2.2. Type Ia Supernovae as Standardized Candles
SNIa have been used as extragalactic distance indicators since Kowal
[42]
first published his Hubble diagram
( = 0.6 mag) for
type I SNe. We now recognize that the old type I SNe spectroscopic
class is
comprised of two distinct physical entities: SNIb/c which are massive
stars that undergo core collapse (or in some rare
cases might undergo a thermonuclear detonation in their cores)
after losing their hydrogen
atmospheres, and SNIa which are most likely thermonuclear
explosions of white dwarfs. In the mid-1980s it
was recognized that
studies of the type I SN sample had been confused by these
similar appearing SNe, which were henceforth classified as type
Ib [59,
94,
102]
and type Ic [36].
By the late 1980s/early 1990s, a strong case was being made
that the vast majority of the true type Ia
SNe had strikingly similar light curve shapes
[11,
46,
47,
48],
spectral time series
[6,
18,
28,
62],
and absolute magnitudes
[47,
54].
There were a small minority of clearly peculiar type Ia SNe (e.g.,
SN1986G
[63],
SN1991bg
[19,
49], and
SN1991T
[19,
78]), but
these could be identified and removed by their unusual spectral
features. A 1992 review by Branch and Tammann
[7]
of a variety of
studies in the literature concluded that the intrinsic dispersion in
B and V maximum for type Ia SNe must be < 0.25 mag,
making them "the best standard candles known so far."
In fact, the Branch and Tammann review indicated that the magnitude dispersion was probably even smaller, but the measurement uncertainties in the available datasets were too large to tell. The Calan/Tololo Supernova Search (CTSS), a program begun by Hamuy et al. [31] in 1990, took the field a dramatic step forward by obtaining a crucial set of high quality SN light curves and spectra. By targeting a magnitude range that would discover type Ia SNe in the redshift range z = 0.01 - 0.1, the CTSS was able to compare the peak magnitudes of SNe whose relative distance could be deduced from their Hubble velocities.
The CTSS observed some 25 fields (out of a total sample of 45 fields) twice a month for over three and one half years with photographic plates or film at the Cerro Tololo Inter-American Observatory (CTIO) Curtis Schmidt telescope, and then organized extensive follow-up photometry campaigns primarily on the CTIO 0.9 m telescope, and spectroscopic observation on either the CTIO 4 m or 1.5 m telescope. Toward the end of this search, Hamuy et al. [31] pointed out the difficulty of this comprehensive project: "Unfortunately, the appearance of a SN is not predictable. As a consequence of this we cannot schedule the followup observations a priori, and we generally have to rely on someone else's telescope time. This makes the execution of this project somewhat difficult." Despite these challenges, the search was a major success; with the cooperation of many visiting CTIO astronomers and CTIO staff, it contributed 30 new type Ia SN light curves to the pool [32] with an almost unprecedented control of measurement uncertainties.
As the CTSS data began to become available, several methods were presented that could select for the "most standard" subset of the type Ia standard candles, a subset which remained the dominant majority of the ever-growing sample [8]. For example, Vaughan et al. [97] presented a cut on the B-V color at maximum that would select what were later called the "Branch Normal" SNIa, with an observed dispersion of less than 0.25 mag.
Phillips [64]
found a tight correlation between the rate at which
a type Ia SN's luminosity declines and its absolute magnitude,
a relation which apparently applied not only to the Branch
Normal type Ia SNe, but
also to the peculiar type Ia SNe. Phillips
plotted the absolute magnitude of the existing set of nearby SNIa,
which had dense photoelectric or CCD coverage, versus the parameter
m15(B), the amount the SN decreased
in brightness in the
B-band over the 15 days following maximum light. The sample showed a
strong correlation which, if removed, dramatically improved the
predictive power of SNIa. Hamuy et al.
[33]
used this empirical
relation to reduce the scatter in the Hubble diagram to
< 0.2 mag in V for a
sample of nearly 30 SNIa from the CTSS search.
Impressed by the success of the
m15(B) parameter, Riess et al.
[79]
developed the multi-color light curve shape method (MLCS), which
parameterized the shape of SN light curves as a function of their absolute
magnitude at maximum. This method also included a sophisticated error model
and fitted observations in all colors simultaneously, allowing a color
excess to be included. This color excess, which we attribute to
intervening dust, enabled the extinction to be measured. Another method
that has been used widely
in cosmological measurements with SNIa is the "stretch"
method described in Perlmutter et al.
[74,
77].
This method is based on the observation that the entire range of SNIa
light curves, at least in the B
and V-bands, can be represented with a simple time stretching (or
shrinking) of a
canonical light curve. The coupled stretched B and V light curves serve
as a parameterized set of light curve shapes
[26],
providing many of the benefits of the
MLCS method but as a much simpler (and constrained) set. This method,
as well as recent implementations of
m15(B)
[24,
65], also allows
extinction to be directly incorporated into the SNIa distance
measurements.
Other methods that correct for intrinsic luminosity differences or limit
the input sample by various
criteria have also been proposed to increase the precision of type Ia
SNe as distance indicators
[9,
17,
93,
95].
While these latter techniques are not as developed as the
m15(B),
MLCS, and stretch methods, they all provide distances that are
comparable in precision, roughly
= 0.18 mag about the
inverse square law, equating to a fundamental precision of
SNIa distances of ~ 6% (0.12 mag), once photometric
uncertainties and peculiar velocities are removed.
Finally, a "poor man's" distance indicator, the snapshot method
[80],
combines information contained
in one or more SN spectra with as little as one night's multi-color
photometry. This method's accuracy depends
critically on how much information is available.