Sharpening the tip of the red giant branch

Abstract

We introduce a modified detection method for measuring the luminosity of the tip of the red giant branch (TRGB) by introducing the composite magnitude T ≡ I - β[(V - I)x̂ - 1.50], where β is the slope of the tip magnitude as a function of color (or metallicity). The method is specifically designed to account for known systematics due to metallicity. In doing so, this simple transformation does away with arbitrary color selections in measuring the tip, and thereby significantly boosts the population of resolved stars that go into defining the TRGB distance. Moreover, this method coincidentally reduces the impact of reddening on the true modulus as well as its final uncertainty. © 2009. The American Astronomical Society. All rights reserved.