Cepheid theoretical models and observations in HST/WFC3 filters: the effect on the Hubble constant Ho

We present a complete theoretical scenario for classical Cepheids in the most commonly used HST/WFC3 filters, going from optical (F555W, F606W and F814W) to near-infrared (F160W) bands. The importance of such a study is related to the recent release of new classical Cepheids observed with HST/WFC3 in 8 distant galaxies where SNIa are hosted. These observations have posed sound constraints to the current distance scale with uncertainties on the Hubble constant Ho smaller than 3%. Our models explore a large range of metallicity and Helium content, thus providing a robust and unique theoretical tool for describing these new and future HST/WFC3 observations. As expected, the Period-Luminosity (PL) relation in F160W filter is linear and slightly dependent on the metallicity when compared with optical bands, thus it seems the most accurate tool to constrain extragalactic distances with Cepheids. We compare the pulsation properties of Cepheids observed with HST/WFC3-IR with our theoretical scenario and we discuss the agreement with the predicted Instability Strip for all the investigated galaxy samples including the case of NGC4258. Finally, adopting our theoretical F160W PL relation for Z=0.02 and log P>1.0, we derive new distance moduli. In particular, for NGC 4258, we derive a distance modulus mu0 = 29.345 +- 0.004 mag with a sigma = 0.34 mag, which is in very good agreement with the geometrical maser value. Moreover, using the obtained distance moduli, we estimate the Hubble constant value, Ho=76.0 +- 1.9 km s-1 Mpc-1 in excellent agreement with the most recent literature values.Comment: 13 pages, 9 figures, 4 tables, MNRAS, accepte

Wide-field multi-color photometry of the Galactic globular cluster NGC 1261

(Abriged)This work studies in more detail the stellar population, including its photometric properties and characteristics, in the rarely studied southern Galactic globular cluster NGC 1261. We focus on the brighter sequences of the cluster's color-magnitude diagram (CMD). Like in our previous works, we rely upon photometry in several passbands to achieve more reliable results and conclusions. We carried out and analyzed new multi-color photometry of NGC 1261 in UBVI reaching below the turnoff point in all passbands in a fairly extended cluster field, about 14'x14'. We found several signs of the inhomogeneity ("multiplicity") in the stellar population. The most prominent of them are: (1) the dependence of the radial distribution of sub-giant branch (SGB) stars in the cluster on their U magnitude, with brighter stars less centrally concentrated at the 99.9 \% level than their fainter counterparts; (2) the dependence of the location of red giant branch (RGB) stars in the U-(U-B) CMD on their radial distance from the cluster center, with the portion of stars bluer in the (U-B) color increasing towards the cluster outskirts. Additionally, the radial variation of the RGB luminosity function in the bump region is suspected. We assume that both the SGB stars brighter in the U and the RGB stars bluer in the (U-B) color are probably associated with blue horizontal branch stars, because of a similarity in their radial distribution in the cluster. We estimated the metalicity of NGC 1261 from the slope of the RGB in U-based CMDs and the location of the RGB bump on the branch. These metallicity indicators give [Fe/H]zw = -1.34 +/- 0.16 dex and [Fe/H]zw = -1.41 +/- 0.10 dex, respectively. We isolated 18 probable blue straggler candidates. They are more centrally concentrated than the lower red giants of comparable brightness at the 97.9 \% level.Comment: 8 pages, 9 figures, accepted for publication in A&

Uncertainties on the theoretical predictions for classical Cepheid pulsational quantities

The expected distribution of Cepheids within the instability strip is affected by several model inputs, reflecting upon the predicted Period-Luminosity relation. On the basis of new and updated sets of evolutionary and pulsational models, we quantitatively evaluated the effects on the theoretical PL relation of current uncertainties on the chemical abundances of Cepheids in the Large Magellanic Cloud and on several physical assumptions adopted in the evolutionary models. We analysed how the different factors influence the evolutionary and pulsational observables and the resulting PL relation. As a result, we found that present uncertainties on the most relevant H and He burning reaction rates do not influence in a relevant way the loop extension in temperature. On the contrary, current uncertainties on the LMC chemical composition significantly affect the loop extension and also reflect in the morphology of the instability strip; however their influence on the predicted pulsational parameters is negligible. We also discussed how overshooting and mass loss influence the ML relation and the pulsational parameters. In summary, the present uncertainties on the physical inputs adopted in the evolutionary codes and in the LMC chemical composition are negligible for the prediction of the main pulsational properties; the inclusion of overshooting in the previous H burning phase and/or of mass loss is expected to significantly change the resulting theoretical pulsational scenario for Cepheids, as well as the calibration of their distance scale. These systematic effects are expected to influence the theoretical Cepheid calibration of the secondary distance indicators and in turn the resulting evaluation of the Hubble constant.Comment: accepted for publication on A&

The intermediate age open cluster NGC 2660

We present CCD UBVI photometry of the intermediate old open cluster NGC2660, covering from the red giants region to about seven magnitudes below the main sequence turn-off. Using the synthetic Colour - Magnitude Diagram method, we estimate in a self-consistent way values for distance modulus ((m-M)0 ~= 12.2), reddening (E(B-V) ~= 0.40), metallicity ([Fe/H] about solar), and age (age ~ 1 Gyr). A 30% population of binary stars turns out to be probably present.Comment: 12 pages, 8 (encapsulated) figures, to be published on MNRA