Eclipsing Binaries in the Local Group: III - Unprecedented Accuracy in Distance Determination to M33 and Calibration of the Cosmic Distance Scale

Details Eclipsing Binaries in the Local Group: III - Unprecedented Accuracy in Distance Determination to M33 and Calibration of the Cosmic Distance Scale Eclipsing Binaries in the Local Group: III - Unprecedented Accuracy in Distance Determination to M33 and Calibration of the Cosmic Distance Scale

Jul 2008

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2008hst..prop11725g&link_type=abstract

HST Proposal ID #11725. Cycle 17

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Scientific paper

The Great Spiral Galaxy in Triangulum {M33} is a crucial calibrator for the Cosmic Distance Scale, and thus for determining the age and evolution of the Universe. M33 is viewed face-on, has a simple geometry, large and diverse stellar populations, and morphologies similar to our Galaxy and other more distant galaxies used for distance determinations. Yet currently the M33 distance {d 830 +/- 120 kpc} still has measurement dispersions of 10-15%. Moreover, the distance to M33 derived from Cepheids, RR Lyrae stars, H2O masers, RGB stars, and EBs is currently discrepant by 15% {Bonanos et al. 2006}. In our work on the LMC and M31 distances we have demonstrated that double-line eclipsing binaries serve as excellent "standard candles." Distances derived from eclipsing binaries are basically geometric and essentially free from many assumptions and uncertainties that plague other less direct methods, such as metallicity differences and calibration zeropoints. The absolute radii of the component stars of eclipsing binaries can be determined to better than a few percent from the time-tested analyses of their light and radial velocity curves. With accurate determinations of radii, temperatures, and ISM absorption it is possible to determine reliable distances. In Cycle 15 we extended our program of using eclipsing binaries as standard candles to M33 using ACS/SBS and WFPC/2 spectrophotometry of a well suited 19th mag O7+O7 eclipsing binary system. Although insightful, ACS/SBC data alone do not provide sufficient accuracy for the unambigous determination of individual temperatures, [Fe/H], and ISM extinction, which are central to distance determination. We propose a 4 orbit follow-up spectrophotometry with the newly installed HST/COS G140L and the repaired HST/STIS G230L and G430L to obtain the single missing key element of this program. These quantities, when combined with the results from existing light and radial velocity curves for the target {cf. Bonanos et al. 2006}, yield stellar masses, radii, luminosities and, most importantly, the distance. Based on our previous experience, we expect to reduce the uncertainty of the M33 distance to less than 5%, thereby leading to a resolution of the currently irreconcilable distances to M33 derived by different methods, and a firm calibration of the Cosmic Distance Scale and the zeropoint of the Hubble Constant {Ho}.;

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