Simulating super-Chandrasekhar white dwarfs

The famous Chandrasekhar limit was derived back in the 1930s, with Subrahmanyan Chandrasekhar receiving the Nobel Prize for this work in 1983. However, this limit was derived under the assumption of ideal Fermi gases at zero temperatures. Further, effects like rotation and magnetic fields are not considered during the derivation. Indeed, there are observations that indicate the violation of this mass limit. Several over-luminous peculiar type Ia supernovae, e.g. SNLS-03D3bb, argue their respective progenitor to be a significantly super-Chandrasekhar WD with a mass-limit higher than the Chandrasekhar limit.

Our group, for more than a decade, has been actively working on the theoretical possibility of these massive, super-Chandrasekhar WDs, more massive than their conventional value/limit. It has been shown in previous work that the magnetic fields of WDs can lead to higher masses being supported by the star. The magnetic field can lead to both classical and quantum effects on the eventual mass of the star. In fact, previous work showed that in the presence of a magnetic field, there can be a series of mass limits, depending on the type of field geometry and profile present within the WD considered.

In the current work, we explore the formation and time-dependent evolution of magnetic WDs (BWDs). For this, we use the Cambridge stellar evolution code, STARS. We explore in detail the evolutionary path taken by a magnetized main sequence (MS) star as it forms a BWD. To fully incorporate the magnetic effects, we have introduced magnetic fields to STARS by modifying the code appropriately.

Through our simulation, we also explore the possible formation scenarios for magnetized, super-Chandrasekhar WDs. We hypothesize two scenarios for the formation of massive WDs. We may obtain highly massive super-Chandrasekhar WDs from the evolution of magnetized, massive MS models (with masses $\approx 10M_\odot$) such that the magnetic field has a significant effect throughout the evolution and the BWD is massive from birth. Another option is the ``accretion” scenario - here, we consider an evolved sub-Chandrasekhar mass BWD with a dormant field, i.e., the magnetic field does not yet affect the stellar structure. Through further evolution of the BWD by mass accretion, say, from a companion star, the magnetic field grows, leading to a higher mass limit.

We explore further the properties of the formed BWDs. An important source of deviation from the Chandrasekhar relation other than the magnetic effect is the finite temperature effect. The cores of WDs are isothermal, but there is a finite temperature gradient in the core-crust interface. A combination of the magnetic field and finite temperature effects lead to low mass WDs having larger radii, and showing deviation from the Chandrasekhar relation. This has interesting observational implications. Finally, we look at how the effect of the decay of the magnetic field can affect the further evolution of the star.