The Key to Understanding RR Lyr Stars: WFPC2 Observations of a Unique LMC EB with a RR Lyr Component

Details The Key to Understanding RR Lyr Stars: WFPC2 Observations of a Unique LMC EB with a RR Lyr Component The Key to Understanding RR Lyr Stars: WFPC2 Observations of a Unique LMC EB with a RR Lyr Component

Jul 2007

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2007hst..prop11223g&link_type=abstract

HST Proposal ID #11223. Cycle 16

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Scientific paper

We are proposing HST/WFPC2 2550-10420A multi-band photometry of an important "unique" LMC eclipsing binary with an RR Lyr component. This binary is the only bona fide eclipsing binary EB with an RR Lyr component. Because of their constant mean luminosities L ~ 45 Lsun; ~ +0.5 mag and easily recognizable light curves, RR Lyr variables have long served as the "cornerstone" of the Pop II distance scale in our galaxy and for Local Group galaxies. However, in spite of their critical importance to astronomy, there is a paucity of fundamental data available for RR Lyr stars. In fact, there are no direct measures of their most fundamental properties - such as Mass, Radius and Luminosity. The astrophysical and cosmological consequences of finding an RR Lyr star in an EB are considerable, because the masses and absolute radii of the stars of eclipsing binaries can be determined to within a few percent from time-tested analyses of their light and radial velocity curves. With accurate temperatures and ISM absorption values, determined from the proposed WFPC2 observations, it is possible to determine reliable stellar luminosities and distances. It is for these reasons that we propose WFPC2 observations of the recently discovered detached LMC eclipsing binary OGLE J052218.07-692827.4 ~18.6-mag; 0 ~+0.27; Porb = 8.9231-d; the RR Lyr primary component has a pulsation period of PRR = 0.564876-d. This important binary star is an integral part of our on-going multi-wavelength study of selected eclipsing binaries in nearby galaxies. Three HST/WFPC2 orbits are requested to determine complementary accurate Teff, log g and ISM absorption A-lambda for the component stars. These quantities will be combined with the fundamental stellar data being determined from our ground-based radial velocity and photometric observations. The combined observations will yield accurate stellar masses, radii, temperatures and luminosities, as well as a direct distance to the binary and LMC-Bar. This RR Lyr/EB thus offers the unprecedented opportunity to: 1 determine directly and for the first time the fundamental physical properties M, R, L of an RR Lyr star, 2 directly calibrate "in situ" the zero-point of the LMC RR Lyr - P - Mv - Z relation and 3 to derive an additional accurate distance to the Bar region of the LMC.;

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