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Type Ia Supernovae as Distance Indicators 1
Bruno Leibundgut
European Southern Observatory
Karl­Schwarzschild­Strasse 2
D­85748 Garching
Germany
bleibundgut@eso.org
1 Introduction
Type Ia Supernovae (SNe Ia) have become the principal distance indicator for the deter­
mination of distances in the Hubble flow and, in connection with their calibration through
Cepheids, the Hubble constant (Hamuy et al. 1996, Riess et al. 1996, Saha et al. 1997).
They also are currently the only serious competitor for a measurement of the cosmological
deceleration (Perlmutter et al. 1995, 1997, Schmidt et al. 1998). In spite of this wide usage
as distance indicators it should be kept in mind that we do not know which stellar systems
evolve into SNe Ia (Branch et al. 1995) and no satisfactory theory of these explosions has
been proposed yet. It is thus very important to understand the sample properties as exactly
as possible. There is little doubt that SNe Ia can be successfully used do measure H 0 de­
spite the current disagreements which are purely due to sample selection and treatment (e.g.
Hamuy et al. 1996, Riess et al. 1996, Saha et al. 1997, contributions by Lukas Labhardt
and Wendy Freedman). A careful investigation of sources of uncertainty and systematics is
quintessential.
2 The Hubble Constant
The small scatter in the Hubble diagram (recession velocity vs. peak magnitude) of appro­
priately chosen samples of SNe Ia has now been shown many times (Tammann & Leibundgut
1990, Branch & Miller 1993, Vaughan et al. 1995, Sandage & Tammann 1995, Hamuy et al.
1996, Riess et al. 1996). The debate centers on the absolute magnitude of SNe Ia and the
compatibility of the nearby, Cepheid calibrated, sample and the one of distant supernovae.
Absolute magnitudes have been measured for a small number of very close supernovae and
is fairly well established (Saha et al. 1997, contribution by Lukas Labhardt). The Hubble
diagram from Hamuy et al. (1996) and Riess et al. (1996) also tie down the line in the Hub­
ble diagram very well. The zero­point of this relation depends on H 0 and M peak alone (e.g.
Leibundgut & Pinto 1992). There are, however, some systematics in determining the correct
location of the line in the Hubble diagram. Depending on its zero­point the Hubble constant
can change by almost 40% for a given absolute magnitude (cf. Leibundgut & Pinto 1992).
This is often ignored. Other sources of uncertainties which affect this measurement are pho­
tometric errors, the determination of the peak magnitude, the light curve shape correction,
if it is to be applied, and extinction. While modern CCD photometry is mostly accurate to
better than 5%, the peak magnitude is often derived by some light curve fitting procedure
1 Contribution to the proceedings of the workshop ''How far can you go?'', held 25.---27. June 1997 in La
Petite Pierre, Alsace, France, available at: http://astro.u­strasbg.fr/howfar/toc.html

(Leibundgut et al. 1991, Hamuy et al. 1996, Riess et al. 1996, Vacca & Leibundgut 1996). It
is very difficult to judge the accuracy of these procedures. For some data sets the coverage of
the light curve peak is dense and the derived peak magnitude is very accurate. In other cases
the peak magnitude has to be extrapolated and systematic uncertainties from assumptions
about light curve families can be introduced (e.g. Hamuy et al. 1994 for SN 1992K). Light
curve shape corrections are controversial, although Riess et al. (1996) presented powerful
statistical arguments why they are tenable. By selecting only a subsample, as proposed by
Vaughan et al. (1995), some of these problems may be avoided. Extinction is either deduced
from external indicators, or treated implicitly (Riess et al. 1996). All these arguments explain
why the measurement of H 0 , although simple in principle, is still debated.
3 Universal deceleration
Even though only a relative distance measurement is required for the determination of q 0 ,
contrary to the absolute measurement needed for the Hubble constant, it is a very tight error
game as well. This is mostly due to the small effect (¸0.2 magnitudes at z=0.3 difference
between a closed and an open universe). Two groups have recently undertaken the task to find
supernovae at redshifts larger than 0.3 (Perlmutter et al. 1995, 1997, Leibundgut et al. 1996,
Leibundgut & Spyromilio 1997, Schmidt et al. 1998). These searches are now contributing
the majority of all supernovae discovered. As of to date (July 1997) there are 86 supernovae
reported in IAU Circulars at redshifts larger than 0.3 amongst which 70 spectroscopically
confirmed to be SNe Ia (29 from the High­z SN Team and 41 by the Berkeley Cosmology
Project). All technical aspects can be controlled fairly well. The CCD photometry can
be performed to almost the same accuracy (¸10%) as with nearby supernovae. The peak
magnitude can be measured quite accurately due to the specific search techniques employed
in these distant searches which ensure observations near maximum light (Perlmutter et al.
1997, Schmidt et al. 1998). K­corrections can be calculated accurately (!5%) for SNe at
redshifts below 0.55 and above 0.9 due to the particular filter distribution (Kim et al. 1995,
Schmidt et al. 1998). By observing the supernovae in two filters and with a sufficient light
curve coverage the light curve correction methods can be used for the correction of extinction
and the peak magnitude.
Perlmutter et al. (1997) published the data of the first seven SNe at z?0.3. Unfortunately,
some of those supernovae lack spectral confirmation and they have been observed only in one
filter severely compromising detection of extinction. Two of the seven supernovae also have
light curve shapes which are not consistent with any SN Ia observed in the nearby universe.
The High­z Team has now also seven supernovae at hand all spectroscopically confirmed to
be regular SNe Ia and with two filter light curves (rest frame B and V).
Time dilation has already been shown to act on the light curves of these distant supernovae
(Leibundgut et al. 1996, Goldhaber et al. 1997). The light curve of SN 1995K could not
be understood in the framework of known SNe Ia if time dilation would not be present
(Leibundgut et al. 1996). The claim, however, that this is proof for the expansion of the
universe has been contested (Segal 1997, Narlikar & Arp 1997).

4 Conclusions
Type Ia Supernovae are probably the best standard candles known in cosmology. They will be
further employed to narrow down the range of the Hubble constant with more observations of
Cepheid distances. What clearly is missing is an understanding of the explosion and radiation
physics which occur SNe Ia. There are still a few thorny problems in SN research. The light
curves are not understood to a level which makes one comfortable to use SNe Ia 'blindly' for
distance measurements. The secondary peak in the red light curves (Suntzeff 1996) and the
occurrence of this bump into the bolometric light curve (Vacca & Leibundgut 1996, Contardo
et al. 1998) are not convincingly explained by theory.
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