Probing the red giant branch phase transition: near-infrared photometry of six intermediate-age large magellanic cloud clusters

This is the first of a series of papers devoted to a global study of the photometric properties of the red stellar sequences in a complete sample of the Large Magellanic Cloud clusters, by means of near-infrared array photometry. Deep J, H, Ks photometry and accurate color-magnitude diagrams down to K ≈ 18.5, i.e., ≈1.5 mag below the red He clump, for six intermediate-age clusters (namely, NGC 1987, NGC 2108, NGC 2190, NGC 2209, NGC 2231, NGC 2249) are presented. A quantitative estimate of the population ratios (by number and luminosity) between red giant branch (RGB) and He-clump stars for each target cluster is provided and discussed in the framework of probing the so-called RGB phase transition (Ph-T). By using the Elson & Fall s-parameter as an age indicator, the observed RGB population shows a sharp enhancement (in both number and luminosity) at s = 36. Obviously, the corresponding absolute age strictly depends on the details of theoretical models adopted to calibrate the s-parameter. Curiously, the currently available calibrations of the s-parameter in terms of age based on canonical (by Elson & Fall) and overshooting (Girardi and coworkers) models provide ages that well agree within 10%, suggesting that the full development of the RGB occurs at t ≈ 700 Myr and is a relatively fast event (δt ≈ 300 Myr). However, the RGB Ph-T epoch derived from the overshooting calibration of the s-parameter turns out to be significantly earlier than the epoch provided by the recent evolutionary tracks by Girardi and coworkers. A new calibration of the s-parameter based on high-quality color-magnitude diagrams and updated models is urged to address the origin of this discrepancy and finally establish the epoch of the RGB Ph-T.