BL Herculis variable
BL Herculis variables are a type of variable star with low luminosity and mass that have a period of less than eight days. They are pulsating stars making up a subclass of Type II Cepheids with light curves that frequently show a bump on the descending side for stars of the shortest periods and on the ascending side for longer period stars. Like other Type II Cepheids, they are very old population II stars found in the galaxy’s halo and globular clusters. Also, compared to other Type II Cepheids, BL Her variables have shorter periods and are fainter than W Virginis variables. Pulsating stars vary in spectral class as they vary in brightness and BL Herculis variables are normally class A at brightest and class F when most dim. When plotted on the Hertzsprung–Russell diagram they fall in-between W Virginis and RR Lyrae variables.
The prototype star, BL Herculis, varies between magnitude 9.7 and 10.6 in a period of 1.3 days. The brightest BL Herculis variables are: VY Pyxidis (7.7 mag max), V553 Centauri (8.2), SW Tauri (9.3), RT Trianguli Australis (9.4), V351 Cephei (9.5), BL Herculis (9.7), BD Cassiopeiae (10.8), and UY Eridani (10.9).
The BL Herculis stars show a wide variety of light curves, temperatures, and luminosity, and three subdivisions of the class have been defined, with the acronym AHB referring to above horizontal branch
- XX Virginis stars (AHB1), with very fast rises to maximum and low metallicity;
- CW stars (AHB2), W Virginis variables, longer periods, the bump on the ascending leg
- BL Herculis stars (AHB3), shorter periods, the bump on the descending leg
- Wallerstein, George (2002). "The Cepheids of Population II and Related Stars". The Publications of the Astronomical Society of the Pacific. 114 (797): 689–699. Bibcode:2002PASP..114..689W. doi:10.1086/341698.
- Soszyński, I.; Udalski, A.; Szymański, M. K.; Kubiak, M.; Pietrzyński, G.; Wyrzykowski, Ł.; Szewczyk, O.; Ulaczyk, K.; Poleski, R. (2008). "The Optical Gravitational Lensing Experiment. The OGLE-III Catalog of Variable Stars. II.Type II Cepheids and Anomalous Cepheids in the Large Magellanic Cloud". Acta Astronomica. 58: 293. arXiv:0811.3636. Bibcode:2008AcA....58..293S.
- "The Masses and Pulsations of BL Herculis Variables" (PDF). Information Bridge. US Department of Energy, Office of Scientific and Technical Information. Retrieved 30 May 2013.
- "General Catalogue of Variable Stars". GCVS. Institute of Astronomy of Russian Academy of Sciences and Sternberg State Astronomical Institute. Retrieved 31 May 2013.
- McNamara, D. H.; Pyne, M. D. (1994). "A Photometric Study of XX Virginis and V716 Ophiuchi". Publications of the Astronomical Society of the Pacific. 106: 472. Bibcode:1994PASP..106..472M. doi:10.1086/133402.
- Diethelm, R. (1983). "A photometric classification of pulsating variables with periods between one and three days". Astronomy and Astrophysics. 124: 108. Bibcode:1983A&A...124..108D.
- Diethelm, R. (1996). "Period changes of AHB1 variables". Astronomy and Astrophysics. 307: 803. Bibcode:1996A&A...307..803D.
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