Interstellar Gas Clouds Research Articles

Estimates of magnetic fields in interstellar gas clouds from 18-cm OH satellite lines

In interstellar clouds, where “thermal” /nonmaser/ OH emission in the 18-cm lines is observed, prominent anomalies of the intensities of the two satellite lines, 1612 and 1720 MHz, are often seen. If a cloud is observed against a background source of radio continuum, then one satellite may be in strong absorption, while the other one is in emission almost as strong as that in the main lines, 1665 and 1667 MHz. In those clouds, for which several positions were observed in the OH lines, the roles of the satellites are sometimes interchanged with the displacement along the cloud. At the same time, the main lines usually have the intensity ratio close to the equilibrium one. This behaviour indicates to deviations of the level populations of OH molecules from the Boltzmann ones.

Solar system and galactic influences on the stability of the earth

In addition to the intrinsic variability of the solar output, there exist a variety of ways in which the stability of the Earth's environment can be affected by the cosmos. The change with time of the shape and orientation of the Earth's orbit and precession of the rotational axis of the Earth caused by the gravitational forces from other members of the Solar System induce predictable variations in the terrestrial climate which can be seen in the geologic record. Encounters of the Solar System with dense interstellar gas clouds can also exert considerable influences on the terrestrial environment as can the occasional nearby explosion of supernovae. However, the most dramatic and, perhaps, the most likely hazards presented to the Earth by the cosmos are from the occasional collisions with substantial asteroids or comets. The energy delivered to the Earth in such an event would be equivalent to 10,000 times the total nuclear weapons stockpile of all nations. Such an event would have, and is thought to have had, enormous consequences for the stability of the biosphere.

Solar system and galactic influences on the stability of the earth

In addition to the intrinsic variability of the solar output, there exist a variety of ways in which the stability of the Earth's environment can be affected by the cosmos. The change with time of the shape and orientation of the Earth's orbit and precession of the rotational axis of the Earth caused by the gravitational forces from other members of the Solar System induce predictable variations in the terrestrial climate which can be seen in the geologic record. Encounters of the Solar System with dense interstellar gas clouds can also exert considerable influences on the terrestrial environment as can the occasional nearby explosion of supernovae. However, the most dramatic and, perhaps, the most likely hazards presented to the Earth by the cosmos are from the occasional collisions with substantial asteroids or comets. The energy delivered to the Earth in such an event would be equivalent to 10,000 times the total nuclear weapons stockpile of all nations. Such an event would have, and is thought to have had, enormous consequences for the stability of the biosphere.

Ionic reactions in atmospheric, interstellar and laboratory plasmas

Abstract The ionic reactions that can occur in ionized gases, and especially those in naturally-occurring low-temperature gaseous plasmas, are discussed. These reactions include several types of positive ion and negative ion reactions with molecules, ion-electron dissociative recombination reactions, electron attachment reactions and ion-ion mutual neutralization reactions. The laboratory techniques used to study and to determine the rate coefficients for these reactions are briefly described. Three case studies are presented which illustrate how the various types of ionic reactions modify the ionic and neutral composition of ionized media. These case studies describe the reactions which (1) lead to the observed ions in the terrestrial atmosphere; (2) produce the neutral molecules and molecular ions that are detected in interstellar gas clouds; and (3) can occur in the laboratory discharge plasmas that are used to etch the surfaces of semiconductors in the preparation of microprocessors.

Gas-phase reactions of silicon ion (Si+ with ammonia and the amines (CH3)xNH3-x(X = 1-3): possible ion-molecule reaction pathways toward SiH, SiCH, SiNH, SiCH3, SiNCH3, and H2SiNH

Rate constants and product distributions have been determined for gas-phase reactions of ground-state Si/sup +/(/sup 2/P) ions with ammonia and the amines (CH/sub 3/)/sub x/NH/sub 3-x/ (x = 1-3) at 296 +- 2 K with the selected-ion flow tube technique. All reactions were observed to be fast and can be understood in terms of Si/sup +/ insertion into N-H and C-N bonds to form ions of the type SiNR/sub 1/R/sub 2//sup +/ (r/sub 1/, R/sub 2/ = H, CH/sub 3/) proceeding in competition (in the case of the amines) with hydride ion transfer to form immonium ions of the type CH/sub 2/NR/sub 1/R/sub 2//sup +/ (R/sub 1/, R/sub 2/ = H, CH/sub 3/). C-N bond insertion appears more efficient than N-H bond insertion. The contribution of hydride ion transfer increases with increasing stability of the immonium ion. The latter reaction leads directly to SiH as a neutral product. Other minor reaction channels were seen which lead directly or indirectly to SiCH and SiCH/sub 3/. Rapid secondary proton transfer reactions were observed for SiNH/sub 2//sup +/ and SiNHCH/sub 3//sup +/ to produce gas-phase SiNH and SiNCH/sub 3/ molecules. With methylamine SiNH/sub 2//sup +/ also appears to produce H/sub 2/SiNH/sub 2//sup +/more » which may deprotonate to form the simplest silanimine, H/sub 2/SiNH, or aminosilylene, HSiNH/sub 2/. Reactions of this type are of interest in molecular synthesis and here are proposed to contribute to the formation of SiH, SiCH, SiNH, SiCH/sub 3/, and H/sub 2/SiNH or HSiNH/sub 2/ in partially ionized interstellar gas clouds containing silicon, ammonia, and methylamines.« less

Selected-ion flow tube studies of reactions of C 2N + ions derived from cyanogen by electron ionization

The chemistry of C 2N + has been explored in a selected-ion flow tube (SIFT) apparatus in which C 2N + was derived by electron impact from cyanogen. Two forms of C 2N + were identified which have distinctly different chemical reactivities. One form appears to be the CCN + isomer, which reacts selectively with H 2, D 2, CO 2, Xe, and O 2. The less reactive form is thought to be the CNC + isomer. Reactions of both forms are also investigated with N 2, CO, HCN, C 2N 2, N 2O, H 2O, D 2O, CH 4, CH 3CN, C 2H 2, OCS, CH 3OH, C 2H 4, H 2S, NH 3, CS 2, and NO. Rate constants and product distributions are reported at 296 ± 2 K. The results exhibit a wide range in reactivity and chemistry. They provide new insight into the reactions of C 2N + in interstellar gas clouds and additional evidence for the formation of the CNC + isomer in the important interstellar reaction of C + with HCN.

Selected-ion flow tube studies of reactions of the carbene cation:C4N+ and their implications for interstellar gas cloud chemistry

The carbene cation:C4N+ was generated in a Selected-Ion Flow Tube (SIFT) apparatus from allyl cyanide by electron impact and reacted at room temperature in helium buffer gas at ca. 0.33 mm Hg with the molecules H2, CO, HCN, CH4, CD4, H2O, HC3N, C2H2, CH3CN, CD3CN, H2S, NH3, ND3, and C4H2. The reactions were observed to exhibit a range in reactivity and a variety of reaction products. The observed products are consistent with expectations based on the carbene character of:C4N+. The results have implications for the role of:C4N+ as a reaction intermediate in the chemistry of interstellar gas clouds. Possible synthetic routes initiated by:C4N+ are proposed for the formation of dicyanoacetylene, cyanodiacetylene, and several acyclic as well as cyclic neutral carbenes.

Compression of a gas cloud by pressure of the intergalactic medium

It is shown that the evolution of an interstellar gas cloud that enters the space between galaxies depends strongly on the pressure of the hot intergalactic medium. If the difference between the pressure P/sub 0/ of the cloud and the ambient hot gas P/sub ext/ is large (P/sub ext//P/sub 0/ approx. 10), then in a wide range of masses and temperatures the cloud will be compressed without limit by the external pressure, being subsequently transformed into a star. For a moderate difference in the pressure (P/sub ext//P/sub 0/ approx. 3) the values of the parameters for which the clouds preserve their individuality are found as functions of the power of the heating source.

Comets and life

Some of the chemical species which have been detected in comets include H 2O, HCN, CH 3CN, CO, CC 2, NH 3, CS, C 2 and C 3. All of these have also been detected in the interstellar medium, indicating a probable relationship between interstellar dust and gas clouds and comets. Laboratory experiments carried out with different mixtures of these molecules give rise to the formation of the biochemical compounds which are necessary for life, such as amino acids, purines, pyrimidines, monosaccharides, etc. However, in spite of suggestions to the contrary, the presence of life in comets in unlikely. On the other hand, the capture of cometary matter by the primitive Earth is considered essential for the development of life on this planet /1/. The amount of cometary carbon-containing matter captured by the Earth, as calculated by different authors, is several times larger than the total amount of organic matter present in the biosphere (10 18g). The major classes of reactions which were probably involved in the formation of key biochemical compounds are discussed. Our tentative conclusions are that: 1) comets played a predominant role in the emergence of life on our planet, and 2) they are the cosmic connection with extraterrestrial life.

Simulations of close encounter between galaxies: behaviour of interstellar gas clouds and enhancement of star formation rate

On presente des simulations numeriques du rapprochement d'une galaxie parente avec un disque isole et d'une galaxie perturbatrice. On s'interesse au comportement des nuages de gaz interstellaires et a l'influence du rapprochement des galaxies sur le taux de formation stellaire

Selected-ion flow tube studies of reactions of the radical cation (HC3N)+• in the interstellar chemical synthesis of cyanoacetylene

The radical cation (HC3N)+• was produced in a Selected-Ion Flow Tube (SIFT) apparatus from cyanoacetylene by electron impact and reacted at room temperature in helium buffer gas with a selection of molecules including H2, CO, HCN, CH4, H2O, O2, HC3N, C2H2, OCS, C2H4, and C4H2. The observed reactions exhibited a wide range of reactivity and a variety of pathways including charge transfer, hydrogen atom transfer, proton transfer, and association. Association reactions were observed with CO, O2, HCN, and HC3N. With the latter two molecules association was observed to proceed close to the collision limit, which is suggestive of covalent bond formation perhaps involving azine-like N—N bonds. For HC3N an equally rapid association has been observed by Buckley etal. with ICR (Ion Cyclotron Resonance) measurements at low pressures and this is suggestive of radiative association. The hydrogen atom transfer reaction of ionized cyanoacetylene with H2 is slow while similar reactions with CH4 and H2O are fast. The reaction with CO fails to transfer a proton. These results have implications for synthetic schemes for cyanoacetylene as proposed in recent models of the chemistry of interstellar gas clouds. Proton transfer was also observed to be curiously unfavourable with all other molecules having a proton affinity higher than (C3N)•. Also, hydrogen-atom transfer was inefficient with the polar molecules HCN and HC3N. These results suggest that interactions at close separations may lead to preferential alignment of the reacting ion and molecule which is not suited for proton transfer or hydrogen atom transfer.

Flow-tube studies of reactions of selected ions with cyanoacetylene

Rate constants and product distributions have been determined for ion/molecule reactions of cyanoacetylene (HC3N) with He+, C+(2P), CH3+, C2+, C2H+, C2H2+, CN+, C2N+, C2N2+, N2+, CO+, H3+, N2H+, HCO+, C2H3+, H3O+, CH3NO2H+, CH3CNH+, (CH3)2COH+, i-C3H7OH2, C3H+, and C4N+. The measurements were performed with the selected-ion flow-tube (SIFT) technique at 296 ± 2 K. The observed reactions exhibit a variety of pathways including charge transfer, proton transfer, condensation, and association. The condensation and association reactions are suited for molecular growth by ion/molecule reactions. Special consideration is given to the active role of cyanoacetylene in the possible molecular synthesis which occurs in dense interstellar gas clouds by ion/molecule reactions.

Terrestrial mass extinctions

That galactic triggering or forcing of terrestrial biologic crises could arise as a result of collisions (or close encounters) of the solar system with intermediate‐sized to large‐sized interstellar clouds of gas and dust is a theory postulated by R. Rampino and B. Stothers of NASA's Goddard Institute for Space Studies. The idea is that episodes of major mass extinctions and impact cratering on the earth during the last 250 million years seem to have a dominant periodicity of 30 million years with essentially identical phasing. According to Rampino and Stothers, this terrestrial periodicity is found to be strongly correlated with the time needed for the solar system to oscillate vertically about the plane of the Milky Way galaxy, which is 33±3 million years based on current astronomical evidence (see Eos, February 28, 1984, p. 75).

Terrestrial mass extinctions, cometary impacts and the Sun's motion perpendicular to the galactic plane

Episodes of mass extinctions on the Earth are now strongly suspected to be cyclical1. We report here that our analysis of the data of Raup and Sepkoski1 suggests that the dominant cyclicity in major marine mass extinctions during at least the past 250 Myr is 30 ± 1 Myr, with the standard deviation of an individual episode being ±9 Myr. We find this terrestrial cycle to be strongly correlated with the time needed for the Solar System to oscillate vertically about the plane of the Galaxy, which is 33 ± 3 Myr according to the best current astronomical evidence. It is argued that galactic triggering or forcing of terrestrial biological crises may arise as a result of collisions (or close encounters) of the Solar System with intermediate-sized to large-sized interstellar clouds of gas and dust, which are sufficiently concentrated towards the galactic plane to produce the observed cyclicity and its scatter. Among other consequences, a nearby interstellar cloud would gravitationally perturb the Solar System's family of comets and thereby increase the flux of comets and comet-derived bodies near the Earth, leading to large-body impacts. We find a dominant cyclicity of 31 ± 1 Myr in the observed age distribution of impact craters on Earth, the phase of this cycle agreeing with that shown by the major biological crises. Our galactic hypothesis can thus simultaneously account for the mean interval between major terrestrial crises and for the 50% scatter of the time intervals about their mean value.

The ambipolar diffusion time scale and the location of star formation in magnetic interstellar gas clouds

The ambipolar diffusion time scale and the location of star formation in magnetic interstellar gas clouds

Masses of interstellar gas clouds

The equilibrium masses of the interstellar gas clouds have been calculated when the clouds are imbedded with magnetic fields. It is found that the masses of these clouds may be of the order of several thousand solar masses. These masses appear to be consistent with the generally accepted level of efficiency for the processes of star formation.

X-ray sources in molecular clouds

Models are calculated for the structure and infrared line emission from a dense interstellar gas cloud containing a compact X-ray source. For constant gas pressure models, the resulting structure consists of nested spherical shells containing, respectively, coronal gas at T>10/sup 6/ K, an H II region with Tapprox.10/sup 4/ K, and H I region with Tapprox.8000 K, and finally an H/sub 2/ region with T<5000 K. Scaling laws are given for the locations of the transitions. Approximately 10% of the X-ray luminosity absorbed in the H/sub 2/ region is converted into infrared emission lines that may be observable. Line ratios are predicted.

Structure of the local interstellar medium and the line of sight to α Oph

The precise physical state and spatial distribution of interstellar matter within ≃100 pc of the Sun has been the subject of intense speculation because of the implications its knowledge would have on interstellar medium theory1,2, soft X-ray and EUV transparency3,4 and, possibly, for very nearby objects, on solar and terrestrial evolution5,6. The general consensus holds that the local interstellar medium out to ≃100 pc from the Sun is exceptionally clear of any condensation of density ≥0.1 cm−3. Presumably, this is due to the effect of a supernova or stellar wind blast wave sweeping through the local interstellar medium in the relatively recent past7,8. I point out here that a surprising number of independent lines of observational evidence unambiguously imply the existence of a large, moderately dense (nH≥1 cm−3), cool (T≃70 K) cloud of interstellar gas lying between the Sun and α Oph, an A5V star located 16.7±0.8 pc away9 in the direction l = 32°, b = 23°.

Formation of complex molecules in TMC-1

The Taurus molecular cloud 1 (TMC-1) is an interstellar gas cloud which contains a remarkable variety of complex molecules, including members of the cyanopolyyne group [HCzn, n=3,5,7,9] and other, possibly related, species, including C4H, C3H4 and C3N. The cyanopolyynes, which are found in several, different interstellar environments, have a notably high abundance in TMC-1 for such a small cloud and their method of formation has been a considerable problem in gas-phase chemistry. Several authors1–3 have presented schemes for the production of the cyanopolyynes but all these schemes break down under the weight of observational and experimental evidence4. We have reinvestigated the mode of cyanopolyyne formation hi TMC-1 and find that their absolute abundance may be reproduced to within a factor of 3 by a model which uses gas-phase reactions of atomic nitrogen with hydrocarbons to produce them. The observed, spatial displacement of the cyanopolyyne and ammonia peaks5 is readily understood in this model and the abundances of many other molecules are found to be in excellent agreement with observations.

Transfer of angular momentum in a galactic disk by the interaction of clouds of interstellar gas

Transfer of angular momentum in a galactic disk by the interaction of clouds of interstellar gas