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

tel-00372504, version 1 - 1 Apr 2009
CHAPTER 2. OBSERVATIONAL COSMOLOGY
expand, instead fueling even more the helium burning), increases the temperature continually,
and as it approaches the Fermi temperature the electrons become once again non-degenerate
and the helium-flash stops.
The star then burns quietly all the helium in the core, creating carbon and oxygen until (as
in earlier phases), the core fuel ends and the reactions continue in an outer shell of helium. Once
again the star expands, and since the helium burning reaction (triple-alpha) is very sensitive to
temperature, the star will enter a pulsating phase, where the balance between internal pressure
and gravity changes as the star expands (and the temperature decreases) or contracts (and the
temperature increases). These pulsations generate a very strong outflow of mass from the stars
surface, which will gradually scatter its envelope in the form of an expanding shell heated by
the hot core, a planetary nebula 3 . The core, constituted of a degenerate carbon and oxygen gas,
continues to contract and cool, and a WD is formed.
The WD’s exotic constitution makes it a strange object: the more massive it is the smaller
it gets (a WD with the mass of the Sun would be roughly the size of the Earth). Besides, a WD
is only stable up to a specific mass, the Chandrasekhar mass (Chandrasekhar 1931) of ∼ 1.4M⊙,
above which the degenerate electron gas pressure cannot counter the gravitational collapse.
That is the key behind the SNe Ia: if we have a WD in a binary system (the single-degenerate
scenario), accreting matter from a companion (Fig. 2.5), its mass will steadily increase up to the
Chandrasekhar limit. At that moment, the density in the core is such that carbon fusion ignites,
leading to the production of 56 Ni. As we have already seen, thermonuclear fusion is unstable
in the degenerate matter, in the sense that it does not allow the moderation of the burning by
expansion. A nuclear instability ensues, followed by a nuclear burning front that propagates
through the star and explodes it completely.
A new SNe Ia then appears in the sky and its luminosity peaks in just a few days. The
consequent decrease of the light curve over several months cannot be powered by the explosion
itself, since temperature decreases too fast, and is explained instead by the radioactive decay of
the 56 Ni produced during the carbon fusion, into its daughter nucleus: 56 Ni → 56 Co → 56 Fe.
While accounting for the relative SNe Ia homogeneity (related to the limit imposed by
the Chandrasekhar mass), not all the physical processes behind this model are yet fully understood.
Several flame propagation and explosion scenarios are proposed (Hillebrandt and
Niemeyer 2000), and despite the huge improvement in recent years, numerical hydrodynamical
simulations of SNe Ia explosions are still incapable of providing an accurate enough description
of observations (Roepke 2008).
Core-collapse supernovæ
The fact that type II or Ib/c supernovæ are absent from the elliptical galaxies, which contain
only old, low-mass stars, makes a strong point on their origin as the gravitational collapse of
a massive star. In the core of their progenitors (stars with mass higher than ∼ 10 M⊙), the
hydrostatical equilibrium holds as long as the fusion reactions burn the available hydrogen and
form heavier and heavier elements, up to iron. The star has then an “onion-layer” structure,
with layers of different composition, from the iron in the core to the hydrogen 4 in the envelope.
Since no further exothermic reactions are possible, as the iron core mass continues to increase, it
reaches the Chandrasekhar mass and the internal (degenerate electron) pressure can no longer
counterbalance gravity. The star collapses until the core attains a large enough density and
3 Historically called this way, since it looks like a planet when viewed through a small telescope.
4 In the case of SNe II. SNe Ib progenitors are thought to have lost their hydrogen layer due to strong stellar
winds or interaction with a binary companion, while SNe Ic would lose their helium layer as well.
26