Other
Scientific paper
May 1994
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1994pasp..106..472m&link_type=abstract
Astronomical Society of the Pacific, Publications (ISSN 0004-6280), vol. 106, no. 699, p. 472-480
Other
5
Hydrogen, Iron, Metallicity, Stellar Gravitation, Stellar Magnitude, Stellar Temperature, Variable Stars, Astronomical Photometry, Periodic Variations, Stellar Color, Stellar Luminosity, Temperature Distribution, Ubv Spectra
Scientific paper
We present Stroemgren photometry of the two Population II variables XX Virginis and V716 Ophiuchi. An analysis of the photometry yields the following physical parameters of the variables: for XX Vir average value of Teff = 6410 K, (Fe/H) = -2.0, and average value of log g = 1.95; for V716 OPH average value of Teff = 6450 K, (Fe/H) = -2.1, and average value of log g = 1.67. The average value of log g values of these two metal-poor variables are considerably smaller than for the metal-strong variable BL Herculis (average value of log g = 2.37). The big surprise in this investigation is that the surface gravity of V716 Oph (P = 1.12 days), the shorter-period variable, is smaller than the surface gravity of XX Vir (P = 1.35 days), the longer period variable. With the aid of the P x (square root of rho) = Q relation we derive a semitheoretical P-L relation for the Type II Cepheids of the form Mv = -1.88 log P + 0.13. This expression is in rough agreement (within the uncertainties of many quantities used in deriving the expression) with the observational data. The relatively gentle slope of the P-L relation (compared with the slope of the P-L relations of other pulsating stars) is probably a consequence of the fact that the masses of the Type II Cepheids are constant or nearly constant, independent of period, and a large variation of their M/R values with period.
McNamara D. H.
Pyne M. D.
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