The
FERRUM Project
Description
The FERRUM project is an international collaboration in atomic
astrophysics initiated by S. Johansson, Lund. It aims at the
production of reliable oscillator strengths (f-values) or
transition probabilities (A-values) for spectral lines of
singly ionized Iron Group Elements of astrophysical significance.
For allowed transitions the goal of the project is to provide:
a
set of lines in the satellite ultraviolet (l
< 3000 Å) region
a set of lines in the optical (l
> 3000 Å) region
lines with a large spread in excitation energy
The
project also includes atomic data for forbidden transitions
based on measurements of radiative lifetimes of metastable
states. The project is presented and described in Johansson
et al, Physica Scripta T100, 71-80, (2002).
Justification
The
three requirements listed for the allowed transitions are
motivated as follows.
Astrophysical spectra may contain absorption and/or emission
lines superimposed on a continuous spectrum produced by stellar
blackbody radiation (BBR). In general, stellar atmospheres
give absorption lines in the continuous spectrum, whereas
gas clouds around or between stars give a pure emission line
spectrum.
Stellar absorption lines are used to determine the chemical
abundance in the star's atmosphere, and that procedure requires
gf-values. Stellar emission lines are used for temperature
and density diagnostics of diluted plasmas, and they might
also be the source of information about the concentration
of various chemical elements. Stellar spectra are recorded
either from the ground (l> 3000
Å) or from space (down to about 900 Å), and they
often lead to independent analyses. This explains the need
for separate data in the optical and satellite UV wavelength
regions.
Most
abundance analyses assume local thermal equilibrium (LTE)
in homogeneous parallel layers of the stellar atmosphere.
Thus, by using lines from energy levels of quite different
excitation energy in the abundance analysis one can test the
validity of the LTE assumption in the stellar model atmosphere
by the Boltzmann distribution law. The abundance derived from
different spectral lines of a given ion should give a level
population that is proportional to the exponential factor
(e-hn/kT) in the Bolzmann
formula. As we should see later the utilization of lines from
high-excitation levels in the abundance analysis may be of
great significance, as intrinsically strong lines (large f-values)
may result in reasonably faint features in the stellar spectrum
and obey the criteria for linear curve of growth analysis.
Methods
t + BF => A-value => f-value
Measurements
t : Radiative
lifetime - measured by laser induced fluorescence technique
at LLC
BF: Branching fractions from relative intensities (I), where
BFik is proportional to Iik, measured
using Fourier Transform Spectroscopy Aik
BFik= Aik/SAil
and ti = 1/SAil
=> Aik = BFik/ ti
Theory
f:
Calculate f-values using different theoretical techniques
Working scheme
The various steps in the working scheme used to achieve the
goal of the FERRUM Project can be listed as follows:
• Measure f-values for some lines in a given transition array (TA) with a spread in excitation energy
• Calculate all lines in the same TA using the orthogonal operator technique
• Compare experimental and theoretical values and estimate the uncertainty of the calculated values
• Make the list of lines in the TA array as complete as possible and add error bars to all f-values
• Insert the f-values for all the lines in a synthetic spectrum and compare to a stellar spectrum
• Investigate whether possible deviations in the previous comparison depend on the synthetic spectrum f-value, model) or the stellar spectrum (blends)
• Adjust the line list by adding/removing the complementary information obtained from the stellar spectrum.
Even
if we stress the necessity of measurements it is important
to underline the significance of incorporating theoretical
data in the line lists. Calculated f-values will always constitute
the bulk of the database, and the experimental values will
be used to normalize them and to assess their accuracy.
People
The people involved in the project have expertise in the fields
of atomic spectroscopy, laser physics, accelerator physics,
stellar spectroscopy and astrophysics. They are listed below
under the various steps in the process of getting oscillator
strengths.
Project Leader: Sveneric
Johansson, Atomic Astrophysics, Lund University.
Radiative
Lifetimes
Laser
Induced Fluorescence (permitted lines):
Lund
Laser Centre: H. Lundberg, Z. Li, S.
Svanberg
Universität Hannover: M. Kock,
R. Schnabel, M. Johanning
Storage Ring (forbidden lines):
Stockholm University: S. Mannervik,
P. Royen, L-O Norlin, A. Derkatch, D. Rostohar, A. Schmit
Lund University: H. Hartman, H. Lundberg
Branching
Ratios
Fourier Transform Spectroscopy:
Atomic
Astrophysics, Lund Univ.: U. Litzén,
H. Sabel, H. Nilsson, C. M. Sikström
Imperial College, London: J. Pickering
Harvard University: P. Smith
Theory
University of Amsterdam: T.
Raassen
Queens University, Belfast: M. Donnelly,
A. Hibbert
Astrophysics
HST Spectroscopy:
NASA/Goddard SFC: D. Leckrone, C.
Proffitt
Lund University: G. Wahlgren
