An Astrometric Calibration of Population II Distance Indicators

Details An Astrometric Calibration of Population II Distance Indicators An Astrometric Calibration of Population II Distance Indicators

Feb 2008

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2008noao.prop...73h&link_type=abstract

NOAO Proposal ID #2008A-0073

Astronomy and Astrophysics

Astronomy

Scientific paper

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 two nights with the RCSP on the CTIO 4m to obtain the necessary spectra and 0.8 night of CTIO 0.9m service mode observing to acquire UBVRI photometry of the three fields.

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