science

The usual place to look for metal-poor stars in the Galaxy is the halo or in dwarf galaxies, but many of the oldest, metal-poor stars are actually expected in the central parts of the Milky Way: the Galactic bulge. Finding these stars in such a dense, dusty and mainly metal-rich environment is not trivial. Dedicated efforts are needed to obtain large samples of metal-poor stars in the inner regions of our Galaxy. 

We therefore decided to start the Pristine Inner Galaxy Survey (PIGS) in 2017. I am leading this sub-survey of the Pristine project, which uses narrow-band CaHK photometry from the Canada-France-Hawaii Telescope to efficiently pre-select metal-poor stars. We have obtained a large sample of low/intermediate-resolution spectroscopic observations at the Anglo-Australian Telescope. I use this to learn about the properties of the metal-poor tail of the inner Galaxy. 

PIGS is on average the most metal-poor inner Galaxy survey (see metallicity distributions on the left compared to a few other surveys) and details about our successful approach are presented in Arentsen et al. (2020b). 

First results on the PIGS kinematics are published in Arentsen et al. (2020a), where we show that the rotation of stars around the Galactic center decreases with decreasing metallicity, until the most metal-poor stars do not show any rotation at all (see figure below). We have expanded this analysis in Arentsen et al. (2023), looking at the detailed orbital properties and finding that even the lowest metallicity stars still show some rotation. We also show that many of the metal-poor stars in the inner Galaxy stay there for most of their life. 

And many more PIGS results are still to come, such as high-resolution spectroscopic follow-up of the most metal-poor and/or carbon-rich stars in PIGS, and a more detailed orbital analysis of the low-resolution sample. 

I have presented a summary of the PIGS results at the ESO Hypatia Colloquium in June 2021, which can be re-watched here. 

I am also interested in the detailed properties of the most metal-poor stars. One interesting subtype of metal-poor stars are the carbon-enhanced metal-poor (CEMP) stars, which are ever more ubiquitous at lower metallicities. Some of these CEMP stars can be explained by binary mass-transfer from a former asymptotic giant branch (AGB) companion, but the most metal-poor ones often do not show those AGB signatures and need to be explained in other ways: they were likely born with their peculiar abundance pattern.

 I worked on the binarity of these CEMP-no stars, and found that a larger number of them are in binary systems than expected. This can have important implications for their relation to the First Stars, and/or for star formation processes at low metallicity. The results were published in Arentsen et al. (2019a). 

I also study how the the properties of CEMP stars vary between different Galactic environments, and what this can tell us about the building blocks of the Milky Way. Recently I published a paper on the occurrence of CEMP stars in the inner Galaxy/bulge region thanks to PIGS (Arentsen et al. 2021), in which I found that there may be fewer CEMP stars in the inner Galaxy compared the rest of the halo. 

I recently reviewed the literature regarding the frequencies of CEMP stars among very metal-poor stars (Arentsen et al. 2022). There are inconsistencies between various published CEMP frequencies, which we propose are related to various sample selection effects and different analysis methods. CEMP stars are very useful for Galactic archaeology, but only if we can meaningfully compare them between different environments, and some more work needs to be done there!