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Astron. Astrophys. 326, 988-994

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The distribution of oxygen on the surface of UMa:
an abundance distribution Doppler image

J.B. Rice1*, W.H. Wehlau2**, and D.E. Holmgren3

1Department of Physics, Brandon University, Brandon, Manitoba R7A 6A9, Canada (rice@brandonu.ca)
2Astronomy Department, University of Western Ontario, London, Ontario N6A 3K7, Canada
3Stellar Department, Astronomical Institute, Academy of Sciences of the Czech Republic, Ondrejov 251 65, Czech Republic

Received 6 January 1997 / Accepted 15 May 1997


An excellent Doppler image of the surface of FORMULA UMa shows a strong, sharply defined belt at the magnetic equator with oxygen abundance five orders of magnitude greater than the abundance in the magnetic polar regions. The centre of abundance-symmetry for oxygen is located quite precisely at latitude +28FORMULA and longitude 349FORMULA. A secondary belt of oxygen abundance is observed tilted at 60FORMULA to 70FORMULA to the primary belt. In this secondary belt the abundance is only enhanced about two orders of magnitude above that of the surrounding abundance plateau in the polar regions. The relative depletion of Cr by about three orders of magnitude in a belt around the magnetic equator is confirmed. It is noted that, qualitatively, the location of the primary belt of oxygen enhancement, its extreme abundance difference compared to the polar plateau and the existence of the secondary oxygen belt, might suggest a chemically separated wind as a likely mechanism for producing the oxygen abundance pattern even though the radiative forces should not be sufficient to selectively expel oxygen in a wind.

Key words: stars: chemically peculiar - stars: abundances - surface distribution - stars: Doppler imaging - oxygen - stars: individual: FORMULA UMa - peculiar A

*Visiting Astronomer: Dominion Astrophysical Observatory, operated by the National Research Council of Canada.
**It is with great sadness that we note here the passing of Bill Wehlau in 1995. Bill began this work with us but missed the enjoyment of seeing the results.

Send offprint requests to: J.B. Rice

© European Southern Observatory (ESO) 1997

Online publication: October 1, 1997
Last change: April 8, 1998