spectra caught the rapid decline for the first
time and unambiguously showed the subsequent constant velocity (Figure 3).
Magnesium is a product of thermonuclear
carbon burning and not oxygen burning.
At the phase of constant velocity, the magnesium line therefore locates the boundary
between carbon and oxygen burning. This
boundary is thought to be where the transition from a subsonic to a supersonic burning
front occurs, and its location is sensitively
controlled by the density under which the
transition occurs. If the transition density is
the origin of the observed spread in the peak
luminosities, it might also drive the luminosity-decline rate relation (Hoeflich et al. 1995).
The time-series GNIRS spectra of SN 2011fe
shows an extended period of constant velocity for the magnesium feature, beginning at
10 days before maximum light and lasting
until the feature disappears at 10 days past
maximum light. Therefore, a single spectrum
obtained at any phase within this range is sufficient to determine the transition density of
a SN Ia. Armed with this insight, we surveyed
the near-infrared spectra in the literature and
measured their near-infrared magnesium velocity in a consistent manner (Figure 3).
Surprisingly, when we plot up the magnesium velocities, as a proxy for the transition
densities, against the decline rate of the supernova light curves, there is no correlation.
The transition density does not seem to have
a strong influence on the peak luminosities.
We need to go back to the drawing board
and rethink the origin of the observed variation in the peak luminosities. It is likely that
the transition density affects the luminosity
on a secondary level, which offers the possibility of improving further the standardization of SN Ia luminosities. We are currently investigating the cosmological utility of these
velocity measurements.
April2013
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Eric Hsiao is a Postdoctoral Researcher at Las
Campanas Observatory, La Serena, Chile. H