COMMISSION 27 OF THE I. A. U.
         INFORMATION BULLETIN ON VARIABLE STARS
                     Number 1910

                                     Konkoly Observatory
                                     Budapest
                                     1981 January 9

                                     HU ISSN 0374-0676


    MIRA'S LINEAR POLARIZATION NEAR THE 1980 LIGHT MAXIMUM.



   Four wide-band (B-filter) linear polarization measurements of Mira (o Ceti)
were carried out over a one week interval which commenced about six weeks after
the 1980 light maximum. (Janet A. Mattei of the A.A.V.S.O. kindly provided a
provisional estimate of the occurrence of Mira's light maximum.) Polarization
measurements were carried out at the Cassegrain focus of the 61-cm telescope at
Columbia University's Harriman Observatory. A description of the polarimeter
and observing procedures, as well as evidence for instrumental accuracy and
precision, have been given by Hayes (1980). The observations are listed in
Table I, with the amount (P) and direction (theta) of polarization being expressed

                    	  Table I


      Journal of Mira's Polarization Degree and Position Angle



		Date 		  P 	   theta
		(UT) 		 (%) 	   (deg)

	1980   Oct.   31.16  	 0.87  	    40.0
	1980   Nov.   02.18  	 0.83  	    38.6
	1980   Nov.   03.12  	 0.83       40.0
	1980   Nov.   06.16  	 0.82  	    40.0

in percentages and equatorial coordinates, respectively. All observations
have a Poisson photon count standard deviation of 0.02% for P, while the
standard deviation of theta is given by 28.7deg (sigma_p /P). 
   Perhaps the most important datum of this report are the position angles.
Serkowski (1971) noted that theta appeared to alternate between high and low values
for one cycle of Mira's maximum to the next, with variations of 20d - 40d being
generally found in the earlier data [vide Shawl (1975)]. Such variations
resemble those found in the RV Tauri star U Mon, which Serkowski (1970) has
attributed to nonspherical pulsation modes. But the position angles obtained
near the 1977 maximum (McLean and Coyne, 1978) differed markedly from the
earlier results. Tomaszewski et al. (1980) reported that the position angles
around the 1978 maximum were also discrepant, but to a lesser extent. The
polarization amounts and positions being reported here bear resemblance to some
of the earlier results reported by Shawl (1975). The 1980 polarization
observations in concert with results derived by other techniques may permit a
resolution of the roles played by nonspherical pulsations and alternative
mechanisms such as grain growth in the extended atmosphere, and the waxing and
waning of large scale convective cells in the lower atmosphere of this star.



DANIEL P. HAYES            	 RICHARD A. RUSSO
Astronomy Department        	 Scienco Education Department
Columbia University         	 Teachers College, Columbia University
New York, New York 10027    	 New York, New York 10027
United States of America    	 United States of America




References:

Hayes, D. (1980) Pub. A.S.P., in press. [BIBCODE 1980PASP...92..661H ]
McLean, I., and Coyne, G. (1978) Ap.J. (Letters), 226, L145. [BIBCODE 1978ApJ...226L.145M ]
Serkowski, K. (1970) Ap.J., 160, 1107. [BIBCODE 1970ApJ...160.1107S ]
Serkowski, K. (1971) Kitt Peak Obs. Contr. 554, p. 107. [BIBCODE 1971CoKit.554..107S ]
Shawl, S. (1975) A.J., 80, 602. [BIBCODE 1975AJ.....80..602S ]
Tomaszewski, L., Landstreet, J., McLean, I., and Coyne, G, (1980) Ap.J., 238,
    935. [BIBCODE 1980ApJ...238..935T ]