DOPPLER IMAGES OF ABUNDANCE FEATURES ON THETA-AURIGAE

Doppler images of the silicon and chromium distribution on the magnetic Ap star THETA-Aurigae are presented. The silicon distribution consists of a small depleted spot which is at latitude +50-degrees and is coincident in longitude with the negative magnetic pole. There is also a ring-like enhanced feature of radius 50-degrees-90-degrees and centred on the depleted spot. This ring is apparently located near the magnetic equator. This distribution is that expected from the selective diffusion of silicon in the presence of a predominantly dipolar magnetic field. The chromium distribution differs markedly from the silicon distribution in that it not only has a depleted spot coincident with the depleted silicon spot at the magnetic pole but also a depleted band near the enhanced silicon band and presumably coincident with the magnetic equator. There is also a large enhanced chromium feature just below the stellar equator and near the expected location of the positive magnetic pole. If chromium diffuses in a manner similar to silicon then its surface distribution is indicative of a magnetic field having a strong quadrupole component. Alternatively, the two distributions could be reconciled by invoking horizontal diffusion. This, however, would require different time-scales for the diffusion of silicon and chromium.