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Context. In many plasmas, long-lived metastable atomic levels are depopulatedby 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 E2transitions). If the atomic transition data are known, these lines areindicators of physical plasma conditions and used for abundance determination.Aims. Transition rates can be derived by combining relative intensities betweenthe decay channels, so-called branching fractions (BFs), and the radiativelifetime 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 withlifetime measurements using a laser probing technique on a stored ion-beam(CRYRING facility,MSL, Stockholm). These quantities are used to derive theabsolute transition rates (A-values). New theoretical transition rates andlifetimes are calulated using the CIV3 code. Results. We report experimentallifetimes of the Sc ii levels 3d2 a3P0,1,2 with lifetimes 1.28, 1.42, and 1.24s, respectively, and transition rates for lines from these levels down to 3d4sa3D in the region 8270-8390 A. These are the most important forbidden [Sc ii]transitions. New calculations for lines and metastable lifetimes are alsopresented, and are in good agreement with the experimental data.

The FERRUM Project: experimental and theoretical transition rates of forbidden [Sc II] lines and radiative lifetimes of metastable Sc II levels