The passage of radiation through the atmosphere of a star is interrupted by numerous interactions, primarily with electrons. These electrons may be free or bound to an atom or ion. The result of a collision with a bound electron may be to liberate the electron altogether, or to excite the electron to a higher bound level within the ion. These three processes are sometimes referred to as scattering, photoionization or photoexcitation, and sometimnes as free-free (ff), bound-free (bf) and bound-bound (bb) collisions.
The modelling of a stellar atmospheres (or any astrophysical object) and its emergent radiation requires a detailed model of how photons interact with electrons. Such processes can be approximately modelled if a relatively small number of basic quantities can be supplied, which carry information about the structure of the atom or ion being considered. These "atomic data" are fundamental to accurate astrophysical modelling. However, because these data are different for every transition in every atom and ion, and because there are billions of such transitions in nature, calculating and storing this data is a major problem in astrophysics
For calculating model spectra of early-type helium stars in LTE, I maintain a database of atomic data for, mostly, blue-visual absorption lines of light elements. The data is sorted with one file for each ion.
For each absorption line included, the database includes the wavelength, oscillator strength, radiative and collisional damping constants (where avaliable), the excitation energy of the lower-level in the transition, the multplet number (from Moore's revised multiplet tables). A reference for each of the oscillator strengths and damping constants are also given. The database is dynamic, with new lines being added as required. Some vetting is applied to ensure that only the most reliable data are included. Consequently the database is quite dynaimc, with corrections and updates being included at arbitrarry times. There are currently plans to make major improvments to the overall format of the data tables.
An article describing the database originally appeared in the CCP7 newsletter (Jeffery C.S., 1991, Newsletter on 'Analysis of Astronomical Spectra', No. 16, p. 17). An archive version of the database may be found on the CCP7 website.
Links to other sources of atomic data are maintained on the CCP7 resources web page.
This page is maintained by:
Simon Jeffery (email@example.com)
Last modified: 08/06/00
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