Tuesday, March 31, 2015

Surviving Population III Stars in the Galaxy

Population III stars are the first generation of stars to form in the universe. At that time, the universe only had hydrogen and helium since stars have yet to exist to forge the heavier elements. As a result, Population III stars are composed entirely of hydrogen and helium (i.e. no metals). Although a number of extremely metal-poor stars have been found, no Population III star has yet been directly observed.

Population III stars are predicted to be extremely massive and hot. These stars have short lifespans as they burnout rapidly. Nevertheless, simulations of the formation of Population III stars show that in the initial mass distribution of these stars, the least massive ones may be less massive than the Sun. If Population III stars with less than 0.8 times the Sun’s mass formed in the early universe, the lower mass and correspondingly longer lifespans means that they should still be present today.

Although these low-mass Population III stars started out metal-free, over the course of their long lives, the can accrete sufficient metal-enriched material from the interstellar medium to pollute and mask their primordial nature. The interstellar medium is comprised of gas and dust. A study by Jarrett L. Johnson (2015) shows while gas can accrete onto low-mass Population III stars, the accretion of dust can be inhibited as pressure from radiation emitted by these stars can push dust particles away. If only gas accretion occurs, it can create a unique chemical signature where there is an enhancement in elements that came from gas accretion and depletion in elements that came from dust accretion.

Carbon and oxygen tend to be in the gas phase while iron tends to be in the dust phase. If low-mass Population III stars are only polluted by gas accretion, then they are expected to be enriched in carbon and oxygen, and depleted in iron. Such a chemical signature is similar to a category of stars known as carbon-enhanced metal poor (CEMP) stars. CEMP stars are some of the most metal-poor stars known and some fraction of them might turn out to be Population III stars that have been polluted by gas accretion.

Jarrett L. Johnson (2015), “The chemical signature of surviving Population III stars in the Milky Way”, arXiv:1411.4189 [astro-ph.GA]