Simultaneous multichannel photometry with BUSCA

Abstract

This thesis deals with the multicolour simultaneous camera BUSCA (Bonn University Simultaneous CAmera), which was build at the Sternwarte of the University of Bonn for the the 2.2m telescope at the Calar Alto Observatory (Spain).<br /> The advantage of this new instrument is the capability of observing simultaneously in four different colour bands ranging from the ultraviolet to the near-infrared. This technique saves on one hand the amount of time needed for a full colour coverage of astronomical objects. In comparison, single filter systems waste all the light except that light passing the band filter. On the other hand, the simultaneous observation in four colours should provide a stability of the colour indices during non-photometric conditions caused by variable extinction, e.g., thin clouds (droplets, ice crystals) or dust. This includes the assumption that thin clouds have a grey colour, which means an equal flux loss in all colour bands. Additionally, a new shutter, the "Bonn Shutter'', was developed. It provides homogeneous illumination over the whole field of view at small exposure times (<0.1s).<br /> As the first science case the first simultaneous observations of the rapidly variable sdB (sub-luminous B) star PG1605+072 are presented. PG1605+072 belongs to a new class of pulsating stars named V361 Hya stars for which more than 30 sdB pulsators are known. The stellar pulsations allow insight into the structure of the stellar atmosphere and therefore indirectly into the evolutionary history of the sdB stars. With this asteroseismological analysis tool it is then possible to determine the stellar mass as well as the envelope mass. PG1605+072 is an ideal target for a photometric and spectroscopic analysis, because it has the longest pulsation periods known for this class of variable stars, and with 50 known pulsation frequencies it possesses by far the richest frequency spectrum.<br /> The second science project deals with the Stroemgren photometry of Galactic globular clusters. One advantage of the Stroemgren photometry is that it can be used as a reliable metallicity indicator. The Stroemgren <i>v</i> filter includes several iron absorption lines and is as well as the <i>m₁=(v-b)-(b-y)</i> index sensitive to the iron abundance. For giant and supergiant stars the iron metallicity can be determined directly from the photometry. In this work two Galactic globular clusters M12 and M71, for which no Stroemgren photometry were available, were observed and analysed with BUSCA. Moreover, M71 shows additional CN variations among the red giant branch (RGB) and main sequence (MS) stars. The origin of these CN variations is not yet understood. Possible explanations may be that CN at the star surface is enriched with processed material (out of the CNO-cycle) from inside the star, that CN is accreted by stellar winds (self-pollution) from, e.g., AGB stars and novae, or that the CN distribution in the molecular cloud in which the globular cluster was born was not homogeneous. In the past the CN variations were detected spectroscopically for only a few RGB and MS stars, so with that photometric approach more RGB stars can be investigated with BUSCA.