Nearby Supernova Factory: Étalonnage des données de SNIFS et ...

tel-00372504, version 1 - 1 Apr 2009
CHAPTER 2. OBSERVATIONAL COSMOLOGY
Magnitude + const
8
10
12
14
16
18
K-7
H-6
J-5
U+1
I-3
R-2
V-1
0 50 100
Days from B max
(a)
B
M max − 5log(H o /65)
−20
−19
−18
−17
−20
−19
−18
−17
−20
−19
−18
−17
B
V
I
1.0 1.5 2.0
Δm 15 (B) obs
Figure 2.6: (a) Light curve in several bands (Johnson-Cousins UBVRI and near-infrared) from a typical
SN Ia (SN2003du). Phase is with relation to B maximum. Adapted from Stanishev et al. (2007); (b)
Brigther-slower correlation for SNe Ia light curves. Adapted from Phillips et al. (1999).
On the other hand, if one wants to know their absolute magnitude (to test our standard
candle assumption), the case is somewhat different, since we need to have a measure of its
distance. Hamuy et al. (1996) observed a number of nearby supernovæ in the smooth Hubble
flow 7 , for which the distance can be estimated via the host galaxy redshift and Hubble’s law.
The found (uncorrected - see next paragraph) values present some dispersion (Fig. 2.7a): scatters
of 0.38 and 0.26 magnitudes for MB and MV (the absolute magnitude in the B and V bands
respectively) were found. These scatters are compatible with results from several teams, which
have been using observations of SNe Ia on galaxies with distances known through different
techniques, like Cepheids measurements, for the determination of the Hubble constant: Sandage
et al. (2006) observes mean absolute magnitudes MBV between -19.3 and -19.55, compatible with
the 0.3 magnitudes range span of the MB measured by other teams, depending on calibration
techniques and observational methods (see Table 2 in Gibson et al. (2000)).
Despite the existence of this scatter (thought to be mainly due to the amount of 56 Ni produced
during the explosion (Timmes et al. 2003)), it is remarkably small: SNe Ia seem indeed
to be good standard candles.
The first step in the SNe Ia standardization was made when, as new supernovæ started
7 Where they are far enough (z > 0.02) so that intrinsic velocities are negligible, and near enough (z < 0.08) so
that the cosmological expansion acceleration effects are negligible and Hubble’s law is valid in first approximation.
28
(b)