Thursday, July 10, 2014

New Class of Totally Metal Stars

Stars form from the collapse of giant gas clouds comprised almost entirely of hydrogen and helium, with heavy elements (i.e. elements heavier than hydrogen and helium) making up only a tiny percentage of the material. In these clouds, dust grains contain a large fraction of the heavy elements. Astrophysicists refer to elements heavier than hydrogen and helium as metals, and the concentration of heavy elements in a star is referred to as the star’s metallicity. For example, the Sun’s metallicity is 0.02, which means 2 percent of its mass is in the form of elements heavier than hydrogen and helium.

Turbulence is a ubiquitous process in nature. A study by Philip F. Hopkins (2014) suggests that given the right conditions in star-forming clouds, dust grains can behave as aerodynamic particles and decouple from the gaseous hydrogen and helium. The presence of turbulence can preferentially concentrate the dust grains in specific regions such that the local abundance of heavy elements can become so high that stars made almost entirely of metal can form. Such stars would have metallicities approaching 1.00. In a star-forming cloud with preferential concentration of dust grains, perhaps one in 10,000 stars could form as a “totally metal” star.

For the preferential concentration of heavy elements to occur in star-forming gas clouds, the dust grains must be of a certain size range such that they are large enough to decouple from the gas but also small enough to “feel” the gas flow. Eddies created by presence of turbulence in these gas clouds would drive these dust grains into regions of lower vorticity (i.e. gaps between eddies). As a result, heavy elements become concentrated separately from gaseous hydrogen and helium. Stars that form in these regions can acquire abnormally high metallicities and might even be “totally metal”.

The process of how a core of dust grains might collapse to form a “metal” star is not entirely clear. Nevertheless, it is reasonable to think that as a core of dust grains collapses under its own gravity, the dust grains would shatter into smaller grains and melt to form gas-phase metals. Such a process would cool the collapsing core and drive a more rapid collapse until a “metal” star is formed. Since planets seem to be ubiquitous around stars, planetary systems should also form around “metal” stars. Given the unique high metallicity conditions, the formation of truly exotic planets might be possible.

With an exceedingly high metallicity, a “metal” star would have a remarkably long lifespan and would appear very unusual. This brings to mind the concept of “frozen stars” proposed by Fred Adams and Gregory Laughlin (1997) in a paper entitled “A Dying Universe: The Long Term Fate and Evolution of Astrophysical Objects”. An exceptionally high metallicity low mass star could sustain internal nuclear fusion at a very slow rate for a thousand trillion years or so, outliving the dimmest red dwarf stars by a factor of a thousand. Such a star can even be cool enough to have water-ice clouds, hence the term “frozen stars”.

- Philip F. Hopkins (2014), “Some Stars are Totally Metal: A New Mechanism Driving Dust Across Star-Forming Clouds, and Consequences for Planets, Stars, and Galaxies”, arXiv:1406.5509 [astro-ph.GA]
- Fred Adams and Gregory Laughlin (1997), “A Dying Universe: The Long Term Fate and Evolution of Astrophysical Objects”, arXiv:astro-ph/9701131