Why are there no known white dwarfs between 1.35 to 1.44 solar masses?

Question

The Chandrasekhar limit for white dwarfs is 1.44 Solar masses, however the heaveist known white dwarf is only 1.35 solar masses. https://earthsky.org/space/smallest-most-massive-white-dwarf/

What's the cause of this difference in mass?

Answer 1

Two reasons. Firstly, the "Chandrasekhar mass" of 1.44 solar masses is based on a pair of unrealistic assumptions, that are not met in practice, which means the true mass limit is more like 1.37 or 1.38 solar masses. Secondly, white dwarfs more massive than about $1.2 M_{\odot}$ are not produced by normal single-star stellar evolution, only through mass transfer in binary systems. This mass transfer may result in the star exploding as a supernova before it grows beyond $1.35M_{\odot}$.

The two assumptions are: (I) that the white dwarf is supported by ideal electron degeneracy pressure. i.e. Point-like, non-interacting fermions. (II) That the structure of the star is governed by Newtonian gravity.

The first assumption fails because the electrons and ions do have Coulomb interactions that make the material more compressible. More importantly, at high densities (and the density increases with mass), the electron Fermi energy eventually becomes high enough to initiate electron capture to make more neutron-rich nuclei. Since the electrons are ultra-relativistic, the star is already marginally stable at this stage, and the removal of electrons causes instability and collapse.

The second assumption fails because more massive white dwarfs are smaller and General Relativity must be used. The General Relativistic formulation of the equation of hydrostatic equilibrium features pressure on the RHS. So the higher the pressure, the steeper the required pressure gradient. Ultimately, this also leads to an instability at a finite size and density that occurs at masses lower than the canonical Chandrasekhar mass.

For typical C/O white dwarfs, both of the instabilities discussed above occur when the white dwarf is at about 1.38 solar masses.

Note that white dwarfs of more than about 1.2 solar masses are not expected to arise from the evolution of a single star. If the C/O core of a star is more massive than this, then it will also become hot enough to ignite these elements. More massive white dwarfs will need to have been produced by accretion in a binary system or by a merger. Then, another factor comes into play, which is the possible detonation of the entire white dwarf, which may also occur above 1.35 solar masses, possibly ignited by the fusion of helium from the accreted material or by pycnonuclear reactions as the C/O core increases in density.

Postscript - there actually are some white dwarfs with estimated masses of $1.35-1.37M_\odot$ in classical novae binary systems (e.g. Hachisu & Kato 2001). These may be systems that are about to go "bang".