Spectral observations were performed at ESO with the 1.4m Coudé Auxiliary Telescope (CAT) and the Coudé Échelle Spectrometer (CES). The detector was ESO CCD #34 with 2048 pixels along the direction of dispersion (the pixels are wide). Table 1 details the observations, i.e. time, phase and wavelength coverage. The resolving power for all spectra was (resolution at 6705 Å: 67.1 mÅ) and signal-to-noise ratio was S/N = 100 to 120.
Fig. 3 shows the variability of the Li I feature; it shows the spectra, as well as the equivalent width and radial velocity of each of the two components of this feature. The quantitative analysis of the spectrum of this star in the regions 6675-6735 Å, 6120-6180 Å and 6615-6675 Å, to our knowledge, is carried out for the first time. In Fig. 4 the normalized spectrum in the region of the lithium blend 6708 Å for the rotational phases 0.055 and 0.419 is shown. The variability of the spectral lines, which change both their position (shift of the line as a whole) and the profile appearance, is evident. These changes are the largest and impressive for the lithium blend 6708 Å. The other lines, in particular 6690.9 Å, 6706.7 Å, 6727.7 Å also reveal some variability. Apparently we observe surface abundance variability not only of lithium, but of other elements too, connected with different geometry and probably physical conditions in the spots and in the non-spotted photosphere.
The quantitative analysis of the spectra of HD 83368 was carried out by the method of synthetic spectra with the help of Tsymbal's code STARSP (Tsymbal, 1994) and Kurucz's atmospheric models (Kurucz, 1993). We used the Kurucz line lists (Kurucz, 1995, CDROM 23) and the VALD list (Piskunov et al., 1995, Kupka et al., 1999), accessible on INTERNET (URL http://cefa-www.harvard.edu/amdata/ampdata/kurucz23/sekur.html and http://www.astro.univie.ac.at/ vald , respectively). The data for the doubly ionized rare earth element (REE) were taken from the list of Reader & Corliss (1980). For Nd III , Pr III , Ce II and Ce III we used the level energy data and gf data provided to us by Cowley (1998), Bord (1998) from Michigan University and by Sugar (1998) from NIST (National Institute of Standards and Technologies). For identification purpose we have also calculated the positions of the lines of the ionized rare elements using the energy level data of NIST (URL http://www.aeldata.nist.gov ) and, for Dy III , the energy levels from the paper of Spector et al. (1997). Due to the small observed spectral region and the insufficient covering of rotation phases we carried out only a preliminary analysis. We have tried to determine the mean chemical composition (in the photosphere and spots) for each observed rotational phase, using one atmospheric model with and metal abundance [M/H]=0.0. The calculated spectra were convolved with the rotation profile with the value of . This and other parameters for the calculations of the synthetic spectrum were chosen in accordance with the data of North et al. (1998). We also tried to calculate the synthetic spectra with other model atmospheres, changing on and on . The best agreement in the abundances computed from Fe I and Fe II lines was achieved for the model . By fitting the calculated synthetic spectra with the observed ones we have found line intensity changes for several elements, depending on the rotational phase.
The values of relative to hydrogen for the different phases are given in Table 2 (Columns 2 to 10). The number of lines used for abundance estimate and the errors for each element are given in Columns 11 and 12 of Table 2. Let us note that the estimated errors on the abundances depend mainly on the line intensities, numbers of lines, blending with other lines, accuracy of gf-values and inhomogeneous surface distribution of the element (and of other elements, due to blending), and therefore depend on the rotation phase too. Because of these difficulties we give only one estimated value of the error for all phases. The procedure of fitting observed and calculated spectra was carried out until the discrepancy for all the analysed lines of each element reached its minimum. The last three columns of Table 2 give, for comparison, the solar abundances (Kurucz, 1993) and the abundances for a similar roAp star, HD 24712, by Ryabchikova et al. (1997). HD 24712 also shows variability of chemical composition versus the rotational phase, but has no measurable Li 6708 Å line. We notice that the abundances and their behaviour for the majority of the elements (Fe, Ca, REE, light elements) are essentially similar to the case of HD 83368. The data for the light elements (C,N,O) in HD 24712 were taken from the paper of Roby & Lambert (1990).
Table 2. The element abundances for each rotation phase from the spectral range 6675-6735 Å.
© European Southern Observatory (ESO) 2000
Online publication: June 5, 2000