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NASA NTRS1987

The physics of interstellar shock waves

This review discusses the observations and theoretical models of interstellar shock waves, in both diffuse cloud and molecular cloud environments. It summarizes the relevant gas dynamics, atomic, molecular and grain processes, radiative transfer, and physics of radiative and magnetic precursors in shock models. It then describes the importance of shocks for observations, diagnostics, and global interstellar dynamics. It concludes with current research problems and data needs for atomic, molecular and grain physics.

Shull, J. Michael↗
NASA NTRS1987

Heating and cooling of the interstellar gas

Basic considerations of the global heating and cooling of the interstellar gas are summarized. The various energy sources are reviewed. Expressions for the rates of a number of typical heating and cooling processes are given. General comments are made about the conditions in the several phases of the interstellar medium in thermal balance.

Black, John H.↗
NASA NTRS1987

VLA and low-frequency VLBI observations of the radio source 0503 + 467 - Austere constraints on interstellar scattering in two media

The radio source 0503 + 467 lies near the Galactic plane (l = 161.0 deg, b = 3.7 deg) and at the edge of the supernova remnant (SNR) HB 9. The VLA observations show that it has a spectrum typical of a compact extragalactic radio source. The resultant small angular size of the source makes it an excellent probe of turbulence in two media: the diffuse, or 'type A', component of interstellar turbulence and a hypothesized region of hydromagnetic turbulence upstream of the supernova remnant. An eight-station VLBI experiment at 326 MHz indicates that the source is less than about 20 milliarcseconds (mas) in angular diameter. A value of 16 mas is most appropriate as an upper limit to the interstellar scattering contribution to the measured angular size. The implications of this upper limit are twofold. First, the galactocentric radial scale to the type-A turbulence is probably less than or equal to about 6 kpc. Second, no evidence is seen for shock-associated turbulence upstream of HB 9. The measurements make it possible to constrain a parameter which is a function of the rms density fluctuation in the upstream region, the outer scale to the density turbulence, and the thickness of SNR foreshock region.

Spangler, Steven R.↗
NASA NTRS1987

Supernovae and the interstellar medium

Repeated supernovae from an OB association will, in a few 10s of Myr, create a cavity of coronal gas in the interstellar medium, with radius greater than 100 pc, surrounded by a dense expanding shell of cool interstellar gas. Such a cavity will likely burst through the gas layer of a disk galaxy. Such holes and 'supershells' have been observed in optical and H I radio emission maps of the Galaxy and other nearby galaxies. The gas swept up in supershell is likely to become gravitationally unstable, providing a mechanism for propagating star formation that may be particularly effective in irregular galaxies.

Mccray, Richard↗
NASA NTRS1987

Infrared emission from interstellar PAHs

The mid-IR absorption and Raman spectra of polycyclic aromatic hydrocarbons (PAHs) and the mechanisms determining them are reviewed, and the implications for observations of similar emission spectra in interstellar clouds are considered. Topics addressed include the relationship between PAHs and amorphous C, the vibrational spectroscopy of PAHs, the molecular emission process, molecular anharmonicity, and the vibrational quasi-continuum. Extensive graphs, diagrams, and sample spectra are provided, and the interstellar emission bands are attributed to PAHs with 20-30 C atoms on the basis of the observed 3.3/3.4-micron intensity ratios.

Allamandola, L. J.↗
NASA NTRS1988

Newly detected molecules in dense interstellar clouds

The last year or so has seen the identification of several new interstellar molecules, including C2S, C3S, C5H, C6H, and (probably) HC2CHO in the cold, dark cloud TMC-1, and the discovery of the first interstellar phosphorus-containing molecule, PN, in the Orion 'plateau' source. Further interesting results include the observations of (C-13))3H2 and C3HD, and the first detection of HCOOH (formic acid) in a (C-13)3H2 cold cloud.

Irvine, William M.↗
NASA NTRS1988

Unresolved velocity structure in diffuse interstellar clouds

Recent high-resolution observations of interstellar absorption lines of CH and CN toward Zeta Oph obtained by Crane et al. (1986), and Palazzi et al. (1988), exhibit line widths that suggest thermal line broadening at high temperature, T about 1200 K. Observations of CO line emission at 2.6 mm toward Zeta Oph (Langer et al.,1987) indicate that the molecular gas resides in four distinct velocity components that span less than 3 km/s in Doppler velocity. Simulated CH and CN absorption line profiles are compared for high-temperature (T = 1200 K) thermal broadening and for a combination of low-temperature (T = 50 K) thermal plus turbulent broadening. It is shown that the two broadening models reproduce existing observations comparably well and are virtually indistinguishable at a lambda/Delta-lambda ratio of about 100,000. The observed differences in the CH and CN line widths may reflect slightly different distributions of those molecules along the line of sight. The simulations use very recent, improved laboratory spectroscopic data on CH (Bernath). Some related consequences of such unresolved velocity structure on the ultraviolet absorption lines of CO are examined. Indirect diagnostics of temperature in the Zeta Oph cloud favor low-temperature thermal plus turbulent broadening, and the implied rate of dissipation of turbulence is in harmony with estimates of the global input of mechanical energy into to interstellar medium.

Black, John H.↗
NASA NTRS1988

Interstellar scattering of the radio source 2013+370

Results are reported from a six-station 1663-MHz VLBI observation of the radio source 2013+370. The image of this source is dominated by interstellar scattering and is observed to be elliptical, with a major axis (FWHM) of 17 marcsec and an axial ratio of of 0.70. A fit of the observations to a model visibility function yields a value for alpha, the spectral index of the power-law density irregularity spectrum, of alpha=3.79 + or - 0.05. Corrections for the finite angular size of the source would probably produce a downward revision of alpha. The present observations are consistent with the interstellar density spectrum being close to Kolmogoroff for those scales which determine the angular broadening.

Spangler, Steven R.↗
NASA NTRS1988

Interstellar grain chemistry and the composition of comets

During the past 15 years considerable progress in observational techniques has been achieved in the middle infrared, the spectral region most diagnostic of molecular vibrations. Spectra of many different astronomical infrared sources are now available. By comparing these astronomical spectra with the spectra of lab ices, one can determine the composition and abundance of the icy materials frozen on the cold dust grains present in the interior of molecular clouds. In the experiments described, the assumption is made that cometary ices are similar to interstellar ices. As an illustration of the processes which can take place as an ice is irradiated and subsequently warmed, the infrared spectra is presented of the mixture H2O:CH3OH:CO:NH3:C6H14 (100:50:10:10:10). Apart from the last species, the ratio of these compounds is representative of the simplest ices found in interstellar clouds.

Allamandola, Louis J.↗
NASA NTRS1988

Interstellar dust and the IUE

Understanding of the physical nature of interstellar dust, as modified by observations of extinction obtained with IUE, is considered. Properties of extinction laws along various lines of sight, with emphasis on the bump, are reviewed. Correlations of the UV and optical/near infrared extinction are discussed. Other diagnostics of interstellar dust, such as the near infrared emission bands, polarization, far infrared thermal emission, and depletions are mentioned, and theories are summarized.

Mathis, John S.↗
NASA NTRS1988

The small scale structure of the interstellar medium in the Orion association: The flotsam of star formation

The small scale dynamics and structure of the interstellar medium of the Orion OB1b, c association was studied using IUE high dispersion SWP and LWR/LWP spectra. The area surveyed centers on the Belt and encompasses the Orion Cloak. The dynamics and abundances of the ionized and warm neutral gas surrounding the central parts of the association are determined using 35 B stars. Results are compared with the picture of the Orion interstellar medium gained using COPERNICUS, especially for the structure and dynamics of the Orion Cloak.

Sonneborn, G.↗
NASA NTRS1989

Commentary on interstellar matter associated with 18 open clusters

Information supplementary to that contained in Section 4 of an article entitled, A CO Survey of Regions Around 34 Open Clusters, (Leisawitz, Bash, and Thaddeus) published in the Astrophysical Journal Supplement Series, Volume 70, Number 4, August 1989 is summarized. The information presented here, which describes the interstellar environments of young clusters and some cluster physical characteristics, comes from observations published in the astronomical literature and the author's carbon monoxide (CO) emission line survey, and may help clarify our understanding of the interaction of massive stars with the interstellar medium.

Leisawitz, David↗
NASA NTRS1988

The formation of interstellar jets

The formation of interstellar jets by convergence of supersonic conical flows and the further dynamical evolution of these jets are investigated theoretically by means of numerical simulations. The results are presented in extensive graphs and characterized in detail. Strong radiative cooling is shown to result in jets with Mach numbers 2.5-29 propagating to lengths 50-100 times their original widths, with condensation of swept-up interstellar matter at Mach 5 or greater. The characteristics of so-called molecular outflows are well reproduced by the simulations of low-Mach-number and quasi-adiabatic jets.

Tenorio-Tagle, G.↗
NASA NTRS1988

Interstellar silicon carbide - How much older than the solar system?

The SiC grains in C2 chondrites have large isotopic anomalies in Si, C, N, Ne, and Xe, and thus are virtually undegraded interstellar matter that survived formation of the solar system. They also contain excess Ne-21, of which only a minor part can have formed during the recent, 1 Myr cosmic-ray exposure of the meteorite. If the excess Ne-21 is cosmogenic, then the presolar exposure age of the SiC is 41 + 20 or - 14 Myr. This age is surprisingly short compared to the predicted lifetime of refractory interstellar grains, 500-1000 Myr. Possible explanations are: (1) the predicted grain lifetime is wrong, (2) the age of SiC has been falsified by neon losses, or (3) the solar system formed in part from atypically young material.

Ming, Tang↗
NASA NTRS1988

Interstellar cloud material in meteorites

In addition to material of apparently circumstellar origin, many primitive meteorites contain material for which an origin in interstellar clouds is indicated. Such material, apparently organic in nature, is revealed by enrichments of deuterium relative to terrestrial material. Astronomical observations have shown that simple organic molecules in dense interstellar clouds have large D/H ratios. The H-isotopic compositions of those molecules are believed to be the result of ion-molecule reactions taking place at low temperatures.

Zinner, Ernst↗
NASA NTRS1987

The local interstellar medium

The structure of the local interstellar medium is reviewed with emphasis on the new observations of the cold neutral component. The properties of the high latitude molecular clouds are discussed as well as their relationship to the 'infrared cirrus' and atomic gas. The molecular clouds are shown to lie preferentially along the loops and filaments of local H I gas, but a detailed examination shows the CO and H I peaks are offset from one another. An expansion velocity of 7 km/s is derived for one of the shells, suggesting an expansion energy of about 8 x 10 to the 47th ergs, which is somewhat smaller than that expected for a supernova explosion. If the pressure in the clumps of the high latitude clouds is in equilibrium with the surrounding gas, the pressure in the shells is 10,000-100,000/cu cm K, an order of magnitude higher than what is generally argued to exist in the interstellar medium.

Blitz, Leo↗
NASA NTRS1988

Interstellar matter within elliptical galaxies

Multiwavelength observations of elliptical galaxies are reviewed, with an emphasis on their implications for theoretical models proposed to explain the origin and evolution of the interstellar matter. Particular attention is given to interstellar matter at T less than 100 K (atomic and molecular gas and dust), gas at T = about 10,000 K, and gas at T = 10 to the 6th K or greater. The data are shown to confirm the occurrence of mass loss from evolved stars, significant accretion from companion galaxies, and cooling inflows; no evidence is found for large mass outflow from elliptical galaxies.

Jura, Michael↗
NASA NTRS1989

Composite interstellar grains

The possibility that interstellar grains are composite collections of very small particles, loosely attached to one another, is investigated. The grains are assumed to include a substantial proportion of vacuum along with solid particles. The optical properties of the composite grains are calculated with the approximate 'Bruggeman mixing rule', and the extinction of the grain is calculated wih standard Mie theory. The derived cross sections are close to those of hollow spheres in which the constituents occupy the proper volume fractions, and the derived optical constants of the mixture also obey the Kramers-Kroenig relations. The calculated interstellar extinction law shows reasonable agreement with observations of the diffuse ISM over the 1500-0.12 micron wavelength range. The predicted albedo is 0.5-0.6. It is predicted that the visual extinction per H nucleus should increase slightly with R(V).

Mathis, John S.↗