Population II Cepheids in Galactic GC

Using linear MESA-RSP models, we determined the red and blue edges of the instability strip for T2CEPs. We found that the models providing the best fit to the observations are able to fit 90% of the stars in our sample. They have the following parameters: M=0.6M_{sun}_, Z=0.0003, and this percentage does not change for two different values of the helium abundance (Y=0.220 and 0.245).

Higher values of Y lower this percentage, and the same effect is observed with lower values of Z. In the future, combining the sample of T2CEPs with the precise parallaxes obtained from GCs will strengthen the geometric calibration of a distance ladder based on Population II stars. This will be useful for determining distances within the Milky Way and for cross-checking distances to Local Group galaxies determined through other methods.