Securing the Distance Scale via a Universal VI Wesenheit Template and Deep Infrared ZAMS

Astronomy and Astrophysics – Astronomy

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Scientific paper

HST, VLBA, and HIP geometric distances to SX Phe, Delta Scuti, RR Lyrae, Type II and classical Cepheid variables are used to construct a universal VI Wesenheit template. The template uses the statistical weight of the entire variable star demographic to establish precise (<5%) distances to nearby galaxies, star clusters, etc. The reddening-free nature of the Wesenheit approach obviates the propagation of uncertainties tied to tentative total/differential extinction corrections, ensuring that further calibration may ensue directly from published or forthcoming geometric-based distances (masers, HST, GAIA, SIM). An empirical JHKs ZAMS established from deep 2MASS photometry and precise HIP parallaxes for nearby stars provides a concurrent means of securing absolute Wesenheit magnitudes for variables in stellar clusters (calibrators). The infrared ZAMS is comparatively insensitive to stellar age and [Fe/H], and yields distances to 7 of 9 benchmark open clusters that agree with the van Leeuwen (2009) revised HIP estimates (the Pleiades and Blanco 1 are discrepant cases, but should not detract from the broader consensus). In sum, the distance scale is secured to a geometrically anchored framework that consists of results from several key surveys (OGLE, NOMAD, ASAS, etc.) united by a universal VI Wesenheit template and deep infrared ZAMS. Future research entails populating the universal Wesenheit template with lower-temperature calibrators (variable red giants, Miras, longer period Cepheids, etc.) using observations acquired from the ARO, SRO (AAVSO), OMM, and DAO, and further characterizing insidious photometric contamination associated with variables occupying crowded regions near the cores of globular clusters and galaxies (including the Milky Way).

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