Independent distance determinations to Milky Way Cepheids in open clusters and associations. 1: The binary Cepheid DL CAS in NGC 129

Abstract

view Abstract Citations (11) References (32) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Independent Distance Determinations to Milky Way Cepheids in Open Clusters and Associations. I. The Binary Cepheid DL CAS in NGC 129 Gieren, Wolfgang P. ; Welch, Douglas L. ; Mermilliod, Jean-Claude ; Matthews, Jaymie M. ; Hertling, Gisela Abstract As an eight-day Cepheid which is both a component of a spectroscopic binary and a member of the open cluster NGC 129, DL Cas is potentially a very accurate calibrator of the period-luminosity (PL) relation and Cepheid mass. From 160 high-precision (sigma less than 1.5 km/s) radial velocity observations made with the CORAVEL and DAO spectrometers -- including 67 new unpublished data -- we have obtained both the orbital and pulsational velocity curves of this binary Cepheid. This body of RV data makes DL Cas one of the best observed Cepheids in our galaxy. Our analysis yields an orbital period of 684.4 +/- 0.4 days which confirms DL Cas as one of the shortest-period binaries containing a Cepheid. We derive new precise orbital elements which replace earlier preliminary values found by Harris et al. Isochrone fitting to the V, B - V data points of Turner et al. yields an age of NGC 129 of (7.6 +/- 0.4) x 107 yr and a Cepheid mass of 5.6 solar mass. Evidence from age, a possible period change, and strip crossing times suggest that DL Cas is a solar-abundance star making its third (redward) crossing through the Cepheid instability strip. Existing observational constraints from our mass function of the DL Cas system and an International Ultraviolet Explorer (IUE) spectrum suggest that the companion is a main sequence star in the mass range from 2.6 to 5.6 solar mass. We use the pulsational velocity curve and published photometry to derive the distance and mean radius of DL Cas with the surface brightness method, finding values of 2034 +/- 110 pc and 66.0 +/- 3.5 solar radius, respectively. The radius we derive indicates that DL Cas is a fundamental-mode pulsator, removing any possible ambiguity in mode identification. The distance corresponds to a mean absolute visual magnitude of (MV) = -4.2 +/- 0.3 mag whose error is dominated by the uncertainty of the absorption correction. Since our very precise distance contributes only 0.12 mag to the error in (MV), improved reddening studies of NGC 129 would make DL Cas a very tight calibrator of the PL relation. Our value of the gamma velocity of the DL Cas system is identical to the mean radial velocity of the stars in NGC 129, strengthening the case for cluster membership. However, our distance for DL Cas, and thus for NGC 129, is significantly larger than the 1670 +/- 13 pc obtained by Turner et al. from Zero Age Main Sequence (ZAMS) fitting of the cluster. Possible causes for this discrepancy, and their implications for Cepheid distance scale calibrations, are discussed. Publication: The Astronomical Journal Pub Date: June 1994 DOI: 10.1086/117019 Bibcode: 1994AJ....107.2093G Keywords: Cepheid Variables; Distance; Milky Way Galaxy; Open Clusters; Star Clusters; Brightness Distribution; Stellar Spectrophotometry; Astronomy; OPEN CLUSTERS AND ASSOCIATIONS: INDIVIDUAL: NGC 129; CEPHEIDS full text sources ADS | data products SIMBAD (2) CDS (1) Related Materials (2) Part 2: 1995AJ....110.2280M Part 3: 1996AJ....111.2059G