Figure 1. Color image of SN 2011fe in M101
Credit: B. J. Fulton/LCOGT/PTF
Figure 2. 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.
Figure 3. Model spectrum fit to the GNIRS spectra of SN 2011fe around
the near-infrared carbon line. The observed spectra are plotted as solid
black curves. The best-fit model spectra are plotted as follows: with
all ions, with carbon only, and with all ions except carbon. These are
plotted as red dotted, green dashed, and blue dash-dotted curves,
respectively. The vertical dotted lines mark the location of the
best-fit carbon velocity. The phases relative to maximum light are
noted.
Gemini Near-Infrared Spectrograph (GNIRS) observations lead to
surprising results on the nature of Type Ia supernovae (SNe Ia).
Time-series near-infrared spectra of SN 2011fe hint that more SNe Ia
harbor unprocessed carbon than previously believed, and what we thought
was the main driver of the luminosity-decline rate Phillips relation may
not be correct.
Understanding the true nature of Type Ia supernovae (SNe Ia) is a
linchpin of contemporary cosmology. Specifically, these explosions are
critical for tracking the expansion history and acceleration of our
universe. Commonly called “dark energy,” the discovery of this universal
acceleration was the basis for the 2011 Nobel Prize in Physics.
Now, researchers using the Gemini Near-Infrared Spectrograph (GNIRS), at
the Gemini North telescope on Hawaii’s Mauna Kea, have taken a major
leap forward in understanding SNe Ia with the first detection of
unprocessed carbon in near-infrared spectra of a normal Type Ia
supernova. The supernova (Figure 1), denoted SN 2011fe, was discovered
by the Palomar Transient Factory (PTF) within hours of its explosion on
August 24, 2011 in the nearby galaxy Messier 101, located about 21
million light years away and a popular target for amateur astronomers.
Carnegie Supernova Project postdoc Eric Hsiao reports that these
near-infrared observations (a month-long time series consisting of nine
spectra with GNIRS and one with SpeX on NASA’s Infrared Telescope
Facility), “…provide an ideal baseline to compare with other objects,”
and adds that, “Previous studies have detected carbon in optical
spectra, but in this work we were able to detect it for the first time
in the infrared and capture its time evolution.” Hsiao, at Las Campanas
Observatory in Chile, also worked with Howie Marion, a post-doc
researcher the Harvard-Smithsonian Center for Astrophysics (CfA) in
Cambridge Mass., and Mark Phillips the Associate Director of Las
Campanas Observatory and team leader for the Carnegie Supernova Project.
The work is accepted for publication in an upcoming issue of The Astrophysical Journal.
Prior to this finding, the supernova community relied on very early
optical spectra for the study of unprocessed carbon, leftover from the
progenitor white dwarf. With this work, the team shows that the near
infrared is a better wavelength region to survey this unprocessed
material. There are hints that the number of SNe Ia harboring
unprocessed carbon may have been grossly underestimated from previous
studies in the optical, a result that would have profound impact on our
understanding of these explosions.
The team’s observations also confirmed the long-standing prediction that
magnesium, a marker for the boundary between carbon and oxygen burning,
would decrease rapidly in velocity and then settle into a constant
velocity as SNe Ia evolve (Figure 2). The variation in the location of
the carbon/oxygen burning boundary between supernovae is believed to be
the main driver of the Phillips relation. While a large range of
magnesium velocities was found, there was surprisingly no correlation
with the supernovae’s peak luminosities. The team acknowledges that more
work needs to be done to understand this unexpected finding.
Watch for a more detailed article in the March issue of GeminiFocus (e-published on April 1, 2013) and see the preprint of The Astrophysical Journal paper by Hsiao et al. on astro-ph at: http://arxiv.org/abs/1301.6287.
In-Depth
The following details are provided for readers desiring more details of a technical nature.
A key ingredient to realizing the full potential of near-infrared SN Ia
cosmology is near-infrared spectroscopy, such that the peak luminosities
can be accurately converted to the rest frame. With the limited size of
the world’s current sample, the time evolution and the diversity of the
near-infrared spectral features are poorly understood. These
uncertainties directly affect the determination of the peak luminosity.
To improve our knowledge of this relatively unexplored wavelength
region, the Carnegie Supernova Project and the CfA Supernova Group have
embarked on a joint program to obtain a statistically significant sample
of near-infrared spectroscopic observations.
Using high quality GNIRS spectra and a more sophisticated spectrum
modeling technique, Hsiao et al. were able to detect carbon, a first in
the near-infrared wavelengths for a normal SN Ia. Figure 3, shows the
comparison between observed and model spectra. The near-infrared carbon
line studied is relatively isolated and ideally located between two
magnesium lines. The team’s model spectra shows that the presence of
carbon is required to produce the observed “flattened” profile near 1.03
micron.
Furthermore, the time-series GNIRS observations indicate that the
influence of carbon increases with time (Figure 3). 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. The team proposes
that the delay in the onset of the near-infrared carbon feature can be
explained simply by the change in the ionization condition. 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.
Source: Gemini Observatory
