Wednesday, October 1, 2014

When Supermassive Stars Explode in the Early Universe

Supermassive stars with ~10,000 to ~100,000 times the Sun’s mass are believed to have formed in the very early universe. These are the first generation of stars in the universe and are entirely comprised of hydrogen and helium. They live very short lives before collapsing directly to form black holes. A team of astrophysicists ran a number of supercomputer simulations and found that some of these supermassive stars die in a rather unusual way. Instead of collapsing to form black holes, supermassive stars in a narrow mass range between 55,000 to 56,000 times the Sun’s mass explode as highly energetic thermonuclear supernovae, leaving nothing behind.

This image is a slice through the interior of a supermassive star of 55,500 solar masses along the axis of symmetry. It shows the inner helium core in which nuclear burning is converting helium to oxygen, powering various fluid instabilities (swirling lines). This snapshot shows a moment one day after the onset of the explosion, when the radius of the outer circle would be slightly larger than that of the orbit of the Earth around the Sun. (Credit: Ken Chen, UC Santa Cruz)

A supermassive star with 55,500 times the Sun’s mass lives for about 1.69 million years before it becomes unstable and starts to collapse. During its pre-collapse phase, the size of the star is slightly larger than the diameter of Earth’s orbit around the Sun and the star has an effective surface temperature of about 70,000 K. The star is also remarkably luminous, with ~1.5 billion times the Sun’s luminosity. With the onset of helium burning in the star’s core, the prodigious amount of thermal photons being generated in the core affects the star’s gravitational field by becoming an additional source of gravity. As a consequence, the core begins to contract, causing the temperature and density in the core to rise rapidly, accelerating nuclear burning.

As the core contracts, helium begins to burn explosively, fusing to carbon, and then to oxygen, neon, magnesium and silicon. The explosive nuclear burning occurs within a span of only several hours and releases ~10 times more energy than the binding energy of the star. This causes the star to halt its collapse and unbind completely in a massive explosion known as a general relativistic supernova (GSN). The amount of energy produced in such an event is ~10,000 times the energy released by a typical supernova. In total, about half the mass of the star is ejected in the form of elements heavier than hydrogen and helium. Mostly elements between carbon and silicon are produced, with only trance amounts of iron group elements.

After getting blasted out into the cosmos, these heavy elements are incorporated in the formation of subsequent generations of stars and planets. The energetic demise of these supermassive stars can be detected by upcoming space-based observatories such as ESA’s Euclid and NASA’s Wide-Field Infrared Survey Telescope (WFIRST). Additionally, indirect observational signatures of GSN explosions might be found by looking for early galaxies that are iron deficient but enhanced with elements from carbon to silicon.

Ke-Jung Chen et al. (2014), “General Relativistic Instability Supernova of a Supermassive Population III Star”, arXiv:1402.4777 [astro-ph.HE]