Science Nugget—The Fingerprints of First Light
  Michael Bolte
  University of California, Santa Cruz

When the idea that the Universe came into existence as an extremely hot and dense fireball was first forwarded, it was not met with wide acclaim. The name by which this theory came to be known, the hot “Big Bang”, was originally used to derisively dismiss the concept. But, in the last three decades, predictions of the Big Bang model have been tested on many different fronts and the theory has a perfect record in confrontations with observations.

One prediction of the Big Bang is that in the early expansion of the Universe there was a brief period, specifically between one second and a few minutes after the beginning, when conditions were right for the formation of protons, neutrons and the nucleosynthesis of the chemical elements. However, the Universe rapidly expanded through this phase and the only elements that were produced were isotopes of hydrogen and helium, along with trace amounts of lithium and beryllium. As the Universe further expanded, it cooled and entered the “Dark Ages.” Eventually, after ~100 million years (z=24) by current estimates, the Universe had cooled enough for gravitational instabilities to locally reverse the overall expansion, and the first structures dropped out of the Hubble flow. As these dark matter and baryon “clouds” compressed and converted gravitational potential energy into heat, the temperature and density once again reach the levels where nucleosynthesis occurs—the first stars and the era of “First Light” were born.

The timescale for the first stars to appear after the Big Bang is established quite precisely from observations of the cosmic microwave background. What is not clear is the exact nature of the first light objects—most importantly, what is the typical mass and the distribution of masses? In the range of possibilities are one solar mass objects, some of which would still be floating around the Universe today, 10-30 solar mass objects which would explode more or less like “standard” core-collapse supernovae, and very massive stars up to ~1,000 solar masses.

These latter objects have a rich and somewhat uncertain range of possible evolution paths, including total disruption in a so-called pair-instability explosion and complete collapse into a black hole with no resulting chemical enrichment of the surrounding gas. The predictions are uncertain in large part because of the primordial composition of the gas from which these first stars form. This affects cooling properties of the gas, mass-loss in the early evolution of the stars, fragmentation characteristics of the clouds, opacity of the gas and nucleosynthesis networks. Although it might seem that all the physics should be more straightforward without the complications of trace elements, much of our understanding of these processes is grounded in observations of the Sun and stars like it.

One powerful approach to identifying the nature of the first stars is to search for distinctive patterns of the elements they produce in their brief lives and (in most cases) cataclysmic deaths. The overwhelming majority of stars in the Galaxy today have a chemical composition that is the result of many generations of stellar nucleosynthesis and interstellar medium recycling. The chemical fingerprints of the first light nucleosynthesis are lost in the subsequent chemical enrichment at the Galaxy.

The solution to this problem is to search for the oldest stars in the Galaxy that were formed from nearly pristine—Big Bang abundances—material. Such objects are called ultra-metal-poor (UMP) stars and, after decades of searching, there are now a handful of stars known with iron abundance less than 1/10000 that measured in the Sun. The pattern of abundances for elements heavier than lithium in these objects reflects the nucleosynthesis and ejection of elements in the first generation of stars. The abundances measured for each UMP star carry the chemical imprint of only a few or even a single progenitor star. By making detailed measurements in a large sample of such objects, we will build a complete picture of the nature of the first light objects that seed the elements of subsequent generations of stars and planets.

Making detailed chemical abundance measurements in stars is a category of “precision astrophysics” that requires high spectral-resolution and high signal-to-noise spectra. The latter requirement is particularly true for the case of highly chemically deficient stars, because the atomic absorptions become very weak as abundances go down. Because UMP stars are extremely rare, they are on average quite faint. Despite a concerted effort using high-resolution spectrometers on 8-meter and 10-meter telescopes in the last 10 years, the small number of target stars bright enough for detailed observations has made this a program for the next generation of facilities.

There are several programs now underway to identify new candidate UMP stars and the unexplored territory is at brightness levels factors of 7 to 20 times fainter than those currently known. Even with the best facilities available today, the follow-up observations required for accurate abundance measurements would require multiple-night exposures for each candidate. Because the first-light objects very likely span a range of properties, it will be necessary to build a large sample of UMP stars with detailed abundance measurements to fully characterize this era. Of the current and planned facilities in the next two decades, the High-Resolution Optical Spectrometer (HROS) on TMT is the one ideal capability for undertaking this program.