Planetary nebulae

A comprehensive review of the contemporary status of the problem of planetary nebulae is presented. The characteristics of their galactic distribution, their evolution and their cosmogony are emphasised. Though the review is based on all available information on the planetary nebulae and their nuclei, the literature cited in detail covers those papers published in the years 1982–1987.     

The fundamental parameters of the central stars of eight WR ring nebulae

We describe work that has recently been completed on deriving the fundamental parameters of eight WR stars through the photoionization modelling of their surrounding nebulae using non-LTE WR flux distributions. The resulting effective temperatures range from 57 000–71 000 K for the WN4-5 stars and <30 000–42 000 K for the WN6-8 stars. The derived stellar parameters are compared with those obtained from stellar emission line modelling. We find good agreement for the hot early WN stars, indicating that the non-LTE WR flux distributions have essentially the correct shape in the crucial far-UV region. We find lower temperatures for the four cooler late WN stars, particularly for the two WN6 stars. For the nebulae surrounding these stars, we find that the model flux distributions produce too much nebular ionization. We suggest that these discrepancies arise because of the lack of line-blanketing in the WR atmospheres. For the WO1 central star of G2.4+1.4, with strong nebular He II 4686 A emission, we derive a temperature of 105 000 K, somewhat less than previous estimates. The positions of our eight WR stars on the H-R diagram are compared with the evolutionary tracks of Maeder (1990) for solar metallicity. In common with previous workers, we find that our derived luminosities are too low, giving an initial mass range of 25–40 M, below that expected for the majority of WR stars.     

Synchrotron Radiation from Galactic Sources: What We Can Learn About Particle Acceleration

I review the observations of galactic synchrotron sources, focusing on shell supernova remnants (SNRs), with particular attention to attributes that constrain the properties of electron acceleration. Radio observations provide information on source fluxes, spectral index, morphology, and polarization. Recent observations give us strong reason to believe that several young SNRs show synchrotron X-ray emission. Even if X-rays are thermal, however, limits can be set on the maximum energy to which electrons can be accelerated without a spectral break, since no galactic SNR is observed to have X-ray emission (due to any source) as bright as the extrapolation from radio frequencies of radio synchrotron emission. If synchrotron X-rays are detected or inferred, their morphology and spectrum provide important information on mechanisms governing acceleration to the highest energies. I describe models of synchrotron emission from SNRs and their comparison with observations. Finally, I describe the tasks ahead for both observers and theoreticians, to make better use of what SNR synchrotron emission tells us about particle acceleration.