large and independent sets of optical spectra have reached the same conclusion that
20-30 percent of SNe Ia harbor unprocessed
carbon. Meanwhile, there had been no detection of carbon in the near-infrared spectra
of a normal SN Ia. The issue appeared settled.
It was up to the theorists to find the right
combination white dwarf binary systems
and explosion mechanism to reproduce the
observed rate of carbon detection.
The situation changed, however, in 2011.
Using the high-quality GNIRS spectra and
a more sophisticated spectrum modeling
technique, we were able to detect carbon in
SN 2011fe, a first in the near-infrared wavelengths for a normal SN Ia. In Figure 2, we
show the comparison between observed
and model spectra. The near-infrared carbon
line we studied is relatively isolated and ideally located between two magnesium lines.
Our model spectra show that the presence of
carbon is required to produce the observed
“flattened” profile near 1.03 microns.
Furthermore, the time-series GNIRS observations indicate that the influence of carbon increases with time (Figure 2). The carbon
line in the optical, on the other hand,
usually disappears very early, requiring
that the supernova be discovered at a
very young age. We propose that the delay in the onset of the near-infrared carbon feature can be explained simply by
the change in the ionization condition.
The “flattened” profile caused by the presence of carbon in SN 2011fe appears to be
common in normal SNe Ia. This suggests
that many SNe Ia harbor unprocessed carbon. Again, the low rate of detection in the
optical may be caused by the difficulty of
obtaining spectra within a few days of the
explosion. Since the conclusion of ubiquitous unprocessed carbon would have profound implications for our understanding of
SNe Ia explosions, we are currently conducting a careful survey of the near-infrared carbon feature in our growing sample of nearinfrared spectra.
The Main Driver of the
Luminosity-decline Rate Relation
A landmark paper by Wheeler et al. (1998)
identified the strong and relatively isolated
absorption feature near 1.05 microns as magnesium. They predicted that the velocity of
this feature would decrease rapidly and then
settle to a constant velocity. The prediction is
finally confirmed 15 years later, as our SpeX
and GNIRS spectra caught the rapid decline
Figure 3.
Time evolution of
SN Ia near-infrared
magnesium velocity.
The magnesium velocity
of the GNIRS SN 2011fe
spectra underwent a
rapid decline and an
extended period of
constant velocity. Note
that SN 1999by is a
spectroscopically peculiar
SN Ia, much like SN
1991bg. The magnesium
velocity of normal SNe Ia
all show similar constant
behavior as that of SN
2011fe.
As the supernova ejecta expands, the
temperature decreases. The optical carbon line in its first ionized state then
gradually recombines into neutral carbon which forms the ever stronger neutral carbon feature in the near-infrared.
Due to this fortuitous delay in its appearance, the near-infrared neutral carbon
feature is potentially a superior probe of
unprocessed material to the more commonly used optical feature.
January2014 2013 Year in Review
GeminiFocus
21