Department of Physics and Astronomy

Atomic, molecular, and optical physics in stars

ABUNDANCES OF ELEMENTS in stars as measured from spectra are a cornerstone of modern astrophysics  -- they are used to understand, among other things, the origin of the elements, the evolution of stars and galaxies, and the formation of planets.  However, stellar abundances are interpreted, not observed.  Though strengths of spectral lines can be routinely observed with a precision of order 1%, the systematic uncertainties in measured abundances and other stellar properties are much higher, at least 10%.  The study of the microphysics affecting spectral lines, specifically atomic, molecular and optical processes, is a major piece in the puzzle towards deriving abundances with accuracy approaching 1% or better.

To achieve accurate stellar abundances we need the best possible physics.  This means accurate descriptions of the microphysics, coupled with realistic modelling of the atmospheric structure in three dimensions and radiative transfer beyond the classical assumption of local thermodynamical equilibrium (LTE).
To achieve accurate stellar abundances we need the best possible physics.  This means accurate descriptions of the microphysics, coupled with realistic modelling of the atmospheric structure in three dimensions and radiative transfer beyond the classical assumption of local thermodynamical equilibrium (LTE).

At Uppsala, together with collaborators from abroad, we study important atomic processes through theoretical and computational modelling.  The calculations are applied to astrophysical models and used to more accurately interpret observations.  Our work includes:

a) Inelastic collision processes and their impact on spectral line formation in cool stars.
Specifically we study the processes:

  • electron-impact excitation
  • hydrogen-impact excitation
  • charge transfer involving hydrogen

These processes affect the populations of atomic levels, and thus the strengths of spectral lines produced, and therefore the abundance of the element inferred from comparison of synthetic spectra with observed spectra.  An example of our calculations is shown in the figure below.

Collision rates for Mg + H processes.  The top panel shows the results of quantum mechanical calculations, while the lower panel shows the results of methods often employed till now based on classical modelling (the so-called Drawin formula).  The ratio of the two methods is seen in the inset in the lower panel.  The figure illustrates the main problem of the classical calculations - they cannot predict data for charge transfer processes or those corresponding to optically forbidden transitions.  These processes have some of the largest rates, as illustrated by the transitions from the 4s 1S state (shown in purple), where the classical calculations cannot predict any of these rates from this state.

The neglect of these processes can lead to large differences in measured abundances.  In the cases of Li and Na, it has been found that the charge transfer processes alone can result in changes to derived abundances as large as 60%.  In Mg, similarly large differences are found, and are a result of a combination of excitation and charge transfer processes.

b) Collisional broadening of spectral lines and their impact on the wings of strong lines
Specifically we study the processes:

  • broadening of metal lines by hydrogen
  • self broadening of hydrogen lines

 The strongest lines in stellar spectra are affected due to perturbations by nearby particles - so-called collisional broadening.  These lines represent unique probes of various properties of stars, including temperature, pressure and chemical composition.  The commonly used theory for collisional broadening (the Unsöld formula) gives values roughly half that needed to reproduce observed spectra.  Our calculations do considerably better, as shown in the figure below.

Two lines of calcium the solar spectrum.  Synthetic spectra with our data (full) and old data (dashed; Unsöld) for collisional broadening due to hydrogen, compared to observed spectra (double line, NSO Data).  The meteoritic abundance is assumed, which matches the accepted solar abundance well.

Based on such comparisons, the uncertainty in our calculations is estimated in most cases at better than 10%.  Calculations can be done for a wide range of spectral lines, and thus strong lines can now be used as spectral diagnostics in late-type stars with increased precision and confidence.

Publications:  http://www.astro.uu.se/~barklem/publications.html
Data: http://www.astro.uu.se/~barklem/data.html
Codes: http://www.astro.uu.se/~barklem/codes.html

Contact

Paul Barklem