Variations in star formation history and the red giant branch tip

We examine the reliability of the tip of the red giant branch (TRGB) as a distance indicator for stellar populations with different star formation histories (SFHs) when photometric errors and completeness corrections at the TRGB are small. In general, the TRGB-distance method is insensitive to the shape of the SFH except when it produces a stellar population with a significant component undergoing the red giant branch phase transition. The I-band absolute magnitude of the TRGB for the middle and late stages of this transition (similar to1.3-1.7 Gyr) is several tenths of a magnitude fainter than the canonical value of M(I)approximate to4.0. If more than similar to30% of all stars formed over the lifetime of the universe are formed at these ages, then the distance could be overestimated by similar to10%-25%. Similarly, the TRGB-distance method is insensitive to the metallicity distribution of stars formed except when the average metallicity is greater than <[Fe/H]>=-0.3. If more than similar to70% of all stars formed have [Fe/H]>-0.3, the distance could be overestimated by similar to10%-45%. We find that two observable quantities, the height of the discontinuity in the luminosity function at the TRGB and the median (V-I)(0) at M-I=-3.5, can be used to test whether the aforementioned age and metallicity conditions are met.