AAVSO: American Association of Variable Star Observers

Period Changes and the Evolution of Type II Cepheids (Abstract)

Volume 48 number 1 (2020)

Horace A. Smith
2406 Burcham Drive, East Lansing, MI 48823; smith@pa.msu.edu
Wayne Osborn
address correspondence to Horace A. Smith, 2406 Burcham Drive, East Lansing, MI 48823; smith@pa.msu.edu
Grzegorz Kopacki
address correspondence to Horace A. Smith, 2406 Burcham Drive, East Lansing, MI 48823; smith@pa.msu.edu
Pradip Karmakar
address correspondence to Horace A. Smith, 2406 Burcham Drive, East Lansing, MI 48823; smith@pa.msu.edu
Barton Pritzl
address correspondence to Horace A. Smith, 2406 Burcham Drive, East Lansing, MI 48823; smith@pa.msu.edu
Nathan De Lee
address correspondence to Horace A. Smith, 2406 Burcham Drive, East Lansing, MI 48823; smith@pa.msu.edu
Charles Kuehn
address correspondence to Horace A. Smith, 2406 Burcham Drive, East Lansing, MI 48823; smith@pa.msu.edu
Aaron LaCluyze
address correspondence to Horace A. Smith, 2406 Burcham Drive, East Lansing, MI 48823; smith@pa.msu.edu
Katie Rabidoux
address correspondence to Horace A. Smith, 2406 Burcham Drive, East Lansing, MI 48823; smith@pa.msu.edu

Abstract

(Abstract only) Type II Cepheids are believed to be evolved, low-mass stars. Observed rates of period change for these variables provide important tests for theoretical models of their evolution. If we consider those short-period type II Cepheids (sometimes called BL Her stars) which are metal-poor, we find that observed rates of period change are broadly consistent with predictions that they are stars evolving from the blue horizontal-branch toward the asymptotic red giant branch. There are, however, irregular variations in the rates of period change that are not predicted theoretically, and some of the details of the evolutionary models do not always match observations. Longer-period type II Cepheids (called W Virginis stars) have sometimes been understood to be stars undergoing loops to the blue from the asymptotic red giant branch caused by thermal pulse instabilities. However, not all theoretical models predict such loops. Some W Virginis variables exhibit period changes consistent with those from the loop models, but others show more irregular period fluctuations.