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

tel-00372504, version 1 - 1 Apr 2009
CHAPTER 2. OBSERVATIONAL COSMOLOGY
Scaled f λ + Constant
4
3
2
1
0
Ca II
Ca II
Si II
Fe II
Co II
Cr II
Fe II
Fe II
Si II
Fe III
Fe II
Cr II
Co II
S II
Cr II
Si II
Si II
Na I Si II Fe II
SN 2001V
day +20
4000 5000 6000 7000
Rest Wavelength (Å)
(a)
FLUX
100
10
1
98aq
96X
90N
94ae
98bu
01el
94D
4000 5000 6000 7000 8000
WAVELENGTH
Figure 2.8: (a) Spectrum and characteristic lines at maximum (up) and 20 days later (bottom) for a
typical SN Ia (SN1998aq). From Matheson et al. (2008). (b) Spectra at maximum luminosity for several
“Branch-normal” SNe Ia. From Branch et al. (2006).
The analysis of SNe Ia spectral evolution divides their lifetime into two phases: the early
photospheric phase, where the outer layers are opaque to radiation and we observe their constitution
in the form of absorption lines, that change as the layers expand and become transparent;
and the nebular phase, several weeks after maximum, when the environment has become transparent
and we observe the emission lines of the inner layers, that can be attributed to Co and
Fe transitions. The nebular phase is dominated by the changing strength of these individual
lines multiplets. Despite the overall uniformity, subtle differences exist in lines strengths and
velocities between different SNe Ia (Hachinger et al. 2006).
Deserving to be placed in a category apart, are a lot of peculiar SNe Ia presenting very
different light curves and spectra from the “typical” ones. These will be presented in § 2.3.3.
While this photometric/spectroscopic division was made for easier presentation, they are
obviously intrinsically connected: a light curve point is nothing more than the integration of
a supernova spectrum by a bandpass filter. It is then clear that we could easily study the
homogeneity and standardization of the SNe Ia class by simply using absolutely flux calibrated
spectra. We will see in § 3 how the SNfactory project intends to do this.
2.3.3 SNe Ia heterogeneity - peculiar objects
Not all supernovæ fall into the “Branch-normal” category, as the ones already introduced.
Some are substantially different, enough to place them into two distinct categories: underluminous
and overluminous. They are connected to two objects observed in 1991 and used as
the “reference” for each subclass: SN1991bg (under-) and SN1991T (overluminous) respectively.
Typical light curves and spectra for these supernovæ are presented in Fig. 2.9.
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(b)