Simmering white dwarfs consume electrons

Prior to its explosion as a type Ia supernova, a white dwarf “simmers”: over a 1000 year period, ¹²C fusion in its core gradually raises its internal temperature to roughly 800 million K. Now JINA researchers have discovered how the white dwarf consumes some of its electrons during this period. In one of two independent efforts to study this phase, MSU graduate student David Chamulak has computed the rate at which the core of the white dwarf becomes neutron-enhanced prior to the explosion. The JINA team found, in agreement with Piro & Bildsten of the Kavli Institute for Theoretical Physics, that the reduction in electron abundance depends how much ¹²C is consumed, which depends on factors such as the temperature of the white dwarf prior to ignition. This electron abundance plays an important role in the supernovae: it controls the amount of radioactive ⁵⁶Ni synthesized in the explosion, and hence controls the peak brightness of the supernovae. Astronomers have been aware for some time that type Ia supernovae in elliptical galaxies are slightly dimmer, on average, than their cousins in spiral galaxies, but the reason for this correlation is unclear. It may be that part of the difference depends on the internal structure of the white dwarf prior to its explosion. The JINA team has computed new effective heating and neutronization rates for this simmering phase. These rates include a new evaluation of the ¹³N electron capture rate computed by MSU researcher Remco Zegers and will be useful for future hydrodynamical simulations of the pre-explosion white dwarf.

First Superburst from a Transient LMXB

Keek et al. (2007) have discovered the first superburst from a neutron star transient. In this system, the crust temperature (solid line) can be inferred from observations of the surface temperature during quiescence, when the accretion is off. Using new calculations of the heating from crust nuclear reactions (Gupta et al. 2007) and a time-dependent neutron star crust model, we computed the temperature at the time of the superburst. Surprisingly, the temperature (dotted line) 80 days into the accretion outburst, when the superburst occurred, is not hot enough to for carbon fusion to begin (red box), posing a challenge to current neutron star models.

Credit: L. Keek, J.J.M. in 't Zand, E. Kuulkers, A. Cumming, E. F. Brown, and M Suzuki.

Phase separation of rp-process ashes

Professor Charles Horowitz (Indiana University) has computed the transition from solid to liquid for a typical neutron star crust. At high densities, the electrostatic potential between ions are much larger than the thermal energy, and the ions arrange themselves in a lattice (visible as diagonal planes in the top part of the image). The composition is taken from recent models of the crust (Gupta et al. 2007). Surprisingly, the material was found to separate, with lighter nuclei (oxygen in this case) diffusing into the liquid portion, and heavier nuclei diffusing into the crystalline portion.

Credit: Charles Horowitz and Don Berry (Indiana University) and Edward F. Brown (Michigan State University and the National Superconducting Cyclotron Laboratory).

²²Ne makes ¹²C laminar flames burn faster in white dwarf supernovae

Type Ia supernovae result from the thermonuclear incineration of a white dwarf star. They are currently the premier standard candle for measuring the geometry of the universe and for probing the properties of the "dark energy" that is making the universe accelerate at an accelerating rate. Despite their importance, many of the details of the explosion remain poorly understood. One unanswered question is how the composition of the white dwarf affects the explosion. Michigan State University graduate student David Chamulak has demonstrated that the enrichment of the white dwarf with ²²Ne (formed during the fusion of helium in the progenitor star) speeds up the flame in the early stages of the explosion. This finding means that distant supernovae, which exploded long ago when the universe was poorer in heavy elements, are somewhat different than nearby supernovae. It remains to be seen whether this translates into any systematic errors in using type Ia supernovae as "standard candles."

Credit: David A. Chamulak, Edward F. Brown (Michigan State University and the National Superconducting Cyclotron Laboratory) , and Francis X. Timmes (LANL). This work was supported by the NSF grant AST-0507456, by the Joint Institute for Nuclear Astrophysics at MSU under NSF-PFC grant PHY 02-16783, and by the US Department of Energy via its contract W-7405-ENG-36 to Los Alamos National Laboratory.

Image credit: High-Z Supernova Search Team, HST, NASA

Sinking of CNO affects X-ray bursts

Many observed neutron stars accrete gas from a companion star. Once enough gas has piled up on the neutron star surface, nuclear reactions ignite and trigger an explosion known to astronomers as an X-ray burst. Graduate student Fang Peng (University of Chicago; now a Sherman Fairchild Postdoctoral Scholar at Caltech) is calculating how the sedimentation of isotopes such as carbon, nitrogen and oxygen affects the “fuel” for these bursts. The picture at right illustrates the problem: over time (represented by the arrows) the heavier nuclei (in green) in the accumulated fuel layer settle downwards and the lightest nuclei, hydrogen (in red), float upwards. By lowering the abundance of hydrogen relative to heavier isotopes, sedimentation changes the ashes of the unstable nuclear reactions. This depletion of hydrogen might make the ashes of the burst more enriched in carbon, a necessary ingredient to explain the recently discovered “superbursts.” Although these initial calculations are unable to follow the spatial structure of the burning through multiple bursts, Dr. Peng is now working with Dr. Alexander Heger, Los Alamos National Laboratory, to incorporate this process into an X-ray burst simulation.

F Peng (University of Chicago), E F Brown (Michigan State University and National Superconducting Cyclotron Laboratory) and J W Truran (University of Chicago); the Joint Institute for Nuclear Astrophysics

Neutron star crusts hotter than previously thought

The drawing illustrates the structure of a neutron star.  Although much of the star (red and yellow regions) is at greater than nuclear density, the crust (brown layer) is made of "normal" nuclei. Many neutron stars accrete hydrogen and helium from a stellar companion like our sun. As the hydrogen and helium accumulates, it fuses to heavier elements in an explosion known as a type I X-ray burst. In some systems there are superbursts that occur yearly and are about 1000 times more energetic than a type I X-ray burst. Over millions of years of accretion, the crust of the neutron star is gradually replaced by these "ashes." JINA researchers at MSU, Los Alamos National Laboratory, and University of Mainz, Germany, have now computed the heating in the crust using a realistic model of the relevant nuclear reactions. Interestingly, the amount of heat deposited in the crust by these reactions is much larger (a factor of 5–10) than previously thought. This extra heating may partially explain why how some neutron stars are able to produce superbursts on a yearly timescale: a hot crust helps to ignite the superburst!

Credit: S Gupta (LANL), E F Brown and H Schatz (MSU, NSCL and JINA) , P Möller (LANL), and K-L Kratz (Univ. Mainz)