Astronomy and Astrophysics – Astronomy
Scientific paper
Apr 1994
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1994pasau..11...80m&link_type=abstract
Astronomical Society of Australia, Proceedings, vol. 11, no. 1, p. 80-81
Astronomy and Astrophysics
Astronomy
Pulsars, Stellar Evolution, Stellar Models, Stellar Structure, Supernova Remnants, Brightness, Radio Astronomy, Stellar Magnetic Fields, Stellar Winds
Scientific paper
The postulated association between supernova remnant morphology and supernova type (Type Ia--shell, Type II--plerion or composite), combined with observationally derived birthrates of Galactic supernovae (van den Bergh et al., 1987, Astroys. J., 323, 44), implies that 60 to 85% of Galactic supernova remnants (SNRs) should be plerions or composites. In fact, of the approximately 150 Galactic SNRs discovered to date, approximately 85% are shells. Old remnants are certainly too dissipated to confine the relativistic wind from a central pulsar, but young condensed SNRs should host plerions visible from any viewing geometry. One explanation for the paucity of plerions is that neutron stars are born with a weak magnetic field, only switching on as pulsars after 105 years (when the SNR has largely dissipated) following a period of thermoelectric field growth (Blandford et al., 1983, Mon. Not. R. Astron. Soc., 204, 1025). Another possibility is that all Type II supernovae contain a pulsar surrounded by a plerion and a shell of ejecta. Whether we see the plerion or shell depends on which component has the highest surface brighteness. Supernova remnants whose shell and plerionic components evolve independently have been modeled by Bhattacharya (1990, J. Astrophys. Astron., 11, 125). Three ingredients enter the model: (i) the plerion's synchrotron luminosity evolves due to adiabatic expansion of the nebula (Pacini and Salvati, 1973, Astrophys. J., 186, 249); (ii) shell emission is governed by turbulent amplification of the magnetic field and Fermi acceleration of particles (Gull 1973, Mon. Not. R. Astron. Soc., 161, 47); and (iii) expansion of the plerion and shell are driven by the supernova blast rather than the relativistic wind from the pulsar, with the plerion occupying all the volume within the expanding shell. The model only reproduces a plerionic morphology under the following restricted conditions: the initial period and magnetic field of the pulsar are Crab-like, and the supernova explosion occurs in a low-density ambient medium or the supernova event itself is less energetic than normal.
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