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Advancing the Physics of Cosmic Distances: Conference Summary Advancing the Physics of Cosmic Distances Proceedings IAU Symposium No. 289, 2012 c 2012 International Astronomical Union Richard de Grijs & Giuseppe Bono, eds. DOI: 00.0000/X000000000000000X Advancing the Physics of Cosmic Distances: Conference Summary Richard de Grijs Kavli Institute for Astronomy and Astrophysics, Peking University, Yi He Yuan Lu 5, Hai Dian District, Beijing 100871, China email: [email protected] Abstract. Knowing the distance of an astrophysical object is key to understanding it. However, at present, comparisons of theory and observations are hampered by precision (or lack thereof) in distance measurements or estimates. Putting the many recent results and new developments into the broader context of the physics driving cosmic distance determination is the next logical step, which will benefit from the combined efforts of theorists, observers and modellers working on a large variety of spatial scales, and spanning a wide range of expertise. IAU Symposium 289 addressed the physics underlying methods of distance determination across the Universe, exploring the various approaches employed to define the milestones along the road. The meeting provided an exciting snapshot of the field of distance measurement, offering not only up-to-date results and a cutting-edge account of recent progress, but also full discussion of the pitfalls encountered and the uncertainties that remain. One of the meeting’s main aims was to provide a roadmap for future efforts in this field, both theoretically and observationally. Keywords. gravitational lensing, masers, stellar dynamics, methods: statistical, techniques: interferometric, astrometry, binaries: eclipsing, stars: distances, stars: oscillations, Cepheids, galaxies: distances and redshifts, Local Group, Magellanic Clouds, cosmological parameters, distance scale, large-scale structure of universe 1. Introduction Knowing the distance of an astrophysical object is key to understanding it: without an accurate distance we do not know how bright it is, how large it is, or even (for long distances) when it existed. But astronomical distance measurement is a challenging task, since the only information we have about any object beyond our solar system is its position (perhaps as a function of time) and its brightness (as a function of wavelength and time). In 1997, the Hipparcos space mission provided (for the first time) a significant num- ber of absolute trigonometric parallaxes at milliarcsec (mas)-level precision across the whole sky, which had a major impact on all fields of astrophysics. In addition, dur- ing the past ten years, the use of ground-based 8–10 m-class optical and near-IR tele- scopes (Keck, VLT, Gemini, Subaru) and space observatories (the Hubble Space Tele- scope [HST], Spitzer, Herschel, Chandra, XMM-Newton) have provided an unprecedented wealth of accurate photometric and spectroscopic data for stars and galaxies in the local arXiv:1209.6529v1 [astro-ph.GA] 28 Sep 2012 Universe. Radio observations, particularly with the Very Long Baseline Array (VLBA) and the Japanese VERA array, have achieved 10 micro-arsecond astrometric accuracy. Moreover, stellar models and numerical simulations are now providing accurate predic- tions of a broad range of physical phenomena, which can in principle be tested using accurate spectroscopic and astrometric observations. However, at present, comparisons of theory and observations are mainly hampered by precision (or lack thereof) in distance measurements/estimates. 1 2 Richard de Grijs IAU Symposium 289 highlighted the tremendous amount of recent and continuing research into a myriad of exciting and promising aspects of accurately pinning down the cosmic distance scale. Putting the many recent results and new developments into the broader context of the physics driving cosmic distance determination is the next logical step, which will benefit from the combined efforts of theorists, observers and modellers working on a large variety of spatial scales, and spanning a wide range of expertise. This is a very exciting time in the context of this Symposium. Very Long Baseline Interferometry (VLBI) sensitivity is being expanded allowing, for example, direct mea- surement of distances throughout the Milky Way and even slightly beyond Local Group galaxies. The field will benefit from expert input to move forward into the era of Gaia, optical-interferometer and Extremely Large Telescope-driven science, which (for exam- ple) will allow us to determine Coma-cluster distances without having to rely on sec- ondary distance indicators, thus finally making the leap to accurate distance measure- ments well beyond the Local Group of galaxies. In this Symposium, we managed to bring together experts on various aspects of dis- tance determinations and (most importantly) the underlying physics enabling this (with- out being restrictive in areas where statistical and observational approaches are more relevant), from the solar neighbourhood to the edge of the Universe, exploring on the way the various methods employed to define the milestones along the road. We aimed at emphasising, where possible, the physical bases of the methods and recent advances made to further our physical insights. We thus aimed to provide a snapshot of the field of distance measurement, offering not only up-to-date results and a cutting-edge account of recent progress, but also full discussion of the pitfalls encountered and the uncertainties that remain. We ultimately aimed to provide a roadmap for future efforts in this field, both theoretically and observationally. Although our focus was on techniques of distance determination, this is intimately linked to many other aspects of astrophysics and cosmology. On our journey from the solar neighbourhood to the edge of the Universe, we encountered stars of all types, alone, in pairs and in clusters, their life cycles, and their explosive ends: binary stars, in particular, play an important role in this context, e.g. in pinning down accurate distances to the Pleiades open cluster and Local Group galaxies, as well as in future ground- and space-based surveys; the stellar content, dynamics, and evolution of galaxies and groups of galaxies; the gravitational bending of starlight; and the expansion, geometry and history of the Universe. As a result, the Symposium offered not only a comprehensive study of distance measurement, but a tour of many recent and exciting advances in astrophysics. 2. Key areas of new and/or sustained progress In 16 reviews, 16 invited and 35 contributed talks, as well as more than 50 high-quality posters, the Symposium presented a venue for lively debate, exciting new results and a number of potentially ground-breaking new announcements. 2.1. Has the Pleiades distance controversy finally been resolved? The Pleiades open cluster is a crucial rung of the local distance ladder, whose calibration affects many fundamental aspects of stellar astrophysics. However, the original Hipparcos parallaxes (Mermilliod et al. 1997; van Leeuwen & Hansen-Ruiz 1997; Robichon et al. 1999; van Leeuwen 1999), as well as the recalibrated astrometry (van Leeuwen 2007a,b), yielded distances to the individual member stars and the open cluster as a whole that were systematically lower than those resulting from previous ground-based distance determi- nations. The latter were predominantly based on the main-sequence fitting technique, IAUS289. Advancing the Physics of Cosmic Distances 3 because prior to the successful Hipparcos mission stellar parallaxes at the distance of the Pleiades were too small to be measurable reliably with contemporary instrumentation. Doubt was initially cast on the original Hipparcos analysis, which required advanced mathematical techniques to solve simultaneously for the positions, motions and distances of 118,000 stars. Although the Hipparcos recalibration reduced the discrepany slightly, the difference remains too large for comfort: the variation in distance modulus implied is approximately 0.2–0.3 mag (Pinsonneault et al. 1998), while the difference in parallax required is of order 1 mas, but note that the absolute uncertainty in Hipparcos parallaxes is only 0.1 mas (Arenou et al. 1995; Lindegren 1995). The controversy has, thus, not been fully resolved, and all methods applied to date are affected by their own unique sets of uncertainties (see e.g. Valls-Gabaud 2007). To account for the Hipparcos distance, stellar models would require changes in physics or input parameters that are too radical to be reasonable, e.g. changes in the Pleiades’ characteristic metallicity or helium abundance, or in the mixing length, an age differ- ential between local and Pleiades member stars or an unusual spatial distribution (i.e. depth effects). Most models applied to resolve the Pleiades controversy include many assumptions and simplifications which may well dominate or negate the need for the pro- posed small evolutionary correction between the Pleiades and local stars, e.g. in terms of stellar structure (rotation, convection, magnetic fields), stellar evolution and stellar atmospheres. VLBI observations may come to the rescue in this context: 1% parallax precision for individual Pleiades stars is anticipated (0.5% for objects in Gould’s Belt). The jury is thus still out on the final resolution of the Pleiades controversy, but there appears to be light at the end of the tunnel. 2.2. Further refinements of the distance to the Galactic Centre The exact distance from the Sun to the Galactic Centre, R0, serves as a benchmark for a variety of methods
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