PI: Thomas E Harrison, New Mexico State University, firstname.lastname@example.org
Address: Astronomy Department, Box 30001/MSC4500, Las Cruces, USA
CoI: G. Fritz Benedict, University of Texas, Austin
CoI: Thomas Barnes, University of Texas, Austin
CoI: Michael Feast, University of Capetown
Title: An Astrometric Calibration of Population II Distance Indicators
Abstract: We have a large, 100 orbit, HST project in Cycles 16 and 17 to use the Fine Guidance Sensors (FGS) to obtain parallaxes for four RR Lyrae variables, and two W Virginis stars (``Pop II Cepheids''). The goal of this program is to astrometrically calibrate the K-band Period- Luminosity (PL) relationship. RR Lyrae variables have long been used for understanding old, stellar populations by tracing their chemical and dynamic properties. They have also long been used as distance indicators in the local Group. Yet the current calibration of the zero point in the PL relationship rests on a single FGS parallax: that of RR Lyrae itself. W Virginis stars may have an even greater potential as distance indicators due to their higher luminosities, yet the existing HIPPARCOS parallaxes are insufficient to establish the zero point calibration needed to provide a useful PL relationship. As in previous HST FGS parallax programs, we need to determine spectroscopic parallaxes of the reference frame stars. To accomplish this, we request 0.5 night of CTIO 0.9m service mode observing to acquire UBVRI photometry for the remaining two fields.
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