Chemical Elements as Tracers of the Evolution of the Cosmos

WG3 : Atomic Diffusion in M30

Project lead: Andreas Korn / Alvin

Team members:



The surface abundances of old, metal-poor stars have for a long time been suspected to be modified by internal element transport processes (Michaud et al. 1984, ApJ, 282, 206). Such effects would be pronounced whenever convection or others forms of macroscopic mixing is weak. Among a population of ancient stars, two groups of stars are expected to be most susceptible to atomic diffusion:
main-sequence turn-off and warm horizontal-branch (HB) stars. Warm HB display surface abundances departing greatly from their red-giant counterparts. More recently, turn-off stars in metal-poor globular clusters have also been found to have systematically different
abundances from their red-giant counterparts with deep convection zones, but the effects are generally more subtle (Korn et al. 2007, ApJ 671, 402, Gryuters et al. 2013, A&A 555, 31, Gruyters et al. 2016, A&A 589, 61).

We are continuing the mapping of abundance trends in globular clusters across a range of metallicities. The latest cluster studied, Messier 30, displays the largest trends seen to date reaching 0.3 dex (a factor of two) in iron abundance. Other elements show shallower trends, something that can be explained by stellar-structure models treating all effects of atomic diffusion. Whenever the aim is to get to the birth-cloud abundances of a star, as in Galactic archaeology, such correction are important to consider, both for chemical-evolution studies and age determinations (Dotter et al. 2017, ApJ 840, 99). One of the most affected elements is lithium and atomic diffusion is arguably the physically best motivated contender to explain the cosmological lithium
discrepancy (Korn et al. 2006, Nature 442, 657).

New VLT/FLAMES data allow us to constrain the abundance trends of magnesium and titanium in stars from the turn-off point to the giant-star bump in the very metal-poor globular cluster Messier 30. As the turn-off point is fainter than V=18, these are observations at the limit of what can be done at the VLT with current medium-resolution instrumentation.