Interstellar CH Research Articles

Distribution of hydroxyl clouds in the Galaxy

The distribution of molecular clouds in the Galaxy is investigated on the basis of a survey of the galactic plane in the hydroxyl lines at frequencies 1665 and 1667 MHz. The distances to the clouds are determined from the dependence D(r) of the parameters of the hydroxyl absorption lines on the heliocentric distance. Radial distributions of the molecular clouds in four galactocentric quadrants have been obtained. The observed concentration of the hydroxyl clouds into a molecular ring agrees with the radial distribution of the interstellar CO and CH molecules in the Galaxy. It is shown that the large-scale distribution of the cloud complexes can be described by a model of a two-arm spiral with pitch angles 6/degree/.3 and 5/degree/.4.

Detection of interstellar CH in the far-infrared

The first astronomical detection of CH in the far-infrared has been made. A ground state of rotational transition was observed in absorption against the far-infrared continuum peak of Sgr B2. The lines are resolved at a velocity resolution of 62 km/s, have a line width of roughly 250 km/s, and a line center optical depth of about 0.29. The inferred total column density of CH in the ground state along the line of sight is roughly 1.6 x 10 to the 15th/sq cm. Comparison of the far-infrared profiles to the 3 GHz emission lines confirms that the ground-state Lambda-doublet levels are inverted and gives an accurate estimate of the excitation temperature. The excitation temperature of the 3264 MHz line varies from cloud to cloud along the line of sight, the levels being most inverted in the Sgr B2 molecular cloud. The large intensity of the 3264 MHz line in this cloud relative to other clouds along the line of sight may thus be primarily an excitation effect.

Photodissociation processes in the CH molecule

The photodissociation processes in the CH molecule have been investigated using ab initio self-consistent-field with configuration-interaction (CI) methods. Potential energy curves of all states which dissociate into the lowest four atomic limits have been calculated, as well as those of several higher-lying states with Rydberg character. Transition dipole moments connecting the excited states with the lowest states have been obtained. Direct photodissociation of CH may occur by absorption into the continua of the B 2∑− and 2 2∑+ states, and cross sections for absorption from the X 2Π v″=0 state into these states are reported. Indirect photodissociation of CH may take place by absorption into bound states, which are subsequently dissociated through interactions with continuum states. The A 2Δ vibrational levels couple with the continuum of the X 2Π state through the nuclear rotational operator. The nuclear rotational coupling elements have been obtained from the CI wave functions, and predissociation rates are presented. The C 2∑+ v′=0 state is shown to be predissociated efficiently by spin-orbit coupling with the B 2∑− state. The 2, 3, and 4 2Π states of CH interact strongly through the radial nuclear kinetic energy operator. The nuclear radial coupling elements between these states have been computed by numerical differentiation of the CI wave functions. The coupled equations for the nuclear wave functions have been solved in a diabatic formulation. The calculated cross section for absorption into the coupled states is similar to that found recently for absorption into the 2 2Π and 3 2Π states of the OH molecule: it consists of a series of resonances with asymmetric Beutler–Fano profiles superimposed on a strong continuous background. The 3(F)2∑+ state, as well as the higher-lying (Rydberg) states, may interact with the 2Π states through rotational coupling or with the 2 4∑− state through spin-orbit coupling. Due to uncertainties in the coupling elements, the efficiencies with which these states are dissociated are not well determined, but they are probably small for most states. The photodissociation rate of CH by the unattenuated interstellar radiation field has been computed. The total rate is 9.5×10−10 s−1. The 2 2∑+ and coupled 2Π channels are most effective in the photodissociation of interstellar CH.

An optical measurement of the population inversion of the ground state Lambda doublet of interstellar CH

We have performed very high signal-to-noise ratio optical observations of interstellar CH toward three stars and have measured an inversion temperature of -1.30 +- 0.4 K for the ground-state ..lambda.. doublet levels leading to the 9 cm radio emission of CH toward zeta Per. This result is consistent with the inversion indicated by the radio measurements in this direction.

The C-12/C-13 isotope ratio toward Zeta Ophiuchi

Observations of interstellar CH(+) toward Zeta Oph were performed to determine the C-12/C-13 isotope ratio in this diffuse cloud. The very high signal-to-noise ratio spectra yield 6 sigma detections of the (C-13)H(+) features at 4232 A and 3957 A; a weighted mean C-12/C-13 ratio of 43 + or - 6 (1 sigma) is obtained. The uncertainty includes the contribution of continuum placement errors, statistical channel-to-channel signal fluctuations, and the error introduced in deconvolving the blended isotopic lines at 4232 A. This result indicates a decrease in the local galactic C-12/C-13 ratio by a factor of 2 during the 4.6 billion yr since the formation of the sun.

A reanalysis of the interstellar CH abundance

view Abstract Citations (58) References (52) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS A reanalysis of the interstellar CH abundance. Lien, D. J. Abstract An analysis of historic interstellar CH abundance and electronic state data for the interstellar medium was undertaken to identify the term structures of various electronic states and to identify errors in previous analyses. Attention was devoted to the X(2)Pi, A(2)Delta, B(2)Sigma(-), and C(2)Sigma(+) systems. Error sources were identified as neglect of the Lambda doubling of ground state. The splitting causes a doubling of the oscillator strength of lines from the B-X and C-X electronic systems, blending of two lines appearing as 4300, 3886, and 3143 A lines, the inaccurate identification of which half of the Lambda doublet generated the lines, and no accurate determination of the excitation temperature. Examples are furnished of CH Lambda ground state data from Delta Per, Delta Oph, X Per, and Chi Oph using equivalent widths to show an inverted Lambda doublet, that a radio maser can be detected optically, and a match between the X Per and Delta Per theoretical and measured temperatures. Publication: The Astrophysical Journal Pub Date: September 1984 DOI: 10.1086/162440 Bibcode: 1984ApJ...284..578L Keywords: Abundance; Hydrocarbons; Interstellar Matter; Molecular Spectra; Ultraviolet Astronomy; Line Spectra; Near Ultraviolet Radiation; Oscillator Strengths; Radio Sources (Astronomy); Astrophysics full text sources ADS | data products SIMBAD (4)

Resonant and nonresonant processes in the formation of CH(+) by radiative association

view Abstract Citations (36) References (19) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Resonant and nonresonant processes in the formation of CH(+) by radiative association Graff, M. M. ; Moseley, J. T. ; Roueff, E. Abstract New calculations of the radiative association rate coefficient (RARC) for the formation of CH(+) in the interstellar medium are presented. The method applied uses potential curves based on recent experimental data, takes the initial-population-mechanism and resonance-broadening effects of nonadiabatic processes into account, includes the contribution of the A 1-Pi quasi-bound levels between the two fine-structure dissociation limits, and applies the A 1-Pi-CH(+) radiative-lifetime measurements of Mahan and O'Keefe (1981). The RARC is found to be strongly dependent on temperature, rising to a maximum of about 80 x 10 to the -18th cu cm/sec at 30 K and descending monotonically at higher temperatures (calculated up to 1000 K). The low-temperature enhancement is attributed to resonant processes in the A 1-Pi state. These results are shown to partially account for the discrepancy between the observed and predicted abundance of interstellar CH(+). Publication: The Astrophysical Journal Pub Date: June 1983 DOI: 10.1086/161088 Bibcode: 1983ApJ...269..796G Keywords: Association Reactions; Free Radicals; Hydrocarbons; Interstellar Chemistry; Molecular Clouds; Electron States; Fine Structure; Ionizing Radiation; Reaction Kinetics; Temperature Dependence; Atomic and Molecular Physics full text sources ADS |

The radiative lifetime of the A1Π state of CH+ calculated from long CAS SCF expansions

Abstract A theoretical study of the A 1 Π-X 1 Σ − transition of CH + is presented. Radiative lifetimes (τ) for the A state and the f ν′ν -(A–X) oscillator strengths are calculated, using long mutually non-orthogonal CAS SCF expansions. The convergence of these properties is studied with respect to the size of the active orbital space and the basis set. It is found that very large active spaces and basis sets are needed for obtaining reliably convergent results. The best calculated value for τ(ν′ = 0) of 850 ns and for f 00 of 5.45 × 10 −3 are in good agreement with the results from the most recent experimental investigation, 815 ± 25 ns and (5.66±0.20) × 10 −3 respectively. In the calculation use has been made of experimental excitation energies and potential curves. Compared to the previously most accepted lifetime the present theoretical and the recent experimental values demand an increase of the interstellar CH + abundance.

Vibrationally excited molecular hydrogen in circumstellar clouds and the interstellar CH+ abundance

The production of CH+ in dense interstellar clouds under intense UV irradiation is discussed. A model applicable to the cloud towards the star 20 Tau is described.

Comparisons of interstellar CH[SUP]+[/SUP] and H2

Copernicus observations of H2 toward stars in the Pleiades and 23 Orionis have been obtained because of their importance in understanding the formation of interstellar CH(+). No model of CH(+) equilibrium seems to agree very well with these observations; in particular, the suggestions that CH(+) is formed either from collisions between vibrationally excited H2 and C(+) or from radiative association of C(+) and H2 can apparently be excluded. These data do suggest that there is a correlation between the amounts of rotationally excited H2 and CH(+) which are present. Observations of the Pleiades also help interpretations of the reflection nebulae near these stars.

Interstellar shocks and molecular CH/+/ in diffuse clouds

view Abstract Citations (144) References (45) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Interstellar shocks and molecular CH/+/ in diffuse clouds Elitzur, M. ; Watson, W. D. Abstract The influence of shocks on molecular abundances in diffuse interstellar clouds is investigated numerically, with emphasis on interstellar CH(+) abundances. The time development of the material behind a one-dimensional shock front under interstellar conditions is calculated, taking into account the effects of ram pressure and helium content, beginning at the end of the brief adiabatic compression phase of the shock by computing the radiative cooling of the gas at each point. Calculated abundances of the rotational states of postshock H2, as well as CH(+), CH, OH and H2O are presented, and the column density of CH(+) is approximated, together with line profiles for CH(+) and OH. Observations of the Zeta Ophiuchi clouds, including the velocity shift between CH(+) and the other species, are then accounted for in terms of the interstellar shock model, however it is noted that rather powerful shocks are required to produce the CH(+) column densities observed. Publication: The Astrophysical Journal Pub Date: February 1980 DOI: 10.1086/157730 Bibcode: 1980ApJ...236..172E Keywords: Hydrocarbons; Interstellar Gas; Molecular Ions; Shock Wave Propagation; Abundance; Chemical Reactions; Cooling; Gas Density; Molecular Rotation; Propagation Velocity; Astrophysics full text sources ADS |

Models of the Interstellar Medium

AbstractUp-to-date models of the interstellar medium should account for the existence of at least three phases of the interstellar gas, a hot phase at ˜ 106 K (HICM), a warm phase at ˜ 104 K (WICM) and a cold phase at ≤ 102 K (clouds). Recent observations of interstellar absorption lines are used to derive information about the physical properties and the spatial distribution of these phases. Evidence based on observations of C+ absorption lines indicates that the photoelectric threshold of interstellar dust grains is about 6 eV, much lower than previously thought. If the photoelectric threshold is indeed of this magnitude the WICM can easily be heated to temperatures in the range 6000 to 8000 K by photons of the general interstellar radiation field with λ > 912 Å. The analysis of interstellar CH+ and 0 VI absorption lines suggests that these lines are formed in gas associated with clouds in regions of enhanced pressure that occupy about 10 percent of interstellar space (SN cavities). The CH+ lines are formed in compressed WICM shells around clouds and the 0 VI lines in conductive interfaces between the WICM and the HICM. The velocity characteristics of the observed lines make shock waves a less probable source of the absorbing gases.

The photoionization and photodissociation of CH in the vicinity of the ionization threshold

The Stieltjes-imaging method is applied to the spectrum of CH which is computed by SCF–MO–CI calculations on single excitation series. The calculations use basis sets of up to 50 Slater atomic orbitals. The continuum cross sections obtained are lower than the previous results of Walker and Kelly who used a perturbation theoretical approach. Our computations indicate strong Rydberg series converging to the ground state and the excited states 3Π and 1Π of CH+. If these Rydberg states were predissociated then the dissociation rate of interstellar CH would be an order of magnitude higher than anticipated. We tentatively derive oscillator strengths for the individual states of the Rydberg series converging to 1Σ+ observed by Herzberg and Johns.

Inverting the ground state of interstellar CH

Inverting the ground state of interstellar CH

Radio Observations of Interstellar CH. II

view Abstract Citations (44) References (77) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Radio Observations of Interstellar CH. II Hjalmarson, A. ; Sume, A. ; Elider, J. ; Rydbeck, O. E. H. ; Moore, E. L. ; Huguenin, G. R. ; Sandqvist, Aa. ; Lindblad, P. O. ; Lindroos, P. Abstract The Onsala Space Observatory 25.6 m telescope, equipped with a traveling wave maser preamplifier, has been used to study emission in the three hyperfine transitions of the 211i/2 J = 1/2, A doublet state of interstellar CH. The main line (F = 1-1) has been detected toward more than 100 positions in optically dark nebulae. All three lines have been observed in many directions with relatively strong main line emission, including the Sharpless H ii region S159, the open cluster NGC 2264, and 3C 353. Observations on and off 3C 123, which lies behind Lynds's cloud Ll 500, indicate that the three CH transitions are probably inverted in dark dust clouds. The main line excitation temperature is found to be about -9 K in Ll 500. A statistical analysis of the data indicates that the antenna temperature increases linearly with the cloud opacity class, or the cloud density. An increase is also found for the CH column density, but it seems that the ratio between the CH and the total hydrogen column densities decreases for more opaque clouds. Available optical data agree well with the radio results. Subject headings: interstellar: matter - interstellar: molecules - nebulae: general Publication: The Astrophysical Journal Supplement Series Pub Date: 1977 DOI: 10.1086/190480 Bibcode: 1977ApJS...35..263H full text sources ADS |

Radio observations of interstellar CH. I

view Abstract Citations (108) References (296) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Radio observations of interstellar CH. I. Rydbeck, O. E. H. ; Kollberg, E. ; Hjalmarson, A. ; Sume, A. ; Ellder, J. ; Irvine, W. M. Abstract Results are presented for some eight months of almost continuous high-sensitivity observations of the three hyperfine transitions of the CH ground state in the directions to or in extended areas around more than 30 H I and H II regions, dark dust clouds, OH/IR stars, and other sources. The observations were made with a traveling-wave maser preamplifier on a 25.6-m telescope, giving a zenith-system noise temperature of about 34 K. Detailed discussions of the CH spectra are given in source descriptions and tables; radiative-transfer considerations and possible CH excitation mechanisms are included. Some major findings are that: (1) CH is widespread in the Galaxy; (2) the three CH transitions are observed in emission almost everywhere and exhibit weak maser characteristics; (3) satellite transitions are generally more inverted than the main line; (4) CH, OH, and H2CO seem to be well mixed in Orion-arm and Perseus-arm cloud configurations; (5) the average main-line excitation temperature is about -15 K; (6) the lower satellite transition seems to be a very sensitive far-IR indicator; (7) significant departures from equilibrium line-intensity ratios appear toward H I and H II regions; and (8) the relative abundance of CH with respect to other molecular species seems to diminish systematically from dilute H I clouds to the densest molecular clouds. Publication: The Astrophysical Journal Supplement Series Pub Date: July 1976 DOI: 10.1086/190385 Bibcode: 1976ApJS...31..333R Keywords: Hydrocarbons; Interstellar Matter; Microwave Spectra; Radio Sources (Astronomy); H Ii Regions; Hyperfine Structure; Interstellar Masers; Milky Way Galaxy; Astrophysics full text sources ADS |

Calculations of the lower electronic states of CH3+ - A postulated intermediate in interstellar reactions

view Abstract Citations (23) References (19) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Calculations of the lower electronic states of CH3+: a postulated intermediate in interstellar reactions. Blint, R. J. ; Marshall, R. F. ; Watson, W. D. Abstract Molecular-structure calculations are presented to estimate the photodissociation rate for CH3 + in the interstellar gas. For the average interstellar radiation field, we find that CH3 + probably has a lifetime against photodissociation greater than 1011 s. Thus, in the proposed formation sequence for interstellar CH + that is initiated by a fast C + + H2 radiative association, the postulated (though possibly not essential) rapid photodissociation of CH3 + is unlikely to occur. Restricted Hartree-Fock wave functions for the ground and lowest three singlet states of CH3 + are calculated at various molecular geometries. The energies of the states, the force constants for the ground state, and the oscillator strength of the first excitation are determined from these wave functions. In addition, zero-point energies of CH3 + and CH2D + are calculated for application in proposed reaction schemes that cause deuterium enhancement in interstellar molecules. Subject headings: interstellar: molecules - molecular processes Publication: The Astrophysical Journal Pub Date: June 1976 DOI: 10.1086/154420 Bibcode: 1976ApJ...206..627B Keywords: Deuterium; Electron States; Interstellar Gas; Ionic Reactions; Molecular Structure; Free Radicals; Hartree Approximation; Methyl Compounds; Molecular Ions; Photodissociation; Reaction Kinetics; Astrophysics full text sources ADS |

A new calculation of the radiative association rate of the CH+radical: comparison between semiclassical and quantal results (interstellar abundance)

Ab initio potential curves and internuclear-distance-dependent oscillator strengths of CH+ are used to calculate the radiative association rate coefficient of this radical. A semiclassical treatment is developed which takes into account such collisional effects as non-adiabatic transitions between molecular states and tunnelling through the centrifugal barrier. A quantal formulation of the emission process is given, which takes into account the effect of the centrifugal potential on the vibrational nuclear wavefunctions. The value obtained for the radiative association rate coefficient cannot explain the discrepancy between the observed and theoretically deduced abundances of the interstellar CH+ radical.

Line spectra in interstellar clouds. II - CH and CH+ in Ophiuchus

view Abstract Citations (38) References (20) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Line spectra in interstellar clouds. II. CH and CH+ in Ophiuchus. Chaffee, F. H., Jr. Abstract Observational evidence is presented that suggests that velocity differences of the order of 1 km s ' exist between interstellar CII and CH + in four stars in Ophiuchus. Such velocity stratification would be consistent with the formation mechanism proposed by Black and Dalgarno and by Watson in which CH is formed near the center of interstellar clouds where the density of molecular hydrogen, n(H2), is high, and CH + is formed in the outer regions where n(112) is negligible. The velocity difference between CII + and Ca ii in Oph reported by Hobbs is corroborated, but the interpretation that this implies separate clouds for interstellar molecular and atomic constituents is questioned. Subject headings: interstellar matter - nebulae, individual Publication: The Astrophysical Journal Pub Date: July 1975 DOI: 10.1086/153699 Bibcode: 1975ApJ...199..379C Keywords: Free Radicals; Interstellar Gas; Line Spectra; Molecular Spectra; Nebulae; Chemical Reactions; Data Reduction; Radial Velocity; Reaction Kinetics; Spectral Line Width; Stellar Motions; Tables (Data); Astrophysics full text sources ADS | data products SIMBAD (5) Related Materials (2) Part 1: 1974ApJ...189..427C Part 3: 1977ApJ...213..394C

Observations of interstellar CH and a study of its chemistry and excitation

Observations of interstellar CH and a study of its chemistry and excitation