| 2. SCIENTIFIC OBJECTIVES
2.1 THE ORIGIN OF THE ELEMENTS 2.1.1 PROMPT EMISSION FROM SUPERNOVAE & NOVAE Because the sites of explosive nucleosynthesis, novae and supernovae are optically thick to gamma rays, only the delayed gamma-ray line emission from the decay of synthesized radionuclei can be observed. Furthermore, this is possible only for sites that become at least partially transparent on time scales less than the radioactive decay mean lives. The most luminous lines from individual events are the 56Ni and 56Co lines of Type Ia supernovae. Such supernovae are required to make ~0.6 Mo of 56Ni during their explosion to provide both the energy to unbind the white dwarf and to power the light curve. The ejecta from these supernovae also have higher velocities than Type II's because the characteristic 1051 erg of kinetic energy is distributed within an object about 10 times less massive. The flux at maximum in the prominent lines of 56Co in a typical Type Ia supernova is about 3x10-5 (10 Mpc/D)2 photons cm-2 s-1 occurring about 50 to 150 days after the explosion. These lines are quite broad, however, with typical velocities of about 5000 km/s. The full width is thus about 30 keV. An enduring goal has been to measure Type Ia supernovae in the Virgo cluster at about 20 Mpc where the event rate is high. To study these supernovae and learn anything save the well-known fact that they made some 56Ni, one needs broad line sensitivities no worse than a few times 10-6 photons cm-2 s-1. Lacking adequate sensitivity to do this, one must await the occasional nearby event. Ideally one would like not only to see the lines, but to resolve their velocity structure and get the velocity distribution and mass of 56Ni made in the explosion. This constrains the explosion mechanism (detonation or deflagration) and provides information on mixing of the inner and outer layers of the supernova. Similar information can be obtained by watching the time dependent transparency of the event, but to do this one must begin to measure the flux quite early and continue measuring it for a long time. SN 1987A was typical of Type II supernovae, though about a factor of 10 brighter at maximum in the gamma-ray lines of 56Co (because it was a blue supergiant instead of a red one). The peak flux at 55 kpc was about 10-3 photons cm-2 s-1. This means that observations of Type II supernovae will be restricted, for the next decade or so, to improbable occurrences in the local group of galaxies. Type Ib supernovae are also massive stars, but lack hydrogen envelopes. They produce about 5 times less 56Ni than Type Ia, but expand almost as rapidly. Thus their signal is intermediate between Types II and Ia. A mission having broad line sensitivity of a few times 10-6 photons cm-2 s-1 might detect one in our galaxy would be very bright in the decay lines of 56Co and thus CGRO provides a fail-safe against missing the next galactic event. |