A catalogue of helium abundance indicators from globular cluster photometry

We present a survey of helium abundance indicators derived from a comprehen sive study of globular cluster photometry in the literature. For each of th e three indicators used, we conduct a thorough error analysis, and identify systematic errors in the computational procedures. For the population rati o R=N-HB/N-RGB, we find that there is no evidence of a trend with metallici ty, although there appears to be real scatter in the values derived. Althou gh this indicator is the one best able to provide useful absolute helium ab undances, the mean value is Y approximate to 0.20, indicating the probable presence of additional systematic error. For the magnitude difference from the horizontal branch to the main sequenc e Delta and the RR Lyrae mass-luminosity exponent A, it is only possible to determine relative helium abundances reliably. This is due to continuing u ncertainties in the absolute metallicity scale for Delta, and uncertainty i n the RR Lyrae temperature scale for A. Both indicators imply that the heli um abundance is approximately constant as a function of [Fe/H]. According t o the A indicator, both Oosterhoff I and II group clusters have constant va lues independent of [Fe/H] and horizontal branch type. In addition, the two groups have slopes d log[P-ab]/d[Fe/H] that are consistent with each other , but significantly smaller than the slope for the combined sample.