Non-Periodic Phenomena in Variable Stare
IAU Colloquium, Budapest, 1968
PHOTOMETRIC SEARCH FOR PERIODICITY AMONG MAGNETIC
STARS
KAZIMIERZ STEPIEN
Warsaw University Observatory, Warsaw, Poland
ABSTRACT
The results of the search for periodicity indicate that in almost
every case when a careful photometric investigation is carried out
periodicity can be found. There is no evidence of period variations of
the magnetic stars. The in-phase as well as antiphase relationship
between the light and magnetic curves was observed for different stars.
After extensive observations of a number of magnetic stars Babcock
divided them into three groups. The stars from two of them were supposed
to vary irregularly. Since that many members of these two groups were found
to vary periodically. The problem of periodicity of other magnetic stars became
open again. On the other hand the theory of oblique rotator requires all the
magnetic stars to be periodic and it was often used as an argument against
this theory that only a few magnetic stars were periodic while the rest seemed
to vary irregularly. In addition photometric observations showed a periodicity
of variations of many magnetic stars while the magnetic observations did not
follow these periods. Hence, it seemed important for better understanding
of phenomena taking place in these stars to reconsider the problem of
periodicity among the magnetic stars both from magnetic and photometric
point of view. Such an attempt was undertaken at the Lick Observatory.
Here, the results of the photometric part of program are shortly presented.
Originally 23 stars were chosen mainly from the list of stars observed
spectroscopically by G. W. Preston of the Lick Observatory. Later one of the
comparison stars turned out to be a variable itself and was also included into
the program. Of these 24 stars 10 had previously known photometric periods.
The observations were carried out with the 24 inch reflector of the
Lick Observatory and the standard UBV equipment. Two comparison stars were
usually chosen for each variable and the constancy of light of these stars was
checked against each other. On average 75 observational points in three colors
were obtained for each magnetic star. The mean error of one point was about
0.005 of a magnitude. The variability and periods were confirmed for all 10
stars which were observed in the past. Only in the case of 21 Per the variations
fit to the period of around 3 days instead of 1.7 days as previously found. The
presence of secondary maxima on V curves of 73 Dra and HD 224801 was confirmed
and such a maximum was found for HD 71866. Of the remaining 14 stars two,
HD 2453 and HD 9996, were found to be constant within the error of measurement
during the period of observations. For one star, mu Lib, the number of observations
was insufficient to try to find a period. The star did not show large variations
and the observations were difficult because of rather large negative declination
of the star. In all the other cases it was possible to find periods, of light
variations although for a few stars the results are uncertain due to the small
amplitude of variations and a low number of observations. List of observed
stars and their periods are given in Table 1.
Table 1
List of observed stars and their periods
Name or HD P Name or HD P Name or HD P
(days) (days) (days)
2453 - 71866 6.80 153882 6.01
9996 - 49 Cnc 5.43 10 Aql 9.78
10783 4.13 kappa Cnc 6.91 21 Aql 1.7
21 Per 2.88 17 Com A 5.09 192678 18.
19216 7.7 111133 11. 73 Dra 20.3
32633 6.43 78 Vir 12.3 215038 2.04
53 Cam 8.03 mu Lib - 215441 9.49
15 Cnc 4.12 52 Her 0.96 224801 3.74
A few stars (like HD 224801, 73 Dra, HD 71866) were observed often enough
in the past so the discussion of a constancy of their periods seemed to be
justified. It was always possible to phase all the existing photometric
data into one curve without necessity of introducing any period variations.
Hence, one can draw the conclusion that at the present the magnetic stars
do not present evidences of variations of period. For some stars concurrent
or close in time series of magnetic observations were obtained by G. W. Preston
and the magnetic curves were formed. In these cases it was possible to discuss
the phase relationship between light and magnetic curves. It turned out that
the minimum of the magnetic curve can coincide with the minimum of the light
curve as well as with the maximum. Sometimes it depends on the color discussed.
Hence, the suggestion that the light curve and the magnetic curve are always
in antiphase does not hold any more.
Discussing the problem of irregularity among the magnetic stars one must
keep in mind a few important facts. This investigation showed that chances of
detecting light variables with large amplitudes are rather small. In no case
the amplitude of a new discovered variable exceeded in any color 0.04 of a
magnitude and typically it was of the order of 0.01-0.02 of a magnitude.
It seems that most of variables to be discovered have such amplitudes or even
smaller. There is no doubt that a periodic variable with the amplitude of 0.01
of a magnitude and a period of several weeks or more will be very hard to
detect. New techniques should be employed permitting diminishing the error of
measurement. Periodic variations can be detected only if the amplitude
of variations is at least a few times larger than the probable error of one
observational point (otherwise a very large number of observations is required
to apply the statistical analysis). The same considerations hold for magnetic
measurements except that here, contrarily to the photometry, the probable
error of one observational point varies strongly from one star to another
ranging from 30 gauss to about 1000 gauss. Because many stars show magnetic
variations of the order of a few hundred gauss it is clear that the detecting
of periodicity is often difficult if not impossible. Again new techniques
should be looked for to permit to measure the magnetic fields of stars with
broad spectral lines. This investigation strongly suggests that the percentage
of periodic magnetic stars, probably with constant periods, is high.
More detailed discussion of the photometric behaviour of magnetic stars will
appear in the Astrophysical Journal, vol. 154.
It is the pleasure of the author to thank the staff of the Lick Observatory
and particularly Dr. George W. Preston for hospitality and help during his stay
at the Lick Observatory where this investigation was carried out.
DISCUSSION
Herbig: Does there remain any one well established case of irregular variability
among the magnetic variables?
Stepien: I think we cannot say for the present unambiguously that a given
magnetic star is irregular. Even if long series of observations exist and
the period cannot be found it does not yet prove that the star is really
irregular. An example of such a star is 78 Vir. Babcock obtained a large
amount of measurements for this star and the periodicity could not be found.
Now, it seems that the period of around 12 days may satisfy the data
although a large scatter is present. However, because the amplitude is not
large, such a scatter is expected. Besides the scatter, some irregular
fluctuations may exist in many stars obscuring existing periodicity.
There must be long series of observations, variations must be much
larger than the error of one measurement and such variations must be
confirmed by another investigator (preferably with another equipment)
before the conclusion about irregularity is reached. Taking all of this
into account I can say I do not know any star which was proved beyond
doubt to be irregular.
Plagemann: Your observations that the maximum of light do not always
coincide with a minimum of magnetic field - which of the theories
discussed by Dr. Jarzebowski would this tend to support or reject?
Stepien: It does not influence any particular theory of magnetic stars more
than the other. If the light and magnetic curves were always in antiphase
it would be very difficult to explain this fact on the ground of the basic
theory of magnetohydrodynamics because the equations governing the motion
and the behaviour of matter do not depend on the sign of the magnetic field.
These equations must be used in any attempt of theoretical explanation of
light variations. As a result the preference of the light and magnetic
curves to being in antiphase would cause equal troubles for every theory
of magnetic stars and a lack of such a preference removes these troubles
from all of them.