The solar chemical composition is an important ingredient in our understanding
of the formation, structure and evolution of the Sun and our solar system.
Furthermore, it is an essential reference standard against which the elemental
contents of other astronomical objects are compared, be it other stars, planets,
gas clouds in the interstellar medium or whole galaxies. The chemical composition of
a star like the Sun is inferred from its radiation spectrum, which provides the elemental
fingerprints in the form of absorption lines. To convert the strength of a
spectral line to an elemental abundance requires detailed modelling of the
stellar atmosphere and the processes between atoms and radiation that shape
the spectrum. For the Sun, a major complication arises because of convection
— the bubbling motion akin to boiling water —
which modifies the structure of the solar atmosphere where the spectrum is formed.
The theoretical foundation for the new study is a more realistic
model of the solar atmosphere based on 3-dimensional hydrodynamical simulations
compared with previous 1-dimensional modelling. This
challenging endeavour has been pioneered and systematically refined
by the MPA-group together with their colleagues in Denmark, USA and Australia.
In addition, the interactions between the radiation and the gas have been
followed in detail. Finally new and carefully selected input data for the
considered spectral lines have been employed throughout.
The refined analysis tools and improved atomic data have enabled the team to reach
an unprecedented accuracy in the measured solar chemical composition.
Furthermore, rather than tackling just one or a handful of elements at a time
as customary done in the field, the new study reports elemental abundances for
all 71 elements that can be determined using solar spectroscopy. The ambitious nature
of this undertaking is reflected in the ten years it has taken to complete the project.
The most surprising and radical finding is that the contents of carbon, nitrogen,
oxygen and neon -- the four most abundant elements after hydrogen and helium -- are
only about two-thirds of what they previously were thought to be.
The new 3D modelling, non-equilibrium spectrum calculations, improved
atomic data and more robust selection of spectral lines all play a significant
role for these elements and conspire to act in the same direction.
The new results are supported
by the excellent agreement between the predictions of the 3D solar model and
various observational diagnostics.
The study will be published in the prestigious journal Annual Reviews of
Astronomy and Astrophysics later this year, and should become the de facto
standard in astronomy for many years to come.
The revised solar chemical composition can be validated through a comparison with
the abundances in the most pristine meteorites that has remained largely unaltered
since the formation of the solar system. Although the meteorites can not
be used to measure meaningful abundances of H, He, C, N, O and Ne as they
have been partly evaporated in meteorites, the agreement for the remaining elements is excellent.
Furthermore, the new solar abundances have resolved a long-standing conundrum
why the Sun, that was born 4.5 billion years ago contained more heavy elements
than the present-day interstellar medium and young, massive stars in the Galactic
neighborhood. The overall content of elements heavier than He in the Milky Way
should steadily increase with time as stars die and spew out their nuclear-processed
ashes from which subsequent generations of stars and formed. The new solar values
make the Sun normal in this respect.
Everything is, however, not rosy with this changed astronomical yardstick.
The lower content of heavy elements imply changes in the deep interior of
solar and stellar structure models. The predicted sound speed
variations as a function of depth are now in stark conflict with those inferred
observationally from the oscillations of the Sun. Because sound waves of
different frequencies penetrate to different depths the sound speed variations
can be mapped, a technique dubbed helioseismology. The revised solar chemical
composition has already spurred a flurry of studies devoted to finding a resolution to
the discrepancy. Several possible explanations
have been put forward but unfortunately most can already be ruled out.
Whatever the final solution will be, studies of both the Sun and other stars
will then be on firmer footing. With stars as widely used probes of the cosmos,
this will also mean a better understanding of the universe as a whole.
Martin Asplund, Nicolas Grevesse, Jacques Sauval and Pat Scott,
"The chemical composition of the Sun",
2009, to appear in Annual Reviews in Astronomy and Astrophysics