1 Introduction

Symbiotic stars are interacting binary systems in which a cool giant transfers material to a hot companion, responsible for ionization of the gas surrounding the system. Since symbiotic stars are generally embedded in gaseous and dusty envelopes, scattering in the circumbinary nebulae is a likely source for the polarization. Linear polarization of the stellar radiation is, in general, an indication of deviation from spherical symmetry. Therefore, it is to be expected that the symbiotic stars should possess a significant degree of polarization, since most of them present extended asymmetric atmospheres, circumbinary envelopes and/or dusty circumstellar envelopes, disks, etc. The polarization should be variable with time according to the activity of the star or to the orbital motion. The study of this variability should give a better insight into the envelope structures or their binary nature. Serkowski (1970) reported the first polarimetric study of symbiotic stars. Linear polarization observations of the three symbiotics R Aqr, AG Peg and RS Oph, showed that the measured polarization was intrinsic and variable with time in all those objects. Schulte-Ladbeck, Magalhães (1987) found that 9 out of 23 (39%) of the observed symbiotic stars to be intrinsically polarized. Schulte-Ladbeck et al. (1990) (hereafter SLAMS) found evidence for the presence of intrinsic polarization, by means of the wavelength dependence and/or time variability, in 21 out of 39 objects (54%). In order to learn more about the polarizing sources, SLAMS searched correlations between the linear polarization and other properties of the symbiotic systems. Due to circumstellar scattering around the late-type component, they expected to find correlations with IR types S, D and D' and spectral types of the cool component. There are 48% S-type objects showing intrinsic polarization, 88% D-type polarized objects and 2 polarized objects out of 4 D'-type objects studied. A higher probability to find polarized D-type symbiotic persists in the larger sample.

   

Table 1: List of objects

Object	Cool	IR-Type	Orbital Period
	Component		P(days)	References
V748 Cen (*)	M4	S	566.5	(1)
Hen 1103	M0	S
KXTrA	M6	S
CL Sco	M	S	624.7	(2)
AR Pav (*)	M3.7	S	604.5	(3)
FN Sgr	M4	S
Hen 1761	M5	S
RR Tel	M5	D
CD $-43^{\circ }$14304	K5-M0	S yellow	1448	(4)
AG Peg	M2	S	816.5	(5)

Notes:

(*) eclipsing binary.

(1) van Genderen et al. (1974).

(2) Kenyon (1986).

(3) Bruch et al. (1994).

(4) Schmid et al. (1998) (circular solution).

(5) Fernie (1985).

Later spectral types would favor, through larger mass loss, the presence of polarization but SLAMS found that the number of polarized symbiotic stars correlated with the spectral type of the cool component is not clear.

Piirola (1988a) made an extensive multicolor polarimetry of CH Cyg. He found drastic changes in the wavelength dependence of the polarization on a time scale of months and years and small variations in the course of days and weeks. He proposed that the main mechanism is scattering by dust particles. The variations were interpreted in terms of formation or growth of dust particles and changes in the density, geometry and particle size distribution of the circumstellar envelope. Very small particles produce polarization increasing steeply toward short wavelength in agreement with the Rayleigh law $(P\sim {\lambda}^{-4})$ and larger dust particles produce a flat wavelength dependence with a drop in the ultraviolet polarization. Possible connections of polarization variations to the orbital motion were suggested by Piirola.

The purpose of this paper is to present new polarization measurements of ten symbiotic stars; to analyze the wavelength behavior of both the percentage of polarization and the position angle; to detect temporal variations and to fit our results with the proposed polarizing mechanisms that could be working in symbiotic stars. In our sample of 10 observed stars, 4 have no previous polarization data (V748 Cen, Hen 1103, CL Sco and FN Sgr); 3 were considered intrinsically polarized (Hen 1761, RR Tel and AG Peg) and 3 represent unpolarized objects or else bordeline cases (KX TrA, AR Pav and CD  $-43^{\circ }$14304). The majority of the stars in our sample are S-type symbiotics; only one, RR Tel is a D-type symbiotic with a Mira variable as cool component and CD  $-43^{\circ }$14304 is considered an S-type yellow symbiotic.

   

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	4
	R	-.43	-.85	.96	.10	121.6	2.9	4
	I	-.39	-1.29	1.35	.27	126.5	5.6	4
August 1997	U	-.50	-2.04	2.10	.85	128.1	11.1	15
10684.52/86.49	B	-.30	-1.53	1.56	.20	129.5	3.7	18
	V	-.22	-.73	.76	.12	126.6	4.3	18
	R	-.28	-.57	.64	.12	122.0	5.3	18
	I	-.22	-.40	.46	.15	120.5	9.3	18
April 1998	U	-.83	-1.92	2.10	.12	123.3	1.7	29
10921.80/24.80/	B	-.92	-1.86	2.07	.14	121.8	2.0	29
27.76/28.72	V	-.48	-.64	.80	.05	116.8	1.9	29
	R	-.52	-.64	.83	.06	115.4	2.0	29
	I	-.41	-.27	.49	.05	106.4	3.1	29
Hen 1103
July 1995	U	.89	-1.07	1.39	.62	-25.1	12.0	18
9907.61/08.60/	B	.19	.56	.59	.46	35.5	18.9	18
09.57	V	.99	1.70	1.96	.44	29.9	6.3	18
	R	.28	2.45	2.47	.34	41.8	3.9	13
	I	.22	2.06	2.08	.24	42.0	3.2	13
March 1996	U	1.40	2.72	3.06	.15	31.3	1.4	4
10162.82	B	1.49	3.47	3.78	.22	33.4	1.6	4
	V	.79	2.59	2.70	.49	36.5	5.1	4
	R	.74	3.09	3.18	.44	38.3	3.9	4
	I	.42	2.38	2.41	.47	40.0	5.5	4
August 1997	U	.60	1.69	1.80	.08	35.2	1.3	16
10684.54/87.53	B	.50	1.99	2.05	.16	38.0	2.3	16
	V	.36	1.71	1.75	.23	39.0	3.8	16
	R	.49	1.69	1.76	.24	36.9	3.9	16
	I	.31	1.93	1.95	.26	40.5	3.8	16
April 1998	U	1.26	1.99	2.36	.13	28.8	1.6	18
10922.81/28.76	B	1.13	2.25	2.52	.09	31.6	1.0	18
	V	.86	2.12	2.29	.07	33.9	.9	18
	R	.83	2.17	2.32	.06	34.5	.8	18
	I	.59	1.85	1.94	.08	36.1	1.2	18

 

Table 2: continued

Star/Date/JD	Band	PX(%)	PY(%)	P(%)	ME	PA	ME	N
2440000+
KX TrA
May 1994	U	.86	.92	1.26	.14	23.5	3.2	12
9503.71/06.69	B	.94	1.01	1.38	.10	23.5	2.1	12
	V	.99	1.36	1.68	.10	26.9	1.8	12
	R	.98	1.10	1.48	.07	24.2	1.3	12
	I	1.01	.95	1.39	.13	21.7	2.7	12
July 1995	U	.82	.90	1.22	.27	23.9	6.3	28
9904.55/05.63/	B	1.09	1.38	1.76	.24	25.8	3.9	28
06.61/08.65	V	1.02	.91	1.37	.14	20.8	3.0	28
	R	.89	.75	1.16	.10	20.1	2.5	28
	I	.51	.69	.86	.08	26.6	2.8	28
March 1996	U	.98	.90	1.33	.15	21.3	3.3	4
10162.86	B	1.42	.75	1.61	.23	14.0	4.0	4
	V	.97	.93	1.34	.04	21.9	.8	4
	R	1.07	1.05	1.50	.04	22.2	.7	4
	I	.63	.67	.92	.05	23.4	1.6	4
August 1997	U	.82	.60	1.02	.12	18.1	3.4	16
10686.57/88.49	B	1.04	.82	1.33	.18	19.2	3.9	16
	V	.85	.72	1.12	.08	20.2	2.1	16
	R	1.04	.76	1.29	.07	18.1	1.6	16
	I	.71	.62	.95	.10	20.6	3.0	16
April 1998	U	1.09	.70	1.29	.17	16.3	3.7	16
10921.87/28.80	B	1.12	1.00	1.51	.07	20.9	1.4	16
	V	1.08	1.00	1.47	.04	21.4	.9	16
	R	1.04	1.08	1.50	.03	22.9	.6	16
	I	.84	1.00	1.30	.04	24.9	.9	16
CL Sco
6-11 July 1995	U	1.19	1.46	1.89	.24	25.4	3.6	32
9904.59/05.68/	B	1.09	1.03	1.50	.23	21.8	4.3	32
9906.63/07.65/	V	.54	.92	1.07	.18	29.7	4.8	32
9908.66/09.65	R	.36	.68	.77	.15	31.1	5.7	32
	I	.01	1.27	1.27	.13	44.8	2.9	32
27 July 1995	U	-.68	1.82	1.94	.20	55.2	3.0	8
9925.59	B	-.64	1.49	1.62	.47	56.6	8.1	8
	V	-.01	.83	.83	.25	45.3	8.3	8
	R	-.02	1.43	1.43	.12	45.5	2.5	8
	I	.28	1.01	1.05	.24	37.3	6.4	8
March 1996	U	.27	2.40	2.42	.09	41.8	1.1	4
10162.88	B	.42	2.97	3.00	.12	40.9	1.1	4
	V	.22	2.10	2.11	.04	42.0	.5	4
	R	.33	2.10	2.12	.03	40.5	.5	4
	I	-.19	1.61	1.62	.06	48.4	1.0	4
August 1997	U	.10	.70	.71	.35	40.9	13.0	8
10687.61	B	1.03	1.04	1.46	.12	22.5	2.3	8
	V	.46	1.12	1.21	.17	33.9	4.0	8
	R	.22	1.41	1.43	.17	40.7	3.5	8
	I	-.05	1.10	1.10	.15	46.3	3.9	8
April 1998	U	.17	-.02	.17	.08	-2.9	12.5	20
10922.84/24.83/	B	.32	.02	.33	.08	2.1	6.9	20
28.84	V	.12	.80	.81	.03	40.8	1.0	20
	R	.09	.87	.87	.02	42.0	.8	20
	I	.03	.99	.99	.03	44.2	1.0	20

 

Table 2: continued

Star/Date/JD	Band	PX(%)	PY(%)	P(%)	ME	PA	ME	N
2440000+
AR Pav
July 1995	U	.21	1.35	1.37	.16	40.6	3.4	16
9904.73/06.76	B	.87	1.68	1.89	.19	31.4	2.9	16
	V	.55	1.52	1.62	.14	35.1	2.5	16
	R	.76	.83	1.12	.15	23.7	3.9	16
	I	.61	.50	.79	.11	19.6	3.9	16
July 1996	U	-1.32	.38	1.38	.11	82.0	2.3	6
10276.73	B	-1.28	.68	1.45	.08	76.1	1.5	6
	V	-.87	.66	1.10	.22	71.6	5.6	6
	R	-.38	.52	.64	.26	63.1	11.0	6
	I	-.42	.51	.66	.22	64.9	9.1	6
July 1997	U	-.30	.23	.38	.08	71.1	5.8	18
10633.68/34.70/	B	.25	-.25	.36	.07	157.5	5.3	18
36.68	V	.60	-.14	.62	.03	173.2	1.3	18
	R	.64	-.14	.66	.03	173.9	1.4	18
	I	.67	-.05	.67	.04	178.0	1.8	18
August 1997	U	.04	.70	.70	.17	43.3	6.8	8
10687.68	B	.57	.12	.59	.06	185.8	3.0	8
	V	.75	.05	.75	.04	181.8	1.4	8
	R	.79	.04	.79	.02	181.6	.9	8
	I	.67	-.04	.67	.03	178.4	1.4	8
April 1998	U	-1.06	.13	1.07	.06	86.5	1.5	8
10924.86	B	-.81	.08	.81	.04	87.3	1.6	8
	V	-.14	.09	.17	.02	73.0	3.7	8
	R	-.04	.12	.12	.03	55.0	6.0	8
	I	-.02	.15	.15	.04	48.1	7.7	8
June 1998	U	-1.00	-.04	1.00	.04	91.1	1.1	6
10985.67	B	-.40	-.31	.51	.04	108.8	2.2	6
	V	.22	-.06	.22	.02	-7.1	2.4	6
	R	.26	-.01	.26	.02	-0.5	1.7	6
	I	.39	.02	.39	.03	1.5	2.0	6
FN Sgr
May 1994	U	.81	-.03	.81	.09	-1.1	3.3	8
9505.84	B	1.22	.13	1.23	.10	2.9	2.3	8
	V	.96	.05	.96	.08	1.4	2.5	8
	R	.44	.02	.45	.07	1.4	4.5	8
	I	.33	.04	.33	.08	3.9	6.8	8
9 July 1995	U	2.82	-.60	2.89	1.11	-6.0	10.5	4
9907.76	B	2.36	-.75	2.48	.47	-8.8	5.4	4
	V	1.33	-.55	1.44	.55	-11.2	10.4	4
	R	.63	1.37	1.51	.62	32.7	11.3	4
	I	.50	1.19	1.29	.47	33.7	10.1	4
27 July 1995	U	-.11	.58	.59	.20	50.5	9.4	8
9925.64	B	.24	.23	.33	.17	22.2	13.5	8
	V	.14	.08	.16	.08	14.9	12.9	8
	R	.27	-.02	.27	.07	-1.8	6.9	8
	I	.14	-.06	.15	.09	-11.2	14.9	8
April 1998	U	.47	-.57	.74	.07	-25.2	2.8	8
10924.88	B	1.12	-.21	1.14	.06	-5.3	1.6	8
	V	.78	.12	.79	.03	4.3	1.0	8
	R	.76	.09	.76	.02	3.4	.9	8
	I	.63	.16	.65	.05	7.2	2.2	8

 

Table 2: continued

Star/Date/JD	Band	PX(%)	PY(%)	P(%)	ME	PA	ME	N
2440000+
Hen 1761
October 1994	U	1.08	-.16	1.10	.10	-4.3	2.5	32
9639.57/40.58/	B	1.06	-.11	1.06	.08	-3.1	2.2	32
41.56/42.55	V	.93	.23	.96	.04	7.0	1.3	32
	R	.91	.26	.95	.02	7.9	.6	32
	I	.80	.20	.82	.02	7.0	.7	32
6-9 July 1995	U	.76	.48	.90	.20	16.3	6.2	32
9904.78/05.83/	B	.84	.48	.96	.12	15.0	3.4	32
06.80/07.84	V	.99	.44	1.08	.09	11.9	2.5	32
	R	.72	.26	.77	.08	9.9	3.0	32
	I	.76	.23	.79	.06	8.4	2.3	32
24 July 1995	U	.17	.00	.17	.22	0.0	26.1	4
9922.77	B	.43	.67	.80	.17	28.5	6.0	4
	V	.67	.10	.67	.02	4.3	.9	4
	R	.80	.22	.83	.02	7.6	.8	4
	I	.74	.09	.74	.04	3.4	1.4	4
September 1995	U	1.13	.62	1.29	.17	14.3	3.7	8
9990.51	B	.98	.34	1.04	.09	9.6	2.5	8
	V	.86	.09	.86	.02	3.1	.6	8
	R	.98	.21	1.00	.02	6.0	.6	8
	I	.75	.26	.80	.02	9.5	.7	8
August 1997	U	.79	.38	.87	.06	12.8	2.1	20
10684.67/87.71/	B	.86	.34	.92	.05	10.7	1.6	20
88.74	V	.82	.30	.87	.02	10.1	.5	20
	R	.84	.32	.90	.02	10.6	.5	20
	I	.73	.28	.78	.02	10.6	.7	20
April 1998	U	.73	.31	.80	.16	11.4	5.6	16
10927.89	B	.75	.18	.77	.12	6.8	4.6	16
	V	.90	.29	.95	.02	9.0	.8	16
	R	.85	.24	.88	.03	7.9	1.0	16
	I	.79	.25	.83	.03	8.9	1.2	16
RR Tel
October 1994	U	.64	-.33	.72	.04	17.3	1.4	12
9640.57/41.54/	B	.64	-.34	.72	.05	166.1	2.1	12
42.53	V	.58	-.24	.62	.08	168.5	3.6	12
	R	.57	-.04	.57	.02	178.2	1.2	12
	I	.06	-.11	.12	.04	149.1	9.8	12
July 1995	U	1.13	-.21	1.15	.31	174.8	7.5	6
9907.86/09.84	B	.74	.34	.82	.25	12.4	8.5	6
	V	.53	.12	.55	.20	6.4	9.8	6
	R	.61	.02	.61	.20	.9	8.9	6
	I	.24	-.15	.28	.14	163.9	13.1	6
August 1997	U	.40	-.10	.41	.07	172.8	4.5	21
10684.76/86.73/	B	.34	-.18	.38	.09	166.0	6.6	21
88.75	V	.25	.10	.27	.04	10.8	3.9	21
	R	.39	.04	.39	.03	2.7	2.1	21
	I	.15	.26	.30	.05	30.3	4.6	21
April 1998	U	.42	-.29	.51	.06	162.7	3.4	8
10928.87	B	.42	-.27	.50	.04	163.6	2.4	8
	V	.54	.08	.55	.04	4.2	2.1	8
	R	.58	-.02	.58	.02	179.1	1.1	8
	I	.47	.61	.77	.08	26.1	2.8	8

 

Table 2: continued

Star/Date/JD	Band	PX(%)	PY(%)	P(%)	ME	PA	ME	N
2440000+
CD- $43^{\circ}$14304
October 1994	U	.43	-.34	.55	.14	160.8	7.1	22
9638.71/39.58/	B	.49	-.21	.54	.60	168.3	24.1	23
40.63/41.58/	V	.41	-.08	.42	.17	174.7	11.0	23
42.56	R	.47	.05	.47	.68	3.2	27.7	23
	I	.39	.02	.39	.04	1.7	2.5	23
July 1995	U	.90	.19	.92	.38	6.1	11.2	30
9904.81/05.85/	B	.91	.03	.91	.30	1.0	9.2	30
06.83/08.80/	V	.53	-.10	.54	.18	174.8	9.4	30
09.78	R	.49	-.16	.51	.13	171.1	7.2	30
	I	.27	-.15	.31	.10	165.6	8.9	30
September 1995	U	.46	-.29	.54	.17	163.7	8.5	8
9992.66/93.63	B	.47	-.12	.48	.11	172.9	6.5	8
	V	.33	-.26	.42	.03	160.7	2.0	8
	R	.38	-.21	.44	.02	165.5	1.4	8
	I	.23	-.27	.35	.04	155.2	3.2	8
August 1997	U	.29	-.22	.37	.11	161.7	8.6	8
10686.78	B	.44	-.30	.53	.09	162.9	4.6	8
	V	.39	-.34	.51	.02	159.4	1.2	8
	R	.33	-.38	.50	.02	155.5	1.0	8
	I	.23	-.31	.38	.06	153.3	4.3	8
April 1998	U	.69	.86	1.11	.42	25.6	10.5	8
10928.90	B	.33	-.76	.83	.30	146.7	10.0	8
	V	.32	-.10	.34	.09	171.0	7.8	8
	R	.45	-.07	.45	.04	175.6	2.8	8
	I	.15	.02	.15	.12	3.0	19.7	8
AG Peg
October 1994	U	.18	-.31	.36	.18	150.2	13.1	12
9640.64/41.61/	B	.07	-.07	.10	.08	157.8	18.7	12
42.60	V	.06	-.17	.18	.08	144.0	12.1	12
	R	-.03	-.00	.03	.04	93.2	24.0	12
	I	.04	-.04	.06	.05	157.6	19.6	12
July 1995	U	.31	.28	.42	.37	20.8	20.7	18
9904.86/06.85/	B	.06	.01	.06	.27	5.7	39.2	18
07.90/08.86/	V	-.28	-.02	.28	.21	91.8	18.2	18
09.87	R	-.60	.09	.61	.28	85.6	12.1	18
	I	-.35	.13	.37	.15	79.5	11.0	18
September 1995	U	-.15	.29	.33	.22	58.4	16.9	8
9992.68/93.66	B	.02	-.14	.14	.13	139.8	21.0	8
	V	-.09	-.18	.20	.03	122.2	3.7	8
	R	-.03	-.12	.12	.02	128.5	4.9	8
	I	-.13	-.08	.15	.04	105.5	7.0	8
August 1997	U	-.01	-.06	.06	.08	132.4	26.6	12
10684.73/86.71	B	-.12	-.06	.14	.07	103.2	13.3	12
	V	-.02	-.10	.11	.04	129.4	9.8	12
	R	-.12	-.08	.14	.02	106.3	5.0	12
	I	-.09	-.08	.12	.04	110.5	10.4	12