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Description
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<1.5km/s) radial velocity observations made with the CORAVEL and DAO spectrometers - including 67 new unpublished data - we have obtained both the orbital and pulsational velocity curves of this binary Cepheid.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+/-0.4days 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)x10^7^yr and a Cepheid mass of 5.6M_{sun}_. 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 IUE spectrum suggest that the companion is a main sequence star in the mass range from 2.6 to 5.6M_{sun}_. 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+/-110pc and 66.0+/-3.5R_{sun}_, 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+/-0.3mag whose error is dominated by the uncertainty of the absorption correction. Since our very precise distance contributes only 0.12mag 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+/-13pc obtained by Turner et al. from ZAMS fitting of the cluster. Possible causes for this discrepancy, and their implications for Cepheid distance scale calibrations, are discussed.
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