21 March 2016 News

Using gravitational waves to search for Population III stars

Possible Population III stars in the CR7 galaxy. From Nature 526, 46–47.
Possible Population III stars in the CR7 galaxy. From Nature 526, 46–47.

Following the announcement earlier in February that gravitational waves were discovered with the Advanced Laser Interferometer Gravitational-Wave Observatory (aLIGO), a completely new window to investigate astrophysical processes, which are otherwise invisible to observations in the electromagnetic spectrum, has been opened. Now researchers are suggesting that these enigmatic signals could be used to learn more about Population III (Pop III) stars - a class of star theorised to have been the 'first-born' stars created in the universe.

Stars were originally divided into two populations by Walter Baade in the 1940s and were assigned a designation depending on their metal content; Population I (Pop I) are classed as metal-rich, while Population II (pop II) stars are metal-poor. However, as it was discovered that even the most metalpoor Pop II stars have metallicities far above that of the gas left over from the Big Bang, it was theorised that an early class of stars must have around to produce the metals that are observed in Pop II stars today, as this cyclic process of stellar birth and death gradually increases the metallicity available for newly forming stars.

If this is indeed the case, then it is surmised that Population III stars are composed entirely of primordial gas – hydrogen, helium and very small amounts of lithium and beryllium and is therefore pristine material left over from the Big Bang. Nonetheless, despite intense searches, no Population III stars have ever been observed, so their existence is entirely hypothetical at present. As the oldest population of stars, the majority of Pop III stars would have exhausted their fuel supplies long ago and if they could be observed they would take the form of stellar remnants such as neutron stars, white dwarfs or black holes.

With the recent detection of gravitational waves, an international team of astronomers headed by Tilman Hartwig, from Sorbonne Universités, Paris have developed a model to simulate the formation of the first stars and to track the binary stellar evolution of the individual systems until these massive compact remnants merge as black hole binaries. As Pop III stars are more massive than Pop I stars, it follows that Pop III black holes are more massive than Pop I black holes and that any resulting gravitational waves are easier to detect. After analysis of their models, the team find a ∼ 1% probability that the current detection GW150914 is of primordial origin, in addition their studies estimate that aLIGO will detect roughly 1 primordial black hole – black hole (BH – BH) merger per year when modifications to aLIGO have been implemented later this year, although this rate depends sensitively on the primordial initial mass function of the Population III star.

Conversely, the team state that by turning this around, the detection of black hole mergers with a total binary mass of ∼ 300 solar masses would enable them to constrain the primordial initial mass function, hence allowing researchers to calculate just how massive Population III stars might have been. For comparison the post-merger black hole that gave rise to the GW150914 event has a mass of about 62 times the Sun's mass.

In astronomy, the initial mass function (IMF) is an empirical function that describes the distribution of initial masses for a population of stars. Understanding the origin of the IMF is crucial as it includes the basic physics that determine our observable universe, from the generation of the chemical elements, to the overall formation and evolution of galaxies. However, scientists do not yet have a clear understanding of the physics that determine the distribution of stellar masses and a number of problems that need to be addressed in order to develop a complete theory for the origin of the IMF still remain outstanding.

The idea of Pop III compact binaries as gravitational-wave sources is not a new one and previous simulations have suggested that the formation of Pop III star binaries and multiple star systems are frequent, however, the initial mass of the Population III star is heavily debated and throws into question just how easily these objects might be detected. However, now that gravitational waves have been conclusively identified, the existence of Pop III massive stars may also be conclusively proved too.

Further information on this research can be found at http://arxiv.org/pdf/1603.05655.pdf

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