Browsing by Author "GIEREN, WP"
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- ItemA DISTANCE TO THE CEPHEID HV-829 IN THE SMALL MAGELLANIC CLOUD(1993) BARNES, TG; MOFFETT, TJ; GIEREN, WPWe present a distance determination to the Cepheid variable HV 829 in the Small Magellanic Cloud using the surface brightness technique. The distance so obtained is independent of any other astrophysical distance scale; in particular, it is independent of the Cepheid PL (and PLC) relations. It is also independent of any assumptions concerning reddening in the SMC. The distance obtained to HV 829 is 61 +/- 6 kpc, which corresponds to a distance modulus of 18.9 +/- 0.2 mag. Because HV 829 may not lie at the centroid distance for the SMC, this may not be representative of the mean distance to the SMC, but it does provide additional evidence for the ''long distance scale'' for the SMC.
- ItemCOMPARISON OF THE OPEN CLUSTER AND SURFACE-BRIGHTNESS DISTANCE SCALES FOR GALACTIC CLASSICAL CEPHEIDS(1993) GIEREN, WP; FOUQUE, PWe have derived a new period-luminosity (PL) relation for galactic Cepheids in open clusters and associations from the Zero-Age Main-Sequence (ZAMS)-fitting method, using the most reliable cluster photometric data and Cepheid reddening data available, and homogeneous treatment of absorption corrections. For about half of the cluster Cepheid sample (17 stars) we also derive distances and absolute magnitudes from the surface-brightness method. From a comparison we find that the surface-brightness PL relation has a zero point about 0.15 mag brighter than the cluster ZAMS-fitting PL relation, but in view of the sources of systematic error involved in both methods, this discrepancy is clearly not significant. Rather, we conclude from our results that the two currently most important methods to derive distances to galactic Cepheids produce consistent results, and that neither of the two methods contain systematic errors in excess of approximately 0. 10 mag. The results of this paper imply a distance to the Large Magellanic Cloud of mu0 (LMC) = 18.65 +/- 0.10 mag. We identify four stars in our cluster/association Cepheid sample for which the discrepancies between ZAMS-fitting and surface brightness M(V) are so large ( > 0.6 mag) that serious doubt is cast on their membership to the proposed host clusters or associations. These cases deserve further study.
- ItemINDEPENDENT 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, GAs 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.
- ItemINDEPENDENT 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, DLContaining 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.
- ItemON THE DIFFICULTY OF DETERMINING THE COLOR-TERM IN THE CEPHEID PLC RELATION(1993) FOUQUE, P; GIEREN, WPWe present here a new tentative to determine the color-term of the Cepheid period - luminosity - color relation, using a large sample of 100 galactic Cepheids, whose distances are known from the visual surface brightness method (approximately 10% accuracy). The derived coefficient seems lower (between 0 and 1) than both its observed value in the Magellanic Clouds (about 2.1) and the semi-theoretical value of about 3.4. Systematic effects due to metallicity are investigated, and discarded as a possible explanation of the discrepancy. Statistical effects are reviewed, and a formalism useful to other topics is presented. Statistical corrections remove only part of the discrepancy we find. We therefore still recommend not to use the PLC relation to determine extragalactic distances, but to prefer a simple PL relation, as long as the question of the PLC color-term is not clearly resolved.
- ItemREEXAMINING THE BEAT CEPHEID TU CAS - NO 2ND OVERTONE, NO MODE SWITCHING(1992) MATTHEWS, JM; GIEREN, WP; FERNIE, JD; DINSHAW, NSimultaneous photometry and radial-velocity measurements of the double-mode Cepheid TU Cas were obtained in 1987, to check on the mode behavior of this star after a decade-long gap in the observing record. We have performed least-squares fits of these data to isolate the primary (2.14 d) and secondary (1.52 d) variations. The residuals of our solution rule out the existence of any tertiary mode with V and velocity amplitudes greater than 0.004 mag and 0.7 km s-1, respectively, contradicting Faulkner's [ApJ, 218, 209 (1977)] report of such a mode with amplitude 0.05 mag. We also do not detect a significant change in the relative amplitudes of the modes compared to previous data, cited as evidence for mode switching in this star by earlier investigators. In addition, we use the components of our fit to the fundamental and first-overtone pulsations in TU Cas as a rigorous test of various Fourier parameters which have been proposed as mode discriminants.
- ItemSURFACE BRIGHTNESS DISTANCE DETERMINATIONS TO THE LARGE MAGELLANIC CLOUD CEPHEID VARIABLES HV-899 AND HV-2257(1993) GIEREN, WPWe present new VRI light curves for the long-period LMC Cepheids HV899 and 2257 and combine these data with the CORAVEL radial velocity curves of Imbert to derive the distances to these variables using the surface brightness method. Assuming metallicities of [Fe/H]=(-0.4 +/- 0.2) for both supergiants, we find metallicity-corrected distances of (65.2 +/- 4.3) kpc for HV899 and (65.7 +/- 4.3) kpc for HV2257, which correspond to true distance moduli of (19.07 +/- 0.15) and (19.09 +/- 0.15) mag, respectively. These distance moduli are about 0.4 mag larger than those predicted for the LMC by the galactic Cepheid calibration of the surface brightness method of Gieren, Barnes & Moffett [which predicts mu0(LMC) approximately 18.65 mag]. While very low metallicities for the Cepheids ([Fe/H]less than or similar to -1.0) might explain this discrepancy, the present results provide evidence of a large distance modulus for the LMC of near 19.0.
- ItemTHE CEPHEID PERIOD-LUMINOSITY RELATION FROM INDEPENDENT DISTANCES OF 100 GALACTIC VARIABLES(1993) GIEREN, WP; BARNES, TG; MOFFETT, TJWe have determined distances to 100 galactic classical Cepheids on a homogeneous system with an improved version of the surface brightness technique, using the extensive northern Cepheid observations of Moffett and Barnes, and the southern Cepheid observations of Gieren. The period-luminosity relations in the V bandpass obtained for the northern and southern Cepheid samples are found consistent, and the absolute magnitudes have been combined into a two-hemisphere PL(V) relation which is M(V) = - 1.371 - 2.986 log P. +/- 0.095 +/- 0.094 This is the currently most precise Cepheid PL(V) relation from the surface brightness technique. Typically M(V) can be obtained from this relation for Cepheids with periods in the range 3 to 45 days to better than +/-0.06 mag, if all the scatter in the relation is observational, or +/-0.27 mag, if all the scatter is cosmic in origin.
- ItemTHE DISTANCE TO THE LARGE MAGELLANIC CLOUD CLUSTER NGC 1866 FROM ITS CEPHEID MEMBERS(1994) GIEREN, WP; RICHTLER, T; HILKER, MWe have derived distances to four Cepheid variables of similar periods (2.6-3.5 days) in the field of the young LMC cluster NGC 1866 using the visual surface brightness modification of the Baade-Wesselink method. For three of these variables, consistent distances are obtained which support their cluster membership and yield a true cluster distance modulus of 18.47 +/- 0.20 mag. The distances have been corrected for the nonsolar metallicity of -0.6 dex recently determined for NGC 1866 by Hilker et al. A fourth Cepheid, HV 12204, is shown to have a bright blue companion star which invalidates the Baade-Wesselink analysis; this Cepheid is likely to be a cluster member in a spectroscopic binary system.