolometer system covering 350, 650 and 850 GHz. The spectrometer is equipped with an XF type digital auto-correlator with four channels each covering up to 512 MHz with 1024 bins, which leads to 2 GHz coverage. The control system is designed to be capable of remote control from several sites via network connection, from the base facility at San Pedro de Atacama (2,400 m altitude) or even from Japan.

The Atacama Submillimeter Telescope Experiment (ASTE)

We have detected the high-excitation lines of carbon-chain molecules such as C4H2 (J ¼ 100;10Y90;9), C4 H( N ¼ 9Y8, F1, F2), l-C3H2 (41,3Y31,2), and CH3CCH (J ¼ 5Y4, K ¼ 2) toward a low-mass star-forming region, L1527. In particular, the F1 line of C4 Hi s as strong as 1.7 K( TMB). The rotational temperature of C4H2 is determined to be 12:3 � 0:8 K, which is higher than that in TMC-1 (3.8 K). Furthermore, the column density of C4H2 is derived to be about 1/4 of that in TMC-1, indicating that carbon-chain molecules are abundant in L1527 for a star-forming region. Small mapping observations show that the C4H, C4H2, and c-C3H2 emissions are distributed from the infalling envelope to the inner part. Furthermore, we have detected the lines of C5H, HC7N, and HC9N in the 20 GHz region. Sincethecarbon-chainmolecules aregenerallydeficientinstar-forming cores, theaboveresultscannot simply beexplained by the existing chemical models. The following hypothesis is proposed. If the timescale of the prestellar collapse in L1527 were shorter than those of the other star-forming cores, the carbon-chain molecules could survive in thecentralpartofthecore.Inaddition,regenerationprocessesofthecarbon-chainmoleculesduetostarformationactivities would play an important role. Evaporation of CH4 from the grain mantles would drive the regeneration processes.Thepresentobservationsshow newchemistryinawarmanddenseregionneartheprotostars,whichisnamed ‘‘warm carbon-chain chemistry (WCCC).’’ Subject headingg ISM: abundances — ISM: individual (L1527) — ISM: molecules — stars: formation

/pdf/abundant-carbon-chain-molecules-toward-the-low-mass-3sx3t43i6s.pdf

Abundant Carbon-Chain Molecules toward the Low-Mass Protostar IRAS 04368+2557 in L1527

We completed the first spectral line survey toward a cold, dark cloud TMC-1 (cyanopolyyne peak) in a frequency range between 8.8 and 50.0 GHz by the 45-m radio telescope of Nobeyama Radio Observatory. We detected 414 lines of 38 molecular species including 11 new molecules, such as C6H, CCO, C3O, CCS, C3S, HNCCC, HCCNC, HC3NH + , HCCCHO, CH2CN, and cyclic C3H (c-C3H), which were previously reported; only one significant line, U49536, remains unidentified. 177 out of 414 lines are reported for the first time. Some half of the detected molecules are linear carbon-chain species and their derivatives. We also discovered three new rarer isotopomers: CC34S, C3S, and HDCS. Spectral charts are presented together with the observed line parameters of all detected lines. Improved molecular constants of several carbon-chain molecules obtained during the course of identification are also presented.

Satoshi Yamamoto

Papers

A survey of CCS, HC3N, HC5N, and NH3 toward dark cloud cores and their production chemistry

Abundances of HCN and HNC in dark cloud cores

The Atacama Submillimeter Telescope Experiment (ASTE)

Abundant Carbon-Chain Molecules toward the Low-Mass Protostar IRAS 04368+2557 in L1527

A 8.8-50 GHz Complete Spectral Line Survey toward TMC-1: I. Survey Data.