Extragalactic distance scale
Research is done by Wolfgang Gieren and his collaborators in Chile, Europe and the USA on the improvement of the calibration of the extragalactic distance scale, in the framework of the Araucaria Project which was started in Concepcion in 2000. The project focusses on improving stellar standard candles including Cepheids, eclipsing binaries, RR Lyrae stars, red clump stars, red giants and blue supergiants by determining their dependences on environmental properties like metallicity, and the age of stellar populations. These dependences need to be very well known to calibrate the different stellar methods accurately enough to achieve distance determinations to nearby galaxies to 1% or better. The knowledge of the distances of nearby galaxies, particularly to the Large Magellanic Cloud which is the most important anchor for the extragalactic distance scale, in tandem with a precise knowledge of how the calibration of methods like the Cepheid period-luminosity relation are influenced by the metallicity and age of stellar populations constitute the main source of systematic uncertainty on the Hubble constant as derived with the classical Cepheid-Supernova Ia method. Our research is therefore fundamental for achieving a reduction of the uncertainty on the Hubble constant to 1 percent which is perhaps the most important goal for cosmology in present-day astrophysics, especially in view of the tension between results for H0 from the Planck satellite data and those based on the classical Cepheid-Supernova method. Besides of laying the foundation for a significant reduction of the error on the Hubble constant, knowledge of the distances to nearby galaxies is extremely important for any kind of physical studies of stars, and stellar populations in these galaxies.
Currently another focus of the Araucaria Project lies on the determination of sub-percent distances to nearby eclipsing binary systems by using interferometry in tandem with spectroscopy. These distances will provide a crucial check on the distances delivered by the Gaia satellite, and their potential systematic uncertainties. A byproduct of this work is the determination of stellar masses and radii accurate to better than one percent which is extremely important for improving stellar evolutionary models. The project is also accumulating very accurate near-infrared photometry and radial velocity measurements for large samples of different classes of pulsating variable stars in the Milky Way to take advantage of the upcoming Gaia distances to these stars, in order to calibrate absolute period-luminosity relations for these classes of variable stars and improve the calibration of the infrared Baade-Wesselink method to measure individual distances to Cepheids, a method which has the potential to deliver one percent distances to galaxies out to a few Megaparsecs.
The Araucaria Project has been extremely successful over the last two decades, based on high-quality data from the largest telescopes and best instruments available in the observatories located in Chile and complemented with data from space-based telescopes like HST, and theoretical work. Several scientific highlights were published in Nature, including the most accurate-ever determination of the LMC distance in 2019. For more detailed information on the project, the scientists involved, and the results produced by the project over the years the reader is referred to the webpage of the Araucaria Project https://araucaria.camk.edu.pl/