Non-Periodic Phenomena in Variable Stars
IAU Colloquium, Budapest, 1968
INSTABILITY OF LIGHT CURVES AND PERIODS OF LONGPERIOD
CEPHEIDS BELONGING TO THE SPHERICAL COMPONENT OF THE
GALAXY
O. P. VASILJANOVSKAJA and G. E. ERLEKSOVA
Astrophysical Institute, Academy of Sciences, Tadjik SSR, Dushanbe
(Read by Shakhovsky)
The longperiod Cepheids belonging to the spherical component of the Galaxy
represent a special group of Cepheids like the cluster variables. From our
investigation of the light curves of these Cepheids we notice that they are
divided into two groups having light curves (Vasiljanovskaja et al. 1966,
1968) deviating from the standard light curves of classical Cepheids. The first
group is characterized by assymmetrical light curves, in most cases with an
hump on the descending branch. The second group is characterized by smooth
symmetrical or almost symmetrical light curves. The division of Cepheids into
two groups was confirmed by the investigations of period changes
(Vasiljanovskaja and Erleksova, 1969a) and the statistical relations between
light amplitude and logarithm of period, intrinsic colour index and logarithm
of period, period change and light amplitude (Vasiljanovskaja and Erleksova,
1969b).
One of the peculiarities of the spherical component Cepheids is the
instability of the shape of their light curves. The first group with
periods longer than 17d shows variability of the light curve from cycle
to cycle on the descending branch, in the region of the hump. Real
changes of the shape and the size of the hump are shown by the Cepheids
W Vir (P = 17d), MZ Cyg (P = 21d) and TW Cap (P = 29d). These changes
were traced by photoelectric or photographic observations taken by one
author only. For example, let us examine the variations of the light
curve of TW Cap. A variation of the hump on the descending branch was
found by Soloviev (1955) from his photographic observations. Wallerstein
(1958) noted a large scatter in the radial velocity curve. Wallerstein
considered the scatter as consequence of irregular variations in the light
curve from cycle to cycle. The normal light curve of TW Cap is represented
in Figure 1, expressed in fractions of the amplitude (Max = 0, Min = 100).
One can see that the whole descending branch of the light curve is changing.
A reliable change of the light curves in the first group with periods shorter
than 17d is not observed. However, we note that the photographic light curves
of ST Tau (P = 4d) obtained by various authors have different descending
branches. We were not able to reduce all observations into one photometric
system and to take into account the period changes. Therefore the observed
discrepancies may not be real.
Cepheids of the second group with periods longer than 20d show variations
of the light curve more frequently than Cepheids of group I. It is established
from photoelectric observations that the light curves of the Cepheids RS Pav
(P = 20d), RU Cam (P = 22d) before the sudden fall in the light amplitude
in the year 1964, RX Lib (P = 25d) show alternating cycles reminding of
RV Tau light curves. The maximum magnitude remains almost constant. The shape
of the light curve and the amplitudes change insignificantly from cycle to
cycle. The normal light curve near the primary maximum is sharper and narrower
(Figure 2 - broken line) than near the secondary maximum (Figure 2 -- solid line).
We take RU Cam as an example, because RU Cam behaved like a Cepheid of Group II
in the course of 60 years.
Fig. 1
The other peculiarity of the spherical component Cepheids is the instability
of their periods. Sudden changes of the period are typical features of all
investigated Cepheids with periods longer than 2.4d. In the cases of SZ Cas,
MZ Cyg and CC Lyr sudden changes of the period are accompanied by progressive
changes. The investigation of the period changes of long period variables and
the comparison with Cepheids show that progressive and sudden changes of the
periods take place at the same time. The causes of the progressive and sudden
period changes are different and independent. The interpretation of the progressive
period change as an evolutionary effect is very probable.
Fig. 2
Fig. 3
Fig. 4
The parabolic term in the elements show the progressive period change quite
convincingly. The influence of the parabolic term was excluded during the
study of the sudden changes. We assume that the sudden period changes can be
derived from the mechanism of stellar variability and therefore we compared
the sudden period changes with the characteristics of stellar variability:
logarithm of period, light amplitude DeltaB and color amplitude Delta(B-V).
Figure 3 shows the lg(abs(mean(DeltaP))/P) versus lg P relation. Figure 4 shows
the abs(mean(DeltaP))/P versus DeltaB relation. In the Figures dots
refer to Cepheids of Group I, circles to Cepheids of Group II.
Our statistical investigations (1969a) show that the mechanism of
period-changes contains an element of a random process. We notice that the
sudden period changes in long period variables are similar to the above
mentioned changes in Cepheids. However, the period changes in long period
variables are larger than those in Cepheids.
REFERENCES
Soloviev, A. V., 1955, Bull. SAO No. 13.
Vasiljanovskaja, O. P., Erleksova, G. E., Shakhovskaja, N. I. 1966. Bull. Dushanbe
No. 48.
Vasiljanovskaja, O. P., Erleksova, G. E., 1968. AC No. 469.
Vasiljanovskaja, O. P., Erleksova; G. E., 1969a. Bull. Dushanbe (in press).
Vasiljanovskaja, O. P., Erleksova, G. E., 1969b. Bull. Dushanbe (in press).
Vasiljanovskaja, O. P., Kiselev, N. N., Kiseleva, T. K. 1969c. Bull. Dushanbe (in press).
DISCUSSION
Fitch: Oosterhoff has found for 4 or 5 Cepheids of about 2 days period
that cycle-to-cycle variations are periodic and caused by the excitation
of a second pulsation-mode. Have you made any effort to see whether the
cycle-to-cycle variations you observed in these longer period cepheids
are periodic?
Shakhovsky: The variability of the shape of light curves is not periodic,
it presents differences between the subsequent cycles like the RV
Tau-type stars.