Evolution of Iron-Core White Dwarfs

Figure 1: A typical white dwarf packs as much mass as the Sun into a volume the size of Earth. With so much mass packed into such a small object, white dwarfs are incredibly dense. A teaspoon if its material would weight many tons.

George C. Jordan et al. (2012) and Isern J. et al. (1991) have proposed ways that can lead to the formation of iron-core white dwarfs (WDs). A study by J. A. Panei et al. (2000) examined the structure and evolution of iron-core WDs, and found that iron-core WDs are markedly different from carbon-oxygen WDs which make up the majority of WDs. The study modelled the evolution of iron-core WDs with various masses and compositions. Also included for comparison in the study are standard carbon-oxygen WD models.

Iron-core WDs with masses of 0.4, 0.5, 0.6, 0.7, 0.8, 0.9 and 1.0 solar mass are adopted. Firstly, the models examined pure iron cores comprising 99 (pure iron model), 75, 50 and 25 per cent of the WD’s mass, with a carbon-oxygen envelop for the last 3 cases. The models then examined WDs with a homogeneous composition of iron and carbon-oxygen by adopting a mass fraction for iron of 0.25, 0.50 and 0.75. In all cases, an outer helium envelope comprising 1 per cent of the WD’s mass is included.

The models show the evolution of iron-core WDs to be very different from carbon-oxygen WDs. Compared with carbon-oxygen WDs of the same mass; iron-core WDs have smaller radii and greater surface gravities because of their denser interiors. In particular, iron-core WDs cool a lot faster than carbon-oxygen WDs. This is because iron nuclei are much heavier than carbon or oxygen, resulting in iron having a much lower specific heat per gram. As a consequence, iron-core WDs are poor at storing heat and given the same amount of time, can cool to much lower luminosities than carbon-oxygen WDs. For iron-core WDs, crystallization of iron also occurs much earlier and at much higher luminosities than carbon-oxygen WDs of the same mass.

Figure 2: Radii in terms of effective temperature corresponding to pure iron (full lines) and carbon-oxygen (dot-dashed lines) WD models, and to WD models with a pure iron core containing 50 per cent of the total WD’s mass (dashed lines) for WD masses of 0.4, 0.5, 0.6, 0.7, 0.8, 0.9 and 1.0 solar mass (the higher the mass the smaller the radius). Source: J. A. Panei et al. (2000).

Figure 3: Same as Figure 2, but now dashed lines show the results for homogeneous WD models with an iron abundance by mass of 0.5. Source: J. A. Panei et al. (2000).

Figure 4: Age versus luminosity relation corresponding to pure iron (full lines) and carbon-oxygen (dotted lines) WD models, and to WD models with a pure iron core containing 50 per cent of the total WD’s mass (dashed lines) for WD masses of 0.4, 0.5, 0.6, 0.7, 0.8, 0.9 and 1.0 solar mass. Note that pure iron models cool down ~5 times faster than carbon-oxygen WDs. At high luminosities in this figure, the discontinuity in the slope of pure iron WD models is a result of crystallisation. Source: J. A. Panei et al. (2000).

Figure 5: Same as Figure 4, but now dashed lines show the results for homogeneous WD models with an iron abundance by mass of 0.5. Source: J. A. Panei et al. (2000).

References:

- J. A. Panei, L. G. Althaus & O. G. Benvenuto, The evolution of iron-core white dwarfs”, MNRAS (2000) 312 (3): 531-539

- George C. Jordan et al. (2012), “Failed-Detonation Supernovae: Sub-Luminous Low-Velocity Ia Supernovae and Their Kicked Remnant White Dwarfs with Iron-Rich Cores”, arXiv:1208.5069 [astro-ph.HE]

- Isern J., Canal R., & Labay J. (1991), “The outcome of explosive ignition of ONeMg cores - Supernovae, neutron stars, or ‘iron’ white dwarfs?”, ApJ, 372, L83