Browsing by Author "WELCH, DL"

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    INDEPENDENT DISTANCE DETERMINATIONS TO MILKY-WAY CEPHEIDS IN OPEN CLUSTERS AND ASSOCIATIONS .1. THE BINARY CEPHEID DL CAS IN NGC 129
    (1994) GIEREN, WP; WELCH, DL; MERMILLIOD, JC; MATTHEWS, JM; HERTLING, G
    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 < 1.5 km/s) radial velocity observations made with the CORAVEL and DAO spectrometers-including 67 new unpublished data-we have obtained both the orbital and pulsational velocity curves of this binary Cepheid.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 10(7) yr and a Cepheid mass of 5.6 M.. 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 5.6 M.. 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 +/- 3.5R., 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 [M(V)] = -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 ZAMS fitting of the cluster. Possible causes for this discrepancy, and their implications for Cepheid distance scale calibrations, are discussed.
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    INDEPENDENT DISTANCE DETERMINATIONS TO MILKY-WAY CEPHEIDS IN OPEN CLUSTERS AND ASSOCIATIONS .2. CF CAS IN NGC-7790
    (1995) MATTHEWS, JM; GIEREN, WP; MERMILLIOD, JC; WELCH, DL
    Containing three Cepheids and an eclipsing binary, the open cluster NGC 7790 has the potential to be a Rosetta Stone for refining the zero point of the extragalactic distance scale. Unfortunately, ZAMS fitting of this relatively sparse, heavily reddened cluster is fraught with difficulty, and even modern determinations of its distance modulus based on comparable data and techniques differ by over 0.3 mag. To provide an independent calibration of the distance to NGC 7790, we have performed a surface brightness analysis of the Cepheid CF Cas (P similar or equal to 4.88 days), which is widely believed to be a member of the cluster. For this analysis, we use new high-quality radial-velocity data obtained with the DAO Radial Velocity Spectrometer, as well as archival velocities and photometry. Our new measurements increase the available set of published RV data for CF Cas by over a third. We derive the distance and radius of CF Cas to be 3130 +/- 160 pc and 40.6 +/- 2.0R., respectively. This radius is consistent with fundamental-mode pulsation for a Cepheid of this period, removing a possible ambiguity in mode identification which could bias a period-luminosity calibration incorporating CF Gas. We go on to estimate the absolute magnitude ([M(V)] similar or equal to -3.05 +/- 0.10 mag) and effective temperature (log T-eff similar or equal to 3.74 +/- 0.02) of CF Gas, although we caution that these values are sensitive to the assumed reddening (unlike our surface-brightness distance and radius). Evolutionary and Wesselink relations yield very similar values for the mass of CF Gas, near 5 M.. (C) 1995 American Astronomical Society.