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A sub-percent distance scale from binaries and Cepheids

Periodic Reporting for period 2 - CepBin (A sub-percent distance scale from binaries and Cepheids)

Reporting period: 2018-05-01 to 2019-10-31

We aim at improving the accuracy of the determination of the Hubble constant, that is a measure of the rate of expansion of the universe. We will achieve this goal through a very accurate measurement of the distances to the Magellanic Clouds (two nearby galaxies), using eclipsing binary systems (pairs of stars that periodically pass in front of each other). This will in turn allow us to calibrate the Period-Luminosity relation (the Leavitt law) of Cepheids present in these two galaxies. Cepheids are of prime importance to determine extragalactic distances and calibrate the Hubble constant.

Our results will not only pave the road for 1 % H0 determination, but will also will provide the opportunity to
improve the calibration of a number of secondary methods for distance determination, and as a result
provide a homogeneous catalog of precise distances to nearby galaxies which will be important for many
fields of modern astrophysics. The unique catalog of precise (1-2 %) physical parameters for some 200
binary systems (e.g. 400 red giant stars) emerging from this project, including 20 classical Cepheids, will
lead to an improved understanding of stellar evolution at late stages, and of stellar pulsation theory. Our
much improved understanding of the basic physics of Cepheids will also boost our confidence in the
calibration of the extragalactic distance scale we will achieve in this project.
We already measured precision stellar parameters for several eclipsing binary systems in the Large Magellanic Cloud and in the Milky Way.
Based on these data we measured very accurate distanced to the studied systems. We also measured the effect of metal content on the brightness
of Classical Cepheids. We demonstrated that this effect is very small and does not affect cosmic distance determinations with Cepheids.
We also improved calibration of the Baade - Wesselink method for distance determination with classical Cepheids.
We also measured precision distances to several nearby galaxies.
Our distance determinations are the most accurate measurements reported so far. The form strong basis for
trully accurate determination of the Hubble constant.