Transitions Of CS Research Articles

A 2000M☉Rotating Molecular Disk around NGC 6334A

We present millimeter and centimeter wave spectroscopic observations of the H II region NGC 6334A. We have mapped the source in several transitions of CO, CS, and NH3. The molecular emission shows a distinct flattened structure in the east-west direction. This structure is probably a thick molecular disk or torus (2.2 × 0.9 pc) responsible for the bipolarity of the near-infrared (NIR) and radio continuum emission which extends in two lobes to the north and south of the shell-like H II region. The molecular disk is rotating from west to east (ω ≈ 2.4 km s-1 pc-1) about an axis approximately parallel to the radio and NIR emission lobes. By assuming virial equilibrium, we find that the molecular disk contains ~2000 M☉. Single-component gas excitation model calculations show that the molecular gas in the disk is warmer and denser (Tk ≈ 60 K, n ≈ 3000 cm-3) than the gas to the north and south (Tk ≈ 50 K, n ≈ 400 cm-3). High resolution (~5'') NH3 (3, 3) images of NGC 6334A reveal several small (~0.1 pc) clumps, one of which lies southwest of the radio continuum shell, and is spatially coincident with a near-infrared source, IRS 20. A second NH3 clump is coincident with an H2O maser and the center of a molecular outflow. The dense gas tracers, CS J = 5 → 4 and 7 → 6, peak near IRS 20 and the H2O maser, not at NGC 6334A. IRS 20 has a substantial far-infrared (FIR) luminosity LFIR ~ 105 L☉, which indicates the presence of an O 7.5 star but has no detected radio continuum (F6 cm < 0.02 Jy). The combination of dense gas, a large FIR luminosity and a lack of radio continuum can best be explained if IRS 20 is a protostar. A third clump of NH3 emission lies to the west of IRS 20 but is not associated with any other molecular or continuum features. The star formation activity in the region has moved west of NGC 6334A to IRS 20 and the H2O maser position. We suggest that NGC 6334A, IRS 20, and the H2O maser spot are part of a protocluster of stars which is condensing from the massive molecular disk. The similarity between the structure around NGC 6334A and other large (r ~ 1 pc), massive (M ~ 103 M☉), rotating disks (K3-50A and G10.6-0.4) suggests that this may be a common mechanism by which open clusters form.

Dense Gas and Star Formation: Characteristics of Cloud Cores Associated with Water Masers

We have observed 150 regions of massive star formation, selected originally by the presence of a water maser, in the J = 5-4, 3-2, and 2-1 transitions of CS, and 49 regions in the same transitions of C$^{34}$S. Over 90% of the 150 regions were detected in the J = 2-1 and 3-2 transitions of CS and 75% were detected in the J=5-4 transition. We have combined the data with the J = 7-6 data from our original survey (Plume et al. 1992) to determine the density by analyzing the excitation of the rotational levels. Using Large Velocity Gradient (LVG) models, we have determined densities and column densities for 71 of these regions. The gas densities are very high (the mean log of the density is 5.9), but much less than the critical density of the J=7-6 line. Small maps of 25 of the sources in the J = 5-4 line yield a mean diameter of 1.0 pc. The mean virial mass is 3800 solar masses. The mean ratio of bolometric luminosity to virial mass (L/M) is 190, about 50 times higher than estimates using CO emission, suggesting that star formation is much more efficient in the dense gas probed in this study. The gas depletion time for the dense gas is roughly 1.3 x 10^7 yr. We find no statistically significant linewidth--size or density--size relationships in our data. Instead, both linewidth and density are larger for a given size than would be predicted by the usual relationships. We find that the linewidth increases with density, the opposite of what would be predicted by the usual arguments. We estimate that the luminosity of our Galaxy (excluding the inner 400 pc) in the CS J = 5-4 transition is 15 to 23 L_sun, considerably less than the luminosity in this line within the central 100 pc of NGC 253 and M82. In addition, the ratio of far-infrared luminosity to CS luminosity is higher in M82 than in any cloud in our sample.

Racemization Barriers of Helicenes: A Computational Study1

The racemization barriers of pentahelicene up to nonahelicene have been computed with AM1, MNDO, and PM3. All methods lead to Cs transition states which have lower energy than those with C2v symmetry. The barriers calculated by AM1 match the experimental values best for all helicenes. The reliability of the results has been confirmed by ab initio methods using the B3LYP functional with the 3-21G basis set as implemented in the GAUSSIAN94 package. Furthermore, 12 methyl-substituted helicenes have been computed with the AM1 method. The racemization barriers of 1-methyl-substituted penta- and hexahelicene are at least as high as that of the next higher unsubstituted helicene. A second methyl group in the 1‘-position increases the barrier further, while methyl groups in the 2-position do not have a severe influence on the racemization barrier.

Pulsed-nozzle Fourier-transform microwave investigation of the large-amplitude motions in HBr–CO2

Microwave spectra of H79Br–CO2 and H81Br–CO2 and their D and 18O isotopomers have been measured using a pulsed-nozzle Fourier-transform microwave spectrometer. The spectra are consistent with a T-shaped Br–CO2 geometry, as concluded previously by Zeng et al. [Y. P. Zeng, S. W. Sharpe, S. K. Shin, C. Wittig, and R. A. Beaudet, J. Chem. Phys. 97, 5392 (1992)] from an investigation of the rotationally resolved infrared spectrum of the asymmetric C=O stretching vibration of the complex. Only b-type Ka=1←0 transitions are observed, with the symmetry-allowed a-type ΔKa=0 transitions being too weak to be detected. The absence of a strong a-type spectrum implies that the HBr axis is nearly parallel to the b-inertial axis of the complex, which itself is parallel to the C∞ axis of the CO2. The Ka=1←0 energy level spacing is approximately 1.2 GHz larger than that predicted from the infrared rotational constants due to an additional contribution to the splitting arising from the hindered-rotation tunneling of the HBr through a Cs or C2v transition state. Because the Bose–Einstein statistics of the spin-zero oxygen nuclei allow only symmetric tunneling states for Ka even and antisymmetric tunneling states for Ka odd, no doubling of the lines is observed. No evidence was obtained for this tunneling motion in the infrared spectrum of Zeng et al., since the tunneling state selection rules are symmetric↔symmetric and antisymmetric↔antisymmetric for the band studied. A dynamical modeling of the 79Br and 81Br nuclear quadrupole coupling constants gives an equilibrium ∠CBrH angle of ∼103° and an HBr zero-point bending amplitude of ∼24°. The implication of this study on the interpretation of experiments on the photoinitiated reaction of H atoms with CO2 using an HBr–CO2 precursor are discussed.

Modeling Line Profiles of Protostellar Collapse in B335 with the Monte Carlo Method

view Abstract Citations (141) References (24) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Modeling Line Profiles of Protostellar Collapse in B335 with the Monte Carlo Method Choi, Minho ; Evans, Neal J., II ; Gregersen, Erik M. ; Wang, Yangsheng Abstract A collapsing dark cloud core, B335, is modeled as an inside-out collapse. The radiative transfer code uses the Monte Carlo method and new collision rates for CS. Line profiles for several observed transitions of CS and H2CO are computed. We confirm that a double-peaked spectrum with a stronger blue peak is a good qualitative signature of infall. Compared to the models using the large velocity gradient method by Zhou et al., our best-fit model using the Monte Carlo method has 20% smaller infall radius, 70% larger CS abundance, and 30% larger H2CO abundance. Also, infall radii found from CS and H2CO agree better than they did in the modeling by Zhou et al. Our model also produces off-center spectra close to the observations. Publication: The Astrophysical Journal Pub Date: August 1995 DOI: 10.1086/176002 Bibcode: 1995ApJ...448..742C Keywords: HYDRODYNAMICS; ISM: GLOBULES; ISM: INDIVIDUAL ALPHANUMERIC: B335; ISM: MOLECULES; METHODS: NUMERICAL; STARS: FORMATION full text sources ADS | data products SIMBAD (1)

OBSERVATIONS OF PHOTOEVAPORATING INTERSTELLAR CLOUDS

ABSTRACT In the first part of this thesis, we present a tabulation of parameters for 50 globules, most of which are bright-rimmed clouds, observed in transitions of 12CO, 13CO, CS, and HCO+ with a 1 arcminute diameter beam. H-alpha images for many bright-rimmed clouds and long-slit H-alpha spectra of two such clouds are presented. The observations are interpreted in terms of the rocket effect and D-critical ionization fronts. The elongated clouds (the so called elephant trunk globules and cometary clouds) tend to point toward their source of ionization, although a possible counter-example is presented. They often have velocity gradients along their lengths; a few examples are newly presented along with others collected from the literature. Mechanisms for forming elongated clouds are discussed. Also, means of determining the three-dimensional geometry of an H II region are discussed. The hypothesis that bright rims implode clouds and induce star formation is reviewed and found to be controversial. In the second part, we present images of the Orion Trapezium made with a unique adaptive optics system that uses either starlight or Rayleigh-backscattered laser light to correct for atmospheric wavefront distortion. The H-alpha and I band images reveal the region around the bright Trapezium stars, which has been studied extensively at radio wavelengths. Approximately one half of the stars in this region are positionally associated with knots of ionized gas, known as pigs, which are interpreted as photoevaporating envelopes of low mass stars. Disks are not observed directly but are inferred from the relatively low extinction to the stars inside the pigs. The comet-like morphology of the pigs is the result of an equilibrium between photoionization, recombination, and shadowing; interaction with the wind from theta-1C ~Ori is not required. The sizes of the ionized component of the envelopes increase with distance from the source of ionization, theta-1C Ori. The size-distance relationship suggests that the mass loss rates from the pigs are all approximately the same. The actual value of the mass loss rate is model dependent, but is ~1.2 x 10-7 M⊙ yr-1. In the third and last part, we extend the laser beacon concept to a polychromatic beacon in order to infer the wavefront tilt by measuring the differential tilt caused by dispersion. The general concept of a polychromatic beacon is to make the atmosphere produce a second beacon of a different color at altitude. Two mechanisms capable of producing the color conversion are Raman scattering in the troposphere, and resonant florescence in the mesosphere. Because the differential tilt is quite small, high precision is required and thus also high photon return rates. Our analysis indicates that existing lasers may satisfy the theoretical requirements; however, the installation and operation of such lasers may be prohibitively expensive using current technology.

CS multitransitional study of density distribution in star-forming regions. 2: The S140 region

The S140 molecular cloud was observed in five transitions of CS with resolutions of 11 to 45 arcsec. The data were analyzed with both the LVG and microturbulent models of radiative transfer to derive the density structure. It was found that the CS emission comes from three components of gas: a spherical component centered on the infrared cluster, an arc component along the ionization front between the S140 H II region and the dense molecular cloud core, and a high-velocity component from the dense part of a molecular outflow. The spherical component contributes most to the CS emission and was analyzed in more detail than the other components. Using a temperature distribution derived from an analysis of the dust emission from S140, we fit a power-law density distribution of n(r) = n(sub i)(r/r(sub i))(exp -alpha) to the spherical component. The best fit was for n(sub i) = 1.4 x 10(exp 6) (density at r(sub i) = 0.026 pc) and alpha = 0.8. The density (n(sub i)) was found to be greater than or equal to the density required to account for the dust emission, depending on the dust opacity laws adopted. The presence of optical emission (Dinerstein, Lester, & Rank 1979) suggests a clumpy structure for the dense gas. Considerations of the virial mass and the lowest amount of column density required to produce dust emission put the volume filling factor (f(sub nu)) of the dense gas at approximately 0.14-0.5. We compared S140 with other regions of star formation where the density structure has been derived from excitation analysis. Source-source variations in density gradients and clumpiness clearly exist, ranging from alpha = 2 and f(sub nu) approximately 1 in B335 to alpha approximately 0, f(sub nu) approximately 0.1 in M17. There is a tendency for more massive star-forming regions to have a flatter density distribution, a more clumpy structure, and a large number of young stars. The implications of this tendency are discussed.

Structure of Dense Cores in M17 SW. I. A Multitransition CS and C 34S Study

view Abstract Citations (47) References (51) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Structure of Dense Cores in M17 SW. I. A Multitransition CS and C 34S Study Wang, Yangsheng ; Jaffe, Daniel T. ; Evans, Neal J., II ; Hayashi, Masahiko ; Tatematsu, Ken'ichi ; Zhou, Shudong Abstract We present results of a multitransition CS and C34S study of the M17 SW molecular cloud core. Fully sampled maps were obtained in the CS J = 1 → 0, 2 → 1, and 7 → 6 lines and the C34S J = 2 → 1 line with 1 8"-36" resolution. Velocity channel maps reveal the clumpy emission from the dense gas on scales of ∼0.2 pc (20"). The overall agreement in the cloud morphology among maps of different CS and C34S transitions suggests that all CS and C34S lines originate in the same dense gas. Excitation and opacity effects probably cause the modest differences between the maps. We carried out a detailed excitation analysis of the multitransition data. The J = 2 → 1 and J = 7 → 6 transitions of CS, analyzed with a large velocity gradient (LVG) radiative transfer model, produced 250 pixel maps of the volume density and the CS column density over an area of about 1.8 pc × 2.4 pc. Peaks in the CS and the C34S line temperature maps are maxima in column density, but not in density. The density maps shows a fairly uniform, high density (n ∼105.7 cm-3) throughout the cloud core. An independent estimate of the gas densities from analysis of the C34S observations confirms the CS results. Along with other evidence, these results imply a clumpy cloud model in which the CS emission arises from structures smaller than our beam. We compared the observed CS maps with a specific clumpy cloud model with 179 clumps decomposed from the C18O J = 2 → 1 maps (Stutzki & Güsten 1990). Model channel maps of CS were synthesized based on the clump parameters listed in Stutzki & Güsten (1990) and were compared with the observed maps. The gas densities used in the models were derived from the clump column densities (based on the C18O J = 2 → 1 emission) and sizes. Most of the dominant clumps had densities near 105 cm-3. The resulting synthesized map does not reproduce the observed CS J = 7 → 6 emission along the eastern ridge of the core. By assuming a constant gas density for all clumps, we were able to synthesize CS channel maps which reproduce the observed cloud morphology and the line intensities reasonably well. A mean clump density in the models of about 5 × 105 cm-3 (about 5 times higher than the density derived from C18O) matches the observed CS line ratios and a CS/H2 abundance ratio of about 4 × 10-9 fits the observed line intensities of the J = 1 → 0, 2 → 1, and 7 → 6 transitions of CS. The discrepancy between densities derived from C18O and CS can be resolved if the clumps have internal density structure. Either smooth density gradients in clumps with sizes just below our angular resolution or a continuation of high-contrast clumping to still smaller scales could account for the difference. While we cannot rule out either of these pictures, it is noteworthy that the scale of the C18O observations (∼0.15 pc) is the largest for which the density discrepancy can be resolved with smooth density gradients in unresolved clumps. Publication: The Astrophysical Journal Pub Date: December 1993 DOI: 10.1086/173521 Bibcode: 1993ApJ...419..707W Keywords: ISM: H II REGIONS; ISM: INDIVIDUAL MESSIER NUMBER: M17; ISM: MOLECULES; ISM: STRUCTURE; RADIO LINES: ISM full text sources ADS | data products SIMBAD (2)

The excitation and kinematics of DR21(OH) from observations of CS

We have mapped the young star formation region DR21(OH) in the J=2-1 and 1-0 transitions of CS and C 34 S, and have obtained spectra with high signal-to-noise ratios towards the three main continuum sources within the core. We find that the CS emission corresponds closely with the thermal continuum, and that all three continuum sources are well separated in velocity as well as spatially. We suggest that wing emission in the CS lines is associated with an outflow originating from DR21(OH)Main, which also excites the observed CH 3 OH masers. Our CS line profiles show clear self-absorption redshifted by about 0.5 km s −1 relative to the rest velocity of the cloud. In order to account for this feature, we also suggest that there is residual contraction of the outer, less dense, parts of the cloud

Embedded star clusters associated with luminous IRAS point sources

A 16 arcmin 2 region toward each of 20 bright IRAS sources in the second and third quadrants of the Galaxy have been imaged at J, H, and K bands and mapped in the CS(J=2-1) transition. The presence of one or two OB stars associated with each of these IRAS sources had been previously inferred from radio continuum observations, and the near-infrared images have revealed that a cluster of stars has formed in at least 19 of the 20 regions. These results strongly suggest that high-mass stars almost always form in clusters and not in isolation. The number of observed cluster members brighter than m H =15.5 mag vary between 15 and 91 stars

Evidence for protostellar collapse in B335

We have observed five rotational transitions of H2CO and CS toward the Bok globule, B335, with high spatial and spectral resolution. The characteristic shape of the observed profiles provides direct, kinematic evidence of collapse. In addition, we have modeled line profiles of collapsing dense cores with density and velocity structures taken from the theory of Shu and coworkers. Using the age of collapse as the only free parameter, we found that the strengths and profiles of the observed lines can be well fitted by the theoretical model. Our best-fit model gives an age of 1.5 x 10 exp 5 yr, corresponding to an infall radius of 0.04 pc and a total mass of 0.4 solar mass for the central star and disk. Outside the infall radius, there is a static envelope with a r exp -2 density distribution, an average temperature of 13 K, and a turbulent velocity (1/e width) of 0.12 km/s. The CS abundance is 3.6 x 10 exp -9 with about 30 percent uncertainty.

A source model for the L134N molecular cloud

The dark molecular cloud L134N is observed at millimeter wavelengths in the CS (J = 3-2), OCS (J = 7-6), and HDO (1 sub 11-1 sub 10) transitions. The CS (J = 3-2) transition was observed at four positions within the cloud, while the other two transitions were observed at one position each. Fractional abundances in the LTE approximation are calculated for each emission line detected. L134N appears to have a high-density core characterized by NH3, C3H2, and H(C-13)O(+) emission maps. A lower density envelope characterized by C(0-18), CS (J = 2-1), and SO emission surrounds the core. There appears to be a gas-phase oxygen abundance gradient in L134N with atomic oxygen depleted in the high-density core. Observed molecular distributions within L134N can be explained by a model in which chemical and physical processes in icy-dust-grain mantles influence the gas-phase molecular abundances.

The predissociation of CS 2 at 210 nm and the spectra of photofragment CS

The 1B 2( 1Σ + u) state of CS 2 at 210 nm was studied using jet-cooled laser-induced fluorescence and dispersed spectra. The lifetime of the 1B 2( 1Σ + u) state at 210 nm was deduced from the transition linewidth to be 4.4 ps. Hot bands of photofragment CS (A 1Π←X 1Σ +) transitions up to v″ = 6 were observed indicating large vibrational excitation occurring in the photodissociation of CS 2. The fluorescence yield from the Σ g 0 band of the 1B 2 state, located at 47562 cm −1, was observed from 205 to 550 nm. The fluorescence with wavelength shorter than the excitation wavelength results from the photofragment CS ( v″ = 1, 2, 3) absorbing a photon sequentially in the photodissociation of CS 2. The dispersed pattern exhibits four intensity maxima which result from a complex mixture of states at 210 nm excitation.

Physical parameters in extragalactic star forming regions

To use a molecule as a diagnostic of a single star forming region, one has to observe it in different transitions. That is especially true for galaxies, where the beam filling factor is small and unknown. From studies of Galactic objects, the CS molecule is a good tracer of dense gas. Toward the IR galaxies NGC 253, IC 342 and M 82, Mauersberger and Henkel (1989) observed the J=2—1, 3—2 and 5—4 transitions of CS at 3, 2 and 1.3 mm wavelength, repectively. They also measured emission of the isotope C34S, which has an abundance ∼ 1/23 that of the main isotope. This study revealed that the H2 densities in the circumnuclear gas exceed 104, and, in the case of NGC 253, 105 cm−3. In all three cases, the distribution of the dense gas traced out by CS follows the CO distribution.

Physical parameters in extragalactic star forming regions

To use a molecule as a diagnostic of a single star forming region, one has to observe it in different transitions. That is especially true for galaxies, where the beam filling factor is small and unknown. From studies of Galactic objects, the CS molecule is a good tracer of dense gas. Toward the IR galaxies NGC 253, IC 342 and M 82, Mauersberger and Henkel (1989) observed the J=2—1, 3—2 and 5—4 transitions of CS at 3, 2 and 1.3 mm wavelength, repectively. They also measured emission of the isotope C34S, which has an abundance ∼ 1/23 that of the main isotope. This study revealed that the H2 densities in the circumnuclear gas exceed 104, and, in the case of NGC 253, 105 cm−3. In all three cases, the distribution of the dense gas traced out by CS follows the CO distribution.

Density and kinematics of the W49A cloud core

The dense core of the W49A molecular cloud (Miyawaki et al. 1986, Schloerb et al. 1987) has been mapped in 5 different transitions of CS and C34S, in order to determine its density structure. The three lower frequency transitions (CS J=3-2, CS J=5-4 and C34S J=5-4) were observed with the IRAM 30m telescope, and the two highest frequency transitions (CS J=7-6 and C34S J=7-6) with the Caltech Submillimeter Observatory. The beamsizes were in the range 12″ to 20″. As a calibration check, the CS J=7-6 line was observed with both telescopes, and was found to give a consistent temperature scale. The spectra at the peak of the emission are shown in Fig. 1.

Density and kinematics of the W49A cloud core

The dense core of the W49A molecular cloud (Miyawaki et al. 1986, Schloerb et al. 1987) has been mapped in 5 different transitions of CS and C34S, in order to determine its density structure. The three lower frequency transitions (CS J=3-2, CS J=5-4 and C34S J=5-4) were observed with the IRAM 30m telescope, and the two highest frequency transitions (CS J=7-6 and C34S J=7-6) with the Caltech Submillimeter Observatory. The beamsizes were in the range 12″ to 20″. As a calibration check, the CS J=7-6 line was observed with both telescopes, and was found to give a consistent temperature scale. The spectra at the peak of the emission are shown in Fig. 1.

Rotational spectrum of the CCS radical studied by laboratory microwave spectroscopy and radio-astronomical observations

The rotational spectral lines of the CCS radical and its isotopic species, CC(S-34), (C-13)CS, and C(C-13)S, have been observed in the laboratory, and the J(N) = 4(3)-3(2), J(N) = 2(1)-1(0), and J(N) = 3(4)-2(3) transitions for CCS and that of J(N) = 4(3)-3(2) for CC(S-34) have been observed toward a cold dark cloud, L1498. The molecular constants of CCS, CC(S-34), (C-13)CS, and C(C-13)S have been determined from the observed transition frequencies, and the rest frequencies of CCS and CC(S-34) below 300 GHz are listed with their line strengths. The frequencies of the low-N transitions of (C-13)CS and C(C-13)S are calculated for future astronomical observations. 19 refs.

Experimental progress using nonlinear optics for precision measurements of the nuclear weak charge in the 6S–7S Cs transition

We report on the current status of our novel experiment designed for precision measurements of parity violation in the forbidden 6S–7S Cs transition. Pulsed 6S–7S excitation in a longitudinal E -field gives rise to large 7S–6P population inversion and subsequent transient amplification of a probe beam, as large as 100%. Dependence on the relative orientation of the excitation and probe polarizations is measured with a sensitive differential polarimeter. We present a calibration procedure circumventing all complications from nonlinear processes, which can thus be exploited to enhance the parity violating asymmetry.

Faraday communications. A pulsed crossed supersonic molecular beam study of the reaction C(3P)+ OCS(X1Σ+)→ CS(a3Πr, X1Σ+)+ CO(X1Σ+)

The C + OCS → CS + CO reaction has been investigated in a pulsed crossed supersonic molecular beam experiment at a centre-of-mass kinetic energy of 0.32 eV. CS radicals, produced in both the a 3Πr and X 1Σ+ states, have been detected by laser-induced fluorescence (LIF). A newly observed CS transition, A 1Π–a 3Πr, is also reported. Comparison of relative LIF intensities following CS(a 3Πr) and CS(X 1Σ+) excitation, shows that the reaction pathway producing CS(a 3Πr)+ CO(X 1Σ+) is highly efficient.