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Deep Infrared ZAMS Fits to Benchmark Open Clusters Hosting delta Scuti Stars

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Daniel J. Majaess, David G. Turner, David J. Lane
Saint Mary's University, Department of Astronomy and Physics, Halifax, Nova Scotia, B3H 3C3, Canada

Tom Krajci
Astrokolkhoz Telescope Facility, P.O. Box 1351, Cloudcroft, NM 88317

Received December 14, 2010; revised February 23, 2011; accepted February 23, 2011

Abstract

This research aims to secure precise distances for cluster delta Scuti stars in order to investigate their properties via a VI Wesenheit framework. Deep JHKs color-color and zero-age main sequence (ZAMS) relations derived from ~– 700 unreddened stars featuring 2MASS photometry and precise Hipparcos parallaxes (d <~ 25 pc) are applied to establish distances to several benchmark open clusters that host delta Scuti stars: Hyades, Pleiades, Praesepe, alpha Persei, and M67 (d = 47 ± 2, 138 ± 6, 183 ± 8, 171 ± 8, 815 ± 40 pc). That analysis provided constraints on the delta Sct sample’s absolute Wesenheit magnitudes (WVI,0), evolutionary status, and pulsation modes (order, n). The reliability of JHKs established cluster parameters is demonstrated via a comparison with van Leeuwen (2009a) revised Hipparcos results. Distances for seven of nine nearby (d <– 250 pc) clusters agree, and the discrepant cases (Pleiades and Blanco 1) are unrelated to (insignificant) Te – (J–Ks) variations with cluster age or iron abundance. JHKs photometry is tabulated for ~– 3 × 103 probable cluster members on the basis of proper motions (NOMAD). The deep JHKs photometry extends into the low mass regime (~– 0.4 Msolar) and ensures precise (<– 5%) ZAMS fits. Pulsation modes inferred for the cluster delta Scuti stars from VI Wesenheit and independent analyses are comparable (± n), and the methods are consistent in identifying higher order pulsators. Most small-amplitude cluster delta Scuti stars lie on VI Wesenheit loci characterizing n >– 1 pulsators. A distance established to NGC 1817 from delta Scuti stars (d ~– 1.7 kpc) via a universal VI Wesenheit template agrees with estimates in the literature, assuming the variables delineate the n >– 1 boundary. Small statistics in tandem with other factors presently encumber the use of mmag delta Scuti stars as viable distance indicators to intermediate-age open clusters, yet a VI Wesenheit approach is a pertinent means for studying d Scuti stars in harmony with other methods.

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