Download Nitrogen abundances in solar

Nitrogen abundances in solar-type stars
Supervision: Dr. Miguel A. Urbaneja, Univ.-Prof. Dr. Norbert Przybilla
Work focus: non-LTE line formation, quantitative analysis
Solar-type stars are valuable tracers of the chemical evolution of the Milky Way over cosmic
history because of their longevity. They are numerous, accessible at high spectral resolution
out to distances of several kpc with large telescopes, and rather ‘easy’ to be analysed. This
is because their atmospheres are described well under the assumption of local thermodynamic equilibrium (LTE). However, some spectral lines are known to be subject to non-LTE
effects. This means that abundance determinations based on LTE calculations will be in
error (in the case of the Figure by a factor ∼5) as the populations of the levels of the involved
atoms are no longer governed by the Saha- and Boltzmann formulas. Instead, the effects
of the (non-local) radiation field need to be considered in addition to thermalising collisional
processes, requiring atomic models to be constructed to account correctly for all involved
atomic transitions.
For our work on hotter and more massive stars we have developed one of the most
comprehensive databases of model atoms. Because of the universality of atomic physics,
these models need to be adjusted only slightly for the modelling of solar-type stars.
In this work, these small adjustments shall be performed for the existing model atom of
neutral nitrogen (N I). This will be in analogy to the procedure used for O I, where first results
of the application to a solar-type star have been published in a refereed journal (see Figure).
First applications of the model atom for an improved N abundance determination in the Sun
and several solar-type stars are aspired in the course of the work.
Figure: Examples of non-LTE model fits (red
lines) to ESO VLT/XSHOOTER observations
of a solar-type star (black line) in order to
determine oxygen abundances from O I lines.
For comparison, LTE line profiles for the same
abundances are shown as dotted lines. From
Cowley, Hubrig & Przybilla, (2014, MNRAS,
440, 2457–2463).
In the proposed work, similar work will be undertaken to extend the existing model atom for
N I for its use in analyses of solar-type stars, including first applications.
Keywords: optical/near-IR spectroscopy – elemental abundances – stellar atmospheres –
non-LTE radiative transfer – solar-type stars
For further information contact: [email protected]