Non-Periodic Phenomena in Variable Stars
IAU Colloquium, Budapest, 1968
ON THE INTRINSIC LIGHT-POLARIZATION IN SOME LATE TYPE
STARS
R. A. VARDANIAN
Byurakan Astrophysical Observatory, Armenia, USSR
The variability of polarimetric parameters of mu Cephei was discovered in
1957 at the Byurakan Observatory (Grigorian 1958). So it became clear,
that sometimes, besides the interstellar polarization, a rather strong
intrinsic polarization can exist as well. After this discovery the
interest in polarimetric observations of late type stars strongly
increased. During the past ten years the variability of polarimetric
parameters of a number of stars was discovered. (Grigorian 1959;
Serkowski 1966; Shakhovski 1964; Vardanian 1966-67; Zappala 1967.)
At present the existence of intrinsic light-polarization can be
considered as established for a group of late type stars. Among them,
however, not more than 15 stars show a great amount of
light-polarization (P >= 2%).
In this paper we wish to present the results of our polarimetric
observations of some late type stars, in which the degree of light
polarization according to our observations was higher than 2% in the
blue part of their spectra.
175 late type stars (spectral types M, N, R, S) were observed during
1967 and 1968. 65 of them are variable stars. Only six variables showed
a considerable amount of light-polarization. The relevant data are
presented in Table 1. Its columns give respectively: the name of the
star, the galactic coordinates, the maximum and minimum magnitudes, the
period, the spectrum, the mean values of maximum and minimum degrees of
polarization, the mean value of the positional angle of polarization,
the difference of magnitudes corresponding to the maximum and minimum
polarization degree and the mean value of the ratio of the polarization
degrees measured in the blue and yellow parts of the spectrum.
Table 1.
Magnitude
Star l^II b^II Type Period Spectrum P_max P_min Pos. mP_max/mP_min P_blue/P_yellow
max min % % angl.
RX Boo 001deg +68deg SRb 8.6m 11.3m 78d M7e-M8e 2.3 1.7 62deg +0.8m 1.7
AB Cyg 050 -15 SRb 9.5 10.1 520 M4III 4.0 3.2 51 +0.1 1.4
AK Peg 054 -44 SRa 8.9 10.8 195 M5e 3.0 1.2 50 +0.7 -
R Gem 162 +15 6.0 14.0 369.93 S 3.9e- 3.2 0.3 80 +1.5 1.8
-S6.9e
Z Cnc 177 +28 SRb 9.4 10.7 104 M6III 2.0 1.0 45 +0.25 2.0
CD Ser 350 +46 Ib 10.0 11.0 80? M4 3.4 3.2 70 +0.1 1.7
Fig. 1
From the polarimetric data gathered in Table 1 it follows:
a) the variations of polarization degree of these stars are greater than
the observational errors (mean (Delta P) = +- 0.2%).
b) it is very probable that at minimum polarization the star is brighter than
at maximum polarization.
c) the degree of polarization in the blue part of the spectrum (lambda_eff =
= 4500 A) is on the average 70% higher than in the yellow part (lambda_eff =
= 5400 A).
Fig. 1. presents the relation between the mean values of maximum and
minimum degrees of polarization and the wavelengths. The open circles
correspond to the state of fainter radiation of the star.
As it was already shown (Vardanian 1967; Coyne and Kruszewski 1968) in
the case of cold supergiants and specially in the case of mu Cephei such
a correlation was not observed.
Although the number of observed stars is not very large, it is,
nevertheless, possible to make some preliminary quantitative analysis.
In fact as only six of 65 randomly chosen variable stars show in the
blue part of the spectrum a polarization degree greater than 2%, it can
be supposed, that at least 9% of late type stars possess an intrinsic
polarization higher than 2%. It should be noted that the polarization
degree of stars surrounding the variables was within the limits of
observational errors. Moreover, none of the 110 late type stars,
situated in the neighbourhood of the 65 variables, showed a remarkable
degree of polarization. It follows therefrom, that the presence and the
changes of light-polarization of late type stars are connected with the
variability of their brightness.
The search for late type star having an intrinsic light polarization is
still in progress. The observational data together with their detailed
analysis will be published after the programme is finished.
Note added October 24, 1968: The polarization of five more variable
stars, TW Peg, AE Cap, Z Eri, T Aqr and Z Psc was discovered by the
author.
REFERENCES
Coyne, G., and Kruszewski, A., 1968, Astr. J., 73, 29.
Grigorian, K. A., 1958, Byurakan Obs. Contr. 25, 45.
Grigorian, K. A., 1959, Byurakan Obs. Contr. 27, 43.
Serkowski, K., 1966, Inf. Bull. Var. Stars. No. 141. Budapest. (IBVS N°.141)
Shakhovski, N. M., 1964, Astr. Zu. 40, 6.
Vardanian, R. A., 1966, Byurakan Obs. Contr. 37, 23.
Vardanian, R. A., 1967. Rus. Astron. Tsirk. No. 433.
Zappala, R., 1967, Astrophys. J. 148, 81.