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Astron. Astrophys. 345, 137-148 (1999)

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6. Analysis of spectroscopic observations

AW UMa is a F0-2 star, thus it has a pure absorption spectrum dominated by the lines of neutral hydrogen and ionized calcium (H and K line). Metallic lines are quite weak. The secondary component lines, very weak because of the low mass ratio, can be detected only near quadratures. The very rapid orbital motion, rather long exposure times and the lower signal-to-noise ratio caused by the use of photographic plates prevented us from measuring the secondary component lines on the spectra from the Ondejov observatory, but they were well resolved and measured on the two CCD spectra from Toledo.

Our spectroscopic analysis is mainly based on 20 spectra taken at the Ondejov observatory in 1992. We have used the H[FORMULA], H[FORMULA], H[FORMULA] and H8-H14 lines to measure the radial velocities (RV) of the primary component. The blend of H[FORMULA] and Ca II H lines was omitted from the analysis. The RVs were measured by the oscilloscopic method using the software SPEFO. The results are given in Table 6 and Fig. 6.


Table 6. Radial velocities of the primary component in km s-1. The phases were calculated using the ephemeris (2)

[FIGURE] Fig. 6. Radial velocities of the primary component of AW UMa, the fit for H[FORMULA] line and corresponding elements of the spectroscopic orbit

H[FORMULA] line is on the edge of the spectral sensitivity of the Kodak IIaO plate. Since the intensity of hydrogen lines decreases towards the Balmer jump, the most suitable line for RV measurements is H[FORMULA]. The elements of the spectroscopic orbit computed from the measured RVs of this line are given in Fig. 6. It is necessary to note that the deviations from the average RV increase from H[FORMULA] to H14. In both solutions, a zero eccentricity and a period P = 0.43873 days were fixed.

We have measured the halfwidth (HW) and equivalent width (EW) of the H[FORMULA], H[FORMULA], H[FORMULA] and H8 lines (Fig. 7). The HWs of the lines depend on phase in the same manner as shown by Paczynski (1964). They exhibit two maxima and minima as expected for a contact binary. We have found HWs around the maximum I to be slightly larger than around maximum II. The phase dependence of EWs is different. They change throughout the orbital phase only a little, but the minimum around the phase 0.9 seems to be real. The large EW in phase 0.169 can be partly influenced by the long exposure time (91 min.) of the spectrum.

[FIGURE] Fig. 7. EWs and HWs of hydrogen lines. Large symbols denote average values for the particular spectrum

The two spectra from Toledo with high S/N ratio were also used for the RV measurements. The only usable line is H[FORMULA]. We have determined RV = (11[FORMULA]2) km s-1 and RV = (-36[FORMULA]2) km s-1 for the spectra taken on March 3, 1995 (phase 0.703) and March 1, 1996 (phase 0.313), respectively. The corresponding systemic velocities calculated using the difference between measured and expected RV (assuming [FORMULA] = (25.3[FORMULA]0.6) km s-1, see Sect. 7) of the primary component in phases 0.703 and 0.313 were (-13.2[FORMULA]2.1) km s-1 and (-12.6[FORMULA]2.1) km s-1, respectively. The spectra were taken near quadratures, so their simple subtraction (after correction for RV of the primary component) enabled us to measure RVs of the secondary component in both phases simultaneously (Fig. 8). The mass ratio calculated from the ratio of the RVs of the primary and secondary component corrected for the systemic velocity shift is q = 0.084[FORMULA]0.006 and q = 0.088[FORMULA]0.006 for the first and second spectrum, respectively.

[FIGURE] Fig. 8. H[FORMULA] profiles taken in orbital phases 0.313 and 0.703. Subtraction of the profiles clearly shows the presence of the secondary component

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© European Southern Observatory (ESO) 1999

Online publication: April 12, 1999