RR Lyrae stars in globular clusters: better distances from infrared measurements?

An investigation into the suitability of near-infrared photometry for the study of distances to globular clusters has been carried out. Single-phase observations at 2.2 μm of RR Lyrae stars in three clusters have been made. A clear period − [2.2 μm (K) luminosity] relation is observed in all three clusters (M3, M5 and M107), and is of the form log P ∝ −0.45 MK. This relationship can be understood readily from pulsation theory, and arises from a strong dependence of period on temperature. Using distance moduli for the three clusters determined on the precepts of recent studies by Sandage, the absolute K magnitude, normalized to a ‘standard’ period, was calculated for the RR Lyrae stars in M3 the absolute K magnitudes at log P = −0.3 for the stars in M3, 5 and 107 were |-0^ m_. 25, -0^ m_. 26, -0^ m_. 24|⁠, respectively. We tentatively attribute this result to the relatively low sensitivity of infrared magnitudes to temperature over the relevant temperature range. This remarkably small scatter, even though it is dependent on the validity of the optical distance moduli, suggests that infrared photometry of RR Lyrae stars could prove a very powerful tool in the study of distances within the galaxy, and of galactic structure in general.