G. Bono

An eclipsing-binary distance to the Large Magellanic Cloud accurate to two per cent.

In the era of precision cosmology, it is essential to determine the Hubble constant to an accuracy of three per cent or better. At present, its uncertainty is dominated by the uncertainty in the distance to the Large Magellanic Cloud (LMC), which, being our second-closest galaxy, serves as the best anchor point for the cosmic distance scale. Observations of eclipsing binaries offer a unique opportunity to measure stellar parameters and distances precisely and accurately. The eclipsing-binary method was previously applied to the LMC, but the accuracy of the distance results was lessened by the need to model the bright, early-type systems used in those studies. Here we report determinations of the distances to eight long-period, late-type eclipsing systems in the LMC, composed of cool, giant stars. For these systems, we can accurately measure both the linear and the angular sizes of their components and avoid the most important problems related to the hot, early-type systems. The LMC distance that we derive from these systems (49.97 ± 0.19 (statistical) ± 1.11 (systematic) kiloparsecs) is accurate to 2.2 per cent and provides a firm base for a 3-per-cent determination of the Hubble constant, with prospects for improvement to 2 per cent in the future.

On the Helium Content of Galactic Globular Clusters via the R-Parameter*

We estimate the empirical R-parameter in 26 Galactic globular clusters covering a wide metallicity range, imaged by Wide Field Planetary Camera 2 on board the Hubble Space Telescope. The improved spatial resolution permits a large fraction of evolved stars to be measured and permits accurate assessment of radial population gradients and completeness corrections. In order to evaluate both the He abundance and the He-to-metal enrichment ratio, we construct a large set of evolutionary models by adopting similar metallicities and different He contents. We find an absolute He abundance that is lower than that estimated from spectroscopic measurements in H II regions and from primordial nucleosynthesis models. This discrepancy could be removed by adopting a 12C(α,γ)16O nuclear cross section about a factor of 2 smaller than the canonical value, although different assumptions for mixing processes also can introduce systematic effects. The trend in the R-parameter toward solar metallicity is consistent with an upper limit to the He-to-metal enrichment ratio of the order of 2.5. Detailed calculations of central He burning times as a function of the horizontal-branch (HB) morphology suggest that He lifetimes for hot HB stars are on average ≈20% longer than for RR Lyrae and red HB stars. Therefore, the increase in the empirical R-values of metal-poor clusters characterized by blue HB morphologies is due to an increase in the HB lifetime and not due to an increase in the He abundance.

Comparison between Predicted and Empirical ΔVHBbump in Galactic Globular Clusters

We present observational estimates of ΔV in a sample of 28 galactic globular clusters (GGCs) observed by the Hubble Space Telescope. The photometric accuracy and the sizable number of stars measured in each cluster allowed us to single out the red giant branch bump both in metal-poor and in metal-rich GGCs. Empirical values are compared with homogeneous theoretical predictions that account for both H- and He-burning phases over a wide range of metal abundances (0.0001<Z<0.02). We found that, within current observational uncertainties on both iron and α-element abundances, theory and observations are in very good agreement, provided that the metallicity scale by Carretta & Gratton as extended by Cohen et al. is adopted. Interestingly enough, we also found that both theoretical and observed values show a change in the slope of the ΔV-[M/H] relation toward higher metal contents.

Improving the Mass Determination of Galactic Cepheids

We have selected a sample of Galactic Cepheids for which accurate estimates of radii, distances, and photometric parameters are available. The comparison between their pulsation masses, based on new period-mass-radius (PMR) relations, and their evolutionary masses, based on both optical and NIR color-magnitude (CM) diagrams, suggests that pulsation masses are on average of the order of 10% smaller than the evolutionary masses. Current pulsation masses show, at fixed radius, a strongly reduced dispersion when compared with values published in the literature. The increased precision in the pulsation masses is due to the fact that our predicted PMR relations based on nonlinear, convective Cepheid models present smaller standard deviations than PMR relations based on linear models. At the same time, the empirical radii of our Cepheid sample are typically accurate at the 5% level. Our evolutionary mass determinations are based on stellar models constructed by neglecting the effect of mass loss during the He burning phase. Therefore, the difference between pulsation and evolutionary masses could be intrinsic and does not necessarily imply a problem with either evolutionary and/or nonlinear pulsation models. The marginal evidence of a trend in the difference between evolutionary and pulsation masses when moving from short- to long-period Cepheids is also briefly discussed. The main finding of our investigation is that the long-standing Cepheid mass discrepancy seems now resolved at the 10% level either if we account for canonical or mild convective core overshooting evolutionary models.

Star Counts across the Red Giant Branch Bump and Below

We present a new observable?Rbump?which is the ratio between the star counts across the red giant branch (RGB) bump and fainter RGB stars to investigate the occurrence of a deep-mixing phenomenon during these evolutionary phases. The comparison between predicted and empirical Rbump-values, based on a large and homogeneous set of Hubble Space Telescope data, brings out that evolutionary lifetimes predicted by canonical RGB models do account for the bulk of Galactic globular clusters included in our sample (29). This evidence suggests that bump and fainter RGB stars do not show the occurrence of deep mixing, which significantly changes their chemical stratification. A few possible exceptions to this general rule are briefly discussed.

On the Delta V_HB_bump parameter in Globular Clusters

We present new empirical estimates of the DELTAV {sup bump}{sub HB} parameter for 15 Galactic globular clusters (GGCs) using accurate and homogeneous ground-based optical data. Together with similar evaluations available in the literature, we ended up with a sample of 62 GGCs covering a very broad range in metal content (-2.16 dex = 0), might be systematically smaller than predicted.

Linear Nonadiabatic Properties of SX Phoenicis Variables

We present a detailed linear, nonadiabatic pulsational scenario for oscillating blue stragglers (BSs)/SX Phe variables in Galactic globular clusters (GGCs) and in Local Group (LG) dwarf galaxies. The sequences of models were constructed by adopting a wide range of input parameters and properly cover the region of the H-R diagram in which these objects are expected to be pulsationally unstable. Current calculations together with more metal-rich models already presented by Gilliland et al. suggest that the pulsation properties of SX Phe variables are partially affected by metal content. In fact, the pulsation periods for the first three modes are marginally affected when moving from Z = 0.0001 to 0.006, whereas the hot edges of the instability region move toward cooler effective temperatures by approximately 300-500 K. The inclusion of a metallicity term in the period-luminosity-color (P-L-C) relations causes a substantial decrease in the intrinsic scatter and in the individual error of the coefficients. This supports the result recently brought out by Petersen & Christensen-Dalsgaard for δ Scuti stars. Moreover, we find that the discrepancy between our relation and similar theoretical and empirical relations available in the literature is typically smaller than 5%. The comparison between theory and observations in the MV- log P plane as well as in the luminosity amplitude-log P plane does not help to disentangle the long-standing problem of mode identification among SX Phe stars. However, our calculations suggest that the secular period change seems to be a good observable to identify the pulsation mode of cooler SX Phe variables. Together with the previous models we also constructed new sequences of models by adopting selected effective temperatures and luminosities along two evolutionary tracks characterized by the same mass value and metal content (M/M☉ = 1.2, Z = 0.001) but different He contents in the envelope, namely, Y = 0.23 and 0.30. The He content in the latter track was artificially enhanced soon after the central H exhaustion to mimic, with a crude approximation, the collisional merging between two stars. Interestingly enough, we find that the He-enhanced models present an increase in the pulsation period and a decrease in the total kinetic energy of the order of 20% when compared with the canonical ones. At the same time, the blue edge of the fundamental mode for the He-enhanced models is approximately 1000 K cooler than for canonical ones. Moreover, we find that the secular period change for He-enhanced models is approximately a factor of 2 larger than for canonical ones. According to this evidence, we suggest that the pulsation properties of SX Phe variables can be soundly adopted to constrain the evolutionary history of BSs and in turn to single out the physical mechanisms that trigger their formation.

Constraints on the formation of the globular cluster IC 4499 from multiwavelength photometry

We present new multiband photometry for the Galactic globular cluster IC 4499 extending well past the main-sequence turn-off in the U, B, V, R, I and DDO 51 bands. This photometry is used to determine that IC 4499 has an age of 12 ± 1 Gyr and a cluster reddening of E(B ― V) = 0.22 ± 0.02. Hence, IC 4499 is coeval with the majority of Galactic globular clusters, in contrast to suggestions of a younger age. The density profile of the cluster is observed to not flatten out to at least r ∼ 800 arcsec, implying that either the tidal radius of this cluster is larger than previously estimated, or that IC 4499 is surrounded by a halo. Unlike the situation in some other, more massive, globular clusters, no anomalous colour spreads in the ultraviolet are detected among the red giant branch stars. The small uncertainties in our photometry should allow the detection of such signatures apparently associated with variations of light elements within the cluster, suggesting that IC 4499 consists of a single stellar population.

The Araucaria project: A study of the classical cepheid in the eclipsing binary system OGLE LMC562.05.9009 in the large magellanic cloud

We present a detailed study of the classical Cepheid in the double-lined, highly eccentric eclipsing binary system OGLE-LMC562.05.9009. The Cepheid is a fundamental mode pulsator with a period of 2.988 days. The orbital period of the system is 1550 days. Using spectroscopic data from three 4–8-m telescopes and photometry spanning 22 years, we were able to derive the dynamical masses and radii of both stars with exquisite accuracy. Both stars in the system are very similar in mass, radius, and color, but the companion is a stable, non-pulsating star. The Cepheid is slightly more massive and bigger (M1=3.70±0.03 Me, R1=28.6±0.2 Re) than its companion (M2=3.60±0.03 Me, R2=26.6±0.2 Re). Within the observational uncertainties both stars have the same effective temperature of 6030±150 K. Evolutionary tracks place both stars inside the classical Cepheid instability strip, but it is likely that future improved temperature estimates will move the stable giant companion just beyond the red edge of the instability strip. Within current observational and theoretical uncertainties, both stars fi to n a 205 Myr isochrone arguing for their common age. From our model, we determine a value of the projection factor of p=1.37±0.07 for the Cepheid in the OGLE-LMC562.05.9009 system. This is the second Cepheid for which we could measure its p-factor with high precision directly from the analysis of an eclipsing binary system, which represents an important contribution toward a better calibration of Baade-Wesselink methods of distance determination for Cepheids.

On the Pulsation Mode Identification of Short-Period Galactic Cepheids

We present new theoretical period-radius relations for first-overtone Galactic Cepheids. Current predictions are based on several sequences of nonlinear, convective pulsation models at solar chemical composition (Y = 0.28, Z = 0.02) and stellar masses ranging from 3.0 to 5.5 M☉. The comparison between predicted and empirical radii of four short-period Galactic Cepheids suggests that QZ Normae and EV Scuti are pulsating in the fundamental mode, whereas Polaris and SZ Tauri pulsate in the first overtone. This finding supports the mode identifications that rely on the comparison between direct and period-luminosity-based distance determinations, but it is somewhat at variance with the mode identification based on Fourier parameters. In fact, we find from our models that fundamental and first-overtone pulsators attain, for periods ranging from 2.7 to 4 days, quite similar 21-values, making mode discrimination from this parameter difficult. The present mode identifications for our sample of Cepheids are strengthened by the accuracy of their empirical radius estimates as well as by the evidence that predicted fundamental and first-overtone radii do not show, within the current uncertainty on the mass-luminosity relation, any degeneracy in the same period range. Accurate radius determinations are therefore an excellent tool to unambiguously determine the pulsation modes of short-period Cepheids.