By observing the difference in optical depths between absorption spectra toward the two components of double sources we have measured the variations in opacity over lengths of less than 0.1 up to 10 pc inside diffuse interstellar clouds. Significant variations are detected on scales larger than about 0.2 pc, but not less. This may represent the minimum size for diffuse cloud structure. By comparing the variations of Gaussian fitted line parameters we find that variations in the internal velocity field of diffuse clouds explain the data rather better than tiny independent “cloudlets.”

This paper will show that a diffuse ionized warm gas fills a large volume of space in the general direction of Radio Loop II. There are three types of observational evidence: Faraday rotation measures (RM's) of extragalactic sources; emission measures (EM's) derived from the Hα emission line in the diffuse interstellar medium; and magnetic field strengths in HI clouds derived from Zeeman splitting observations.

We present results of a search for small scale H I structure at high galactic latitudes using the NRAO 140 foot telescope, which has a 21' beam at 21 cm. We examined randomly selected 4° x 5° regions, as well as regions of particularly low total column density. The amount of apparent structure is small in directions with total column densities of a few times 1020 cm-2.

In 1981 we conducted an extensive, low-latitutde, HI 21-cm absorption survey from the VLA. The goal of the survey was to answer the following questions:

(a) What is the vertical distribution of the cold HI clouds?

(b) Do HI clouds get systematically warmer at high |z|?

(c) Does the fraction of warm intercloud medium HI increase with high |z|?

(d) How much HI has been missed in previous HI emission surveys because of optical depth effects?

(e) What is the galactic distribution of cool HI clouds?

Two level calculations by others for the linear polarizations expected for molecular emission lines from interstellar clouds are extended by considering a number of coupled rotational states of a diatomic molecule. Higher transitions commonly have strengths comparable with that of the J=1-0 transition so that depolarization due to coupling between the various J-states might be expected to reduce the predicted linear polarization. The significance of the null observational results obtained recently by Wannier, Scoville and Barvainis is assessed. The inclusion of additional states tends to decrease the maximum polarizations, though the calculated polarization is increased under conditions for which “superthermal” excitation occurs.

If the gas-to-dust ratio is sufficiently uniform throughout the local interstellar medium, galaxy counts may provide a useful probe of the large scale structure of the interstellar gas. This idea substantiated by gamma-ray observations has led to the discovery of nearby molecular cloud complexes. The reddening studies indicate that one of them lies between 80 and 140 pc from the sun. From CO observations, its molecular mass is estimated to be a few 103 M⊙.

Interstellar Lyman-α observations toward high latitude stars, which give the column density of HI between us and the star, and 21 cm radio observations, which give the total HI column density to and beyond the star, can be combined to map out the amount of neutral gas as a function of distance from the sun. We present recent observations of the 21 cm and Lyman-α lines made toward ~50 high latitude OB stars, and use these data to discuss the distribution of HI in the solar neighborhood.

We report a parallax of 7.9±0.8 mas for PSR 0950+08, corresponding to a distance of 130±15 pc. The measured pulse dispersion of this pulsar implies an average free electron density of 0.023±0.002 cm-3 along the line of sight. This parallax measurement is subject to systematic errors and questions of interpretation which have not yet been fully explored.

Trying to understand the local interstellar gas in detail may be a hopeless task for a theorist. In the interstellar medium as a whole, we can at least address global properties and perhaps come to some reasonable “time averaged” conclusions such as those of Cox and his collaborators (e.g. Cox and Smith 1974, Cox 1979) or McKee and Ostriker (1977). Even this is quite uncertain of course, both because the ISM gas has structure on scales from at least 1 Pc (and probably much smaller) all the way up to the size of the galaxy, and because none of us are quite sure which physical processes (such as thermal evaporation or heating of cooler gas by magnetohydrodynamic processes) are really important. However, in the local ISM things are significantly worse in that we no longer have even the ergodic hypothesis available to us – rather we have to try and deal with individual events and structures. On the other hand, we do have more detailed observations and hence a laboratory to try to decide on the importance of the various physical processes.

The suggestion that the soft x-ray background arises In part from the Sun being inside a large (R ∼ 100 pc) supernova blastwave is examined by producing models of spherical blastwaves. Such models can produce quantitative fits to both surface brightnesses and energy band ratios (for the lowest energy bands) when t ∼ 105 yr, EO = 5 x 1050 ergs, and nO ≃ 0.004 cm-3.

Such models can be generalized by varying the relative importance of such factors as thermal conduction, Coulomb heating of electrons, and external pressure; by allowing the explosions to occur In pre-existing cavities with steep density gradients, or by examining the effects of large obstructions or other anlsotroples in the ambient medium.

Soft X-ray surveys proved the existence of a local hot bubble surrounding the solar system (Mc Cammon et al 1983). Recently two experiments measured the spectrum of this hot interstellar medium with solid state detectors during rocket flights between 300 and 1000 eV (Rocchia et al 1984). The good resolution of the experiment permitted to measure CV, CVI and OVII line emissivities with good accuracy. We use these data to put some constraints on a model where the emission comes from a supernova remnant expanding in a finite pressure medium (Cox and Anderson 1982). We show the importance of non equilibrium ionization phenomena.

Bubbles are very common structure units in the Galaxy and galaxies. Collection of radio, optical, infrared and x-ray observations of the Cyg superbubble (CSB) region of the sky show that the CSB is not a single bubble object. Between 50 to 75 percent of its x-ray emission can be ascribed to discrete sources. The other 25 to 50% x-ray emission, probably originates from bubbles around 8 OB associations of the region. All bubbles located within the spiral structure of Galaxy, M31 and M33 have diameter ≲ 300pc.

The large distance of stellar association from the galactic plane (GP) combined with picture of the gas distribution within the LSA shows that a Reyleigh-Taylor instability in the LSA can develop and give use to the formation of compact steller clusters, such as the Cyg OB2 association. Development stages of the Reyleigh-Taylor instability, some peculiarities of the dust distribution and departures of the local structure from the galactic grand design suggest the absence of a spiral Shockwave in the LSA.

High resolution IUE spectra of Feige 24 reveal sharp C IV absorption features at both the stellar velocity and the velocity of the interstellar lines. Dupree and Raymond have shown that the ionization zone created by Feige 24 itself accounts for the interstellar components of C IV and Si IV, and that the density of hot white dwarfs in the galaxy implies that similar ionized zones around hot white dwarfs make a significant contribution to the C IV column densities observed toward distant O and B stars. Typical white dwarf velocities of around 30 km/s and C IV ionization zone diameters less than 1 pc imply that the crossing time is comparable to the ionization time, so that non-equilibrium ionization models are required. Models of the resulting elongated trails are presented.

This paper presents some reflexions about the theory of magnetic spinning alignment. It is shown that a classical enhanced thermal Davis and Greenstein mechanism may be considered as a limiting case of Purcell's suprathermal spinning alignment theory when the number of active sites on a given grain becomes very important and/or for very short-lived suprathermal sites.

This result is applied to the interstellar medium and some problems in which it may be important are briefly discussed, with special attention to local implications.

The kinematic properties of gas and stars in the Solar neighborhood are described in terms of the line-of-sight component of a three-dimensional first order Taylor series expansion of the local velocity field. The types of object analysed are (1) 21 cm absorbing clouds (2) intercloud medium (3) main sequence B stars closer than 200 pc (4) B stars of luminosity class ranging from I to IV (5) main sequence A stars (6) K-giant stars. The least squares fitting procedure used to derive the 10 coefficients describing each of the six velocity fields was essentially the same, so that a valid comparison could be made. Marked departures from circular motion are found in most cases, but the only systematic trend is a correlation between ∂u/∂x (u being the velocity component along the x-axis directed towards the Galactic center) and stellar spectral type, where the gas behaves like a medium ‘younger’ than the early type stars. Our analysis of the gas indicated that the standard plane-parallel model provided a good description for the intercloud medium, but was inadequate for the absorbing clouds. A velocity ellipsoid description of the residuals is presented for each type of object. The influence of the Gould belt on local kinematics is discussed.

An ensemble of orbits passing through the solar position have been generated for a specific mass model of the galaxy. These orbits are randomly sampled to form simulated density distributions of tracer stars perpendicular to the galactic disk. The simulated distributions are analyzed in order to determine the sampling errors in a self-consistent derivation of the total amount of matter near the sun (the Oort limit).

The spectrographs aboard Space Telescope and Columbus will provide important new information about the interstellar medium in the immediate vicinity of the sun. The Space Telescope high resolution spectrograph (HRS) will produce resolutions, λ/Δλ, of about 18,000 and 70,000 with high sensitivity between 1200 and 3200 A and greatly reduced sensitivity between 1060 and 1200 A. The highest resolution is adequate to define the multicomponent nature of interstellar absorption lines and to measure thermal line widths exceeding 3 km s-1 . The Columbus mission is in the planning stages. However, it is likely that the spacecraft will contain spectrographs capable of resolutions of 3 x 104 between 912 and 1200 A and 500 between 100 and 900 A. In the longer wavelength region, the very important lines of 0 VI, S VI, H2, H I , and D I are available for study. In the short wavelength region, lines of He I and II, are observable. If the 3x104 resolution spectrograph can provide extended wavelength coverage to 770 A, lines of Ne VIII which are expected from 8x105 K gas are accessible. Astronomers using the ST HRS and Columbus spectrographs will be able to study a wide range of problems relating to cold, warm, and hot gas in the local ISM. Some of the most important observing projects are described.

The Berkeley Extreme Ultraviolet/Far Ultraviolet Shuttle Telescope (BEST) will be launched on the Space Shuttle in November, 1984, as part of the NASA UVX project. The Berkeley spectrometer will make observations of the cosmic diffuse background in the 600-1900 Å band, with a spectral resolution of 10 Å. The sensitivity and spectral resolution of the instrument make it ideal for the study of components of the interstellar medium in the 104 - 106K range.

In the local interstellar medium, the presence of hot gas (log T ~ 5 to 6) has been inferred from measurements of oxygen VI absorption in the ultraviolet, and from diffuse emission in the extreme ultraviolet (EUV) and soft X-ray regions. Here we describe a spectrometer that is very sensitive to gas in this temperature range. The spectrometer uses an array of plane-ruled gratings at grazing incidence in the extreme off plane mount. A set of Kirkpatrick-Baez mirrors focuses the conically diffracted light on to one of two microchannel plate detectors. The field of view of the instrument is 0.2° by 12°. The predicted sensitivity ranges from 50 to 200 ph/(cm2 sec str) with a resolving power (λ/Δλ) of 15 to 50 over the 50 to 700A wavelength band. The instrument is currently under construction for a sounding rocket flight.