High-Velocity Features connected to supernova explosion physics

Type Ia supernovae (SNe Ia) are believed to result from the explosion of a carbon-oxygen white dwarf, induced by interaction with a binary companion.   Spectroscopy of SNe Ia offers clues to the explosion properties from the composition and velocities of the SN ejecta imprinted in spectroscopic absorption features.  A new study by CAASTRO researchers Michael Childress and Brian Schmidt, with Berkeley colleagues Alex Filippenko and Mo Ganeshalingam, examines spectroscopic features with anomalously high velocities which provide important clues to the nature of SN Ia explosions.

SNe Ia synthesise new material through rapid nuclear fusion, generate tremendous energy, and eject this material at thousands of kilometres per second.  The spectrum of a SN shortly after the explosion, when the ejecta are hot and dense, is essentially a blackbody spectrum dominated by absorption features produced by material at the edge of this optically thick zone (the photosphere).  These lines typically manifest at the same velocity, corresponding to the edge of the photosphere, which gradually proceeds inwards to lower velocities as the ejecta expand and cool.

Early spectra of some SNe Ia, however, have exhibited some features with exceptionally high velocities 5,000 to 10,000 km/s faster than the photospheric velocity.  Since the origin of these high velocity features (HVFs) remains a mystery, the present study searched for correlations with other SN behaviour to provide clues to the HVF origin.  The research team found a strong correlation of HVF strength with the decline rate of SN Ia lightcurves, such that HVFs are stronger in the more luminous slow-declining SNe Ia and weaker in the fainter fast-declining SNe Ia.  Surprisingly, they also discovered that SNe Ia with high photospheric velocities lack distinct HVFs.

These results show for the first time a distinct connection between the SN Ia explosion physics and the presence of high velocity features in the SN spectrum:  faint fast-declining SNe Ia have low photospheric velocities and no HVFs; luminous slow-declining SNe Ia tend to have either high photospheric velocities or distinct HVFs, but not both. These trends provide an important new observational constraint on the nature of SNe Ia which will inform future models of the progenitor mechanism.

by M. Childress

Figure Caption:
SN Ia ejecta velocity versus light curve decline rate, with data points labelled by the strength of HVFs in their maximum light spectra (blue filled circles for HVF-weak SNe Ia; black open squares for HVF-strong SNe Ia).

Publication Details:
M. J. Childress, A. V. Filippenko, M. Ganeshalingam, B. P. Schmidt in MNRAS 437 (2014) “High-velocity features in Type Ia supernova spectra”