Sharpening the Tip of the Red Giant Branch

TL;DR

This paper presents a new method for measuring the luminosity of the tip of the red giant branch that accounts for metallicity effects, reducing systematics and increasing the accuracy of distance measurements.

Contribution

The paper introduces a composite magnitude T that corrects for metallicity effects, improving TRGB distance measurements without arbitrary color cuts.

Findings

Reduces systematic errors due to metallicity.

Increases the number of resolved stars used in TRGB measurement.

Lessens the impact of reddening on distance estimates.

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 - beta [(V-I)_o - 1.50], where beta 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.