The measurement of the lithium isotope ratio in stars is extremely
challenging, both from an observational and from a theoretical
perspective. Observers need the highest quality data that modern
telescopes and spectrographs can provide to disentangle the weak
signature of the lighter isotope from the observational noise – in
practice, the 6Li detection limit in metal-poor stellar spectra is
only about 2% of the total lithium content. Moreover, many agents can
influence the shape of spectral line profiles and hence affect the
isotopic ratio, and they have to be modelled correctly.
First, thermal and convective gas motions cause Doppler shifts in the
spectral lines, which call for realistic 3D radiation-hydrodynamic
simulations of the atmospheres of stars, such as the Stagger models
developed at MPA. Second, to correctly model the distribution of atoms
in different states of excitation and ionisation one needs to account
for the strong departures from local thermodynamic equilibrium
(LTE). Such complex line-formation calculations are expensive and they
must run for weeks on powerful multi-core machines. This is why until
now, not all effects could be considered at the same time and
simplifying assumptions have been made.
For the first time, a combined 3D, non-LTE technique has been be
applied to model the lithium, sodium and calcium spectral lines in
four very metal-poor stars. The aim was to constrain the lithium
isotope ratios, while the other neutral elements serve as calibrators
of the unknown projected rotational velocity of the
stars. Surprisingly, the astronomers found that none of the stars
displayed a significant presence of the lighter 6Li, contrary to
evidence put forward in several studies over the past two decades.
The refined models show in particular that the assumptions of LTE
leads to systematic overestimations, even false detections, of 6Li
over a wide parameter space. The most interesting example is the
metal-poor turn-off star HD84937, for which an undisputed detection
has been demonstrated in at least three competing studies, while the
new model shows no significant signs of 6Li.
The findings for all four stars are in gratifying agreement with the
standard Big Bang nucleosynthesis model, which forges only
insignificant amounts of the lighter isotope. What remains to be
properly explained is why the element abundances of 7Li instead fall
short of the primordial prediction. Our 3D, NLTE modelling strengthens
the exciting claim that stars can act as sinks of both lithium
isotopes, slowly draining their atmospheres of these and heavier
elements over time. This slow diffusion process has been theoretically
postulated and has the great potential to explain why the observed
stellar abundances of heavy lithium are lower than expected. Thereby,
both cosmological lithium problems, which have haunted particle
physicists and astrophysicists since the launch of the WMAP satellite,
can find their solution in improved physics of the stellar
atmospheres.
Karin Lind (MPA), Jorge Melendez (Department of Astronomy, University of Sao Paulo, Brasil), Martin Asplund (Mount Stromlo Observatory, Australian National University, Australia), Remo Collet (Mount Stromlo Observatory, Australian National University, Australia), Zazralt Magic (MPA)
Reference
Lind K., Melendez J., Asplund M., Collet R. & Magic Z.,
"The lithium isotopic ratio in very metal-poor stars",
(submitted to A&A)
Further reading
Asplund, M., Lambert, D. L., Nissen, P. E., Primas, F., & Smith, V.V.,
"Lithium Isotopic Abundances in Metal-poor Halo Stars",
2006, ApJ, 644, 229
Cayrel, R., Steffen, M., Chand, H., et al., "Line shift, line asymmetry, and the 6Li/7Li isotopic ratio determination", 2007, A&A, 473, L37
Cayrel, R., Steffen, M., Chand, H., et al., "Line shift, line asymmetry, and the 6Li/7Li isotopic ratio determination", 2007, A&A, 473, L37
