A Search for Stellar Obscuration Events Due to Dark Clouds

We discuss physical conditions in interstellar molecular clouds with visual extinction AV ∼1–10 magnitudes, based primarily on observations of spectral lines of CO and NH3. Diffuse clouds with AV∼1 mag have kinetic energy density which is primarily nonthermal, and is comparable to that of the “intercloud” gas; diffuse clouds have too little mass to bind their internal motions by gravity; their incidence of low-mass cores and low-mass stars is rare. Dark clouds with mean AV∼3 mag have kinetic energy density which is primarily nonthermal, like diffuse clouds; but much greater than that of the intercloud gas, unlike diffuse clouds. Dark clouds are generally gravitationally bound. They are often surrounded by a number of diffuse clouds; in dark clouds low-mass cores and low-mass stars are common. Low-mass cores have mean AV∼10 mag. Their kinetic energy density is mainly thermal, unlike their surrounding dark or diffuse clouds, and much higher than that of the intercloud gas. Cores are generally bound, and are frequently associated with low-mass stars. Several lines of evidence indicate that the nonthermal energy density in most molecular clouds is largely magnetic.

Theoretical predictions by Farebrother et al. and Meijer et al. of rovibrational excitation probabilities in H2 arising from formation by Eley-Rideal processes on a graphite surface are incorporated into a model of the chemistry and excitation of interstellar H2. The model includes the usual radiative and collisional pumping of H2 rotational and vibrational states, in addition to the formation processes. Predictions are made for HH2 rovibrational emission line intensities for representative points in diffuse and in dark interstellar clouds. We find that – if all the interstellar HH2 is formed by this Eley-Rideal process – then the consequences of formation pumping, as distinct from collisional and radiative pumping, should be clearly evident in both cases. In particular, we predict a clear spectral signature of this direct HH2 formation process on graphite, distinct from radiative and collisional pumping; this signature should be evident in both diffuse and dark clouds; but the emissivity for dark clouds is predicted to be some 500 times greater than that in diffuse clouds in which the dense material may be embedded. An observational search for this signature in two dark cloud sources was made, but a preliminary analysis of the data did not yield a detection. The implications of and possible reasons for this preliminary conclusion are discussed.

view Abstract Citations (27) References (17) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Diffuse interstellar clouds associated with dark clouds. Federman, S. R. ; Willson, R. F. Abstract Measurements of CH emission at 9 cm toward the bright stars Alpha Cam and Kappa Cas and from dark clouds near the stars reveal an association between diffuse clouds and dark clouds. Here the term diffuse clouds refers to clouds seen in optical absorption; the term dark clouds refers to opaque regions on the Palomar Sky Survey observed through radio techniques. The radio emission toward Alpha Cam, and possible emission toward Kappa Cas, occurs at the same velocity as the optical CH absorption. The column densities derived from both techniques are similar. A dark cloud near each star also shows emission at the velocity of absorption. The column density of CH is larger in the dark cloud. From these data, it is concluded that for most lines of sight, diffuse clouds are the outer, relatively transparent portions of dark clouds. Publication: The Astrophysical Journal Pub Date: September 1982 DOI: 10.1086/160239 Bibcode: 1982ApJ...260..124F Keywords: Interstellar Matter; Interstellar Radiation; Nebulae; Centimeter Waves; Electromagnetic Absorption; Radicals; Sky Surveys (Astronomy); Astrophysics full text sources ADS | data products SIMBAD (2)

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.

We undertake calculations of the time-dependent structure of shock waves propagating in dark and diffuse interstellar clouds. The results of the timedependent model are compared with those obtained by means of an independent steady-state code and found to agree well at sufficiently late times. Discontinuities in the flow of the neutral fluid are handled by introducing a pseudo-viscosity. Special procedures are adopted to correct for the associated widening of the discontinuity, in order not to distort the role of inelastic collision processes. We find that, in dark clouds, C shocks will tend to predominate, but are unlikely to have attained steady state in the cloud lifetime. On the other hand, in diffuse clouds, steady state may be reached but the discontinuity in the flow of the neutral fluid remains. We find no evidence for the existence of C* shocks, in which the neutral fluid undergoes a continuous transition from supersonic to subsonic flow (in the reference frame of the shock wave). Attention is drawn to the possible importance of these results for the interpretation of H 2 rovibrational line intensities.

Cross sections and branching ratios for the dissociative recombination (DR) reactions of the astrophysically important ions HN2+, HCO+, DOCO+, and SO2+ at reactant kinetic energies from 1 to 1000 meV have been measured using the CRYRING ion storage ring facility at the Manne Siegbahn Laboratory, Stockholm University. Whereas the break-up of the N-N bond leading to NH + N is the major pathway in the DR of HN2+, the analogous reaction in HCO+ almost exclusively leads to H and CO. In the DR of both DOCO+ and SO2+ three-body break-up was observed. Inclusion of the newly measured branching ratios into a standard model on dark interstellar clouds leads to an improvement of the predictions of such models, especially concerning the abundances of nitrogen compounds. The impact of these newly found branching ratios and reaction rates on the chemistry of different astronomical environments like dark clouds, circumstellar envelopes and planetary ionospheres, is discussed.

view Abstract Citations (31) References (19) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS CN in dark interstellar clouds. Crutcher, R. M. ; Churchwell, E. ; Ziurys, L. M. Abstract Observations have been made at 113 GHz of the N = 1-0 transition of CN toward seven positions in six dark clouds. Five clouds were detected. Millimeter CN lines were detected for the first time toward dark clouds outside Taurus. The weakest hyperfine component of N = 1-0 (which has only about 1 percent of the total intensity of N = 1-0 for optically thin LTE conditions) was detected for the first time. The six detected positions were also observed at 226 GHz for N = 2-1 transitions of CN; detections were made at two positions. Evidence for self-absorption was found toward a position in the Rho Oph dark cloud and possibly toward the TMC-1 cloud. Possible non-LTE hyperfine excitation was also observed. The weakness of CN lines is explained by the low level of excitation rather than by low column densities. Comparison with time-dependent astrochemical theory shows that most of our results are compatible with the low-metallicity, steady state (age greater than 100 million yr) abundance calculations. Publication: The Astrophysical Journal Pub Date: August 1984 DOI: 10.1086/162352 Bibcode: 1984ApJ...283..668C Keywords: Cn Emission; Interstellar Chemistry; Molecular Clouds; Radio Sources (Astronomy); Abundance; Hyperfine Structure; Line Spectra; Millimeter Waves; Optical Thickness; Self Absorption; Taurus Constellation; Astrophysics full text sources ADS | data products SIMBAD (6)

We study the evolution of nitrogen (N) in the Galactic halo, thick disc, thin disc, and bulge by comparing detailed chemical evolution models with recent observations. The models used in this work have already been constrained to explain the abundance patterns of α-elements and the metallicity distribution functions of halo, disc, and bulge stars; here, we adopt them to investigate the origin and evolution of N in the different Galactic components. First, we consider different sets of yields and study the importance of the various channels proposed for N production. Secondly, we apply the reference models to study the evolution of both the Galactic discs and bulge. We conclude that: i) primary N produced by rotating massive stars is required to reproduce the plateau in log(N/O) and [N/Fe] ratios at low metallicity, as well as the secondary and primary production from low- and intermediate-mass stars to reproduce the data of the thin disc; ii) the parallel model can provide a good explanation of the evolution of N abundance in the thick and thin discs, and we confirm that the thick disc has evolved much faster than the thin disc, in agreement with the results from the abundance patterns of other chemical elements; and iii) finally, we present new model predictions for N evolution in the Galactic bulge, and we show that the observations in bulge stars can be explained if massive stars rotate fast during the earliest phases of Galactic evolution, in agreement with findings from the abundance pattern of carbon.

Molecular oxygen has been confirmed as the fourth most abundant molecule in cometary material O$_2$/H$_2$O $\sim 4$ %) and is thought to have a primordial nature, i.e., coming from the interstellar cloud from which our solar system was formed. However, interstellar O$_2$ gas is notoriously difficult to detect and has only been observed in one potential precursor of a solar-like system. Here, the chemical and physical origin of O$_2$ in comets is investigated using sophisticated astrochemical models. Three origins are considered: i) in dark clouds, ii) during forming protostellar disks, and iii) during luminosity outbursts in disks. The dark cloud models show that reproduction of the observed abundance of O$_2$ and related species in comet 67P/C-G requires a low H/O ratio facilitated by a high total density ($\geq 10^5$ cm$^{-3}$), and a moderate cosmic ray ionisation rate ($\leq 10^{-16}$ s$^{-1}$) while a temperature of 20 K, slightly higher than the typical temperatures found in dark clouds, also enhances the production of O$_2$. Disk models show that O$_2$ can only be formed in the gas phase in intermediate disk layers, and cannot explain the strong correlation between O$_2$ and H$_2$O in comet 67P/C-G together with the weak correlation between other volatiles and H$_2$O. However, primordial O$_2$ ice can survive transport into the comet-forming regions of disks. Taken together, these models favour a dark cloud (or "primordial") origin for O$_2$ in comets, albeit for dark clouds which are warmer and denser than those usually considered as solar system progenitors.

Interstellar dark clouds are known as the source of star and planetary system formation, and the study of chemical composition in dark clouds is important for understanding the process of evolution of matter in the universe toward the planets and to life. The results of the first molecular spectral line survey toward a typical dark cloud, TMC-1 (Taurus Molecular Cloud-1) have been reported. The observations were completed in the frequency range 8800–50000 MHz, and the results of detailed data analysis are presented. This is the first complete spectral line survey toward a cold, dark cloud ever made. We used the 45 m mm-wave telescope and a very large acousto-optical radiospectrometer with 32000 output channels of the Nobeyama Radio Observatory, NAOJ. We detected 404 lines from 38 molecules including 11 new molecules such as C6H, CCO, CCCO, CCS, CCCS, HNCCC, HCCNC, HCCCNH+, HCCCHO, CH2CN and cyclic C3H, all of them are short-lived organic compounds unknown before our detection. We also detected three new isotopomers, CC34S, CCC34S and HDCS. These results provide the basic general composition of cold, dark clouds for the first time. We also discuss chemical and physical evolution of dark clouds.

view Abstract Citations (36) References (33) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS A comparison of optical and radio wavelength observations of CH in the diffuse interstellar medium. Lang, K. R. ; Wilson, R. F. Abstract We report radio wavelength measurements and upper limits for CH emission from diffuse interstellar clouds in the direction of nearby bright stars whose optical wavelength spectra exhibit the absorption lines of CH. The F = 1-1 A-doublet transition at 3335.481 MHz was detected in the direction of 10 stars, and the F = 1-0 and F = 0-1 transitions at 3263.794 and 3349.193 MHz were detected in the direction of four stars. No departures from equilibrium line intensity ratios were observed. The velocities of the CH lines observed at optical and radio wavelengths are compatible with the assumption that in most cases the radio and optical transitions originate in the same diffuse interstellar cloud. The ratio of the column densities of CH inferred from the 10 measurements in the two spectral regions is NRAD/NopT = 1.68 + 0.34, where the error bar denotes one standard deviation. These measurements ensure the compatibility of the reduction procedures in the two regions, and indicate that the oscillator strengths and excitation temperatures used in the reductions cannot be in error by more than a factor of 2. The CH molecules must be well distributed within the diffuse interstellar clouds, and if clumping exists it is not substantial. The central velocities and the velocity dispersions of the CH and CH + profiles in the direction of Oph agree, suggesting that the two molecular species coexist throughout the diffuse interstellar cloud, and that neither molecule originates in separate compact or circumstellar regions. The column density, NoH, of the CH in the diffuse interstellar medium is strongly correlated with the extinction of starlight m = 3.0 , and the total hydrogen column density, (NH+ ), with Nc = 6.3 x l0' = (NH + 2NH2) for 0.6 mag and for extinctions smaller than 2 mag. The relative abundance NcH/NH is an order of magnitude higher than that predicted by the grain-formation theory of CH. When the linear relation found for extinctions less than 2 mag is extrapolated to the higher extinctions characteristic of dark clouds, column densities are predicted which are an order of magnitude higher than those observed. This difference may be due to uncertainties in the extinction estimates of dark clouds. Estimates of the relative column densities of CH and carbon monoxide, CO, obtained from radio wavelength measurements of the diffuse interstellar clouds agree with those obtained from observations at optical and ultraviolet wavelengths with NcH/Noo 0.3. The relative abundance of CH and OH in the diffuse interstellar medium is comparable to that in dark clouds with Nc /No 0.3, but the dark clouds appear to have a relative overabundance of CO or a relative underabundance of CH and OH with Nc /Nco 10- in dark clouds. Subject headings: interstellar: abundances - interstellar: molecules - radio sources: lines Publication: The Astrophysical Journal Pub Date: August 1978 DOI: 10.1086/156356 Bibcode: 1978ApJ...224..125L Keywords: Absorption Spectra; Hydrocarbons; Interstellar Matter; Molecular Absorption; Radio Sources (Astronomy); Abundance; Diatomic Molecules; Gaseous Diffusion; Interstellar Extinction; Line Spectra; Molecular Diffusion; Stellar Luminosity; Astrophysics; Interstellar Clouds:Molecules; Interstellar Clouds: Radio Observations full text sources ADS | data products SIMBAD (19)

Interstellar clouds are concentrations of cold (T ≲ 100 K) neutral gas (cf. Spitzer 1978) which are immersed within an intercloud medium. It is worthwhile to distinguish between diffuse clouds (roughly those with E[B-V] ≳ 0.5) and dark clouds (those with E[B-V] ≳ 0.5). This distinction is useful in the sense that diffuse clouds are relatively warm (T ∼ 100 K), they are composed mostly of atomic species except for hydrogen which can be appreciably molecular, and they are dynamically controlled by their interaction with the intercloud medium. Dark clouds are relatively cold (T ∼ 10 K), they contain a rich variety of molecules, and self-gravity is important in their evolution. Because the interstellar extinction is a rapid function of wavelength, most ultraviolet observations have been of diffuse clouds. The IUE satellite is sufficiently powerful that observations of some dark clouds are possible, and an important area of future research will be to delineate more quantitatively the similarities and differences between diffuse clouds and dark clouds.With ultraviolet observations, considerable progress has been made in understanding the physical characteristics of clouds including determinations of their densities, temperatures, chemical compositions and dynamics (cf. Spitzer and Jenkins 1976). Because particular progress has been made on understanding the abundances within diffuse clouds and because of the limitations of space, we restrict this review to a discussion of abundances within diffuse clouds. These abundance measurements provide a set of fundamental astrophysical data.

view Abstract Citations (39) References (30) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Hydrogen Clouds and the MACHO/EROS Events Henriksen, Richard N. ; Widrow, Lawrence M. Abstract We propose that the recently reported MACHO/EROS events correspond to gravitational amplification by dark clouds rather than compact objects. These clouds must be very dense, with M ~ 0.1 M_sun_ and R <~ 10^14^ cm. In all likelihood, the clouds will be members of a family of objects with different sizes and masses. We therefore expect events of longer duration than the ones reported by the MACHO and EROS groups but with light curves very different from the ones derived assuming a point-mass lens. We suggest that one such event has already been observed in radio measurements of the quasar 1502 + 106. The abundances of free electrons, metals, complex molecules, and dust grains are constrained to be very small, suggesting that the clouds are formed from a primordial mixture of hydrogen and helium. Cosmic rays and background ultraviolet radiation ionize a halo around the cloud. Radio waves from distant sources will be scattered by the electrons in this halo, an effect which may have already been observed in quasars such as 1502 + 106. We argue that dark clouds are a viable alternative to compact objects for baryonic dark matter in the halo. Publication: The Astrophysical Journal Pub Date: March 1995 DOI: 10.1086/175336 arXiv: arXiv:astro-ph/9402002 Bibcode: 1995ApJ...441...70H Keywords: Cosmic Rays; Dark Matter; Galactic Halos; Gravitational Lenses; Helium; Hydrogen; Hydrogen Clouds; Ultraviolet Radiation; Abundance; Baryons; Light Curve; Magellanic Clouds; Quasars; Radio Waves; Astrophysics; COSMOLOGY: DARK MATTER; COSMOLOGY: GRAVITATIONAL LENSING; Astrophysics E-Print: 11 pages, LaTex full text sources arXiv | ADS | data products SIMBAD (1) NED (1)

We present a theoretical investigation of the chemistry of fluorine-bearing molecules in diffuse and dense interstellar gas clouds, combining recent estimates for the rates of relevant chemical reactions with a self-consistent model for the physical and chemical conditions within gas clouds that are exposed to the interstellar ultraviolet radiation field. The chemistry of interstellar fluorine is qualitatively different from that of any other element, because unlike the neutral atoms of any other element found in diffuse or dense molecular clouds, atomic fluorine undergoes an exothermic reaction with molecular hydrogen. Over a wide range of conditions attained within interstellar gas clouds, the product of that reaction, hydrogen fluoride, is predicted to be the dominant gas-phase reservoir of interstellar fluorine nuclei. Fluorine is the heavy element that shows the greatest tendency toward molecule formation; in diffuse clouds of small extinction, the predicted HF abundance can even exceed that of CO, even though the gas-phase fluorine abundance is 4 orders of magnitude smaller than that of carbon. Our model predicts HF column densities of ~1013 cm-2 in dark clouds and column densities as large as 1011 cm-2 in diffuse interstellar gas clouds with total visual extinctions as small as 0.1 mag. Such diffuse clouds will be detectable by means of absorption-line spectroscopy of the J = 1-0 transition at 243.2 ?m using the Stratospheric Observatory for Infrared Astronomy (SOFIA) and the Herschel Space Observatory (HSO). The CF+ ion is predicted to be the second most abundant fluorine-bearing molecule, with typical column densities a factor of ~102 below those of HF; with its lowest two rotational transitions in the millimeter-wave spectral region, CF+ may be detectable from ground-based observatories. HF absorption in quasar spectra is a potential probe of molecular gas at high redshift, providing a possible bridge between the UV/optical observations capable of probing H2 in low column density systems and the radio/millimeter-wavelength observations that probe intervening molecular clouds of high extinction and large molecular fraction; at redshifts beyond ~0.3, HF is potentially detectable from ground-based submillimeter observatories in several atmospheric transmission windows.

We present the first identification in interstellar space of the thioformyl radical (HCS) and its metastable isomer HSC. These species were detected toward the molecular cloud L483 thanks to observations carried out with the IRAM 30m telescope in the λ 3 mm band. We derive beam-averaged column densities of 7 × 1012 cm-2 for HCS and 1.8 × 1011 cm-2 for HSC, which translate to fractional abundances relative to H2 of 2 × 10-10 and 6 × 10-12, respectively. Although the amount of sulfur locked by these radicals is low, their detection allows to put interesting constraints on the chemistry of sulfur in dark clouds. Interestingly, the H2CS/HCS abundance ratio is found to be quite low, ~ 1, in contrast with the oxygen analogue case, in which the H2CO/HCO abundance ratio is around 10 in dark clouds. Moreover, the radical HCS is found to be more abundant than its oxygen analogue, HCO. The metastable species HOC, the oxygen analogue of HSC, has not been yet observed in space. These observational constraints are confronted with the outcome of a recent model of the chemistry of sulfur in dark clouds. The model underestimates the fractional abundance of HCS by at least one order of magnitude, overestimates the H2CS/HCS abundance ratio, and does not provide an abundance prediction for the metastable isomer HSC. These observations should prompt a revision of the chemistry of sulfur in interstellar clouds.