Research Byte

Published in the RSAA Lunations
Vol1 Issue35 1–31 December 2022

The first stars formed from the primordial gas of mostly hydrogen and helium.
These first stars of Population III could grow more massive than the subsequently formed Population II and Population I stars because the inefficient cooling rate of the primordial gas requires more mass in order to gravitationally collapse to form the first stars. Population II stars formed under the more efficient cooling rate of an interstellar medium that is enriched in heavy elements synthesized by the Population III and later generation stars. Population I stars, like the Sun, formed from the more heavy element enriched gas as a result of chemical evolution through many cycles of stellar birth, and death by supernova explosion.
 
A massive Population III star is short-lived and ends its life as a supernova by the transformation of energy into matter. In particular, for a very massive star in the range from 150 to 300 Msun, the thermal energy in the core produces electron-positron pairs causing the loss of pressure support, and the star is totally destroyed by the supernovae explosion, leaving no compact remnant behind. The heavy elements synthesized by this star, including all the iron in the core, are ejected into the interstellar medium. The iron-rich ejecta has a very low abundance ratio, X/iron, where X is a heavy element X synthesized mostly outside the iron core.
 
Until recently, there was no evidence that Population III stars ever existed. However, a team of astronomers, that included Bruce Peterson at the RSAA, and was led by Yuzuru Yoshii at the University of Tokyo, who spent several years at Mt. Stromlo during 1987 to 1993, have found the potential signature of Population III pair-instability supernova ejecta in the spectrum of the quasar ULAS J1342+0928 at z=7.54, only 700 million years after the Big Bang.

The Broad Line Region of this quasar is enriched in iron by a factor of 20 relative to the solar abundance ([Fe/H]=1.36), while the magnesium to iron abundance ratio i very low, by a factor of 10 relative to the solar ratio ([Mg/Fe]=-1.11). The team concluded that such an unusual abundance feature cannot be explained by the standard view of chemical evolution that considers only the contributions from standard supernovae. They propose that the massive iron enrichment and very low magnesium to iron abundance ratio observed in the Broad Line Region are the ancient traces from a pair instability supernova explosion of a very massive Population III star at the high-mass end of the progenitor star's mass range of 150-300 solar masses.
 
Ref: ApJ,937,61

Bruce Peterson

_{Picture: Yuzuru Yoshii and Bruce Peterson. Yoshii attributes his many interactions with the staff members at Mt. Stromlo during his time here as leading him to where he is today (Emeritus Professor at the University of Tokyo and Laureate Professor at the University of Arizona). Yoshii recalls with nostalgia, living in an ANU house in Garran across the street from Matthew Colless, and occasionally 
giving Matthew a ride in his car to and from Mt. Stromlo.}