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

TL;DR

This study analyzes the evolved stellar populations in Leo II dwarf galaxy using near-infrared and optical data, revealing insights into AGB star properties, metallicity distribution, and model predictions versus observations.

Contribution

It provides a detailed near-infrared analysis of AGB stars in Leo II, comparing observations with population synthesis models and highlighting discrepancies in O-rich star predictions.

Findings

Models overpredict high-luminosity O-rich TP-AGB stars.

Carbon star numbers and luminosities match models well.

Minimum mass for carbon star progenitors is about 1 solar mass.

Abstract

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. 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. 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 HST 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 TP-AGB stars of higher luminosity due to a likely under-estimation 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 ~1 M_sun.