# Browsing by Subject "stars: variables: cepheids"

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Item 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 NardettoShow more 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.Show more Item Calibrating The Cepheid Period-Luminosity Relation From The Infrared Surface Brightness Technique II. The Effect Of Metallicity And The Distance To The LMC(2011-10) Storm, Jesper; Gieren, Wolfgang; Fouque, Pascal; Barnes, Thomas G.; Soszynski, I.; Pietrzynski, G.; Nardetto, Nicolas; Queloz, D.; Barnes, Thomas G.; Nicolas NardettoShow more Context. The extragalactic distance scale builds directly on the Cepheid period-luminosity (PL) relation as delineated by the sample of Cepheids in the Large Magellanic Cloud (LMC). However, the LMC is a dwarf irregular galaxy, quite different from the massive spiral galaxies used for calibrating the extragalactic distance scale. Recent investigations suggest that not only the zero-point but also the slope of the Milky Way PL relation differ significantly from that of the LMC, casting doubts on the universality of the Cepheid PL relation. Aims. We want to make a differential comparison of the PL relations in the two galaxies by delineating the PL relations using the same method, the infrared surface brightness method (IRSB), and the same precepts. We furthermore extend the metallicity baseline for investigating the zero-point dependence, by applying the method to five SMC Cepheids as well. Methods. The IRSB method is a Baade-Wesselink type method to determine individual distances to Cepheids. We apply a newly revised calibration of the method as described in an accompanying paper (Paper I) to 36 LMC and five SMC Cepheids and delineate new PL relations in the V, I, J, & K bands as well as in the Wesenheit indices in the optical and near-IR. Results. We present 509 new and accurate radial velocity measurements for a sample of 22 LMC Cepheids, enlarging our earlier sample of 14 stars to include 36 LMC Cepheids. The new calibration of the IRSB method is directly tied to the recent HST parallax measurements to ten MilkyWay Cepheids, and we find a LMC barycenter distance modulus of 18.45 +/- 0.04 (random error only) from the 36 individual LMC Cepheid distances. In the J, K bands we find identical slopes for the LMC and Milky Way PL relations and only a weak letallicity effect on the zero points (consistent with a zero effect), metal poor stars being fainter. In the optical we find the Milky Way slopes are slightly shallower than the LMC slopes (but again consistent with no difference in the slopes) and small effects on the zero points. However, the important Wesenheit index in V, (V - I) shows a metallicity effect on the slope and on the zero point which is likely to be significant. Conclusions. We find a significant metallicity effect on the W-VI index gamma(W-VI) = -0.23 +/- 0.10 mag dex(-1) as well as an effect on the slope. The K-band PL relation on the other hand is found to be an excellent extragalactic standard candle being metallicity insensitive in both slope and zero-point and at the same time being reddening insensitive and showing the least internal dispersion.Show more