ASTRONOMY & ASTROPHYSICS AUGUST 2000, PAGE 197 SUPPLEMENT SERIES Astron. Astrophys. Suppl. Ser. 145, 197–214 (2000) Linear polarization of a group of symbiotic systems? E. Brandi1,2,L.G.Garc´ıa1, V. Piirola3,F.Scaltriti4, and C. Quiroga1,5 1 Facultad de Ciencias Astron´omicas y Geof´ısicas, Universidad Nacional de La Plata, 1900 La Plata, Argentina 2 Member of the Carrera del Investigador Cient´ıfico, Comisi´on de Investigaciones Cient´ıficas de la Provincia de Buenos Aires (CIC), Argentina 3 Tuorla Observatory, University of Turku, Finland 4 Osservatorio Astronomico di Torino, Pino Torinese, Italy 5 Fellow of the Consejo Nacional de Investigaciones Cient´ıficas y T´ecnicas (CONICET), Argentina Received February 21; accepted March 23, 2000 Abstract. We report linear polarization measurements of expected that the symbiotic stars should possess a signifi- a set of symbiotic stars, made at several epochs during cant degree of polarization, since most of them present ex- the period 1994-1998. Evidence of intrinsic polarization tended asymmetric atmospheres, circumbinary envelopes is looked for from the wavelength dependence of the po- and/or dusty circumstellar envelopes, disks, etc. The po- larization degree and position angle in UBVRI bands. larization should be variable with time according to the The results have also been analysed to search for tempo- activity of the star or to the orbital motion. The study of ral variability of polarization. Several objects have shown this variability should give a better insight into the enve- a polarization spectrum different from that produced by lope structures or their binary nature. Serkowski (1970) interstellar dust grains and/or polarimetric variations on reported the first polarimetric study of symbiotic stars. time scales as short as several days or months, indicating Linear polarization observations of the three symbiotics the presence of polarization component of circumstellar R Aqr, AG Peg and RS Oph, showed that the measured origin. polarization was intrinsic and variable with time in all those objects. Schulte-Ladbeck, Magalh˜aes (1987) found Key words: stars: binaries: symbiotic — stars: binaries that 9 out of 23 (39%) of the observed symbiotic stars to eclipsing — stars: variable stars — polarization be intrinsically polarized. Schulte-Ladbeck et al. (1990) (hereafter SLAMS) found evidence for the presence of in- trinsic polarization, by means of the wavelength depen- dence and/or time variability, in 21 out of 39 objects (54%). In order to learn more about the polarizing sources, 1. Introduction SLAMS searched correlations between the linear polariza- tion and other properties of the symbiotic systems. Due Symbiotic stars are interacting binary systems in which a to circumstellar scattering around the late-type compo- cool giant transfers material to a hot companion, responsi- nent, they expected to find correlations with IR types S, ble for ionization of the gas surrounding the system. Since D and D’ and spectral types of the cool component. There symbiotic stars are generally embedded in gaseous and are 48% S-type objects showing intrinsic polarization, dusty envelopes, scattering in the circumbinary nebulae 88% D-type polarized objects and 2 polarized objects out is a likely source for the polarization. Linear polarization of 4 D’-type objects studied. A higher probability to find of the stellar radiation is, in general, an indication of de- polarized D-type symbiotic persists in the larger sample. viation from spherical symmetry. Therefore, it is to be Later spectral types would favor, through larger mass loss, the presence of polarization but SLAMS found that Send offprint requests to: E. Brandi the number of polarized symbiotic stars correlated with e-mail: [email protected] the spectral type of the cool component is not clear. ? Based on observations taken at Complejo Astron´omico El Leoncito (CASLEO), operated under an agreement be- Piirola (1988a) made an extensive multicolor polarime- tween the Consejo Nacional de Investigaciones Cient´ıficas y try of CH Cyg. He found drastic changes in the wave- T´ecnicas de la Rep´ublica Argentina, the Secretar´ıa de Ciencia length dependence of the polarization on a time scale of yTecnolog´ıa de la Naci´on and the National Universities of La months and years and small variations in the course of Plata, C´ordoba and San Juan. days and weeks. He proposed that the main mechanism 198 E. Brandi et al.: Linear polarization of a group of symbiotic systems Table 1. List of objects five bands UBVRI, following the design principles by Object Cool IR-Type Orbital Period Piirola (1973, 1988b). The combination of the dichroic Component P (days) References beam splitters and the filters cemented to the field lenses of the dichroic filter set matches closely the standard V748 Cen (*) M4 S 566.5 (1) UBVRI system. The final response of the five photomul- Hen 1103 M0 S KXTrA M6 S tipliers corresponds to equivalent wavelengths 0.36, 0.43, CL Sco M S 624.7 (2) 0.53, 0.65 and 0.86 µm. AR Pav (*) M3.7 S 604.5 (3) During each observing run a set of zero and high po- FN Sgr M4 S larized standards stars were observed to determine both Hen 1761 M5 S the instrumental polarization and the coordinate transfor- RR Tel M5 D mation into the equatorial system respectively. The net ◦ CD −43 14304 K5-M0 S yellow 1448 (4) polarization of telescope and instrument, typically ∼ 0.01 AG Peg M2 S 816.5 (5) percent, was subtracted from all the data. Notes: (*) eclipsing binary. 3. Results (1) van Genderen et al. (1974). (2) Kenyon (1986). We have obtained polarimetric data for 10 symbiotic (3) Bruch et al. (1994). systems whose main characteristics are included in (4) Schmid et al. (1998) (circular solution). Table 1. The results of our observations are presented (5) Fernie (1985). in the Table 2 where we give for each object, in the different epochs, the average polarization in the UBVRI is scattering by dust particles. The variations were inter- bands, in terms of the weighted mean values of the Stokes preted in terms of formation or growth of dust particles parameters PX and PY, the corresponding degree of and changes in the density, geometry and particle size dis- linear polarization in percent P (%), the position angle tribution of the circumstellar envelope. Very small parti- in degrees PA, and the corresponding mean errors. The cles produce polarization increasing steeply toward short number of observations, N, is also indicated. Weighting wavelength in agreement with the Rayleigh law (P ∼ λ−4) according to the inverse square of the estimated error and larger dust particles produce a flat wavelength depen- for each observation is applied. The estimated error is dence with a drop in the ultraviolet polarization. Possible taken either from the least-square fit of the double cosine connections of polarization variations to the orbital mo- curves to the eight integrations in the different positions tion were suggested by Piirola. of the λ/2 plate, or from the photon statistics, whichever is greater. The purpose of this paper is to present new polariza- tion measurements of ten symbiotic stars; to analyze the The multicolour polarization data were used to deter- wavelength behavior of both the percentage of polariza- mine the wavelength dependence of the degree of polariza- tion and the position angle; to detect temporal variations tion and the position angle. In order to investigate whether and to fit our results with the proposed polarizing mech- the observed polarization is purely interstellar, or a sig- anisms that could be working in symbiotic stars. In our nificant contribution due to intrinsic polarization exists, sample of 10 observed stars, 4 have no previous polariza- we compared the observed spectral distributions of the tion data (V748 Cen, Hen 1103, CL Sco and FN Sgr); 3 polarization with that given by the Serkowski’s law for were considered intrinsically polarized (Hen 1761, RR Tel the wavelength dependence of the interstellar polarization and AG Peg) and 3 represent unpolarized objects or else (Serkowski et al. 1975). bordeline cases (KX TrA, AR Pav and CD −43◦14304). All the symbiotic stars were observed in different The majority of the stars in our sample are S-type sym- epochs and several of them show changes in the wave- biotics; only one, RR Tel is a D-type symbiotic with a length dependence of the polarization on time scales of Mira variable as cool component and CD −43◦14304 is days or months. Plots of the measured polarization vs. considered an S-type yellow symbiotic. wavelength are presented for each star at the different epochs. 2. Observations 3.1. Results on individual objects The observations were obtained during the interval May 1994 – June 1998 with the five channel photopolarime- V748 Centauri ter of Turin Observatory, attached at the 2.15-m tele- scope of CASLEO (San Juan, Argentina). The instrument V748 Cen has been classified as a R CrB variable, with allows simultaneous measurements of polarization in the a photographic amplitude of about 2 mag and a period E. Brandi et al.: Linear polarization of a group of symbiotic systems 199 Table 2. Weighted mean values Star/Date/JD Band PX(%) PY(%) P (%) ME PA ME N 2440000+ V748 Cen July 1995 U .23 −1.59 1.60 .18 139.1 3.2 19 9906.54/07.58/ B −.30 −1.62 1.64 .16 129.8 2.8 19 08.58/09.54 V −.10 −1.13 1.14 .14 132.5 3.4 19 R −.28 −.45 .53 .09 119.2 4.7 19 I −.26 −.11 .29 .09 101.2 8.9 19 March 1996 U .18 −2.62 2.63 .20 137.0 2.2 4 10162.78 B .24 −2.83 2.84 .39 137.4 3.9 4 V −.17 −1.06 1.08 .10 130.4 2.6