EGAS41 - Swansea University

41 st EGAS CP 210 Gdańsk 2009
The FERRUM project: combining laboratory and astrophysical
studies reveals rates of forbidden transitions in [Fe II]
S. Mannervik 1,∗ , J. Gurell 1 , H. Hartman 2 , R. Blackwell-Whitehead 2 , H. Nilsson 2 ,
E. Bäckström 1 , L.O. Norlin 3 , P. Royen 1
1 Department of Physics, Stockholm University, AlbaNova University Center, SE-10691
Stockholm, Sweden
2 Lund Observatory, Box 43, SE-22100 Lund, Sweden
3 Department of Physics, Royal Institute of Technology, AlbaNova University Center, SE-10691
Stockholm, Sweden
∗ Corresponding author: mannervik@fysik.su.se,
Iron is a highly abundant element in many astrophysical objects and the Fe II spectrum
has therefore been studied in much detail through the years. More than 1000 energy levels
have been identified [1]. The complex spectrum of Fe II makes the observed spectrum
line rich, and the lines are observed in both emission and absorption.
Forbidden lines are very rarely observed in spectra from laboratory sources, due to the
levels’ sensitivity to collisions. The low radiative rates make the collisional deexcitation
the dominating process. However, in the spectra of astrophysical low density plasmas,
such as nebulae and HII regions, the forbidden lines are often the strongest lines in the
spectrum. The sensitivity to collisions make forbidden lines invaluable probes of physical
conditions, e.g. density and temperature, which in many cases can be determined by
simple line ratio measurements.
The experimental studies of forbidden transitions in Fe II started in 2001 when the
lifetimes of the levels a 6 S 5/2 and b 4 D 7/2 were measured by laser probing a stored ion
beam [2,3]. The experimental study was continued in 2003 when the same experimental
technique was used to measure the lifetimes of two additional metastable levels, a 4 G 9/2
and b 2 H 11/2 [4]. The lifetimes of a 6 S 5/2 , b 4 D 7/2 and a 4 G 9/2 were also combined with
branching fractions (BFs) for forbidden lines derived from astrophysical spectra in order
to deduce experimental A-values for forbidden transitions.
In the present work we have extended our experimental work and measured two new
levels. The resulting lifetimes of the 3d 6 ( 3 G)4s a 4 G 11/2 and the 3d 6 ( 3 D)4s b 4 D 1/2 levels
are τ = 0.75 ± 0.10s and τ = 0.54 ± 0.03s respectively. A-values for two forbidden decay
channels at 4243.97 and 4346.85Å from the 3d 6 ( 3 G)4s a 4 G 11/2 upper level are 1.05 ±0.15
and 0.25 ± 0.05s −1 , respectively. Both the lifetimes and the transition probabilities are
compared to previously published calculated values [5,6].
References
[1] S. Johansson, Phys. Scr. T134, 014013 (2009)
[2] S. Mannervik, Phys. Scr. T105, 67 (2003)
[3] D. Rostohar, A. Derkatch, H. Hartman, S. Johansson, H. Lundberg, S. Mannervik,
L.-O. Norlin, P. Royen, A. Schmitt, Phys. Rev. Lett. 86, 8 (2001)
[4] H. Hartman, A. Derkatch, M.P. Donnelly, T. Gull, A. Hibbert, S. Johansson, H.
Lundberg, S. Mannervik, L.-O. Norlin, D. Rostohar, P. Royen, P. Schef, Astron. and
Astrophys. 397, 1143 (2003)
[5] R.H. Garstang, Mon. Not. R. Astron. Soc. 124, 321 (1962)
[6] P. Quinet, M. Le Dourneuf, C.J. Zeippen, Astron. and Astrophys. Suppl. 120, 361
(1996)
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