On the central helium-burning variable stars of the LeoI dwarf spheroidal galaxy

TL;DR

This study analyzes the properties of central helium-burning variable stars in LeoI, revealing a large sample of RR Lyrae and Cepheids, with findings on their period distributions and implications for stellar evolution and galaxy history.

Contribution

It provides the largest sample of Cepheids and RR Lyrae stars in LeoI, comparing their properties with other dwarf galaxies and exploring stellar evolution models.

Findings

Cepheid period distribution peaks at longer periods in LeoI.

The sample includes a mix of Anomalous and short-period classical Cepheids.

The transition mass between star types is approximately 2.1 solar masses.

Abstract

We present a study of short period, central helium-burning variable stars in the Local Group dwarf spheroidal galaxy LeoI, including 106 RR Lyrae stars and 51 Cepheids. So far, this is the largest sample of Cepheids and the largest Cepheids to RR Lyrae ratio found in such a kind of galaxy. The comparison with other Local Group dwarf spheroidals, Carina and Fornax, shows that the period distribution of RR Lyrae stars is quite similar, suggesting similar properties of the parent populations, whereas the Cepheid period distribution in LeoI peaks at longer periods (P \sim 1.26d instead of ~0.5d) and spans over a broader range, from 0.5 to 1.78d. Evolutionary and pulsation predictions indicate, assuming a mean metallicity peaked within -1.5<= [Fe/H]<=-1.3, that the current sample of LeoI Cepheids traces a unique mix of Anomalous Cepheids (blue extent of the red--clump, partially electron degenerate central helium-burning stars) and short-period classical Cepheids (blue-loop, quiescent central helium-burning stars). Current evolutionary prescriptions also indicate that the transition mass between the two different groups of stars is MHeF \sim 2.1 Mo, and it is constant for stars metal-poorer than [Fe/H]\sim-0.7. Finally, we briefly outline the different implications of the current findings on the star formation history of LeoI.