Project

The Araucaria Project is a collaboration between astronomers from institutions in Chile, the US and Europe. Its principal aim is to provide an improved calibration of the local extragalactic distance scale. In the process of setting up the extragalactic distance scale, the greatest difficulty leading to the currently largest contribution on the systematic uncertainty of the Hubble constant lies in the determination of accurate absolute distances to nearby galaxies. The principal reason for this persisting difficulty is in the ill-known dependences of stellar standard candles, used to measure the distances of nearby galaxies, on the environmental properties of their host galaxies (metallicity, age of the stellar populations). The Araucaria Project is an effort to remedy this situation for several of the most important stellar candles, including Cepheid variables, RR Lyrae stars, red clump giants, and blue supergiants which all have the potential to provide accurate distance determinations to nearby galaxies once their environmental dependences are well calibrated. Our strategy is to determine precise distances using the various stellar candles to each of a sample of eight galaxies with widely different environmental parameters (NGC6822, IC1613, WLM and NGC3109 in the Local Group; NGC55, NGC247, NGC300 and NGC7793 in the Sculptor Group), and carefully analyze the distance differences between the different stellar candles, and how they vary with the environmental parameters of their host galaxies. All our target galaxies can be resolved into their stellar components with the modern telescopes and instruments available to the project.


Four different types of datasets are being obtained in the project: wide-field images of the target galaxies in a number of filters, to identify sizable samples of the different distance indicators we are scrutinizing, and study their time-dependent properties; followup near-infrared imaging of selected sub-samples of these objects at the ESO VLT and NTT, and at the LCO Magellan telescopes to exploit the usefulness of near-infrared properties of various stellar candles for distance determination; multiobject spectroscopy of blue supergiant candidates identified from our wide-field images with VLT/FORS2, and subsequent quantitative spectroscopy of the confirmed blue supergiants to derive their distances from their flux-weighted gravities and wind momenta; and HST/ACS images for the assessment of crowding effects and for improved stellar photometry. The Araucaria Project enjoyed a Large Programme status at ESO (2003-2005).



We expect that as a result of the project, we will be able to clearly identify the most accurate stellar methods of distance measurement, and provide calibrations of their dependences on metallicity and age which will allow to determine the distances of nearby galaxies to 5 percent or better. This "local" step is mandatory if we want to take full advantage of the results of the HST Key Project on the Extragalactic Distance Scale. The Araucaria Project is one of the principal science activities of the Chilean FONDAP Center for Astrophysics, a joint effort of the three principal astronomical institutions in the country located at the Universidad de Chile and Pontificia Universidad Catolica de Chile in Santiago, and at the Universidad de Concepcion.