The evolved stars of Leo II dSph galaxy from near-infrared UKIRT/WFCAM observations

We present a study of the evolved stellar populations in the dwarf spheroidal galaxy LeoII, based on JHKs observations obtained with the near-infrared array WFCAM at the UKIRT telescope. Combining the new data with optical data, we derived photometric estimates of the distribution of global metallicity [M/H] of individual red giant stars from their V - Ks colours. Our results are consistent with the metallicities of red giant branch (RGB) stars obtained from CaII triplet spectroscopy, once the age effects are considered. The photometric metallicity distribution function has a peak at [M/H] = -1.74 (uncorrected) or [M/H] = -1.64 +/- 0.06 (random) +/-0.17 (systematic) after correction for the mean age of LeoII stars (9 Gyr). The distribution is similar to a Gaussian with σ[M/H] = 0.19 dex, corrected for instrumental errors. We used the new data to derive the properties of a nearly complete sample of asymptotic giant branch (AGB) stars in LeoII. Using a near-infrared two-colour diagram, we were able to obtain a clean separation from Milky Way foreground stars and discriminate between carbon- and oxygen-rich AGB stars, which allowed us to study their distribution in Ks-band luminosity and colour. We simulate the JHKs data with the TRILEGAL population synthesis code together with the most updated thermally pulsing AGB models, and using the star formation histories derived from independent work based on deep Hubble Space Telescope photometry. After scaling the mass of LeoII models to the observed number of upper RGB stars, we find that present models predict too many O-rich thermally pulsing AGB (TP-AGB) stars of higher luminosity due to a likely underestimation of either their mass-loss rates at low metallicity, and/or their degree of obscuration by circumstellar dust. On the other hand, the TP-AGB models are able to reproduce the observed number and luminosities of carbon stars satisfactorily well, indicating that in this galaxy the least massive stars that became carbon stars should have masses as low as ~1Msolar.