Anchoring the Universal Distance Scale Via a Wesenheit Template
Daniel J. Majaess
Arne A. Henden
Astrokolkhoz Telescope Facility, Cloudcroft, NM 88317
A VI Wesenheit diagram featuring SX Phoenicis, δ Scuti, RR Lyrae, and type II and classical Cepheid variables is calibrated by means of geometric-based distances inferred from HST, Hipparcos, and VLBA observations (n = 30). The distance to a target population follows from the offset between the observed Wesenheit magnitudes and the calibrated template. The method is evaluated by ascertaining the distance moduli for the LMC (μ0 = 18.43 ± 0.03 σx) and the globular clusters ω Cen, M54, M13, M3, and M15. The results agree with estimates cited in the literature, although a nearer distance to M13 is favored (pending confirmation of the data’s photometric zero-point) and observations of variables near the core of M15 suffer from photometric contamination. The calibrated LMC data are subsequently added to the Wesenheit template since that galaxy exhibits precise OGLE photometry for innumerable variables of differing classes, that includes recent observations for δ Scuti variables indicating the stars follow a steeper VI Wesenheit function than classical Cepheids pulsating in the fundamental mode. VI photometry for the calibrators is tabulated to facilitate further research, and includes new observations acquired via the AAVSO’s robotic telescope network (e.g., VY Pyx: <V> = 7.25 and <V> – <I> = 0.67). The approach outlined here supercedes the lead author’s prior first-order effort to unify variables of the instability strip in order to establish reliable distances.