This paper provides a brief review on ionization and radiation models of an optically thin, hot plasma and their application to the interpretation of observations of the Local Bubble. The basic structure and atomic physics ingredients of the models are outlined. Some general characteristic results of ionization equilibrium and non-equilibrium calculations are shown. Recent progress in plasma model codes is reviewed. The interpretation of the observations of absorption lines and emission spectra from the Local Bubble is discussed in the framework of ionization equilibrium and non-equilibrium ionization models with particular reference to the fast adiabatic cooling model of Breitschwerdt and Schmutzler.

In this contribution, we discuss some topics in the study of dust in the local interstellar medium. The spectrum of local clouds has recently been measured using ISO and COBE, confirming in general the models of excitation and cooling of dust illuminated by the the interstellar radiation field. We discuss in some detail the spatial correlation of interstellar dust and gas and the idea that infrared emission traces the total column density of the interstellar medium, with the particular application to the formation of H2. We also show that dust is present in the nearby Loop I shell, with properties similar to average, suggesting that the walls of the Local Bubble would also have relatively ‘normal’ dust content.

Galactic filaments called cirrus are used here to study the froth structure of the interstellar medium. Using the Leiden/Dwingeloo HI survey we observe that a significant fraction of intermediate latitude cirrus are distant clouds (RGal < 6 kpc) and that some cirrus extend far above (or below) the Galactic plane (1 kpc). Among them, a supershell centered at l = 15°, b = –12° is studied.

We have reprocessed the IRAS and DIRBE full-sky 100 μm maps, using Hi as a zero-point, to produce a map of Galactic extinction. This map is crosscorrelated with APM galaxy counts over the full APM region, including dusty regions of the sky not used to compute the APM correlation function in Maddox et al.(1990a). The angular correlation function, w(θ), of galaxy counts is computed with and without the extinction correction, and is used to suggest that our map accounts for ~ 90% of the dust-induced correlation in the APM region on scales from 2 – 10°. This is the first time that a limit has been placed on the correlation function of the extinction not represented in a given map.

I review some of the evidences for dust in the Local Bubble and in galactic halos and show that a general mechanism based on radiation pressure is capable of evacuating dust grains from regions dominated by massive star energy input and thus originate huge dusty halos. A Monte Carlo/particle model has been developed to study the dust dynamics above HII chimneys and the results, among other findings, show that dust can travel several kpc away from the plane of the parent galaxy. The cosmological implications of extragalactic dust are briefly outlined.

We present the mass distribution of interstellar grains measured in situ by the Galileo and Ulysses space probes as cumulative flux. The derived in situ mass distribution per logarithmic size interval is compared to the distribution determined by fitting extinction measurements. Large grains measured in situ contribute significantly to the overall mass of dust in the local interstellar cloud. The problem of a dust-to-gas mass ratio that contradicts cosmic abundances is discussed.

Dust grains in low density gas are subjected to sputtering and shattering in fast supernovae shocks. Although there has been extensive modelling of the dust destruction, still little is known about the grains which survive. Jones et al. (1996) have modeled the effect of the destruction processes on the size distribution of grains. They predict that fast J shocks are efficient in grinding large dust grains into smaller particles. We present observations of the dust emission around the nearby star Spica. Comparison of the observations to the infrared emission of ’standard’ cirrus suggest that the abundance of very small grains is significantly enhanced.

We estimate the 1.8 MeV luminosity of the Sco-Cen association due to radioactive decay of 26A1 to (4 – 15) 10−5ph cm−2 s−1. We propose a low surface brightness, limb brightened bubble for the 1.8 MeV intensity distribution. The detectibility of this distribution with existing γ-ray telescopes is discussed.

26Al radioactivity is believed to originate predominantly from massive stars, ejected into interstellar medium in wind phases and/or supernova events. With its million-year decay time, penetrating γ-rays from 26Al decay measure the massive-star history averaged over a time scale of ≃million years, thus extending times cales accessible otherwise. The COMPTEL 1.809 MeV all-sky data from 5 years of observations show irregularities and features at intermediate latitudes, which may have a more local origin (≃ 1 kpc). We find that the large scale emission can be characterized by a Galactic scale height of ≃ 130 pc, and a Galactocentric scale radius of ≃ 5 kpc, with features from spiral structure. Catalogues from massive-star related objects do not significantly improve the description of COMPTEL data above this. Emission associated with nearby structures such as the Gould Belt, Loop I, or stellar aggregates, is indicated, yet cannot be clearly detected. Combined with our imaging results, this suggests that 26A1 yields from massive star ensembles depend on specifics of those stars and their history. Further 26A1 γ-ray studies are underway to help mapping of the massive star history in the solar vicinity.

In the first place, I will summarize the evolution of our picture of the ISM over the past several decades in order to see the importance of superbubbles for ISM structure. In accordance with the progress of observations, the picture of the ISM has dramatically changed from the 1960s. Then, I will review the evolution of theoretical models of superbubbles including the idea of chimney structures of the ISM.

We investigate the gravitational fragmentation in expanding shells by applying an instability ’thermometer’ similar to the Toomre parameter for instabilities in self-gravitating disks. For Sedov–like evolving systems the onset of instability is mainly depending on the density of the ambient medium and the sound speed of the shell matter, whereas the energy injection rate is less important. Shells evolve towards gravitational instability, if the density gradient of the ambient medium is shallower than an isothermal profile, otherwise they become more stable. For density gradients flatter than ∝ r −1, the fragmentation becomes non-linear on the same time scale as the gravitational instability needs to start. In a homogeneous ambient medium the typical size of gravitationally unstable shells is 1 kpc for a gas density of n = 1 cm−3 and decreases to 10 pc for n = 104 cm−3.

The fragmentation of expanding shells and subsequent star formation are analyzed using an analytical model and computer simulations. We discuss the role of the sound speed in the ambient interstellar medium and the influence of the finite thickness of the gaseous disk.

We report on the detection of X-ray emission from the supernova remnant (SNR) G359.1-0.5 in a deep ROSAT PSPC pointed observation. The diffuse emission is well confined within the radio shell of the SNR. Its spectrum can be represented by a thermal plasma model at T ~ 1 keV and Fx ~ 2×10−11 erg cm−2 s−1 (0.1–2.0 keV) modified by the column absorption of NH ~ 3 × 1022 cm−2. This result supports the view that the source is at a large distance consistent with that from radio observation, and helps to resolve a puzzle about the SNR in previous observations.

The Cepheus bubble is a giant shell with a diameter of about 120pc, discovered by Kun (1987) from IRAS sky flux maps. We have imaged this 10° × 10° region in the CO J=l-0 line, using the FCRAO Quarry system with sample spacing equal to the 45” FWHM beam width. We estimate the total molecular gas mass to be ~ 105 solar masses and a total kinetic energy ~ 1051 ergs. We find 49 O and B0-B2 type stars in the Cepheus bubble region, most belong to the CepOB2 association. 120 IRAS point sources, selected according to color criteria representative of young stellar objects, fall within our map. Many of these sources are associated with the globules and filamentary structures seen in the CO emission, similar to the situation found in IC1396 (Patel et al. 1995). These stars, which are still associated with dense molecular gas plausibly represent the third and youngest generation of stars in this region.

We present ASCA SIS observations of the wind-blown bubble NGC 6888. Because the ASCA SIS is sensitive to higher energy photons and has a higher spectral resolution compared to the ROSAT PSPC, we are able to detect a T ≈ 8×106 K plasma component besides the T ≈ 1.5×106 K component known from previous PSPC observations. The existence of a high-temperature component, the observed limb-brightened X-ray surface brightness profile, and the observed level of X-ray surface brightness cannot be satisfactorily explained by currently available models. Reducing heat conduction at the contact discontinuity may raise the central temperature and produce a limb-brightening; however, the expected X-ray surface brightness is still considerably higher than the observed surface brightness.

Preliminary results of the 4He relative abundance determination from Radio Recombination Lines observations at different frequencies in the Orion, Rosette and W3 HII regions are presented. The Orion HII region has been mapped at both 22 GHz and 36 GHz with the same beamwidth (2 arcmin) using the Medicina and Puschino radio telescopes respectively. The RRLs parameters, together with their variation with frequency and with distance were determined by centering the map on the star θ 1OriC. Three positions were observed in the Rosette HII region at 8.3 GHz leading to the first detection in this region of the transition He92α. The derived 4He relative abundance is considerably greater than the ones obtained from previous measurements. The W3 HII region was observed at 36 GHz and the 4He/H value derived was compared with previous measurements performed with higher spatial resolution.

The status of halo studies using UV absorption lines is assessed. Sin absorption is essentially only seen from gas with z ≤ 1 kpc, and shows a modest flow toward the galactic disk. CIV shows diffuse absorption in general only from gas with z ≥ 1 kpc with predominantly negative velocities. Three exceptional cases of gas showing CIV absorption without absorption by neutral components are discussed. Outflowing gas has not been detected thusfar.

High latitude observations find C3+, N4+, and O5+, while shadowing by intervening hydrogen clouds indicates the presence of soft X-ray emitting gas in the halo of our galaxy. This paper reports on detailed hydrocode simulations of one of the possible source types: isolated supernova remnants evolving in the tenuous ambient medium of the halo. Their O5+ column densities are similar to those observed from the halo. The N4+, C3+, and X-ray properties are also simulated and discussed. The calculations may also be of interest for the Local Bubble, external galaxies, and non-thermal pressure estimations.

We present new spectroscopic results which constrain the distance of a large IR emitting filament near the LMC, as well as some new results for a local region with widely distributed detections of Intermediate Velocity Clouds (IVCs). We present spectroscopic observations of stars toward both regions which constrain the distances and kinematics of the interstellar features. New optical high resolution (λ/δ λ ⋍ 106) spectra have been obtained for the star HD 22252, and are used to estimate approximate distances, temperatures, and pressures within the cloud. An unusual local region of the interstellar medium with several IVCs is also reported, with multiple components of absorption from -80 < υ LSR < 70 km s−1 seen toward several stars in the region.

The boundaries of high-velocity-clouds that hit the Galactic plane are sources of strong X-ray emission indicating gas temperatures of a few million Kelvin. We show that numerous individual magnetic reconnection processes in the interaction region between an HVC and the ionized Reynolds layer may cause the gas heating via dynamical Ohmic dissipation. We present results of numerical plasmaneutral gas fluid simulations performed to study the dynamical reconnection process and to calculate the conversion of magnetic energy into heat.