The Cousins VRI System: A Consistency Test of Equatorial and Southern Standard Stars

Astronomy and Astrophysics – Astronomy

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Stars: Fundamental Parameters, Surveys

Scientific paper

We present new VRI measurements of southern-hemisphere stars which are on the standard-star system of Landolt [AJ, 88, 439 (1983)]. Using our data and previously published data, we study the relationship between Landolt and "SAAO" standards in the Cousins VRI system. Our work expands on a study done by Menzies et al. [MNRAS, 248, 642 (1991)] and extends also to the standard stars of Graham [PASP, 94, 244 (1982)]. To within moderate errors, the Graham and SAAO systems turn out to be identical. A zero-point difference in V between the data of Landolt and Menzies et al. is found to be in the latter data set. For relatively blue stars in V- R, we confirm a deduction by Menzies et al. that there is a scale-factor difference between the Landolt and southern-hemisphere standards. Especially when recent work by Bessell [PASP, 107, 672 (1995)] is considered, the red-star V-R transformation of Landolt's data turns out to be uncertain. At a given Landolt color for red stars, the range of possibilities for transformed colors is about 30 mmag. For R-I, we find that if data used by Bessell for very red stars are excluded, all standard-star data we have tested adhere very closely to a single system. Our work adds to the list of photometric data sets for which rms errors per datum are known to be decisively less than 10 mmag, and we therefore suggest that "millimagnitudes" should become a generally accepted unit in photometry. We conclude our discussion by suggesting that the "Landolt subsystem" should be retained at present, and that it should be tested further before a definitive conversion is made to the SAAO system.

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