Eclipsing Binaries in the Local Group: II - Calibration of the Zeropoint of the Cosmic Distance Scale and Fundamental Properties of Stars in M33

Details Eclipsing Binaries in the Local Group: II - Calibration of the Zeropoint of the Cosmic Distance Scale and Fundamental Properties of Stars in M33 Eclipsing Binaries in the Local Group: II - Calibration of the Zeropoint of the Cosmic Distance Scale and Fundamental Properties of Stars in M33

Jul 2006

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2006hst..prop10919g&link_type=abstract

HST Proposal ID #10919. Cycle 15

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Scientific paper

The Great Spiral Galaxy in Triangulum M33 is potentially 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 +/- 110 kpc still has measurement dispersions of 10-15%. We have demonstrated, in our work on the LMC and M31 distances, that double-line eclipsing binaries can 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. We are extending our program of using eclipsing binaries as standard candles to determine an accurate distance to M33. As a first step, we are proposing to carry out HST/ACS spectrophotometry of a well suited ~19th mag ~O7 + ~O7 eclipsing binary system in M33 that has been previously observed from the ground. HST/ACS prism/grism low-resolution spectrophotometry 118-850 nm is the only missing key element of this program and is used to determine more reliable values for T_eff, [Fe/H], and ISM extinction. These quantities, when combined with the results from existing light and radial velocity curves for the target, yield the stellar masses, radii, luminosities and, importantly, the distance. The proposed HST/ACS program can be carried out effectively with only 1 HST orbit. Based on our previous experience, we expect to reduce the uncertainty of the M33 distance to better than 5-7%, thereby leading to a firmer calibration of the Cosmic Distance Scale and the zeropoint of the Hubble Constant Ho.

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