about Pristine

What is the Pristine survey? (for a general audience)

What was the Universe like just after the Big Bang when the first stars were forming to shine their light and end the age of darkness? To get an answer to this question, astronomers use different methods. One is to look far into the Universe and back in time, to see the first stars and galaxies growing. Another option is to look carefully at our own Galaxy where the oldest stars still alive still carry information from the early Universe. To find these oldest messengers among the overwhelming population of younger stars is no easy task. 

Just after the Big Bang, the Universe was filled with hydrogen and helium and a bit of lithium. No heavier elements were around, as these are synthesized in the hot interiors of stars, and those did not exist yet. Our Sun has ~2% of heavier elements ("metals")  in its atmosphere, as can be seen when we make a spectrum of its light. We thus know from this fact alone that it has been formed as part of a later generation, including in its atmosphere the products of the stars that lived and died before it. In searching for the oldest stars we instead search for stars with more pristine atmospheres than our Sun. The more pristine the atmosphere, the earlier the generation in which this star was born. Studying stars of different generations allows us to understand the history of the Galaxy, an area of research also sometimes dubbed Galactic archaeology.

The Pristine survey, led by Dr. Else Starkenburg (Kapteyn Astronomical Institute, University of Groningen, the Netherlands) and Dr. Nicolas Martin (CNRS, University of Strasbourg, France), is searching for the most pristine stars. The survey is able to search for these rare stars with unprecedented efficiency. Instead of taking spectra of millions of stars, we employ a special colour filter on the Canada-France-Hawaii Telescope to search for stars with relatively pristine atmospheres. Having already photographed over 6000 square degrees on the sky with this filter, we have an impressive dataset available from which we select the most metal-poor candidates for more detailed spectroscopic studies, as well as study the build-up and structure of the Milky Way galaxy.

The Pristine survey in more detail

The search for the most metal-poor stars has justly been compared to finding a needle in a haystack: In a typical Galactic halo field, only one in ~80,000 stars is expected to have [Fe/H] < –4 (Youakim et al., 2017). Once uncovered though, they allow us to investigate the initial mass function and yields of the First Stars. At present only ~13 stars are known to have [Fe/H] < –4.5. These lowest metallicity stars are observed to have peculiar abundance patterns - most notably in carbon, but also in other elements - and provide us with clues about the nature of the First Stars and their exotic supernovae events. Moreover, the small samples of [Fe/H] < –3 stars (several hundreds) have indicated trends of low-metallicity star abundances with disk height, in the halo and the bulge. If confirmed, such trends will give us valuable insight into the early formation of the Milky Way. 

To very efficiently increase the numbers of metal-poor stars, investigate their distribution, and hunt for the oldest stars, we have designed the Pristine survey (Starkenburg, Martin et al., 2017), using a narrow-band Ca H&K filter (see Figure 1) with the 1-degree field-of-view imager MegaCam on the CFHT. Any very metal-poor stars will show weaker Ca H&K features, setting them apart from more metal-rich stars of the same temperature. This method, using only photometry, works extremely well (see Figure 2). Taking advantage of the excellent narrow-band filter, the CFHT 4-meter telescope and the wealth of relevant expertise in the Pristine science team, we show a factor five increase in efficiency to find extremely metal-poor stars with respect to previous surveys with the same purpose (Youakim et al., 2017). An overview of our publications in recent years can be found here. The first data release of the Pristine survey is presented in Martin, Starkenburg et al. (2023). 

The Pristine survey footprint measures ~6100 square degrees at present, with the survey still on-going. Deliberately included in this footprint are a range of Galactic latitudes (30 < b < 73) and substructure features such as stellar streams to probe different Galactic halo environments. We additionally target a number of Milky Way satellites/dwarf galaxies for detailed study. Finally, the Pristine survey has expanded into the bulge of the Galaxy, a promising hunting ground for very old stars according to cosmological models.

Ongoing science projects (not exhaustive):

• Increasing the number of known (very/extremely) metal-poor stars 

• Finding the most metal-poor stars existent in the Galaxy

• Follow-up of Pristine photometry with low-/intermediate-/high resolution spectroscopy

  • In the near future with WEAVE

• Study of Milky Way satellites with Pristine

• Study of the metal-poor inner Galaxy (PIGS)

• Kinematics of very/extremely metal-poor stars

• ...