Dark Dust Research Articles

The C3H2 2(20)-2(11) transition: absorption in cold dark clouds.

The first observations of the 2(20)-2(11) transition of cyclopropenylidene (C3H2) at 21.6 GHz are described. In all cases where it has been detected, the line appears in absorption, showing that this transition is "refrigerated" (i.e., Tex < 2.7 K) in cold dust clouds. These results are compared with those for the 1(10)-1(01) transition of C3H2, and the consequences for the excitation of C3H2 qualitatively discussed.

Hyperion: 13-day rotation from Voyager data

The rotation period of Hyperion is determined here on the basis of a light-curve derived from low-resolution Voyager images taken over a two-month span. The data indicate a coherent 13-day spin period over 61 days, with the spin axis nearly parallel to the orbital plane. Since Hyperion is not in synchronous rotation, it cannot be used to test the hypothesis that dark dust spiralling inward on retrograde orbits causes the pronounced leading side/trailing side albedo asymmetry observed on the surface of Iapetus.

Spiral structure and star formation. I - Formation mechanisms and mean free paths

The wide variety of optical appearances observed in spiral galaxies has encouraged the growth of two theoretical approaches to explain the spiral patterns exhibited by such young objects as OB associations, giant H II regions, and dark dust lanes. These approaches are related to the density wave theory of spiral structure and 'stochastic, self-propagating star formation'. Levinson and Roberts (1981) tried to reconcile these two theoretical approaches, and considered a disk filled with discrete gas clouds. The present investigation is concerned with refinements and extensions of the model of Levinson and Roberts. Attention is given to gravitational forces and dynamical propagation, cloud-cloud collisions, supernova interactions, computational models, a theory concerning the interactions of the gas cloud and stellar association systems, the time evolution of the gas cloud-stellar association systems, and aspects of collisionally triggered star formation.

Surface materials on unusual planetary object chiron

We present the first JHK near-infrared colorimetry giving some indication of the surface materials on the peculiar object 2060 Chiron. The colors appear to exclude relatively clean bright ice surfaces, such as those of Europa, Ganymede, Rhea, Dione, and theoretically studied micronscale clean ice particles. The colors are consistent with those of numerous outer solar system asteroids, which in all measured cases have: albedos of only a few percent, C−, RD−, or DM-type spectra, and no known H 2O ice absorption features. The colors are also consistent with theoretical colors for certain size distributions of dirty ice grains. These and additional VJHK colorimetric data suggest that the spectrally dominant surface is C-type material, probably dark carbonaceous-like silicate dust, with a possible admixture of ice grains on a microscale. If Chiron has the low albedo common to such material on known interplanetary bodies, its diameter would be about 310 to 400 km, placing Chiron among the eight largest “asteroids”. Relationships to comet observations are discussed.

A high-resolution study of the gas flow in barred spirals

The considered investigation is concerned with conditions in that type of spiral galaxy which is known as a 'barred spiral'. The lack of star formation and the smoothness of the dark dust lanes may indicate that the bar region is pervaded by a more homogeneous interstellar medium with a lower viscosity than that characteristic of the outer regions. The effects of such a reduced viscosity are investigated on the basis of high-resolution time-dependent gasdynamical equations. Calculations modeling the gas flow in a given bar potential are presented. The calculations were performed on grids with various mesh sizes and consequently had different resolutions and numerical viscosities. A reduction in the numerical diffusion and viscosity was accompanied by changes in the flow pattern, including the appearance of a strong shock in the bar region and the occurrence of postshock outflow.

Optical and Theoretical Studies of Giant Clouds in Spiral Galaxies

An optical study of four spiral galaxies, combined with radiative transfer models for transmitted and scattered light, has led to a determination of the opacities and masses of numerous dark patches and dust lanes that outline spiral structure. The observed compression factors for the spiral-like dust lanes are in accord with our expectations from the theory of gas flow in spiral density waves. Several low density (102 cm-3) clouds containing 106 to 107 M⊙ were also studied. We discuss these results in terms of recent theoretical models of cloud and star formation in spiral galaxies. The long-term evolution of giant molecular clouds is shown to have important consequences for the positions and ages of star formation sites in spiral arms.

The Core of a Quiescent Cloud, L183

Simultaneous observation of the J,K=1,1 and 2,2 inversion transitions of ammonia (NH3) with high spatial resolution (≲1 arc min) offers a powerful method of probing the core region of interstellar clouds for evidence of molecular clumping and of prevailing physical conditions which could lead to star formation. We have therefore used the Effelsberg 100-m radiotelescope to make an extensive study of the central region of the nearby dark dust cloud L183 (also known as L134N) in the NH3 (1,1) transition; the spatial resolution was 40 arcsec. The core region as mapped in the NH3 (1,1) transition with a velocity resolution of 0.08 km s-1 consists of two elongated condensations separated by about 2 arcmin in north-south direction (see Fig. 1). The central part of the NH3 cloud has an approximate dimension of 6′ (N-S) by 2′ (E-W) corresponding to a linear extent of 0.17 × 0.06 pc at an assumed distance of 100 pc. The measured velocity structure of the NH3 cloud seems to reflect the double peaked nature of the cloud in that it increases from 2.30 km s-1 in the south to about 2.5 km s-1 at the northern end of the southern NH3 peak, and then decreases again to 2.3 km s-1 towards the north. The intrinsic linewidths of NH3 (corrected for hyperfine blending) do not vary significantly with position and are between 0.2 and 0.3 km s-1. The two ammonia peaks are part of a central molecular ridge from which we have observed NH3 (2,2) emission at 9 positions (see Fig. 1). The rotation temperature T21 as determined from the optical depths of the (1,1) and (2,2) transitions is ∼9K for all positions, and hence the kinetic temperature Tkin seems close to this value as well, i.e. ∼10K throughout the central part of L183. A more detailed account is being publsihed elsewhere (Ungerechts, Walmsley and Winnewisser).

Gas dynamics in barred spirals - Gaseous density waves and galactic shocks

Steady-state gasdynamical studies, previously limited to tightly wound normal spiral galaxies, are extended to models of barred spirals with a 5% to 10% perturbing potential. The models show that a strong wave manifestation is an important constituent of the bar structure in many barred spirals and that a density-wave shock wave can form a bar structure as pronounced as the narrow bars often evident in optical photographs of barred spirals. The dark narrow dust lanes often observed along the leading edges of bar structures are identified as tracers of shocks, and it is found that strong shocks along a bar structure during even a small part of a galaxy's lifetime might easily deplete a large enough proportion of the gas to cause a lack of gas in the inner annuli encompassing the bar by the time of the present epoch. It is emphasized that even moderate-amplitude barlike perturbations in the disk can drive large noncircular gas motions, typically 50 to 150 km/s.

Radio observations of interstellar CN toward diffuse clouds, dark clouds, black clouds, and circumstellar clouds

Emission at 113 GHz from the CN molecule has been searched for in a large number of interstellar regions, primarily dark dust clouds. Lines have been detected in four dark clouds, the first time CN has been observed in this type of object. Comparative CO observations were also performed. The CN/CO abundance ratio varies from cloud to cloud, even among objects which are apparently otherwise similar. This variation suggests that the chemistry of dark clouds may be time-dependent. A previously reported detection of CN emission from a diffuse cloud was not confirmed. Several black clouds and circumstellar clouds were reobserved to obtain better line parameters and to serve as comparative interstellar chemical systems.

Observations of strongly deuterated molecules - Implications for interstellar chemistry

view Abstract Citations (37) References (16) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Observations of strongly deuterated molecules: implications for interstellar chemistry. Turner, B. E. ; Zuckerman, B. Abstract The J = 1 - 0 transitions of DCO(+), N2D(+), DCN, and DNC in 13 interstellar clouds has been surveyed. In the dark dust clouds L63 and L134N, the apparent ratio DCO(+)/HCO(+) exceeds unity. While the apparent ratio DNC/HNC is of order unity in these cluds, N2D(+)/N2H(+) is significantly smaller. In other sources outside the galactic center, DCO(+)/HCO(+) and DNC/HNC are typically a few hundredths and somewhat larger than DCN/HCN. None of these deuterated species is detected in the galactic-center region. These results are analyzed in terms of current ion-molecule reaction schemes. Possible differences between DCO(+)/HCO(+) and N2D(+)/N2H(+) are considered and used to place restrictions on physical conditions and abundances in deuterated sources. Predictions are made for NH2D/NH3 and HDO/H2O and compared with recent observations. Ion-molecule chemistry, as at present understood, appears consistent with existing observations of deuterated interstellar molecules. Publication: The Astrophysical Journal Pub Date: October 1978 DOI: 10.1086/182797 Bibcode: 1978ApJ...225L..75T Keywords: Cosmic Dust; Deuterium Compounds; Galactic Nuclei; Interstellar Chemistry; Abundance; Ionic Reactions; Molecular Interactions; Reaction Kinetics; Astrophysics; Deuterium:Interstellar Clouds; Interstellar Clouds:Molecules full text sources ADS |

Radiation transport and non-LTE analysis of interstellar molecular lines. II - Carbon monosulfide

view Abstract Citations (24) References (11) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Radiation transport and non-LTE analysis of interstellar molecular lines. II. Carbon monosulfide. Liszt, H. S. ; Leung, C. M. Abstract We present numerical solutions to the coupled equations of radiative transfer and statistical equilibrium in the rotational transitions of carbon monosulfide (CS), similar to those in our earlier paper on carbon monoxide. Static, spherical, and microturbulent models are constructed over a wide range in density, abundance, and temperature, and the results are compared with observations of massive clouds near H ii regions and of dark dust clouds and globules. The optical depths found for CS are quite small in all cases, typically < 2, and the J = 1 0 line is found to be a weak maser in the rarer isotopic species in some denser clouds. No single value of the abundance fits all massive clouds, but typical central densities and relative abundances are 5 x l0 < < 2 x l0 , 2 x 10-10 < nos/ < 2 x l0- . Mean densities are a factor 5-10 lower because strong density gradients are required to reproduce the small spatial extent of the observed CS emission. The observations are dominated by instrumental effects, and it is difficult to determine whether source-to-source abundance variations actually exist. In the four globules which show emission in the two lowest transitions, ncs! 5-10 >c 10-10 and 2 5-20 x l0 . The observational results for L134 and the Taurus dark cloud are greatly discrepant and cannot be modeled for any reasonable choice of parameters. Subject headings: interstellar: abundances - interstellar: molecules Publication: The Astrophysical Journal Pub Date: December 1977 DOI: 10.1086/155693 Bibcode: 1977ApJ...218..396L Keywords: Abundance; Carbon Compounds; Interstellar Matter; Molecular Spectra; Radiative Transfer; Spectrum Analysis; Sulfides; Electron Transitions; Hydrogen Clouds; Molecular Rotation; Thermodynamic Equilibrium; Astrophysics full text sources ADS |

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 |

Radiation transport and non-LTE analysis of interstellar molecular lines. I - Carbon monoxide

We present numerical solutions to the coupled equations of radiative transfer and statistical equilibrium in the 10 lowest rotational transitions of carbon monoxide. Theoretical models are constructed for giant molecular clouds associated with H II regions and for simple dark dust clouds, using static, spherical models and a microturbulent velocity field. Results are presented for a range of CO abundances and optical depths. Available observational evidence is summarized and detailed comparisons are made in terms of intensity, shape, and width of the computed line profiles. Our calculations reproduce successfully a large body of observations of /sup 12/CO and /sup 13/CO in the j=1--0 and j=2--1 transitions, as long as the relative abundance of /sup 12/CO to molecular hydrogen (/sup 12/CO)/(H/sub 2/)approx. = (1--4) x 10/sup -5/. Consequently, the /sup 12/CO optical depths in the j=1--0 line are smaller than previously assumed, by a factor of 10, giving tauapprox. = (1--4). A non-LTE analysis of the excitation mechanism is made and the importance of line interlocking at small optical depths is discussed. Enhancement of the /sup 13/CO line under non-LTE conditions and large saturation effects in the /sup 12/CO line (even at tauapproximately-greater-than1) help to reduce the intensity ratio of /supmore » 12/CO//sup 13/CO in the j=1--0 transition to the observed value of 4, even with an isotopic ratio of approx.40. Other implications of these results are discussed. (AIP)« less

Studies of the dusty H II region NGC 2024

view Abstract Citations (8) References (27) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Studies of the dusty H II region NGC 2024. Balick, B. Abstract Optical spectra obtained at 44 positions in the extremely dusty H II region NGC 2024 including the dark dust lanes and adjacent bright emission regions have been obtained. Lines of low-ionization species such as H, S(+), N(+), and O(+) are readily observed; however, lines from high excitation species such as He(+) and O(++) are weak or absent. The optical line ratios, and hence the reddening, show no evidence of variations as a function of nebular position or dust extinction. When compared with available radio and infrared observations and combined with results from model calculations, the present data can be satisfactorily understood without invoking an additional agent (dust) which is mixed with the gas and which affects the transfer of ionizing radiation. Publication: The Astrophysical Journal Pub Date: August 1976 DOI: 10.1086/154583 Bibcode: 1976ApJ...208...75B Keywords: Astronomical Spectroscopy; Cosmic Dust; H Ii Regions; Nebulae; Astronomical Maps; Emission Spectra; H Lines; Helium Ions; Oxygen Spectra; Radiative Transfer; Astrophysics 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 |

Study of CO in LYNDS 134

Observations of the J=1..-->..0 transitions of /sup 12/C/sup 16/O, /sup 13/C/sup 16/O, /sup 12/C/sup 18/O were made for the dark dust cloud Lynds 134 using the 4.6-m telescope at Aerospace Corporation. Results indicate that within the dust cloud there are four molecular clouds with radial velocities of +0.1, +0.7, +2.7, +4.00.3 km sec/sup -1/ with respect to the local standard of rest, of which the two at +0.7 and +2.7 km sec/sup -1/ are most prominent. These two clouds are approximately isothermal, with excitation temperature approx.12 K. A comparison of the CO data with 21-cm line data indicates that these two clouds coincide in position, velocity, and linewidth with 21-cm absorption features. The ratio H/H/sub 2/ in the +2.7-km sec/sup -1/ cloud is estimated to be > or =7.7 x 10/sup -3/. We only attempted to map /sup 12/C/sup 18/O in the cloud at +2.7 km sec/sup -1/ and there the ratio of /sup 13/C/sup 16/O//sup 12/C/sup 18/O is approx.6.5. (AIP)

Cyanoacetylene in dense interstellar clouds

view Abstract Citations (135) References (32) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Cyanoacetylene in dense interstellar clouds. Morris, M. ; Turner, B. E. ; Palmer, P. ; Zuckerman, B. Abstract Results are presented for a survey of interstellar cyanoacetylene (HC3N), which was conducted with the 11-m NRAO telescope and involved seven rotational transitions ranging from J = 5-4 to J = 14-13. A total of 17 sources were detected, including six near the galactic center, nine associated with H II regions or far-IR sources, on dark dust cloud, and the molecular envelope of a carbon star. The J = 5-4 line is mapped over a large area in Sgr B2, the source having the strongest lines of HC3N, and the presence of two distinct clouds is indicated. Upper limits on the intensity of the J = 10-9 lines in the same source are obtained from species containing C-13. Excitation calculations are applied to five sources for which sufficient data exist, a core-halo model is proposed for Sgr B2, and it is suggested that the intensity of the J = 1-0 line in this source is the result of weak maser action. The cyanoacetylene abundances are compared with those of other known molecular species containing the CN radical, and possible chemical formation processes are considered for HC3N. Publication: The Astrophysical Journal Pub Date: April 1976 DOI: 10.1086/154252 Bibcode: 1976ApJ...205...82M Keywords: Acetylene; Cyano Compounds; Interstellar Gas; Molecular Spectra; Abundance; Astronomical Maps; Cyanoacetylene; Graphs (Charts); H Ii Regions; Molecular Rotation; Radial Velocity; Tables (Data); Astrophysics full text sources ADS |

A comparative study of H2CO, OH, and H I in the dark Taurus dust cloud

A comparative study of H2CO, OH, and H I in the dark Taurus dust cloud

Microwave Spectroscopic Mapping of Gaseous Nebulae. III. - Gen, Helium, and Carbon in Orion a I

view Abstract Citations (11) References (22) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Microwave Spectroscopic Mapping of Gaseous Nebulae. III. - Gen, Helium, and Carbon in Orion a I Chaisson, E. J. Abstract Recombination lines of hydrogen and helium have been detected at five locations distributed throughout the interior portion of Ori A. The average values of 9200 K for the temperature and of 0.10 for the relative ionized helium abundance are essentially constant throughout the region mapped. Carbon recombination lines that are thought to originate in a partially ionized region have also been mapped at five locations. Carbon-line emission is found to be most intense near the dark dust lane toward the northeast. Subject headings: abundances, nebular - nebulae - Orion Nebula - radio lines Publication: The Astrophysical Journal Pub Date: December 1973 DOI: 10.1086/152518 Bibcode: 1973ApJ...186..545C full text sources ADS | Related Materials (3) Part 1: 1973ApJ...182..767C Part 2: 1973ApJ...183..479L Part 4: 1973ApJ...186..555C

Interstellar Cs: Observations of New Transitions and Isotopic Species, and a Study of its Excitation

view Abstract Citations (111) References (35) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Interstellar Cs: Observations of New Transitions and Isotopic Species, and a Study of its Excitation Turner, B. E. ; Zuckerman, B. ; Palmer, Patrick ; Morris, M. Abstract The J = 1-0 and 2-1 transitions of 12C32S, 12C345, and 12C33S have been observed in interstellar sources. CS is often observed in the direction of infrared sources, and is quite widespread in occurrence, with typical projected densities of 1014 cm - . The strongest sources of CS are those of strongest emission in other molecules at millimeter wavelengths (HCN, CO, U89.2) and lie in the general direction of the brightest H ii regions. CS was also detected in new regions apparently associated with far-infrared sources, which include NGC 2264, H2-3, W3, NOC 7338, R CrA, and some regions near the galactic center. The 1-0 transition of CS has also been detected in at least one optically dark dust cloud. If the abundance ratio CS/H2 CO is similar in dark clouds and in infrared/H ii region clouds, it is possible to deduce firm upper limits for the total density in several dark dust clouds (typically n < 4 x 10 cm-3). Isotopic abundance ratios of carbon and sulfur appear to have terrestrial values, with the exception that in the galactic-center region the 12C/'3C ratio may be only about half the terrestrial value. Present analysis suggests that the excitation of interstellar CS can in most cases be explained by collisions obeying dipole selection rules. Assuming statistical equilibrium, the populations of the CS energy levels are calculated including the effects of collisions, a blackbody radiation field, and spontaneous emission. When the hard-sphere collisions obey no selection rules, population inversions can occur in the lower transitions. (This situation does not appear necessary to explain the observations in most sources, and can probably be ruled out in some of them.) Infrared pumping to the first excited vibrational state could also be important, and may produce certain anomalies in the ground-state populations. Subject headings: abundances, nebular - infrared sources - molecules, interstellar - nebulae - radio lines Publication: The Astrophysical Journal Pub Date: November 1973 DOI: 10.1086/152482 Bibcode: 1973ApJ...186..123T full text sources ADS |