Report to the 1999 General Assembly for 1996-99
During the three-year period the activities of the Commission were carried out by correspondence. The activities were confined primarily to consideration of conferences to be supported by IUPAP, and membership of the Commission.
Conferences sponsored by the Commission 1996-1999:
New Developments in the Field
Optical counterparts of gamma-ray bursters
The discovery of optical counterparts to some gamma ray bursts (M. Metzger et al., Nature 387, 878, 1997, and M. Djorgovska et al., Nature 387, 876, 1997) has confirmed that these are cosmological events. Time evolution of gamma ray, optical and radio emissions supports a fire-ball model for the explosions. However, it is still not clear what causes the underlying explosive event - coalescence of two neutron stars and a 'hypernova' explosion being the favored alternatives.
There is now clear evidence for extrasolar planets orbiting some solar-like stars (e.g., the review by A.P. Boss, Physics Today, September 1996). The presence of the planets is inferred from their effect on the motion of the parent star, with Doppler shifts corresponding motions at about 1 m/s now being achieved. Most of these planets have Jupiter-like masses. There has been a very recent report of an Earth-like planet inferred from a gravitational lensing event.
A coordinated international program for continuous, precise observations of the radio velocity at the solar surface provides information on the properties of the modes of oscillation of the Sun, and hence of the internal structure of the Sun (e.g., the reviews in Science, Volume 272, 1996). In particular, splitting of the modes provides information on the differential rotation inside the Sun, showing that the latitudinal differential rotation ceases below the base of the solar convection zone.
Very-Long-Baseline-Interferometry from Space
The angular resolution radio wavelengths increased markedly with the launch of the Japanese spacecraft HALCA which allows VLBI with Earth-based telescopes. The available resolution is 0.3 milliarcseconds. An early radio image was that of a jet in quasar 1156+295.
Black Hole at the center of the Milky Way
Stellar motions very close to the center of our Galaxy imply the presence of a central black hole with a mass of 2.6 million solar masses (Eckart et al., Monthly Notices of the Royal Astronomical Society, 284, 576, 1997).
The reported discovery of neutrino oscillations by Super-Kamiokande is the strongest evidence so far for neutrino mass. The implications for neutrino astrophysics, in particular for the solar neutrino deficiency problem and for the role of neutrinos in supernova explosions, are still unclear.
Hubble Deep Fields
The Hubble Deep Fields in the north, and more recently in the south, have provided evidence on the early epoch of galaxy evolution, including the early phase of heavy-element formation (e.g., Ferguson et al., Physics Today, April 1997).