Astron. Astrophys. 345, 137-148 (1999)
3. Orbital period changes of AW UMa
3.1. Times of minimum light
Several UBV photometric observations of AW UMa were taken during
eclipses. These observations were analyzed using the procedure
introduced by Kwee & van Woerden (1956) to determine the times of
minima. We have determined the times of 13 primary and 11 secondary
minima. Most of them (20) were already published (Pribulla et al.,
1997). The heliocentric times of our new 4 minima of AW UMa together
with all available photoelectric minima from the literature are listed
in Table 3.
Table 3. Photoelectric times of minima of AW UMa. Errors () are given in 0.0001 days. Julian dates are -2 400 000. The last 9 minima were excluded from the study of variations of the minima times
References: 1 - Paczynski (1964), 2 - Kalish (1965), 3 - Dworak & Kurpinska (1975), 4 - Woodward et al. (1980), 5 - Eaton (1976), 6 - Hrivnak (1982), 7 - Hart et al. (1979), 8 - Istomin et al. (1980), 9 - Kurpinska-Winiarska (1980), 10 - Mikolajewska & Mikolajewski (1980), 11 - BBSAG 47, 12 - Pribulla et al. (1997), 13 - Bakos et al. (1991), 14 - Heintze et al. (1990), 15 - BBSAG 88, 16 - Demircan et al. (1992), 17 - BBSAG 97, 18 - Müyesseroglu et al. (1996), 19 - Yim & Jeong (1995), 20 - this paper, 21 - Ferland & McMillan (1976), 22 - Srivastava & Padalia (1986), 23 - Oprescu (1997), 24 -Udalski (Rucinski, 1992), 25 - BAV-M 59, 26 - BAA 81, 27 - BBSAG 109.
Our UBV photometry shows that while the primary minima of AW UMa
occur almost simultaneously in all three passbands, the secondary
minima in the U passband occur later than in the B and V passbands.
The mean delay of the times of secondary minimum in the U passband in
comparison with the average times of the secondary minima in the B and
V passbands is
(96 22) s.
We have used the minima from Table 3 (except last 9) to find
the systematic delay of the secondary minima with respect to their
expected position in phase 0.5 given by primary minima. The resulting
(54 27) s is even larger
than the = 36 s found by
Demircan et al. (1992) and incompatible with an explanation by the
internal light-time effect (Liu et al., 1990). Another phenomenon must
be responsible for the shift. The large error in its determination
suggests that the phenomenon is highly variable.
3.2. O-C diagram and its explanation
Woodward et al. (1980) found that the orbital period of AW UMa
decreases. According to Hrivnak (1982), the observed change can be
either sudden or continuous and can be caused by mass transfer from
the more massive to the less massive component.
Altogether, we have used 108 minima (given in Table 3) to
study the period change. The times of 9 minima deviated too much from
the general trend (listed at the end of Table 3) and were
therefore excluded from this study.
Fig. 2. diagram for AW UMa constructed using the ephemeris (2)
A linear ephemeris (2) was applied to construct the
diagram (Fig. 2). The long-term
period decrease can be explained as:
Two sudden period changes . The first jump was detected by
Woodward et al. (1980). The second possible jump was detected by the
authors (Pribulla et al., 1997). Fitting the data by a broken-line, we
derived three linear ephemerides. The ephemeris valid for
24621 we have:
and finally for
24621 we have:
The sum of the squares of residuals for
the three linear fits is 0.00031816 d2. The first sudden
period change =
(7.30.2) 10-6 occurred at
JD 2 442 650105; the second one
(7.20.7) 10-6 at JD
2 448 850115. The time interval
between two possible sudden period changes was
6200150 days (= 17 years).
Continuous period change . A parabolic fit provides the
The sum of the squares of residuals is 0.00053518 d2.
The total change of the period is =
(1.420.06) 10-5. While
the largest residuals from the quadratic fit occurred in 1993-4, the
activity of the system reached its maximum four years earlier (see
Sect. 5.2). Therefore, the increase of the residuals was not caused
only by the activity of the system.
Continuous period change and the light-time effect . The
parabolic fit to the (O-C) data (continuous period change) shows its
greatest residuals at the two times cited in case (1) above as
possible sudden period changes. These residuals can be reduced by
invoking a light-time effect due to an additional body in the system,
superimposed on the continuous period change. The times of minima can
be computed as follows:
sin i is the
projected semi-major axis, e is the eccentricity,
is the longitude of the periastron,
is the true anomaly.
is the quadratic ephemeris of the
minima in an eclipsing binary and c is the velocity of the
A preliminary period of the light-time effect was found by Fourier
period analysis of residuals from the quadratic fit as
65001200 days. The parameters of the
fit obtained by simultaneous fitting of the light-time effect and
parabolic ephemeris obtained by the differential corrections method
are given in Fig. 2. The sum of squares of residuals is 0.00033223
d2. The period of the body
= 6250340 nearly coincides with the
time interval between possible jumps (case 1). The body responsible
for such a light-time effect would cause the systemic velocity to
change with an amplitude of 0.9 km s-1, which is much
smaller than the observed one (see Sect. 7).
To summarize, the data can be interpreted as two sudden period
changes or as one continuous period change (quadratic fit). In the
latter case, the residuals show a systematic effect which can be
interpreted as the light-time effect due to another body. Although the
sum of the residuals is smaller in the first case, the resulting fits
for both cases differ only slightly (see Fig. 2). The present data are
not sufficient to decide which interpretation is correct.
© European Southern Observatory (ESO) 1999
Online publication: April 12, 1999