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

tel-00372504, version 1 - 1 Apr 2009
CHAPTER 2. OBSERVATIONAL COSMOLOGY
2008), to infer the galaxy cluster mass function: direct counting, x-ray flux/temperature,
the Sunyaev-Zeldovich effect or weak gravitational lensing.
2.3 Supernovæ
A supernova explosion is the energetic display of a star’s transition to a new phase of its
evolution, or its death. This is one of the most energetic events in the universe, creating a new
short-lived object in the sky, that outshines its host galaxy during a time-frame of a few weeks.
Their extreme intrinsic luminosity make supernovæ observable up to cosmological distances, and
thus standardizable probes of dark energy.
I will first present the divisions of the broad supernovæ “label” and corresponding origins,
and then detail the present usage of supernovæ in cosmology.
2.3.1 Classification and origins
Historically, supernovæ classification has been based on their spectral features. Minkowski
(1941) divided them into two groups, depending on the existence of hydrogen lines on their
spectra at maximum light: type I supernovæ do not show H lines, whether type II do. This
division was latter extended (Filippenko 1997), as it became noticeable that there were more
spectral and photometric differences inside each group.
The type I group is divided into 3 sub-families: the type Ia supernovæ (SNe Ia) with strong
Si ii absorption lines; and the Ib and Ic, both without Si ii and respectively with or without
He I. The type II divides into: II-l and II-p, based on their light curve 2 shapes (linear or with
a plateau); II-n distinguished by narrow emission lines and slowly decreasing light curves; and
II-b which represent a special type of supernova whose early time spectra is similar to type II
and late time to Ib/c. This classification and corresponding spectra and light curve examples
can be seen in Fig. 2.3 and 2.4.
Despite what one could think from this “standard” classification, type Ib/c supernovæ are
closer to type II than to type Ia. This can be seen from the similar late time spectra (Fig. 2.4a),
and from measurements of the explosion rates per host galaxy (Cappellaro et al. 1999): we do
not observe any Ib/c or II supernovæ on elliptical galaxies, whether Ia are observed in all galaxy
types. Furthermore, SNe Ia present an overall homogeneous spectroscopic and photometric
behavior, contrary to the other types. These evidences (as well as the observation of the crossover
II-b type, linking SNe II with SNe Ib/c), reflect the different physical mechanisms, and
hence the different origins, between the SNe Ia and all the other SNe.
Assuming a supernova originates from a single stellar object, only two physical mechanisms
can explain the observed energy output: either through the release of the nuclear energy by an
explosive reaction (thermonuclear supernovæ); either through the release of the gravitational
binding energy when a star collapses to a compact object (core-collapse supernovæ).
Thermonuclear supernovæ
The homogeneity of the class of SNe Ia, allied to the facts that no hydrogen or helium exists
in their spectra, and no remaining object is found in its remnants, provides a strong hint that
their progenitor may be a carbon-oxygen white dwarf (WD) star (Woosley and Weaver 1986).
A WD is the “residual after-life” of stars whose main-sequence mass is lower than 10M⊙.
During the main part of its life, the star burns hydrogen transforming it into helium. The
2 A light curve is the representation of the supernova’s brightness variation with time.
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