Browsing by Subject "galactic cepheids"

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    Calibrating The Cepheid Period-Luminosity Relation From The Infrared Surface Brightness Technique I. The P-Factor, The Milky Way Relations, And A Universal K-Band Relation
    (2011-10) Storm, Jesper; Gieren, Wolfgang; Fouque, Pascal; Barnes, Thomas G.; Pietrzynski, G.; Nardetto, Nicolas; Weber, M.; Granzer, T.; Strassmeier, K. G.; Barnes, Thomas G.; Nicolas Nardetto
    Aims. We determine period-luminosity relations for Milky Way Cepheids in the optical and near-IR bands. These relations can be used directly as reference for extra-galactic distance determination to Cepheid populations with solar metallicity, and they form the basis for a direct comparison with relations obtained in exactly the same manner for stars in the Magellanic Clouds, presented in an accompanying paper. In that paper we show that the metallicity effect is very small and consistent with a null effect, particularly in the near-IR bands, and we combine here all 111 Cepheids from the Milky Way, the LMC and SMC to form a best relation. Methods. We employ the near-IR surface brightness (IRSB) method to determine direct distances to the individual Cepheids after we have recalibrated the projection factor using the recent parallax measurements to ten Galactic Cepheids and the constraint that Cepheid distances to the LMC should be independent of pulsation period. Results. We confirm our earlier finding that the projection factor for converting radial velocity to pulsational velocity depends quite steeply on pulsation period, p = 1.550-0.186 log(P) in disagrement with recent theoretical predictions. We find PL relations based on 70 MilkyWay fundamental mode Cepheids of M(K) = -3.33(+/- 0.09)(log(P)-1.0)-5.66(+/- 0.03), W(VI) = -3.26(+/- 0.11)(log(P)-1.0)-5.96(+/- 0.04). Combining the 70 Cepheids presented here with the results for 41 Magellanic Cloud Cepheids which are presented in an accompanying paper, we find M(K) = -3.30(+/- 0.06)(log(P) - 1.0) - 5.65(+/- 0.02), W(VI) = -3.32(+/- 0.08)(log(P) - 1.0) - 5.92(+/- 0.03). Conclusions. We delineate the Cepheid PL relation using 111 Cepheids with direct distances from the IRSB analysis. The relations are by construction in agreement with the recent HST parallax distances to Cepheids and slopes are in excellent agreement with the slopes of apparent magnitudes versus period observed in the LMC.
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    High-Resolution Spectroscopy For Cepheids Distance Determination - III. A Relation Between Gamma-Velocities And Gamma-Asymmetries
    (2008-10) Nardetto, N.; Stoekl, A.; Bersier, D.; Barnes, T. G.; Barnes, T. G.
    Context. Galactic Cepheids in the vicinity of the Sun have a residual line-of-sight velocity, or gamma-velocity, which shows a systematic blueshift of about 2 km s(-1) compared to an axisymmetric rotation model of the Milky Way. This term is either related to the space motion of the star and, consequently, to the kinematic structure of our Galaxy, or it is the result of the dynamical structure of the Cepheids' atmosphere. Aims. We aim to show that these residual gamma-velocities are an intrinsic property of Cepheids. Methods. We observed eight galactic Cepheids with the HARPS*** spectroscope, focusing specifically on 17 spectral lines. For each spectral line of each star, we computed the gamma-velocity (resp. gamma-asymmetry) as an average value of the interpolated radial velocity (resp. line asymmetry) curve. Results. For each Cepheid in our sample, a linear relation is found between the gamma-velocities of the various spectral lines and their corresponding gamma-asymmetries, showing that residual gamma-velocities stem from the intrinsic properties of Cepheids. We also provide a physical reference to the stellar gamma-velocity: it should be zero when the gamma-asymmetry is zero. Following this definition, we provide very precise and physically calibrated estimates of the gamma-velocities for all stars of our sample [in km s(-1)]:-11.3 +/- 0.3 [R TrA],-3.5 +/- 0.4 [S Cru],-1.5 +/- 0.2 [Y Sgr], 9.8 +/- 0.1 [beta Dor], 7.1 +/- 0.1 [zeta Gem], 24.6 +/- 0.4 [RZVel], 4.4 +/- 0.1 [l Car], 25.7 +/- 0.2 [RS Pup]. Finally, we investigated several physical explanations for these gamma-asymmetries like velocity gradients or the relative motion of the line-forming region compared to the corresponding mass elements. However, none of these hypotheses seems to be entirely satisfactory to explain the observations. Conclusions. To understand this very subtle gamma-asymmetry effect, further numerical studies are needed. Cepheids' atmosphere are strongly affected by pulsational dynamics, convective flows, nonlinear physics, and complex radiative transport. Hence, all of these effects have to be incorporated simultaneously and consistently into the numerical models to reproduce the observed line profiles in detail.