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Astron. Astrophys. 357, 920-930 (2000)

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5. The rare earth elements (REE)

Special attention was paid to the line identification, taking into account the lines of the rare earths, in the computation of the synthetic spectrum especially in the region of Li 6708 Å. This kind of work is yet to be completed, but some preliminary results permit us to assign some earlier unidentified lines to rare earth elements, once- and twice-ionized. For instance, we have identified the feature 6690.96 Å as a Nd III line and two features, 6706.70 Å and 6727.67 Å, with Pr III lines. The intensities of these three lines vary, like the lithium blend, but with some phase shift relative to the latter. So, in Fig. 4 we show the spectra at two phases, nearest to the maxima of the surface magnetic field: 0.419 when the Li line has maximum intensity, and 0.055, in which all three lines of REE mentioned above are intensified. The line 6690.9 Å is possibly a blend of Nd III 6690.96 Å, Er II 6690.00 Å and Ca I 6691.02 Å. The appearence of its profile depends on the phase. We note the difference from the Li line profiles which have constant width at various phases, a fact which indicates the constant size of the Li line forming region (two spots). On the contrary, since the width of the rare earth element lines changes, then the sizes of the visible part of their abundance patches changes too.

Besides 8 spectra in the lithium region near 6708 Å we have 2 additional spectra: in the region 6120-6180 Å (phase = 0.320) and in the region 6610-6670 Å (phase = 0.768). For the last phase we see two components of the line Eu II 6645.06 Å (Fig. 6). Also two components with different intensities are observed in the Ba II , Nd III and Fe II lines (Fig. 7) for the phase 0.320. We have calculated the line profile of Eu II 6645.06 Å with magnetic splitting for the field of 2 kG and 11 kG, taking into account spot structure (see Table 3 in the Sect. 6).

[FIGURE] Fig. 6. The line profile of Eu II 6645.06 Å. Bold line: the observed spectrum at the phase 0.768, dashed line: the calculated one with magnetic splitting for the field of 2 kG (Zeeman splitting), and thin line for 11 kG (Paschen-Back effect), both with taking into account spot structure.

[FIGURE] Fig. 7. The spectrum of HD 83368 in the region 6140-6175 Å. Continuous line: the observed spectrum at phase of 0.320; dashed line: the computed spectrum using Kurucz model atmosphere with [FORMULA]. The spectrum has been convolved with a rotation profile for [FORMULA]. It is evident the splitting of the Ba II 6141.71 Å and Nd III 6145.07 Å. If we adopt the Nd abundance from the line Nd III 6145.07 Å, we obtain enhanced intensities of Nd II lines in the calculated spectrum relative to the observed spectrum.


Table 3. Data about REE spots taken into account for the calculated spectra at Fig. 8b (phase=0.689) and at Fig. 6 (phase=0.768).

The similar intensity ratios of the components for Fe II and Ba II at phase 0.320 indicate a similar spot (or ring) distribution of these ions near the poles with the lithium spots. The Nd III line 6145.07 Å shows an opposite intensity ratio of the two components at the same phase - probably the surface distribution of this ion differs from that of Fe II and Ba II (Fig. 7). Although the Nd III line 6145.07 Å is blended with Fe II line 6145.610 Å, the last one affects the shape of its profile insignificantly.

For computing the synthetic spectra we used the relative line strengths of Pr III from the list of Sugar (1974), and the absolute values of gf were assigned assuming equal abundances from Pr III and Pr II . For Pr II the data of Kurucz (1993) were used. The Pr abundance, determined in the region 6120-6180 Å at phase 0.320, gives a good agreement between the calculated and the observed lines of Pr II and Pr III in the lithium region at phase 0.331. For two lines of Nd III , 6145.07 Å and 6690.96 Å, belonging to the same multiplet, we have computed the relative strengths, supposing the L-S moment sum rule. Accepting the value [FORMULA] for the line 6145.07 Å (Cowley & Bord, 1998), we have found for the line 6690.96 Å a value [FORMULA], that is closed to data of Bord [FORMULA] (Bord, 1998) for this line. Using these gf values, we have obtained the same Nd abundance for the nearest phases 0.320 (line 6145.07 Å) and 0.331 (line 6690.96 Å). However this Nd abundance gives too strong intensities for the lines of Nd II (see Fig. 7), computed with the gf data from Kurucz (1993). The disagreement between the abundances derived from Nd II and Nd III consists in 0.8-1.5 dex. Similar enhanced Pr III abundances were obtained using the new values of [FORMULA] computed by Bord for four lines of Pr III : 6160.24 Å (-0.827), 6161.22 Å (-0.728), 6706.70 Å (-1.285), 6727.63 Å (-3.289). Probably, the differences in the abundances of once- and twice-ionized rare earth elements are connected with the specific conditions of ionization near the magnetic poles.

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© European Southern Observatory (ESO) 2000

Online publication: June 5, 2000