Carbon monoxide frosts in the interstellar medium

Summary The presence of solid carbon monoxide (CO) on interstellar grains was confirmed observationally in 1984 with the detection of infrared absorption at 4.67m wavelength in several molecular clouds. Subsequent observations suggest that solid CO is ubiquitous in the quiescent molecular cloud environment. In some lines of sight, the degree of frosting on to grains is sufficient to reduce appreciably the abundance of CO remaining in the gas, a result of considerable astrophysical significance: in addition to its importance as a tracer of molecular material, CO is vital to the production of many polyatomic molecules by gas phase reaction schemes, and its depletion could have a dramatic effect on the abundances of more complex carbon-bearing molecules. The infrared spectrum of solid CO provides an important diagnostic of the chemical composition and thermal evolution of grain mantles, leading to the prediction that CO₂ is also present in solid form.As it is now some six years since observations of interstellar solid CO were first reported, this is an appropriate time to review the topic and to suggest some directions for future research. The introduction (Sect. 1) attempts to place the subject in its broader astrophysical context. The infrared observations and their implications are discussed in detail in Sect. 2. The question of the degree of CO depletion implied by the observations of both solid state and gas phase CO is re-examined in Sect. 3. We assess the possibility of CO detection by means of solid state absorption or luminescence in the ultraviolet in Sect. 4. Future prospects are summarised in the final section.This article was processed by the author using the Springer-Verlag TEX Theaar macro package 1988.     

Carbon molecules as possible carrier of the diffuse interstellar bands

Douglas (1977) proposed linear carbon molecules as one of the carriers of the diffuse interstellar bands. In particular, he suggested that either the species C₅, C₇, or C₉ should produce the most intense interstellar 443 nm band. We have performed laboratory experiments to investigate whether the basic assumption of this hypothesis is fulfilled, namely whether a species of carbon molecules exhibits a strong absorption in the vicinity of 443 nm. For this purpose, we studied the UV-VIS spectra of large carbon molecules applying the matrix isolation technique. We found that in fact a carbon molecule with such an absorption does exist. A rather preliminary interpretation of our data suggests that this band is produced by the linear molecule C₇. Because the laboratory spectra are distorted by matrix-effects, a conclusive comparison with the interstellar absorptions is not yet possible.Paper presented at a Workshop on The Role of Dust in Dense Regions of Interstellar Matter, held at Georgenthal, G.D.R., in March 1986.     

Observations and analysis of MT Her

Previously reportedBV photometry of MT Her is analysed with the aid of efficient curve-fitting techniques. There is insufficient overall data to allow a definitive parametrization of the binary, but we can confidently rule out the possibility of a classical Algol.A provisional model is advanced which consists of two close unevolved dwarfs: the primary a late F-type and the secondary a mid K-type. The system is located at a distance of about 250 pc in a region of fairly heavy interstellar extinction near the galactic disk. It may be a short-period RS CVn-type binary, but this possibility, as the provisional model itself, requires spectrographic confirmation.     

Interstellar gas depletion and dust parameters

It is shown that on theoretical grounds the relative abundances of the elements in the interstellar gas phase should be correlated with the wavelength of maximum interstellar polarization _max. If these correlations can be determined by observations, then there is the possibility to determine the relative abundances of the heavier elements within the mantles as well as within the cores of interstellar dust grains, at least in principle.The observational data available up to now confirm the existence of such correlations between _max and the interstellar gas phase abundances of titanium, iron, magnesium, and carbon. Statements about the chemical composition of the dust particles are not yet possible. For this there are observations of the interstellar gas depletion needed, especially in such lines of sight where _max has extreme values.Paper presented at a Workshop on The Role of Dust in Dense Regions of Interstellar Matter, held at Georgenthal, G.D.R., in March 1986.     

Interstellar molecule formation in grain mantles: The laboratory analog experiments,results and implications

Laboratory and theoretical studies have been made of the effects of ultraviolet photolysis of interstellar grain mantles which consist of combinations of hydrogen, oxygen, carbon and nitrogen — dirty ice. It is shown that processes involving photolysis (photoprocessing) of interstellar grains are important during most of their lifetime even including the time they spend in dense clouds. A laboratory designed to simulate the interstellar conditions is described. This is the first time such a laboratory has been able to provide results which may be directly scaled to the astrophysical situations involving interstellar grains and their environment. The evolution of grain analogs is followed by observing the infrared absorption spectra of photolyzed samples of ices deposited at 10 K. The creation and storage of radicals and the production of molecules occur as a result of reactions within the solid. A large number of molecules and radicals observed in the interstellar gas appear in the irradiated ices. Energy released during warm-up is seen from visible luminescence and inferred from vapor pressure enhancement which occurs during warming of photolyzed samples relative to unphotolyzed samples. The evolution of a grain and its role as a source as well as a sink of molecules is pictured as a statistical process within dense clouds. The gradual accretion on and photolysis of an individual grain provides the stored chemical energy the release of which is sporadically triggered by relatively mild events (such as low velocity grain-grain collisions) to produce the impulsive heating needed to eject or evaporate a portion of the grain mantle. An extremely complex and rather refractory substance possessing the infrared signatures of amino groups and carboxylic acid groups and having a maximum mass of 514 amu has been produced at a rate corresponding to a mass conversion rate of interstellar grains of between 2% and 20% in 10⁷ yr. The shape and position of the astronomically observed 3.1 m band is duplicated in the laboratory and is shown to be a natural consequence of the processing of grain mantles.Invited contribution to the Proceedings of a Workshop onThermodynamics and Kinetics of Dust Formation in the Space Medium held at the Lunar and Planetary Institute, Houston, 6–8 September, 1978.     

Laboratory comparisons of organic materials to interstellar dust and the Murchison meteorite

Spectra of objects which lie along several lines of sight through the diffuse interstellar medium (DISM) reveal an absorption feature near 3.4 micrometers, which has been attributed to saturated aliphatic hydrocarbons on interstellar grains. The similarity of the absorption bands near 3.4 micrometers (2950 cm-1) along different lines of sight indicates that the carrier of this band lies in the diffuse dust. Several materials have been proposed as "fits" to the 3.4 micrometers feature over the years. A comparison of these identifications is presented. These comparisons illustrate the need for high resolution, high signal-to-noise observational data as a means of distinguishing between laboratory organics as matches to the interstellar material. Although any material containing hydrocarbons will produce features in the 3.4 micrometers region, the proposed "matches" to the DISM do differ in detail. These differences may help in the analyses of the chemical composition and physical processes which led to the production of the DISM organics, although ISO Observations through the 5-8 micrometers spectral region are essential for a definitive identification. A remarkable similarity between the spectrum of the diffuse dust and an organic extract from the Murchison meteorite suggests that some of the interstellar organic material may be preserved in primitive solar system bodies. The 3.4 micrometers absorption feature (in the rest frame) has recently been detected in external galaxies, indicating the widespread availability of organic material for incorporation into planetary systems.     

Origin of the diffuse interstellar absorption bands

Evidence is presented that zero phonon lines of defect centres in MgO and CaO are responsible for the diffuse interstellar bands at 5362, 5705, 6425.7, and 6699.4 Å. Phonon sidebands of these lines are identified with the diffuse bands at 5535, 6177, 6196, 6284, and 6314 Å. These features arise in interstellar MgO and CaO particles with sizes 50 Å. Infrared spectral features due to interstellar MgO and CaO are discussed.     

Experimental investigations of astronomically important interstellar silicates

In this paper methods and results of laboratory experiments for the investigation of the silicate component of interstellar dust are reviewed. In Section 2 basic properties expected for astronomically important interstellar silicates (AIIS) are discussed. Chemical constraints coming from the abundance of elements, from the depletion in the interstellar gas and from theoretical calculations of the condensation processes point to magnesium silicates. Some basic structural properties of interstellar silicates, the expected high degree of lattice disorder and spectral features expected for interstellar silicate grains are discussed. In Section 3 a review on laboratory investigations of AIIS is given. Physical and chemical methods for producing amorphous silicates are summarized. Important measurements of optical data for AIIS are listed. Spectral characteristics of amorphous silicates produced in order to simulate the interstellar dust silicates are discussed. From the comparison of the observed MIR silicate bands with those of the experimentally produced silicates it is concluded that at least two types of dust silicates exist in interstellar space: molecular-cloud silicate (suggested to be of pyroxene-type) and late-type star silicate (suggested to be of olivine-type). The mass absorption coefficient at the 10 m peak of both types of silicate grains amounts to 3000 cm² g^–1 and the ratio of 20 to 10 m peaks amounts to about 0.5. Finally, open questions in connection with laboratory experiments are mentioned and recommendations for future experiments are given.Paper presented at a Workshop on The Role of Dust in Dense Regions of Interstellar Matter, held at Georgenthal, G.D.R., in March 1986.     

Revised interstellar neutral helium/hydrogen density ratios and the interstellar UV-radiation field

The data deduced from the UV-spectroscope on theCopernicus satellite strongly suggest that the most important ionization source in interstellar space near the solar system is a UV radiation field originating from B-stars. Adopting this hypothesis, we have used the ionization state of several elements in the interstellar medium observed byCopernicus to determine the required radiation field. From this, the degree of ionization of elements that could not be observed byCopernicus is estimated.It is shown that this interpretation of thecopernicus data can be made consistent with neutral interstellar hydrogen densities inferred from extraterrestrial L observations and with electron densities deduced from pulsar dispersion measures. Furthermore, it is shown that the ratio of neutral interstellar helium to neutral interstellar hydrogen is likely to be 2 to 3 times as large as the cosmic abundance ratio of these elements. The possibility that this ratio is about 10 times as large, meaning equal interstellar neutral hydrogen and helium densities near the solar system, cannot be ruled out. It would, however, require an interstellar radiation temperature near 9000 K. A comparison of the intensity of the interplanetary back scattered He 584 Å and the H 1216 Å radiation would lead to a direct determination of this ratio provided the solar radiation at these lines is known.     

The determination of physical parameters for circumstellar clouds

The possibility of the detection of circumstellar clouds around hot stars have been shown based on the analysis of observed equivalent widths of resonance line 2852 Mgi of interstellar origin. An attempt has been made to combine two methods, earlier suggested — Method of Doublet 2800 Mgii and Method of Depression — for the determination of the parameters of circumstellar clouds. The sequence of the determination of these parameters is illustrated on the application of this combined method for circumstellar clouds around five hot stars. The value of the mean absorption coefficientk ₀ for the system of resonance lines in the region of depression 2400 Å, is obtained. An application of the conception of spectral classification of circumstellar clouds has been realized.     

Complex organics in space from Solar System to distant galaxies

Recent observational and experimental evidence for the presence of complex organics in space is reviewed. Remote astronomical observations have detected \(\sim \)200 gas-phased molecules through their rotational and vibrational transitions. Many classes of organic molecules are represented in this list, including some precursors to biological molecules. A number of unidentified spectral phenomena observed in the interstellar medium are likely to have originated from complex organics. The observations of these features in distant galaxies suggests that organic synthesis had already taken place during the early epochs of the Universe. In the Solar System, almost all biologically relevant molecules can be found in the soluble component of carbonaceous meteorites. Complex organics of mixed aromatic and aliphatic structures are present in the insoluble component of meteorites. Hydrocarbons cover much of the surface of the planetary satellite Titan and complex organics are found in comets and interplanetary dust particles. The possibility that the early Solar System, or even the early Earth, have been enriched by interstellar organics is discussed.     

Cumulenic and heterocumulenic anions: potential interstellar species?

A recent theoretical investigation by Terzieva & Herbst of linear carbon chains, C _n where n  ≥ 6, in the interstellar medium has shown that these species can undergo efficient radiative association to form the corresponding anions. An experimental study by Barckholtz, Snow & Bierbaum of these anions has demonstrated that they do not react efficiently with molecular hydrogen, leading to the possibility of detectable abundances of cumulene-type anions in dense interstellar and circumstellar environments. Here we present a series of electronic structure calculations which examine possible anionic candidates for detection in these media, namely the anion analogues of the previously identified interstellar cumulenes C _n H and C _{n −1}CH₂ and heterocumulenes C _n O (where n  = 2–10). The extraordinary electron affinities calculated for these molecules suggest that efficient radiative electron attachment could occur, and the large dipole moments of these simple (generally) linear molecules point to the possibility of detection by radio astronomy.     

Polyatomic molecules in diffuse clouds

The consequences on the chemistry in diffuse clouds of the injection of small hydrocarbon molecules is explored. It is assumed that the injection arises from the erosion of carbonaceous grains at cloud boundaries, and that the injected species take part in conventional interstellar chemistry. The models indicate that for plausible injection rates the daughter species of the injected parent molecules should appear in significant amounts, at least during the injection period, and that daughter species can appear in gas that is otherwise almost entirely atomic. Therefore, injection of hydrocarbons is a mechanism that is capable of providing polyatomic molecules in detectable amounts in diffuse atomic regions of the interstellar medium. The implications of this result for the carriers of the unidentified diffuse interstellar bands are briefly described.     

Diffuse band extinction and polarization in core-mantle grains

Diffuse band shapes in both extinction and polarization are calculated for interstellar coremantle particles for varying size distributions of mantle thickness. It is shown that no matter whether the source of the bands is in the silicate cores or the accreted icy mantles the polarization shapes are highly asymmetric for all mantel thicknesses. The extinction band shapes are significantly less asymmetric although the effect is clearly present. The only apparent possibility for producing symmetric band shapes in the dust grains is in the very small bare particles in interstellar space which, if they are aligned and produce the 2200 band, must exhibit a strong polarization effect in this region.Work supported in part by NASA Grant NGR-33-011-043.Paper presented at the Symposium on Solid State Astrophysics, held at the University College, Cardiff, Wales, between 9–12 July, 1974.     

Pseudo-resonance absorption lines in planetary nebulae: Discovery?

The possibility of the formation of pseudo-resonance absorption lines in planetary nebulae is predicted only theoretically. However, this has not been confirmed by direct observations. In the present article an attempt has been made to show, as a result of careful analysis of IUE spectral recordings for a group of planetary nebulae, that the existence of one pseudo-resonance line 1300 SiIII is without doubt at least in spectra of three planetary nebulae: NGC 2610, NGC 3587, and NGC 6891. The presence of this line in the spectra of the other three planetary nebulae, NGC 6210, IC 3568, and IC 4776, seems to be probable. The role of the interstellar selective absorption, the blending effect by the resonance lines of SiII,Oi, andSi in the formation of the pseudo-resonance line 1300 SiIII as well as the possibility of formation of this line in the photosphere of central star of nebula were analysed.     

Interstellar communication: The colors of optical SETI

It has recently been argued from a laser engineering point of view that there are only a few magic colors for optical SETI. These are primarily the Nd:YAG line at \(1{,}064\,\hbox {nm}\) and its second harmonic (532.1 nm). Next best choices would be the sum frequency and/or second harmonic generation of Nd:YAG and Nd:YLF laser lines, 393.8 nm (near Fraunhofer CaK), 656.5 nm (\(\hbox {H}\alpha \)) and 589.1 nm (NaD2). In this paper, we examine the interstellar extinction, atmospheric transparency and scintillation, as well as noise conditions for these laser lines. For strong signals, we find that optical wavelengths are optimal for distances \(d\lesssim \,\hbox {kpc}\). Nd:YAG at \(\lambda =1{,}064\,\hbox {nm}\) is a similarly good choice, within a factor of two, under most conditions and out to \(d\lesssim 3\,\hbox {kpc}\). For weaker transmitters, where the signal-to-noise ratio with respect to the blended host star is relevant, the optimal wavelength depends on the background source, such as the stellar type. Fraunhofer spectral lines, while providing lower stellar background noise, are irrelevant in most use cases, as they are overpowered by other factors. Laser-pushed spaceflight concepts, such as “Breakthrough Starshot”, would produce brighter and tighter beams than ever assumed for OSETI. Such beamers would appear as naked eye stars out to kpc distances. If laser physics has already matured and converged on the most efficient technology, the laser line of choice for a given scenario (e.g., Nd:YAG for strong signals) can be observed with a narrow filter to dramatically reduce background noise, allowing for large field-of-view observations in fast surveys.     

Symbiotic star AS 201: A planetary nebula

Optical spectrum of AS 201 is presented: this object shows emission lines of moderate excitation (exc. class 5) and absorption spectrum of a G2III star. For the interstellar extinction and distance the values c=0.45 and d=1.5 kpc were estimated. The hot components of AS 201, Cn 1-1 and M 1-2 are located on the Harman-Seaton sequence in the region of evolved central stars of PN. It is suggested that the hot components of some SS are evolved nuclei of PN; high el. density of their nebulae may be explained due to mass loss from the respective cool stars of the binaries.Paper presented at the IAU Colloquium No. 93 on Cataclysmic Variables. Recent Multi-Frequency Observations and Theoretical Developments, held at Dr. Remeis-Sternwarte Bamberg, F.R.G., 16–19 June, 1986.     

Optical counterparts of cosmological GRBs

The possibility of observing of optical counterparts of cosmological GRBs has been considered assuming that the counterparts are due to the -radiation interaction with the interstellar medium.     

Formation of E-cyanomethamine in a nitrile rich environment

Recently a new molecule, cyanomethamine, has been detected towards Sagittarius B2(N)(Sgr B2(N)). Studying the formation mechanisms of complex interstellar molecules is difficult. Hence,a theoretical quantum chemical approach for analyzing the reaction mechanism describing the formation of interstellar cyanomethamine through detected interstellar molecules and radicals(NCCN+H) is discussed in the present work. Calculations are performed by using quantum chemical techniques, such as Density Functional Theory(DFT) and M?ller-Plesset perturbation(MP2) theory with a 6-311G(d,p)basis set, both in the gas phase and in icy grains. The proposed reaction path(NCCN+H+H) has exothermicity with no barrier which indicates the possibility of cyanomethamine formation in the interstellar medium.     

Effect of interstellar density fluctuations on signal dispersion measure

The effect of electron number density fluctuations in the interstellar medium on signals from pulsars is studied in terms of the frequency dependent signal dispersion. It is shown that if the density fluctuations are representative of long wavelength (1100 pc) [or large scale length (1100 pc)] disturbances in the interstellar gas, then the observed signal dispersion is not a measure of the integral of the electron number density in the line of sight. Evidence has been presented elsewhere for believing that such long wavelength disturbances should exist in the interstellar gas, so this result indicated that some care must be exercised in the interpretation of signal dispersion measurements from pulsars.