A neutron star is an ultra-dense remnant core leftover from the violent demise of a massive star. It packs roughly as much mass as the Sun in an incredibly tiny volume measuring just several kilometres across. A spoonful of its material would contain a mass of roughly a billion tons. If the neutron star has a sufficiently close stellar companion, it can strip material from the companion in a process known as accretion. The accreted material can lead to the formation of an ocean on the neutron star. This ultra-dense and exotic ocean is comprised of elements with atomic number Z = 6 and larger. Most of these elements are formed from nuclear burning of the accreted hydrogen and helium from the companion star. Here, the ions behave like a liquid, hence the term “ocean”. Nonetheless, it is in no way like the oceans on Earth. The densities, pressures and temperatures are so extreme that they are only comprehensible numerically.
Figure 1: Artist’s impression of an accreting neutron star. Material stripped from the companion star forms an accretion disk around the neutron star. Image credit: NASA / Goddard Space Flight Centre / Dana Berry.
The ability to observe the sky in X-rays using space-based instruments has led to the discovery of superbursts. These energetic outbursts recur on timescales of years and are believed to be driven by the unstable ignition of a carbon-enriched layer on a neutron star. To ignite a superburst, a carbon-enriched layer needs to contain a carbon mass fraction of roughly 20 percent. However, such a carbon-enriched layer is difficult to produce in most theoretical models. Besides requiring enough carbon, models for superbursts also require large ocean temperatures of roughly 600 million K. Such high temperatures are difficult to attain from standing heating models of neutron stars.
A study by Medin & Cumming (2011) suggests that the preferential freezing of heavier elements at the base of the ocean on an accreting neutron star can substantially enrich the ocean with lighter elements such as oxygen and carbon. At the base of the ocean, the increasing pressure from the continuous accretion of material onto the neutron star forces the preferential freezing of heavier elements. The separation of lighter elements from heavier elements releases energy and provides an additional source of heating for the ocean. After the preferential freeze-out of heavier elements, the remaining fluid becomes lighter than the fluid immediately above it and acts as a source of buoyancy which drives convective mixing of the ocean. Convection distributes the heat throughout the ocean in the form of a convective flux. The extra heat input can raise the temperature of the ocean up to the required ignition temperature of around 500 to 600 million K to produce a superburst.
In the study, a 300 million K ocean consisting of a mixture of iron (Z = 26) and selenium (Z = 34), and a mixture of oxygen (Z = 8) and selenium (Z = 34) is examined. At the base of the ocean, the preferential freezing of heavier elements enhances the abundances of lighter elements in the ocean. For example, a mixture of oxygen and selenium with initial 2 percent oxygen by mass can be enriched to almost 40 percent oxygen by mass. Although oxygen was chosen as the light element in this study, models with carbon (Z = 6) were also investigated and shown to yield similar enrichment results. The carbon mass fraction can be brought up by enrichment to the required ~20 percent for superburst ignition.
Figure 2: Phase diagram for crystallization of an iron/selenium mixture (top panel) and an oxygen/selenium mixture (bottom panel) in a 300 million K ocean on a neutron star. The stable liquid region of each phase diagram is labelled as “L”, the stable solid region(s) are labelled as “S” or “S1” and “S2”, and the unstable region is filled with plus symbols. Additionally, in each panel the composition at the top of the ocean is marked by a vertical dashed line, the ocean-crust boundary is marked by a horizontal dotted line, the composition of the liquid at the base of the ocean is marked by a filled square, and the composition of the solid(s) in the outer crust are marked by filled circles. Medin & Cumming (2011).
Figure 3: Thermal profile of an ocean on an accreting neutron star. The ocean is composed of a mixture of oxygen and selenium. The solid line represents the thermal profile when the convective flux (i.e. energy released at the base of the ocean from the separation of lighter elements from heavier elements) is included in the total heat flux. The dashed line represents the thermal profile when the convective flux is ignored (i.e. the total heat flux is due only to the heat emanating from the neutron star’s interior). Medin & Cumming (2011).
Medin & Cumming, “Compositionally Driven Convection in the Oceans of Accreting Neutron Stars”, ApJ 730:97 (10pp), 2011 April 1.