Browsing by Author "Fouque, P"

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    An improved calibration of Cepheid visual and infrared surface brightness relations from accurate angular diameter measurements of cool giants and supergiants
    (1997) Fouque, P; Gieren, WP
    We have calibrated optical and near-infrared surface brightness - colour relations for cool giant and supergiant stars using high-precision angular diameters of these stars determined from Michelson interferometry. We find that the giant and supergiant relations are undistinguishable over a wide range of intrinsic colours. We independently determine the slopes of these relations obeyed by Cepheid variables and find that in all the diagrams considered, these agree very well with the slopes derived from the stable giants and supergiants. Forcing the slopes to the values derived from the Cepheids, we determine a very precise value of the zero point of the surface brightness - colour relations valid for Cepheid variables, which is 3.947 +/- 0.003. This value is in agreement with the one derived from the Cepheid effective temperature scale of Pel (1978), and from the lunar occultation angular diameter of the Cepheid zeta Gem (Ridgway et al. 1982).
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    Cepheid period-radius and period-luminosity relations and the distance to the large Magellanic Cloud
    (1998) Gieren, WP; Fouque, P; Gomez, M
    We have used the infrared Barnes-Evans surface brightness technique to derive the radii and distances of 34 Galactic Cepheid variables. Radius and distance results obtained from both versions of the technique are in excellent agreement. The radii of 28 variables are used to determine the period-radius (PR) relation. This relation is found to have a smaller dispersion than in previous studies, and is identical to the PR relation found by Laney & Stobie from a completely independent method, a fact which provides persuasive evidence that the Cepheid PR relation is now determined at a very high confidence level. We use the accurate infrared distances to determine period-luminosity (PL) relations in the V, I, J, H, and K passbands from the Galactic sample of Cepheids. We derive improved slopes of these relations from updated LMC Cepheid samples and adopt these slopes to obtain accurate absolute calibrations of the PL relation. By comparing these relations to the ones defined by the LMC Cepheids, we derive strikingly consistent and precise values for the LMC distance modulus in each of the passbands that yield a mean value of mu(0)(LMC) = 18.46 +/- 0.02.
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    Very accurate distances and radii of open cluster Cepheids from a near-infrared surface brightness technique
    (1997) Gieren, WP; Fouque, P; Gomez, M
    We have obtained the radii and distances of 16 galactic Cepheids supposed to be members in open clusters or associations using a new optical and two near-infrared calibrations of the surface brightness (Barnes-Evans) method. We find excellent agreement of the radii and distances produced by both infrared techniques, which use the V, V-K (K on the Carter system) and the K, J-K magnitude-color combinations, respectively, with typical random errors that are as little as similar to 2%. We discuss possible systematic errors in our infrared solutions in detail and conclude that the typical total uncertainty of the infrared distance and radius of a Cepheid is about 3% in both infrared solutions, provided that the data are of excellent quality and that the amplitude of the color curve used in the solution is larger than similar to 0.3 mag. The optical V, V-R distance and radius of a given Cepheid can deviate by as much as similar to 30% from the infrared value because of large systematic and random errors caused by microturbulence and gravity variations: these affect the optical but not the infrared colors.