arXiv subjects

G. Bono

Publications and source records attributed to G. Bono.

At least 181 records · Page 10Linked to original sources

arXiv2006

The RR Lyrae distance scale from near-infrared photometry: current results

We present new observational results on the RR Lyrae K-band Period-Luminosity relation (PLK). Data on the Galactic globular clusters NGC 3201 and NGC 4590 (M68), and on the Large Magellanic Cloud cluster Reticulum are shown. We compare the observed slopes of the PLK relations for these three clusters with those predicted by pulsational and evolutionary models, finding a fair agreement. Trusting on this finding we decided to adopt these theoretical calibrations to estimate the distance to the target clusters,finding a good agreement with optical-based RR Lyrae distances, but with a smaller formal scatter.

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arXiv2005

The Chemical Composition of Cepheids in the Milky Way and the Magellanic Clouds

We have measured the elemental abundances of 68 Galactic and Magellanic Cepheids from FEROS and UVES high-resolution and high signal-to-noise spectra in order to establish the influence of the chemical composition on the properties of these stars (see Romaniello et al. 2005). Here we describe the robust analytical procedure we have developed to accurately determine them. The resulting iron abundances span a range between ~ -0.80 dex for stars in the Small Magellanic Cloud and ~ +0.20 dex for the most metal-rich ones in the Galaxy. While the average values for each galaxy are in good agreement with non-pulsating stars of similar age, Cepheids display a significant spread. Thus it is fundamental to measure the metallicity of individual stars.

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arXiv2005

The Metallicity Dependence of the Cepheid Period-Luminosity Relation

We have assessed the influence of the stellar iron content on the Cepheid Period-Luminosity (PL) relation by relating the V band residuals from the Freedman et al (2001) PL relation to [Fe/H] for 68 Galactic and Magellanic Cloud Cepheids. The iron abundances were measured from FEROS and UVES high-resolution and high signal-to-noise optical spectra. Our data indicate that the stars become fainter as metallicity increases, until a plateau or turnover point is reached at about solar metallicity. This behavior appears at odds both with the PL relation being independent from iron abundance and with Cepheids becoming monotonically brighter as metallicity increases (e.g. Kennicutt et al 1998, Sakai et al 2004).

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arXiv2005

Reddening distribution across the center of the globular cluster Omega Centauri

We present new medium-band uvby Stromgren and broad-band VI photometry for the central regions of the globular cluster Omega Cen. From this photometry we have obtained differential reddening estimates relative to two other globular clusters (M13 and NGC288) using a metallicity-independent, reddening-free temperature index, [c]=(u-v)-(v-b) - 0.2(b-y), for hot horizontal-branch (HB) stars (T_e> 8,500K). We estimate color excesses of these hot HB stars using optical and near-infrared colors, and find clumpy extinction variations of almost a factor of two within the area of the cluster core. In particular, the greatest density of more highly reddened objects appears to be shifted along the right ascension axis when compared with less reddened ones. These findings complicate photometric efforts to investigate the star formation history of Omega Cen.

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arXiv2005

Near-Infrared Observations of RR Lyrae Variables in $ω$ Centauri

We present Near-Infrared (NIR) $J$ and $K_s$-band observations for 181 RR Lyrae stars in the Galactic Globular Cluster $ω$ Cen. The comparison between predicted and empirical slopes of NIR Period-Luminosity (PL) relations indicates a very good agreement. Cluster distance estimates based on NIR PL relations agree quite well with recent determinations based on different standard candles, giving a true mean distance modulus $μ= 13.71\pm0.05$, and $d=5.52\pm0.13$ kpc.

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arXiv2005

Classical Cepheids as Age Indicators

Theoretical Period-Age and Period-Age-Color relations for different chemical compositions have been recently derived using an updated homogeneous set of evolutionary and pulsational models. We apply these relations to Cepheids in the Magellanic Clouds to constrain the recent star formation history of these dwarf galaxies. Finally, we also compare the radial distribution of classical Cepheids with young clusters and star forming regions.

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arXiv2005

The Carina dSph galaxy: where is the edge?

Recent cosmological N-body simulations suggest that current empirical estimates of tidal radii in dSphs might be underestimated by at least one order of magnitude. To constrain the plausibility of this theoretical framework, we undertook a multiband (U,B,V,I) survey of the Carina dSph. Deep B,V data of several fields located at radial distances from the Carina center ranging from 0.5 to 4.5 degrees show a sizable sample of faint blue objects with the same magnitudes and colors of old, Turn-Off stars detected across the center. We found that the (U-V,B-I) color-color plane is a robust diagnostic to split stars from background galaxies. Unfortunately, current U,I-band data are too shallow to firmly constrain the real extent of Carina.

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arXiv2005

Spectroscopic Survey of the Galaxy with Gaia II. The expected science yield from the Radial Velocity Spectrometer

The Gaia mission is designed as a Galaxy explorer, and will measure simultaneously, in a survey mode, the five or six phase space parameters of all stars brighter than 20th magnitude, as well as providing a description of their astrophysical characteristics. These measurements are obtained by combining an astrometric instrument with micro-arcsecond capabilities, a photometric system giving the magnitudes and colours in 15 bands and a medium resolution spectrograph named the Radial Velocity Spectrometer (RVS). The latter instrument will produce spectra in the 848 to 874 nm wavelength range, with a resolving power R = 11 500, from which radial velocities, rotational velocities, atmospheric parameters and abundances can be derived. A companion paper (Katz et al. 2004) presents the characteristics of the RVS and its performance. This paper details the outstanding scientific impact of this important part of the Gaia satellite on some key open questions in present day astrophysics. The unbiased and simultaneous acquisition of multi-epoch radial velocities and individual abundances of key elements in parallel with the astrometric parameters is essential for the determination of the dynamical state and formation history of our Galaxy. Moreover, for stars brighter than V=15, the resolving power of the RVS will give information about most of the effects which influence the position of a star in the Hertzsprung-Russell diagram, placing unprecedented constraints on the age, internal structure and evolution of stars of all types. Finally, the RVS multi-epoch observations are ideally suited to the identification, classification and characterisation of the many types of double, multiple and variable stars.

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arXiv2005

Pulsation and evolutionary masses of classical Cepheids. I. Milky Way variables

We investigate a selected sample of Galactic classical Cepheids with available distance and reddening estimates in the framework of the theoretical scenario provided by pulsation models, computed with metal abundance Z=0.02, helium content in the range of Y=0.25 to 0.31, and various choices of the stellar mass and luminosity. After transforming the bolometric light curve of the fundamental models into BVRIJK magnitudes, we derived analytical relations connecting the pulsation period with the stellar mass, the mean (intensity-averaged) absolute magnitude, and the color of the pulsators. These relations are used together with the Cepheid observed absolute magnitudes in order to determine the "pulsation" mass -M_p- of each individual variable. The comparison with the "evolutionary" masses -M_(e,can)- given by canonical (no convective core overshooting, no mass-loss) models of central He-burning stellar structures reveals that the M_p/M_(e,can) ratio is correlated with the Cepheid period, ranging from about 0.8 at logP=0.5 to about 1 at logP=1.5. We discuss the effects of different input physics and/or assumptions on the evolutionary computations, as well as of uncertainties in the adopted Cepheid metal content, distance, and reddening. Eventually, we find that the pulsational results can be interpreted in terms of mass-loss during or before the Cepheid phase, whose amount increases as the Cepheid original mass decreases. It vanishes around 13M_(sun) and increases up to about 20% at 4M_(sun).

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arXiv2005

On the Horizontal Branch of the Galactic Globular NGC2808

We present new UV (F218W) data for stars in the central region of the Galactic globular cluster NGC2808, collected with the WFPC2 camera on board of the Hubble Space Telescope. These data together with F439W and F555W-band data and previous ground based observations provide a multifrequency coverage of the cluster stellar population extending up to a distance of 1.7 times the cluster core radius. We discuss this complete sample of stars, which includes 764 Red Giant Branch (RGB) stars brighter than the Horizontal Branch (HB) luminosity level, 1239 HB stars, 119 Asymptotic Giant Branch (AGB), and 22 AGB-manque' stellar structures. As already known, we find that blue HB stars separate into three distinct groups. However, our multiband photometry indicates that several stars in the two hotter HB groups show a flat spectrum, thus suggesting the binarity of these objects. Artificial star experiments suggest that at most 50% of them might be photometric blends. Moreover, at variance with previous claims one finds that canonical Zero Age Horizontal Branch (ZAHB) models do reach effective temperatures typical of observed hot HB stars. We also show that the ratio between HB and RGB stars brighter than the HB luminosity level steadly increases when moving from the cluster center to the periphery, passing from R=1.37 +/- 0.14 in the cluster core to R=1.95 +/- 0.26 in the outer cluster regions. We discuss the possible origin of such a radial gradient in the context of the Blue Tails phenomenon, advancing some suggestions concerning the clumpy stellar distribution along the HB.

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arXiv2005

Limit-cycle behavior in one-zone convective models

We present the results of a detailed set of one-zone models that account for the coupling between pulsation and convection following the original prescriptions of Stellingwerf (1986). Motivated by the arbitrary nature of the input parameters adopted in this theoretical framework, we computed several sequences of models that cover a substantial fraction of the parameter space and a longer integration time. We also included a turbulent pressure term and found that this physical mechanism plays a crucial role in the pulsation characteristics of the models by removing the sharp discontinuities along the light and the velocity curves showed by models that do not account for turbulent pressure. Finally, we investigated the vibrational and the pulsational stability of completely convective models. We consider as the most important finding of the present work the identification of a well-defined region in the parameter space where they approach limit-cycle stability. Several numerical experiments performed by adopting different values of the adiabatic exponent and of the shell thickness indicate that the coupling between pulsation and convection is the key driving mechanism for LPVs, a finding supported by recent theoretical predictions.

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arXiv2005

Predicted and empirical radii of RR Lyrae stars

We present new theoretical Period-Radius-Metallicity (PRZ) relations for RR Lyrae stars. Current predictions are based on a large set of nonlinear, convective models that cover a broad range of chemical abundances and input parameters. We also provide new and homogeneous estimates of angular diameters for a sample of field RR Lyrae stars using a recent calibration of the Barnes-Evans surface brightness relation. Predicted and empirical radii are, within the errors, in reasonable agreement, but in the short-period range the latter present a larger scatter. As a working hypothesis we suggest that this discrepancy might be due to the occurrence either of nonlinear features such as bumps or a steep rising branch. New distance determination for RR Lyr itself is in very good agreement with HST trigonometric parallax and with pulsation parallax.

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arXiv2005

Near-Infrared Observations of RR Lyrae variables in Galactic Globular Clusters: I. The case of M92

We present near-infrared J,H, and K-band time series observations of the Galactic Globular Cluster (GGC) M92. On the basis of these data, we derived well-sampled light curves for eleven out of the seventeen cluster RR Lyrae variables, and in turn, accurate mean near-infrared (NIR) magnitudes. The comparison between predicted and empirical slopes of NIR Period-Luminosity (PL) relations indicates a very good agreement. Cluster distance determinations based on independent theoretical NIR $PL$ relations present uncertainties smaller than 5% and agree quite well with recent distance estimates based on different distance indicators. We also obtained accurate and deep NIR color-magnitude diagrams (CMDs) ranging from the tip of the Red Giant Branch (RGB) down to the Main Sequence Turn-Off. We detected the RGB bump and the NIR luminosities of this evolutionary feature are, within theoretical and empirical uncertainties, in good agreement with each other.

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arXiv2005

On the anomalous red giant branch of the globular cluster Omega Cen

We present three different optical and near-infrared (NIR) data sets for evolved stars in the Galactic Globular Cluster Omega Cen The comparison between observations and homogeneous sets of stellar isochrones and Zero-Age Horizontal Branches provides two reasonable fits. Both of them suggest that the so-called anomalous branch has a metal-intermediate chemical composition (-1.1 < [Fe/H] < -0.8) and is located ~500 pc beyond the bulk of Omega Cen stars. These findings are mainly supported by the shape of the subgiant branch in four different color-magnitude diagrams (CMDs). The most plausible fit requires a higher reddening, E(B-V)=0.155 vs. 0.12, and suggests that the anomalous branch is coeval, within empirical and theoretical uncertainties, to the bulk of Omega Cen stellar populations. This result is supported by the identification of a sample of faint horizontal branch stars that might be connected with the anomalous branch. Circumstantial empirical evidence seems to suggest that the stars in this branch form a clump of stars located beyond the cluster.

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arXiv2005

Discovery of more than two thousand white dwarfs in the globular cluster Omega Centauri

We present deep multiband (F435W, F625W, and F658N) photometric data of the Globular Cluster Omega Cen collected with the Advanced Camera for Surveys on board of the Hubble Space Telescope. We identified in the (F435W-F625W, F435W) plane more than two thousand White Dwarf (WD) candidates using three out of nine available pointings. Such a large sample appears in agreement with predictions based on the ratio between WD and Horizontal Branch (HB) evolutionary lifetimes. We also detected ~ 1600 WDs in the (F658N-F625W, F625W) plane, supporting the evidence that a large fraction of current cluster WDs are $H_α$ bright.

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arXiv2005

White Dwarfs in Omega Centauri: Preliminary Evidence

We present accurate and deep multiband B,R,Halpha data for the globular cluster Omega Cen collected with the Advanced Camera for Surveys on HST. The photometric catalogue includes more than one million stars. By adopting severe selection criteria we identified more than 600 bona fide White Dwarfs (WDs). Empirical evidence suggests that a small sample of WDs are Halpha-bright. The comparison between WD isochrones and observations shows a reasonable agreement at fainter magnitudes and a mismatch at the brighter ones.

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arXiv2004

Variable stars as standard candles and stellar tracers

We review the current status of classical Cepheid and RR Lyrae distance scales. We discuss the most recent theoretical and empirical findings together with pros and cons of different methods. The use of Cepheids and RR Lyrae stars as stellar tracers are also discussed. Preliminary findings concerning the impact that GAIA will have on the calibration of distance determinations and astrophysical parameters are also presented.

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arXiv2004

Classical Cepheid Pulsation Models. X. The Period-Age Relation

We present new Period-Age (PA) and Period-Age-Color (PAC) relations for fundamental and first overtone classical Cepheids. Current predictions rely on homogeneous sets of evolutionary and pulsation models covering a broad range of stellar masses and chemical compositions. We found that PA and PAC relations present a mild dependence upon metal content. Moreover, the use of different PA and PAC relation for fundamental and first overtone Cepheids improves the accuracy of age estimates in the short-period (log P < 1) range (old Cepheids), because they present smaller intrinsic dispersions. At the same time, the use of the PAC relations improves the accuracy in the long-period (log P >= 1) range (young Cepheids), since they account for the position of individual objects inside the instability strip. We performed a detailed comparison between evolutionary and pulsation ages for a sizable sample of LMC (15) and SMC (12) clusters which host at least two Cepheids. In order to avoid deceptive uncertainties in the photometric absolute zero-point, we adopted the homogeneous set of B,V,I data for clusters and Cepheids collected by OGLE. We also adopted the same reddening scale. The different age estimates agree at the level of 20 % for LMC clusters and of 10 % for SMC clusters. We also performed the same comparison for two Galactic clusters (NGC6067, NGC7790) and the difference in age is smaller than 20 %. These findings support the use of PA and PAC relations to supply accurate estimates of individual stellar ages in the Galaxy and in external Galaxies. The main advantage of this approach is its independence from the distance.

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