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Paper: Photometric monitoring of eta Carinae, and the general problem of accurate photometry of S Dor variables
Volume: 233, P Cygni 2000: 400 Years of Progress
Page: 39
Authors: Sterken, C.; Freyhammer, L. M.; Arentoft, T.; van Genderen, A. M.
Abstract: We review the particular problems of ground-based photometry of the composite object eta Carinae for which high-precision long-term photometric monitoring is so hard to accomplish. eta Car, by its appearance as an ultrabright extended object, by its anomalous spectral nature, and by its most outspoken variability, is the single most difficult stellar object to measure photometrically and to monitor over a long time interval. The problems belong to several levels: very limited availability of an astrophysically appropriate photometric system, the presence of strong (and variable) emission lines, the need for a telescope with a suitable f-ratio, and the steadily diminishing possibilities to collect data over a long period of time. We demonstrate that the transformation from our y measurements to V is valid for many stars, but not for eta Car nor for some less peculiar objects. Even pure Johnson V data can not be guaranteed to be accurately tied to the E-region standard stars. The'underestimated' V-magnitude of eta Car based on y-filter photometry is not to be considered a suspicious characteristic of our intermediate-band photometry, but is an inherent element of the photometric approach of discussing V magnitudes on the basis of data obtained with a Stromgren y filter. Aperture corrections of a single isolated measurement remain prone to random errors of the order of 0.005-0.015 mag.
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