Stars with initial masses such that 10 M_☉ ≤  M_(initial)  ≤  100M_☉, where is the solar mass, fuse progressively heavier elements in their centres, until the core is inert iron. The core then gravitationally collapses to a neutron star or a black hole, leading to an explosion—an iron-core-collapse supernova. By contrast, extremely massive stars with M_(initial) ≥ 140M_☉(if such exist) develop oxygen cores with masses, M_(core), that exceed 50M_☉, where high temperatures are reached at relatively low densities. Conversion of energetic, pressure-supporting photons into electron–positron pairs occurs before oxygen ignition and leads to a violent contraction which triggers a nuclear explosion that unbinds the star in a pair-instability supernova. Transitional objects with 100M_☉  3M_☉ of radioactive ^(56)Ni was synthesized during the explosion and that our observations are well fitted by models of pair-instability supernovae. This indicates that nearby dwarf galaxies probably host extremely massive stars, above the apparent Galactic stellar mass limit11, which perhaps result from processes similar to those that created the first stars in the Universe.

Supernova 2007bi as a pair-instability explosion.

Over the past four years, the Nearby Supernova Factory (SNfactory) has obtained hundreds of time-series spectra of Type Ia supernovae (SNe Ia) using its SuperNova Integral Field Spectrograph (SNIFS). This dataset is a promising resource for improving our understanding of SN Ia explosion physics and progenitor models --- key issues that, if resolved, could further improve the utility of and confidence in SNe Ia as reliable cosmological distance indicators. Using a new, automated direct analysis code we have developed (SYNAPPS), we have commenced a systematic study of the SNfactory spectroscopic data set. SYNAPPS solves for a highly parameterized synthetic spectrum consistent with the observed spectrum, enabling line identification and ejecta velocity measurements, while treating line-blending explicitly. These results provide important constraints on stellar explosion models. We present preliminary results including, for example, an investigation of some early-phase spectra, and examine trends in spectral behavior noted previously by others. We also address future directions for code development to bring about a deeper physics analysis of the data.

T. Rollin

Papers

Direct Analysis of Type Ia Supernovae Observed by The Nearby Supernova Factory