Chandrasekhar mass

Iwamoto K., Brachwitz F., Nomoto K., Kishimoto N., Hix R., and Thielemann F.-K. (1999) Nucleosynthesis in Chandrasekhar mass models for Type la supemovae and constraints on progenitor systems and burning-front propagation. Astrophy. J. Suppl. 125, 439—462. 

The critical core mass required to reach all thermonuclear burning stages is called the Chandrasekhar mass. Its value is about 1.4 M0, but it depends on the mean molecular weight per free electron pe — reflecting the balance between pressure and gravity ... 

The real situation may be worse, as the time scale to transport the angular momentum from the white dwarf surface into its degenerate core need to be considered. Clearly, any finite angular momentum redistribution time makes the white dwarf envelope reach critical rotation earlier (cf. Yoon Langer 2002, Yoon et al. 2002). Without any doubt, if Chandrasekhar mass white dwarfs are responsible for Type la supernovae, there must be a way to remove angular momentum from the accreting stars or from the accreted material. At present, it is an open question of how this can be achieved (cf. Livio Pringle 1998). 

Type la SNe are believed to originate from the C-deflagration of a WD reaching the Chandrasekhar mass (1.44 M0) after accretion of material from a young companion in a close binary system. C-deflagration occurs as a consequence of such accretion and... 

High-mass stars have a final mass greater than the Chandrasekhar mass (Sect. 13.1). In these stars, nuclear burning of carbon, oxygen and heavier elements will be followed by collapse as a type II supernova (Sect. 13.2). This is generally thought to occur for stars with initial masses greater than 8 M . 

One challenge encountered by this scenario is that the mass predicted for white dwarf companions in short-period sdB binaries should be < O.5M0. Recent observations have demonstrated several sdB+WD binary systems in which substantially higher masses are required for the unseen companion. In some cases these masses exceed the Chandrasekhar mass limit for white dwarfs (Mch Sect. 13) and are a continuing challenge for stellar evolution theory [53]. 

As said in Sect. 2, the iron core left over following Si burning suffers a dynamical instability as a result of endothermic electron captures and Fe photodid-integration. To a first approximation, this gravitational instability sets in near the classical Chandrasekhar mass limit for cold white dwarfs, Mch = 5.83 T) , Ye being the electron mole fraction. In the real situation of a hot stellar core, collapse may start at masses that differ somewhat from this value, depending on the details of the core equation of state. The reader is referred to [18] (especially Chaps. 12 and 13) for a detailed discussion of the implosion mechanism and for its theoretical outcome and observable consequences. Here, we just briefly summarise the situation. 

A scenario referred to as a sub-Chandrasekhar-mass supernova envisions a C-O WD capped with a helium layer accreted by a companion, and which explodes as the result of a hydrodynamical burning before having reached the Chandrasekhar limit. This type of explosions may exhibit properties which do not match easily the observed properties of typical SNIa events. It cannot be excluded, however, that they are responsible for some special types of events, depending in particular on the He accretion rate and on the CO-sub-Chandrasekhar WD (SCWD) initial mass (e.g. [85]). Unidimensional simulations of He cataclysmics characterized by suitably selected values of these quantities reach the conclusion that the accreted He-rich layer can detonate. Most commonly, this explosion is predicted to be accompanied with the C-detonation of the CO-SC WD. In some specific cases, however, this explosive burning might not develop, so that a remnant would be left following the He detonation. Multidimensional calculations cast doubt on the nature, and even occurrence, of the C-detonation in CO-SC WD (e.g. [86]). 

The calculation of p-process yields from a variety of Chandrasekhar mass Type I supernova models of the deflagration or delayed detonation types, as well as of sub-Chandrasekhar He-detonation models. In the latter case, a... 

Chandrasakhar limit The maximum possible mass of a star that is prevented from collapsing under its own gravity by the degeneracy pressure of electrons. For white dwarfs the Chandrasekhar mass is about 1.4 times the mass of the sun. There is an ana-... 

White dwarf Collapsed remnant of the core of a star whose mass is near 1 solar mass. The mass of a white dwarf is limited to 1.4 solar masses, the so-called Chandrasekhar mass. White dwarfs typically have a radius of about 10" km. 

The final evolution of massive stars (see e.g. the review given by Woosley, Heger, and Weaver, 2002 [366]) becomes more violent than that for low massive stars. A cosmic catastrophe occurs. The gas of the Fe core becomes electron degenerate. As soon as it exceeds about the mass of 1.4 solar masses (this is called Chandrasekhar mass), i.e. the core exceeds the size of the Earth, gravity is too strong to... 

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