Minas C. Kafatos

Supershells and propagating star formation

Stellar winds and repeated supernovae from an OB association will 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. If the association has a typical initial mass function, its supernovae explosions will inject energy into the supershell at a nearly constant rate for about 50 Myr. The supershell loses its interior pressure and enters the snowplow phase when radiative cooling becomes important or when the shell bursts through the gas disk of a galaxy, typically after a few times 10 Myr and with a radius of 100-300 pc. At approximately the same time, the supershell becomes gravitationally unstable, forming giant molecular clouds which are sites for new star formation. There is widespread evidence for supershells in the Galaxy and other spiral and irregular galaxies from 21-cm emission-line surveys, optical emission-line surveys, and studies of supernova remnants. The gravitational instability of the supershells provides a physical mechanism for induced star formation and may account for bursts of star formation, especially in irregular galaxies. 85 references.

Statistical Time-Dependent Model for the Interstellar Gas

We present models for temperature and ionization structure of low, uniform-density (approximately 0.3 per cu cm) interstellar gas in a galactic disk which is exposed to soft X rays from supernova outbursts occurring randomly in space and time. The structure was calculated by computing the time record of temperature and ionization at a given point by Monte Carlo simulation. The calculation yields probability distribution functions for ionized fraction, temperature, and their various observable moments. These time-dependent models predict a bimodal temperature distribution of the gas that agrees with various observations. Cold regions in the low-density gas may have the appearance of clouds in 21-cm absorption. The time-dependent model, in contrast to the steady-state model, predicts large fluctuations in ionization rate and the existence of cold (approximately 30 K), ionized (ionized fraction equal to about 0.1) regions.

TIME-DEPENDENT IONIZATION EQUILIBRIUM AND LINE RADIATION UNDER FLARELIKE CONDITIONS.

The results of calculations of time-dependent ionization equilibrium and line emission are presented and compared with the values obtained under the assumption that steady-state conditions prevail. In the models considered, it is assumed that the electron density is constant ( = 109 cm-3) and that the temperature increases by a factor of 10 from 3 X 106 o K on timescales ranging from 100 to 300 s and decays back to 3 X 106 o K on a timescale ranging from 600 to 1400 s. Ions of oxygen and silicon are considered, and it is found that the spectrum is softer during the rise and harder during the fall than predicted by the steady-state approximation.

Fossil Stroemgren spheres from supernova explosions

Brandt et al. have shown that consistency in the combined observations of the Gum Nebula requires a giant H II region, presumably formed by the Vela X supernova explosion. Morrison and Sartori had concluded on the basis of their He n fluorescence theory of Type I supernovae that a giant H n region would be formed as result of the ultraviolet burst. (Bottcher et al., by integrating over the light curve, expect a smaller H II region.) We present here in brief some consequences of the fluorescence model as illustrated by the Vela X and the Tycho supernovae. We conclude that such giant H n regions might not in general be as easily detectable as the Vela X region. The Tycho region may just be detectable in the 0 II, 0 III forbidden optical lines or as a hole in the 21-cm emission-line profiles (the latter is already suggested in the data). These giant H II regions last appreciably longer than the continuum radio sources within them. Since no very large H II region is likely to be associated with a Type II supernova explosion, detection of giant H II regions around the Galaxy could give us the frequency of Type I explosions.

The unusual ultraviolet variability of the QSO 3C 232

After examining ultraviolet IUE spectra of 18 QSOs with z > 0.35, we find that one object, 3C 232 (z = 0.533), stands out as exhibiting highly unusual spectral and continuum variability. The H 1-Lya shows at least 50% variability on a time scale of roughly a month, while the ultraviolet continuum exhibited approximately a factor of 2 decrease in a day. The magnitude of the UV continuum variability is reminiscent of that seen in the optically violent variables at visual wavelengths. Several spectral features at wavelengths shortward of H I-Lya in the rest frame are identified. A brief discussion COJ?lparing the spectrum of 3C 232 with those of other QSOs in our IUE sample at wavelengths shortward of 1216 A is also presented. In a survey of archival data obtained by the International Ultraviolet Explorer (IUE) of QSOs, representing over 45 spectra, with redshift z > 0.35, we find that one QSO, 3C 232, exhibits both dramatic continuum and spectral variability unlike that seen in any other object in our sample. (The results of our survey will appear elsewhere.) In this Letter we will describe the unusual ultraviolet variability of 3C 232. Also, we will briefly discuss how 3C 232 compares with other QSOs based upon preliminary results of qur survey of spectral features shortward of H I-Lya (1216 A).