Space Astronomy Laboratory
ASTRO - 2
WUPPE PROGRAM CLASS 2.4
The light that emerges from a star near the edge of the visible disk, the so-called limb, is known to be polarized parallel to the limb. This can be easily observed in the case of the Sun by selecting just a small portion of the limb. However, when a spherical star is observed as a single source, all the various directions of polarization combine to average out to zero polarization. In addition, the light coming from the center of the disk is unpolarized and this further dilutes any polarization from the limb.
The "rapid rotators" which WUPPE will be observing are moderately hot normal stars with equatorial rotational velocities in excess of 100 km/sec. For comparison, the Sun's rotational velocity is about 2 km/sec. Rapid rotation causes a gravitationally bound fluid body such as a star to be distorted from the spherical shape it would assume in the absence of rotation. The distorted form is known as an oblate spheroid which has a smaller thickness along the axis of rotation than in the plan of the equator, that is it is flattened at the poles. All else being equal, the local temperature of a star's surface is known to be directly related to the strength of gravity there. At the equator of a rapid rotator some of the strength of gravity is "used up" to provide the centripetal acceleration needed to hold the star together, an effect absent at the poles. Thus the gravity is stronger at the poles and in turn the poles are hotter. For stars of temperatures near 10,000 K, the brightness of the surface in the ultraviolet is very sensitive to the temperature. The poles become very much brighter than the rest of the star and thus the extra polarization at the poles may well overpower the diluting effects of the center of the disk and equatorial regions.
This effect, known as gravity brightening, is believed to cause polarization. In addition, because the star is flattened somewhat, there is more limb area perpendicular to the rotation axis than parallel to it which leads to an enhancement of the net polarization.
WUPPE objects Eta Horologium, HD 61931, and Phi Leonis are believed to have rotational velocities around 300 km/sec, and models computed by University of Wisconsin professor L. R. Doherty suggest that in the ultraviolet the polarization might be as much as one percent. Observations of these stars during the Astro-1 mission should serve to confirm these computation.