Volume 480, Number 2, March III 2008
|Page(s)||575 - 580|
|Section||Atomic, molecular, and nuclear data|
|Published online||25 January 2008|
The FERRUM project: experimental and theoretical transition rates of forbidden [Sc II] lines and radiative lifetimes of metastable Sc II levels*
Lund Observatory, Lund University, Box 43, 22100 Lund, Sweden e-mail: Henrik.Hartman@astro.lu.se
2 Department of Physics, Stockholm University, AlbaNova University Center, 10691 Stockholm, Sweden
3 Department of Applied Mathematics and Theoretical Physics, Queen's University, Belfast BT7 1NN, Northern Ireland
4 Department of Physics, Lund Institute of Technology, Box 118, 22100 Lund, Sweden
5 Department of Physics, Royal Institute of Technology, AlbaNova University Center, 10691 Stockholm, Sweden
Accepted: 11 January 2008
Context.In many plasmas, long-lived metastable atomic levels are depopulated by collisions (quenched) before they decay radiatively. In low-density regions, however, the low collision rate may allow depopulation by electric dipole (E1) forbidden radiative transitions, so-called forbidden lines (mainly M1 and E2 transitions). If the atomic transition data are known, these lines are indicators of physical plasma conditions and used for abundance determination.
Aims.Transition rates can be derived by combining relative intensities between the decay channels, so-called branching fractions (BFs), and the radiative lifetime of the common upper level. We use this approach for forbidden [Sc II] lines, along with new calculations.
Methods.Neither BFs for forbidden lines, nor lifetimes of metastable levels, are easily measured in a laboratory. Therefore, astrophysical BFs measured in Space Telescope Imaging Spectrograph (STIS) spectra of the strontium filament of Eta Carinae are combined with lifetime measurements using a laser probing technique on a stored ion-beam (CRYRING facility, MSL, Stockholm). These quantities are used to derive the absolute transition rates (A-values). New theoretical transition rates and lifetimes are calulated using the CIV3 code.
Results.We report experimental lifetimes of the Sc II levels 3d2 a3P with lifetimes 1.28, 1.42, and 1.24 s, respectively, and transition rates for lines from these levels down to 3d4s a3D in the region 8270-8390 Å. These are the most important forbidden [Sc II] transitions. New calculations for lines and metastable lifetimes are also presented, and are in good agreement with the experimental data.
Key words: atomic data / radiation mechanisms: general / methods: laboratory / stars: individual: Eta Carinae
Based on observations made with the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope Science Institute. STScI is operated by the Association of Universities for Research in Astronomy, Inc. under NASA contract NAS 5-26555. Laboratory data is obtained at the CRYRING facility at the Manne Siegbahn Laboratory (MSL), Stockholm University, Sweden.
© ESO, 2008
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