Astron. Astrophys. 319, 637-647 (1997)

5. Variability of the line parameters: , FWHM

The feature at 6708 Å  is present in all the spectra. As we discussed in the previous sections, the contribution to the blend is probably due to the Li I resonance doublet, to the V I line and to some other unknown element.

The behavior of the profiles of the 6708 Å  blend versus the rotational phase is shown in Fig. 5. It can be seen that the line is an asymmetric blend with a well defined red wing and a more shallow blue wing. The profile of the blend undergoes evident changes during the rotational period. The most simple way to interpret these variations is to suppose that the feature consists of two components, red and blue with somewhat variable intensity and wavelength. The strength of the blue component probably changes in antiphase with the red component. Rotational Doppler effect is visible in the line profiles.

Fig. 5. The variation of the Li feature with the rotational phase. From top to bottom: rotational phases 0.96, 0.94, 0.79, 0.77, 0.73, 0.71, 0.48, 0.41, 0.40, 0.36, 0.34, 0.29, 0.28, 0.04.

Fig. 6 shows the variations of position of the center of gravity of some lines in the Li region, expressed by the rotational radial velocity (km s^-1  ) versus the rotational phase of  CrB. We believe that the variation of the line position is a real effect, but the number of the available observations for the phases .00 - .20, .50 - .70 is insufficient for the determination of the accurate shape of the rotational radial velocity curve. The full amplitude of the variations is consistent with the projected rotational velocity of  CrB (v sin i 2-3 km s^-1, Preston, 1967). The maximum amplitude of the variations is shown by the lines 6702.10 Å  Gd II, and 6704.3 Å  Gd II+Ce II. Our observations indicate that most of the lines vary with the period of rotation. For completeness of presentation of the spectral variability, we have used also other spectral regions: 6645 Å  Eu II, and 6149 Å  Fe II. The behavior of these lines is similar to those of the Li region, but with different amplitudes. The typical scattering from the curve of rotational radial velocity for the Li region is about 0.3 - 0.5 km s^-1.

Fig. 6. Rotational radial velocity versus the rotational phase.

Fig. 7 and 8 show the results of our measurements of the equivalent widths and full width at half maximum FWHM with the rotational phase. The behavior of each line is different. The maximum amplitude of the curve versus rotational phase is shown by the blends 6704 Å  Gd II + Ce II and 6706 Å  Fe I + Ce II, while the lines of Fe I at 6149, 6702 , 6716, and 6715 Å  almost do not vary, and 6702 Å  Gd II shows small variations; therefore the more variable lines are probably those of Ce II. The equivalent width of the Li blend does not vary very much, while the FWHM shows remarkable variations. The FWHM of lines of Fe I and Eu I do not vary in an appreciably way.

Fig. 7. The equivalent widths versus the rotational phase.

Fig. 8. FWHM versus the rotational phase.

The data giving the relations , , and FWHM versus the rotational phase are sufficiently well defined only for few lines. The FWHM is almost constant for the lines examinated, with the exception of 6708 Å. The maximum and the minimum FWHM are observed at rotational phases .3 and .8 respectively. At phase .3 the equivalent width has its maximum value while at phase .8 has a medium value. When the rotational radial velocity is at the maximum and minimum values (i.e. the lithium + other contributors are concentrated at the borders of the stellar disk) the FWHM is at its medium and not at the minimum value, as we should expect. Hence the relations between , , and FWHM are not easily interpretable and possibly indicate the presence of several concentrations of different intensities.

The only two other lines for which we have reasonably well defined curves both for the equivalent width and the rotational radial velocity are 6702 Å  Gd II and the blend 6704 Å  Gd II + Ce II. They show roughly the same behavior, with at its medium value when the rotational radial velocity is at maximum and minimum values, and is maximum when has its medium value; this behavior can be explained by one large spot where Gd and Ce are concentrated.

6706 Å  Fe II + Ce II shows a very well defined curve versus rotational phase, in phase with those for  6702 and 6704 Å , but no rotational radial velocity curve was available.

6716 Å  Fe I shows two curves with smaller amplitude than those for 6702, 6704, and 6706 Å  and a different behavior, with at maximum for varying from its medium to its maximum value and at medium value for medium value of . The full analysis of the correlations between the variations of , , and FWHM need more observations.

We interpret the asymmetric absorption feature  6708 Å  (Fig. 5) as due to the blend of two components with variable center of gravity of each component. A possible explanation is a spotted distribution of lithium and other unknown blending elements on the stellar surface. A source of uncertainty both for the study of the variations of the rotational radial velocity versus the rotational phase and for fixing the position of the 6708 Å  feature is not a sufficient precision in the wavelength scale. We have made corrections to the wavelength scale after a critical analysis of the binary radial velocity curve (Polosukina & Malanushenko, 1995) using the numerous measurements of radial velocity by Neubauer (1944), Oetken & Orwert (1984) and Kamper et al. (1990). The difference between the orbital elements by Oetken & Orwert and Kamper et al. cannot be neglected, because the orbital radial velocity curve presents a very deep and narrow minimum, and the position of the minimum depends on the accuracy in the determination of the period. Since the value of the period is not defined with sufficient precision (only two decimal figures), it is impossible to obtain a very accurate determination of the position of the spectral lines, which is necessary in our study of the Li feature, especially for the observations made in 1990 - 92 which fall near the minimum of the orbital radial velocity curve.

© European Southern Observatory (ESO) 1997

Online publication: July 3, 1998
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