ALBERT

Description: Neutrino losses, such as those driven by the convective Urca process, may affect the evolution of stars in the mass range from 4 to 8 solar masses so as to lead to collapse of their degenerate carbon/oxygen cores. A corresponding hydrodynamic model is computed which leads to the formation of a 1.3 to 1.4 solar mass neutron star with the expulsion of a small fraction of the mass, about 0.l solar mass at about 20,000 km/sec into the overlying hydrogen envelope. This sets the stage for the Ostriker-Gunn mechanism in which Type II supernovae and pulsars are formed.

Description: Wilson (1971) has found that the neutrino transport mechanism is unable to generate a supernova explosion in stars with collapsing iron cores. The present work uses Wilson's analysis to investigate the behavior of the overlying potentially explosive layers which Wilson omitted. The outer boundary of the core of Wilson's models moves in such a manner as to deliver a shock to the base of the envelope. We have numerically followed the progress of such shocks into the envelope of a realistic model obtained from evolutionary calculations. We find that only shocks so strong as to be inconsistent with our treatment are capable of ejecting material. For reasonable shocks the nuclear burning does not proceed rapidly at densities below 1,000 kg/cu cm, and the nuclear energy released is less than the shock energy in all models that come near to ejecting matter.

Description: We report near-infrared (NIR) spectroscopic observations of 12 'branch-normal' Type Ia supernovae (SNe Ia) that cover the wavelength region from 0.8 to 2.5 microns. Our sample more than doubles the number of SNe Ia with published NIR spectra within 3 weeks of maximum light. The epochs of observation range from 13 days before maximum light to 18 days after maximum light. A detailed model for a Type Ia supernovae is used to identify spectral features. The Doppler shifts of lines are measured to obtain the velocity and thus the radial distribution of elements. The NIR is an extremely useful tool to probe the chemical structure in the layers of SNe Ia ejecta. This wavelength region is optimal for examining certain products of the SNe Ia explosion that may be blended or obscured in other spectral regions. We identify spectral features from Mg II, Ca II, Si II, Fe II, Co II, Ni II, and possibly Mn II. We find no indications for hydrogen, helium, or carbon in the spectra. The spectral features reveal important clues about the physical characteristics of SNe Ia. We use the features to derive upper limits for the amount of unburned matter, to identify the transition regions from explosive carbon to oxygen burning and from partial to complete silicon burning, and to estimate the level of mixing during and after the explosion. Elements synthesized in the outer layers during the explosion appear to remain in distinct layers. That provides strong evidence for the presence of a detonation phase during the explosion as it occurs in delayed detonation or merger models. Mg II velocities are found to exceed 11,000 - 15,000 km/s, depending on the individual SNe Ia. That result suggests that burning during the explosion reaches the outermost layers of the progenitor and limits the amount of unburned material to less than 10% of the mass of the progenitor. Small residuals of unburned material are predicted by delayed detonation models but are inconsistent with pure deflagration or merger models. Differences in the spectra of the individual SNe Ia demonstrate the variety of these events.