The first stars in the Universe, so–called Population III or Pop III stars, are believed to have formed from the primordial ‘molecular’ clouds, a metal–free gas available in the very early Universe, in pristine conditions.
Theoretical arguments suggest these stars formed 100 million years after the big bang and they were very low metallicity stars or even independent of metallicity (zero metallicity stars, they have formed out of the only elements to exist hydrogen, helium, and trace amounts of lithium.)
The nature of Pop-III stars is a subject of much discussion. Pop-III stars could be extremely massive and luminous with a mass bigger than 100 times sun’ masses or have masses as low as few-to-several tens of sun’ masses.
Determining the role that Pop-III stars played in the early Universe is an important question. It has been long postulated that the start of the epoch of re-ionisation could have been set off by the first generation of stars; Pop-III stars altered the dynamics of the cosmos by heating and ionizing the surrounding gases, having a significant role in chemically enriching the primordial inter-galactic medium.
The earliest stars produced and dispersed the first heavy elements, paving the way for the eventual formation of solar systems like our own. They also could constitute the seeds of primordial black holes. The collapse of some of the first stars may have seeded the growth of supermassive black holes that formed in the hearts of today galaxies.
Black holes from Pop III seeds would be more massive than the ones formed by Pop-II stars, and would experience a longer than usual accretion phase. So, observing gamma-ray bursts from Pop-III stars would give us a way of learning something about their early Universe progenitors.
Back to the Origins of Population III Stars
Our cosmological simulation have long showed the existence of a first generation of stars in the early universe but if they really lived then we could see them in the very distant universe, but how? The answer is long gamma-ray bursts (LGRBs).
Long gamma-ray bursts (LGRBs) are the brightest transients known in the Universe. According to the standard collapsar model GRBs signpost the birth of a black hole. So detecting such LGRBs and studing them could trace us back to the first stars.
Approximately 300 LGRBs with a measured redshift (z) have been detected to date, largely by the Swift satellite but none of them is from these stars — the most distant confirmed one is at z = 9.4 from Pop-II star. But current models suggest that detection of a GRB afterglow from a Pop-III star is within the capability of current facilities.
An exceptionally high energy burst and long γ–ray duration compared to the rest of the GRB population would be distinctive properties of PopIII progenitors. Nonetheless, these objects have proven to be elusive so far.
In a new paper uploaded on arxiv website, a team of Australian and Italian astronomers used the Australian Telescope Compact Array to explore the hypothesis that particularly long and dim GRBs detected by Swift could originate from Pop-III progenitors.
For the study sample, they selected three GRB sources for which there was no measured redshift. The 3 candidates: are GRB 110210A, GRB 121001A and GRB 111215A. GRB 120401A was observed on 2014 May 3 using the 1.5D array configuration. GRB 121001A and GRB 11215A were observed on 2014 September 13 using the H75 array configuration. For each of these three candidates, they aimed at obtaining a detection. Results? A pop-III GRB would be able to roughly reproduce the X–ray observations but with no genuine detection for any of the target sources.
GRBs from a Pop-III progenitor have proven so far elusive, and therefore belong only to the realm of theoretical simulations. Nonetheless, if these objects do exist, they might not only be observable by the next generation of facilities, but could have been hiding already in our samples, disguised as more “standard” objects.
said David Burlon ~ one of the author of the study.
Our simulations have shown that late time radio observations may be the best diagnostic for distinguishing a Pop-III GRB from a standard one. The former is expected to be orders of magnitude brighter than the latter, and within the reach of current radio facilities.
T. Murphy ~ also one of the authors
Jorryt Matthee ~ PhD student in extragalactic astrophysics at Leiden Observatory, not imply in the study, said:
I have always wondered where we come from. Even as a child I wanted to know where the elements came from: the calcium in my bones, the carbon in my muscles, the iron in my blood. I found out that these were first formed at the very beginning of the universe by the first generation of stars.
Puzzling EvidenceEarly star formation might help explain some puzzling features of the present universe but first we need to find them. Earlier this year, astronomers have discovered a bright distant galaxy which shows evidence of harboring these monster stars but the physical proof of their existence had been inconclusive. Further observations with upcoming space instruments may confirm beyond doubt that what has been observed are Population III stars or not!