Andreas J. Korn

Atomic Diffusion in Old Stars (ADiOS)

We (Korn et al. 2006) recently observed systematic abundance trends between unevolved and evolved stars in the nearby metal-poor globular cluster NGC 6397. The observed abundance trends are in good agreement with predictions from stellar-evolution models which include the effects of atomic diffusion. However, we need to postulate the existence of (turbulent) mixing below the convective envelope to moderate the effects of atomic diffusion. This finding has important consequences for the use of unevolved stars in cosmochemical studies of the Galactic halo. It also helps to understand why atmospheric lithium abundances of halo stars are systematically lower than the amount of lithium known to be produced in Big-Bang nucleosynthesis.

 

We now have several questions that need to be answered to evaluate the impact of Atomic Diffusion and Mixing on stellar abundances and ages.

The metallicity dependence of (turbulent) mixing
Without extra mixing, the effects of atomic diffusion are metallicity dependent. If the postulated mixing also varies with [Fe/H], then the element-specific size of the net effect could well vary with metallicity. This would have an influence on observed abundance trends in the Galactic halo.

Naturally, one would also expect this mixing to vary with other stellar parameters like effective temperature and surface gravity. Note, however, that different degrees of mixing only affect stars hotter than 5800 K differently (see Figure above), while the inward expanding convective envelope erases any differences when the stars evolve along the subgiant branch.

Using the VLT and FLAMES, we are currently investigating abundance trends in NGC 6752, a nearby globular cluster about a factor of three more metal-rich than NGC 6397 (PI Korn).

We are currently broadening the scope of this work: similar, albeit smaller, effects should exists at solar metallicty. To test this, we use FLAMES-UVES to observe stars in M 67, the solar-metallicty solar-age open cluster (PI Gustafsson). Any indication of atomic-diffusion effects would help reconcile the observed colour-magnitude-diagram morphology of M 67 with predictions from stellar structure. This would likely also have implications for the currently hotly debated solar modelling problem (photospheric vs. helioseismic composition).

The nature of (turbulent) mixing
Given the importance of the postulated mixing for the good agreement between theory and observation, we would like to understand what gives rise to it. There are indications that the mixing is related to hydrodynamic effects not captured in the 1D stellar-evolution simulations.

The ADIOS effort tries to make progress in both these fields. It currently consists of Martin Asplund, Paul Barklem, Lionel Bigot, Corinne
Charbonnel, Remo Collet, Frank Grundahl, Bengt Gustafsson, Karin Lind, Lyudmila Mashonkina, Georges Michaud, Nikolai Piskunov, Olivier Richard, Suzanne Talon and Frédéric Thévenin. This effort is led by me.

 

Related publications

• Atomic Diffusion and Mixing in Old Stars II. Observations of Stars in the Globular Cluster NGC 6397 with VLT/FLAMES-GIRAFFE
  Lind K., Korn A.J., Barklem P.S., Grundahl F.,
  Astronomy & Astrophysics 490, 777 (2008) [pdf]
• CIAO? ADiOS! (talk)
  Korn A.J.
  in: XXI Century Challenges for Stellar Evolution (Cefalù, Italy, August 2007), S. Cassisi & M. Salaris (eds.), Memorie della Società Astronomica Italiana, vol. 79/2, 495 (2008) [pdf]
• Atomic Diffusion and Mixing in Old Stars I. VLT/FLAMES-UVES Observations of Stars in NGC 6397
  Korn A.J., Grundahl F., Richard O., Mashonkina L., Barklem P.S., Collet R., Gustafsson B., Piskunov N.,
  Astrophysical Journal 671, 402 (2007) [pdf]
• A Probable Stellar Solution to the Cosmological Lithium Discrepancy
  Korn A.J., Grundahl F., Richard O., Barklem P.S., Mashonkina L., Collet R., Piskunov N., Gustafsson B.
  Nature 442, 657 (2006) [pdf, original, podcast]
• Atomic Diffusion in Old Stars — Helium, Lithium and Heavy Elements (talk)
  Korn A.J. and the ADIOS team
  in: Cool Stars 14 (Pasadena, CA, 2006), in press [pdf]
• Pinning Down Gravitational Settling (talk)
  Korn A.J., Piskunov N., Grundahl F., Barklem P., Gustafsson B.
  in: Chemical Abundances and Mixing in Stars in the Milky Way and its Satellites , L. Pasquini & S. Randich (eds.), ESO Astrophysics Symposia (Springer, Heidelberg), 294 (2006) [pdf]
• New abundances for old stars — atomic diffusion at work in NGC 6397
  Korn A.J., Grundahl F., Richard O., Barklem P.S., Mashonkina L., Collet R., Piskunov N., Gustafsson B.
  The Messenger 125 (2006) [pdf]