## 7. Masses of the contact binary and the third component in the systemThe systemic velocity of AW UMa determined from the Ondejov spectra does not agree with the values found from the Toledo spectra and there are also differences in its determination among other authors. We will go on to show briefly the evidence for a third body, and hereafter and designate the velocities of the mass centres of the binary and triple system, respectively. McLean (see Rensing et al., 1985) changed its original value
=(-177) km s We reanalyzed all the original data using the ephemerides (1)-(3). The correction of the phases for the McLean's (1981) data was as large as 10% of the orbital period. The original and the new values of systemic velocities and amplitudes of the RV curve are given in Table 7.
To find out whether the differences in systemic velocity of binary
are real or caused only by the scatter of observations, we have
applied zero-point shifts to all available RVs of AW UMa. The
individual RVs of the primary component before and after the
correction for are shown in Fig. 9.
The sum of squares of residuals for the original data
(70287 km
This result as well as the study indicate that the variations in are real. We interpret them as evidence for the presence of the third component in the system. It is interesting to note, however, that when previous investigators observed the system over two consecutive seasons, they measured the same each season (Rensing et al. 1985, Rucinski 1992). A similar effect is seen in our two Toledo observations. One might alternatively conclude that the variations in are due to systematic effects between different telescopes rather than to a third body. However, systematic effects seem to be ruled out by the use of radial velocity standard stars (except for our Toledo observations). The agreement in the values of in observations of successive seasons is due to the fact that the period of the third body happens to be comparable to the one-year observing interval. ## 7.1. New spectroscopic elements of the contact binaryAltogether we have 149 determinations of the RV at our disposal.
These data, corrected for particular
and weighted to account for the different qualities of the
spectroscopic observations, were used to obtain a more accurate
spectroscopic orbit of the contact system. The new spectroscopic
elements are given in Fig. 9 (bottom). Adopting a photometric mass
ratio One must be aware, however, that the set of RVs of the primary component is quite inhomogeneous due to the different methods used for finding RVs: cross-correlation (McLean, 1981), synthetised profile (Rensing et al., 1985), broadening function with model profiles (Rucinski, 1992) and line measurements (Paczynski, 1964; this paper). Since some methods do not take into account the secondary component, the RV of the primary is underestimated because of the line profile blending with the secondary (Paczynski,1964; Rensing et al., 1985 and this paper). It is important to note that proximity effects in contact binaries tend to make the light center closer to the companion star than is the center of mass and thus tends to underestimate (Hrivnak, 1988). Rensing et al. (1985), however, state that the distortion effects are not important for AW UMa. ## 7.2. Parameters of the third componentThe third component in the system causes not only changes in the systemic velocity, but also a light-time effect, which can be found in the residuals from the broken-line fit (two sudden period changes). Since the residuals for the third case nearly coincide, henceforth we will assume the first case of period change. We have tried to find a solution taking into account both effects simultaneously. The main problem is the large number of "free" parameters. Some of them, however, are not independent. If we compare the formulae for the light-time effect (Irwin, 1959): and for (e.g. Binnendijk, 1960): we get the amplitudes as follows: Excluding of from the above equations yields: For and Using the equation (10) we constrained the range of possible periods of the third body. Thereafter we have found the value of period of the third body by a period analysis of the systemic velocity and light-time curves separately. The amplitude of residuals is
days
( = 0.0017). The amplitude of the
systemic velocity variations is
= 12.5 km s We have found that the best fit for the third component corresponds
to an orbital period = 398 days
(Fig. 10). The systemic velocity of the triple system is
-15.6 km s
Although the fit is perfect, the corresponding fit in diagram, due to the large scatter of data, is not as good. Hrivnak (1982) estimated that the asymmetry of the LC can cause deviations as large as 0.004 days while the amplitude of the light-time effect caused by the presence of the third body is only about 0.00258 days. The minimum mass of the third component determined from the mass function and the mass of the contact system is = (0.850.13) M. The energy distribution of the third light suggests that the third
component is a white dwarf (see Sect. 4.2). Therefore it is not
expected to be seen on our high dispersion spectra from Toledo.
Nevertheless, due to the problems with the exact photometric solution,
we cannot exclude the possibility that the third component is a main
sequence star. In such a case the ratio of its luminosity and that of
the primary component is 0.0760.040.
Thus it could be detected on high-dispersion spectra. The expected
range of its RVs is -16 to 58 km s © European Southern Observatory (ESO) 1999 Online publication: April 12, 1999 |