New mass limit for white dwarfs
doi:10.1038/nindia.2013.27 Published online 20 February 2013
Astrophysicists at the Indian Institute of Science (IISc) in Bangalore have proposed "a fundamentally new mass limit" for white dwarfs different from what was proposed over 80 years ago known as Chandrasekhar's limit.
Their new finding may have far reaching implications, including "a possible reconsideration of the expansion history of the Universe1."
A white dwarf is formed when a normal star, about five times the mass of the Sun, exhausts its nuclear fuel. In a white dwarf, the inward gravitational force is balanced by the force due to outward pressure created by "degenerate" electrons. [When different states of a particle correspond to the same energy in quantum mechanics, they are called degenerate states.]
In the 1930s, Indian-American astrophysicist Subramanyan Chandrasekhar predicted that the mass of a white dwarf cannot be more than 1.44 times the solar mass (2.864X1030 kg) — the so-called Chandrasekhar limit. He showed that if the white dwarf exceeds this limit — for example by accretion of mass from a companion star — its gravitational force gets stronger leading to contraction and increase in temperature. This initiates fusion reactions resulting in what is known as 'Type Ia supernova' explosion.
In astronomical parlance, such supernovae are called 'standard candles'. Astronomers try to understand the expansion history of the universe by measuring the distance of these supernovae to their host galaxies using the variation of their luminosity with time.
"So far, observations of these supernovae seemed to abide by this Chandrasekhar limit," Banibrata Mukhopadhyay, one of the authors told Nature India. "However, the recent discovery of several peculiar 'Type Ia supernovae' — that are super luminous compared to their standard counterparts — calls for the mass of the white dwarf to be significantly higher than the limit set by Chandrasekhar."
The IISc researchers note that Chandrasekhar's work did not include the effect of a magnetic field in the calculations whereas the Sloan Digital Sky Survey, a project launched in 2000 to map a large part of the universe, has discovered several magnetized white dwarfs with surface fields as high as 109 gauss. "Our main aim here was to obtain the maximum possible mass of such a white dwarf (which is magnetized), and therefore a (new) mass limit," the researchers said.
Besides the pressure due to 'degenerate electrons', which is also amended due to the additional quantum mechanical effects brought in by the magnetic field, an additional outward pressure also arises due to magnetic field in a white dwarf. "Hence, when the white dwarf is magnetized, a larger outward force can be balanced by a larger inward force in its equilibrium allowing it to have more mass," Mukhopadhyay said. "We show that strongly magnetized white dwarfs exhibit a limit of 2.58 solar mass thus significantly violating the Chandrasekhar mass limit of 1.44," he said. "We arrived at this new mass limit by exploiting the effects of the magnetic field in compact objects."
The researchers say these super luminous supernovae are 'new standard candles' for distance measurement. "This may have many far reaching implications, including a possible reconsideration of the expansion history of the universe," Mukhopadhyay says.
"However, it is probably too early to comment whether our discovery has any direct implication on the current dark energy scenario, which is based on the observation of ordinary Type 1A supernovae," he says.
The IISc scientists believe the new limit they propose may perhaps have several consequences. For one, the masses of white dwarfs — currently calculated from luminosities assuming Chandrasekhar's mass-radius relation — may have to be re-examined based on the new mass-radius relation at least for some peculiar objects, for example over-luminous Type 1A supernovae.
Secondly, Mukhopadhyay said "if the peculiar Type Ia supernovae are eventually found to be large in number, then in order to correctly interpret the expansion history of the universe (and then dark energy), one might need to carefully sample the observed data from the supernovae explosions."
- Das, U. et al. New mass limit for white dwarfs: Super-Chandrasekhar type Ia supernova as a new standard candle. Phys. Rev. Lett. 110 (2013) doi: 10.1103/PhysRevLett.110.071102