The Recent Polarization Increase of o And

Limber Observatory, Timber Creek Road, P. O. Box 63599, Pipe Creek TX 78063
email: dam3ma@virginia.edu
Received: 1999 March 7

Since 1986 I have been annually monitoring the UBVRI polarization of o And (B6 III, $v\,sin\,i$ = 260 km s^-1) as part of a larger sampling of bright northern Be stars (McDavid 1994). As may be seen in Figure 1, this star has shown a steady rise in polarization, followed recently by two sharp increases separated by about 3 years.

$\begin{figure}\psfig{figure=oaBi.eps,width=3in}\end{figure}$

Figure 1: Intrinsic B polarization of o And from 1986 to 1998. The degree of polarization is p and the position angle is $\theta$ .

Figure 2 shows that the data points as observed are collinear in the q-u plane, within observational errors, indicating an intrinsic polarization with variable degree at constant position angle. The intrinsic position angle of 108^o derived from a least squares fit is consistent with the interstellar polarization estimate of p_max=0.26%, $\lambda_{max}$ =597 nm, $\theta_{IS}$ =75^o found by Poeckert, Bastien, and Landstreet (1979), as shown by the line from the origin (large asterisk) to the point corresponding to the value of the interstellar B polarization in the q-u plane (small asterisk). I have no explanation for the one errant position angle point (1986) in Figure 1 other than the possibility that the degree of polarization is so small that the position angle is poorly defined.

$\begin{figure}\psfig{figure=quB.eps,width=3in}\end{figure}$

Figure 2: Stokes parameter plot of the observed B polarization of o And from 1986 to 1998. A cross is plotted at the mean of the q,u values with size equal to the average uncertainty $\sigma$ of a single data point, and the solid ellipse shows the corresponding 3 $\sigma$ limits. The dotted ellipse shows the standard deviation of the data set. The large asterisk marks the q-u origin, and the line from there to the small asterisk shows the interstellar polarization.

The main purpose of this note is to encourage all manner of observations of o And as soon as possible to document the current strong Be phase inferred from polarimetry. The present polarization is comparable to the highest ever recorded for this star, which supports the idea of Harmanec et al. (1988) that the shell phases appearing about every 8 years have individual characteristics which vary from one cycle to the next (Arsenijevic et al. 1994).

References
Arsenijevic, J., Markovic-Krsljanin, S., Kubiucela, A., and Jankov, S. 1994, in IAU Symposium 162, Pulsation, Rotation and Mass Loss in Early-Type Stars, eds. Balona, L.A., Henrichs, H.F., and Le Contel, J.M. (Dordrecht, Kluwer), p.236
Harmanec, P., Hadrava, P., Ruzic, Z., Pavlovski, K., Bozic, H., Horn, J., and Koubsky, P. 1988, IBVS 3263
McDavid, D. 1994, PASP, 106, 949
Poeckert, R., Bastien, P., and Landstreet, J.D. 1979, AJ, 84, 812

Last modified: March 7, 1999

David McDavid
dam3ma@virginia.edu