Disk Population of Type II Cepheids in the Galaxy Based on Gaia DR3 Data

Galactic orbital parameters of 519 type II Cepheids classified as BL Her, W Vir and RV Tau variable stars were calculated using Bayesian distances, proper motions, and Gaia DR3 radial velocities. It is shown that at least 27 BL Her type variables out of 100, 17 W Vir type variables out of 308 and 8 RV Tau type variables out of 111 are characterized by a spatial distribution and galactic orbital parameters typical of the population of the thin/old disk of the Galaxy. At the same time, their luminosities are significantly lower than those of classical Cepheids (DCEP), and correspond to the luminosities of type II Cepheids. It is suggested that the region of the instability strip occupied by short-period Cepheids of the BL Her type may contain stars of the old disk with initial masses of about $$0.95 M_{\odot}$$ , which evolved from the main sequence to the stage of the onset of helium burning in the core and reached the instability strip over a time of about 9 Gyr. The presence of evolutionary ‘‘loops’’ explains the existence of a number of BL Her type Cepheids, which demonstrate an evolutionary decrease in period. Thus, both low-mass stars in the phase of evolution of a double-layer source and stars at the stage of the onset of helium burning in the core with a larger initial mass can coexist in the instability strip of BL Her variables. To refine the classification of type II Cepheids, more detailed evolutionary and pulsation calculations and additional comprehensive studies are needed, including, for example, estimates of pulsation radii.