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Paper: Type II Cepheids: Observational Perspective
Volume: 529, RR Lyrae/Cepheid 2019: Frontiers of Classical Pulsators
Page: 305
Authors: Jurkovic, M. I.
Abstract: Type II Cepheids are low-mass (∼0.5 – 0.6 M), old pulsating stars. They are divided into three subgroups according to their pulsation period: BL Herculis (1 – 3 days), W Virginis (4 – 20 days), and RV Tauri (20 – 150 or more days). Type II Cepheids are found in the Milky Way (mainly in the disk), clusters around the Milky Way, the Large and Small Magellanic Clouds, and surrounding galaxies. They form a period-luminosity relation that is separate from the classical Cepheids and other types of pulsating stars. To study dynamical processes in Type II Cepheids in detail, observations need to be continuous and precise. This is challenging given that their pulsation periods can be as long as half a year. Many of the sky surveys (past, present and future) meet these challanges. Another source of precise, and long baseline, data sets come from space telescopes, e.g., INTEGRAL-OMC, Kepler/K2, TESS. The Gaia space mission provides astrometry and spectroscopy together with red and blue photometry, so it stands in a league of its own. The light curve shape changes with the filters used, and this should also be taken into account when analysing the resulting light curves. All of these advances in observations led to the discovery of period doubling, multi-mode pulsation, and quasi-periodic modulation of pulsation in Type II Cepheids. For the first time, Type II Cepheids pulsating in the first overtone were also observed. Combing measurements from separate regions of the electromagnetic spectrum (from ultra violet to far infra red), and calculating the best fit model for the Spectral Energy Distribution of each individual Type II Cepheid in the Magellanic Clouds, the physical parameters (effective temperature and luminosity, as well as the mass and radii) were calculated. Using this data, the period-luminoity relation and the period-radius relation for these stars were derived. The old age of Type II Cepheids makes them suitable for archaeogalactic studies. Since they could be found in galaxies that are far away, and contain old populations, they could become very useful distance calibrators.
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