New identified (³H)4d–(³H)4f transitions of Fe II from UVES spectra of HR 6000 and 46 Aquilae^*,**

¹ Istituto Nazionale di Astrofisica–Osservatorio Astronomico di Trieste, via Tiepolo 11, 34131 Trieste, Italy e-mail: castelli@oats.inaf.it
² Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA
³ Astrophysical Institute Potsdam, An der Sternwarte 16, 14482 Potsdam, Germany

Received: 18 May 2009
Accepted: 4 September 2009

Abstract

Aims. The analysis of the high-resolution UVES spectra of the CP stars HR 6000 and 46 Aql has revealed the presence of an impressive number of unidentified lines, in particular in the 5000–5400 Å region. Because numerous 4d–4f transitions of FeII lie in this spectral range, and because both stars are iron overabundant, we investigate whether the unidentified lines are FeII.

Methods. ATLAS12 model atmospheres with parameters T_eff = 13 450 K,  = 4.3 and T_eff = 12 560 K,  = 3.8 were computed for the individual abundances of the stars HR 6000 and 46 Aql, respectively, to use them as spectroscopic sources to identify FeII lines and determine FeII gf-values. After identifying several unknown lines in the stellar spectra as (³H)4d-(³H)4f transitions of FeII, we derived astrophysical -values for them. The energies of the upper levels were assigned on the basis of both laboratory iron spectra and predicted energy levels.

Results. We determined 21 new levels of FeII with energies between 122 910.9 cm^-1 and 123 441.1 cm^-1. They allowed us to add 1700 new lines to the FeII linelist in the wavelength range 810-15 011 Å. Many of these lines are sufficiently strong to contribute to the spectra of Population I late B-type stars, even when their iron abundance is subsolar. In the 5000–6000 Å region discussed in this paper, the astrophysical and computed -values show good general agreement and greatly improve the synthetic spectrum of both HR 6000 and 46 Aql. However, many features remain unidentified indicating that further work to classify FeII high energy levels has still to be done