Searching the Universe for traces of the first stars
Despite advances in technology, scientists still lack a clear view of the earliest stars to form in the Universe, which are typically missing many of the chemical elements that are present in the subsequent generations of younger stars and are called extremely metal-poor (“EMP”) stars. They have very specific chemical signatures, providing an imprint of the very first stars in the Universe and offer a unique window to investigate the initial stages of the cosmic history.
Given that the early Universe was largely devoid of metals, their very low metal content (metallicity) hints that they are exceptionally old. By measuring the surface composition of EMP stars, one has indeed the opportunity to look back in time and learn about the nature of the early Universe. However, our ability to use them to investigate the early stages of star formation is hampered by their rarity. Only one in ∼ 80, 000 stars meet the criteria ([Fe/H] ≤ -4 dex) and only 12 stars with [Fe/H]≤ – 4.5 are presently known.
The LASTRO team is currently contributing to the international collaboration known as Pristine, a large imaging and spectroscopic survey to identify extremely metal-poor stars in a more efficient way than before.
Pristine is an imaging wide field survey in the Northern Hemisphere of such ancient and metal-poor stars in the stellar halo of the Milky Way, using a new narrow-band filter which efficiently selects this rare population. Pristine builds on the unique combination of the Ca H&K filter and the wide field of MegaCam on the Canada-France-Hawaii telescope to cover a large footprint and yield a pre-selection of EMP stars that is about 5 times better than previous surveys.
This massive survey of the night sky has now covered a total of 6,000 deg2. It is completed by large spectroscopic campaigns that provide detailed analyses of the chemical elements present in each star. At full completion, Pristine will revolutionise the observational landscape of near-field cosmology, delivering thousands of stars with [Fe/H]≤ -3 and a few below ≤ -4.
The ultimate characterisation of the EMP stars identified by Pristine is given by their chemical patterns. This is achieved with high resolution spectroscopy, deriving a large set of chemical abundances such as C, O, Na, Mg, Al, Si, Ca, O, Ti, Sr, Ba, and Eu.
The massive Pristine spectroscopic follow-up campaigns will allow us to address a broad range of open issues.
- What are the physical conditions of the formation of the first stars?
- What is their initial mass function?
- When do low-mass stars form and what is the dominant cooling process responsible for cloud fragmentation in the early Universe?
- What are the relevant nucleosynthesis processes participating to the onset of primordial chemical enrichment?
- What are the effects of first stars on reionization?
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