An Era of Precision Astrophysics: Connecting Stars, Galaxies and the Universe
The golden age of astrophysics is upon us with both grand discoveries (extra-solar planets, dark matter, dark energy) and precision cosmology. Here we argue that fundamental understanding of the working of stars and galaxies is within reach, thanks to the precision measurements that are now becoming possible. We highlight the importance of micro-arcsecond astrometry which forms the basis of model independent distances and masses. Distances are one of the most fundamental external properties of astronomical objects and accurate knowledge of distances can change our perception of astronomical phenomena dramatically. For example, in antiquity, the idea that the Earth revolved around the Sun was rejected because the predicted annual parallax of stars were not observed. In modern times, just before the first distance measure of gamma-ray bursts, there were more theories about the nature of these objects than the number of observed -ray bursts. Some of the strongest motivations to vigorously pursue accurate distance measurements are related to the history and fate of the universe. Two of the three methods available to date the universe are based on accurate distance measurements. The first method is based on the ages of stars, which can only be ascertained if their luminosities (and hence distances) are accurately known. The second method relies on cosmological methods. To first order, the age of the universe is the inverse of Hubble's constant (H0), while further corrections depend on the fate of the universe -closed, critical or openand on the amount and nature of dark energy. Indeed, the previous decadal report stresses that: “the fundamental goal of ... astrophysics is to understand how the universe ... galaxies [and] stars ... formed, how they evolved, and what their destiny will be” (McKee & Taylor, 2001). These age-old questions can be answered to a large extent by science topics that rely significantly on micro-arcsecond astrometry: 1) Galactic archeology: a detailed reconstruction of the formation history of the Milky Way and other Local Group galaxies, 2) the very oldest stars in the Milky Way and the age of the Universe, and 3) H0 and concordance cosmology. In question form we can summarize these goals as: 1) What is the construction history of the Milky Way and other nearly galaxies? 2) what is the age, density and curvature of the Universe? These goals are achievable in the near future by combining micro-arcsecond astrometry from the proposed SIM-Lite mission (Unwin et al. 2008) with data from a modest ground-based spectroscopic observing campaign. The high-quality data like we are advocating for in this white paper will force the biggest reassessment of stellar astrophysics in more than 50 years, and its effects will be very beneficial for many disciplines of astrophysics.