Hipparcos calibration of the TRGB

We confirm that the revised Hipparcos parallaxes are superior to the original ones, and that this improvement is necessary to detect the TRGB. We find a tip absolute magnitude of M_K_=-6.85+/-0.03, in agreement with that expected from previous tip measurements of the Large Magellanic Cloud, Small Magellanic Cloud, and Bulge.

This represents the first geometric calibration of the TRGB and extends previous calibrations, based on metal-poor globular clusters, to solar metallicities. We attempted to use the TRGB to confirm the presence of the Lutz-Kelker bias, with inconclusive results. Attempts to detect the tip in the I band also produced inconsistent results, due to a lack of precise, homogeneous photometry for these bright stars.