arXiv subjects

G. Bono

Publications and source records attributed to G. Bono.

At least 91 records · Page 5Linked to original sources

arXiv2015

On a new theoretical framework for RR Lyrae stars I: the metallicity dependence

We present new nonlinear, time-dependent convective hydrodynamical models of RR Lyrae stars computed assuming a constant helium-to-metal enrichment ratio and a broad range in metal abundances (Z=0.0001--0.02). The stellar masses and luminosities adopted to construct the pulsation models were fixed according to detailed central He burning Horizontal Branch evolutionary models. The pulsation models cover a broad range in stellar luminosity and effective temperatures and the modal stability is investigated for both fundamental and first overtones. We predict the topology of the instability strip as a function of the metal content and new analytical relations for the edges of the instability strip in the observational plane. Moreover, a new analytical relation to constrain the pulsation mass of double pulsators as a function of the period ratio and the metal content is provided. We derive new Period-Radius-Metallicity relations for fundamental and first-overtone pulsators. They agree quite well with similar empirical and theoretical relations in the literature. From the predicted bolometric light curves, transformed into optical (UBVRI) and near-infrared (JHK) bands, we compute the intensity-averaged mean magnitudes along the entire pulsation cycle and, in turn, new and homogenous metal-dependent (RIJHK) Period-Luminosity relations. Moreover, we compute new dual and triple band optical, optical--NIR and NIR Period-Wesenheit-Metallicity relations. Interestingly, we find that the optical Period-W(V,B-V) is independent of the metal content and that the accuracy of individual distances is a balance between the adopted diagnostics and the precision of photometric and spectroscopic datasets.

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arXiv2015

Type II Cepheids in the Milky Way disc. Chemical composition of two new W Vir stars: DD Vel and HQ Car

A robust classification of Cepheids into their different sub-classes and, in particular, between classical and Type II Cepheids, is necessary to properly calibrate the period-luminosity relations and for populations studies in the Galactic disc. Type II Cepheids are, however, very diverse, and classifications based either on intrinsic (period, light curve) or external parameters (e.g., [Fe/H], |z|) do not provide a unique classification. We want to ascertain the classification of two Cepheids, HQ Car and DD Vel, that are sometimes classified as classical Cepheids and sometimes as Type II Cepheids. To achieve this goal, we examine both their chemical composition and the presence of specific features in their spectra. We find emission features in the Hα and in the 5875.64 Å He I lines that are typical of W Vir stars. The [Na/Fe] (or [Na/Zn]) abundances are typical of thick-disc stars, while BL Her stars are Na-overabundant ([Na/Fe]>+0.5 dex). Finally, the two Cepheids show a possible (HQ Car) or probable (DD Vel) signature of mild dust-gas separation that is usually observed only in long-period type II Cepheids and RV Tau stars. These findings clearly indicate that HQ Car and DD Vel are both Type II Cepheids from the W Vir sub-class. Several studies have reported an increase in the Cepheids' abundance dispersion towards the outer (thin) disc. A detailed inspection of the Cepheid classification, in particular for those located in the outer disc, will indicate whether this feature is real or simply an artefact of the inclusion of type II Cepheids belonging to the thick disc in the current samples.

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arXiv2015

New NIR light-curve templates for classical Cepheids

We present new near-infrared (NIR) light-curve templates for fundamental (FU, JHK) and first overtone (FO, J) Cepheids. The new templates together with PL and PW relations provide Cepheid distances from single-epoch observations with a precision only limited by the intrinsic accuracy of the method adopted. The templates rely on a very large set of Galactic and Magellanic Clouds (MCs) Cepheids (FU,~600; FO,~200) with well sampled NIR (IRSF data) and optical (V,I; OGLE data) light curves. To properly trace the change in the shape of the light curve as a function of period, we split the sample of calibrating Cepheids into 10 different period bins. The templates for the first time cover FO Cepheids and the FU short-period Cepheids (P<5 days). Moreover, the zero-point phase is anchored to the phase of the mean magnitude along the rising branch. The new approach has several advantages in sampling the light curve of bump Cepheids when compared with the phase of maximum light. We also provide new estimates of the NIR-to-optical amplitude ratios for FU and FO Cepheids. We perform detailed analytical fits using both 7th-order Fourier series and multi-Gaussian periodic functions. The latter are characterized by a smaller number of free parameters (9 vs 15). Mean NIR magnitudes based on the new templates are up to 80% more accurate than single-epoch measurements and up to 50% more accurate than mean magnitudes based on previous templates, with typical associated uncertainties ranging from 0.015 mag (J) to 0.019 mag (K). Moreover, the errors on individual distances of Small MC Cepheids derived from NIR PW relations, are essentially reduced to the intrinsic scatter of the adopted relations. Thus, the new templates are the ultimate tool to estimate precise Cepheid distances from NIR single-epoch observations, which can be adopted to derive the 3D structure of the MCs.

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arXiv2015

On the alpha-element gradients of the Galactic thin disk using Cepheids

We present new homogeneous measurements of Na, Al and three alpha-elements (Mg, Si, Ca) for 75 Galactic Cepheids. The abundances are based on high spectral resolution (R ~ 38,000) and high signal-to-noise ratio (S/N ~ 50-300) spectra collected with UVES at ESO VLT. The current measurements were complemented with Cepheid abundances either provided by our group (75) or available in the literature, for a total of 439 Galactic Cepheids. Special attention was given in providing a homogeneous abundance scale for these five elements plus iron (Genovali et al. 2013, 2014). In addition, accurate Galactocentric distances (RG) based on near-infrared photometry are also available for all the Cepheids in the sample (Genovali et al. 2014). They cover a large fraction of the Galactic thin disk (4.1 <= RG <= 18.4 kpc). We found that the above five elements display well defined linear radial gradients and modest standard deviations over the entire range of RG. Moreover, the [element/Fe] abundance ratios are constant across the entire thin disk; only the Ca radial distribution shows marginal evidence of a positive slope. These results indicate that the chemical enrichment history of iron and of the quoted five elements has been quite similar across the four quadrants of the Galactic thin disk. The [element/Fe] ratios are also constant over the entire period range. This empirical evidence indicates that the chemical enrichment of Galactic Cepheids has also been very homogenous during the range in age that they cover (~10-300 Myr). Once again, [Ca/Fe] vs. log(P) shows a (negative) gradient, being underabundant among youngest Cepheids. Finally, we also found that Cepheid abundances agree quite well with similar abundances for thin and thick disk dwarf stars and they follow the typical Mg-Al and Na-O correlations.

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arXiv2014

On the distance of the globular cluster M4 (NGC 6121) using RR Lyrae stars: I. optical and near-infrared Period-Luminosity and Period-Wesenheit relations

We present new distance determinations to the nearby globular M4 (NGC~6121) based on accurate optical and Near Infrared (NIR) mean magnitudes for fundamental (FU) and first overtone (FO) RR Lyrae variables (RRLs), and new empirical optical and NIR Period-Luminosity (PL) and Period-Wesenheit (PW) relations. We have found that optical-NIR and NIR PL and PW relations are affected by smaller standard deviations than optical relations. The difference is the consequence of a steady decrease in the intrinsic spread of cluster RRL apparent magnitudes at fixed period as longer wavelengths are considered. The weighted mean visual apparent magnitude of 44 cluster RRLs is $\left =13.329\pm0.001$ (standard error of the mean) $\pm$0.177 (weighted standard deviation) mag. Distances were estimated using RR Lyr itself to fix the zero-point of the empirical PL and PW relations. Using the entire sample (FU$+$FO) we found weighted mean true distance moduli of 11.35$\pm$0.03$\pm$0.05 mag and 11.32$\pm$0.02$\pm$0.07 mag. Distances were also evaluated using predicted metallicity dependent PLZ and PWZ relations. We found weighted mean true distance moduli of 11.283$\pm$0.010$\pm$0.018 mag (NIR PLZ) and 11.272$\pm$0.005$\pm$0.019 mag (optical--NIR and NIR PWZ). The above weighted mean true distance moduli agree within 1$σ$. The same result is found from distances based on PWZ relations in which the color index is independent of the adopted magnitude (11.272$\pm$0.004$\pm$0.013 mag). These distances agree quite well with the geometric distance provided by \citep{kaluzny2013} based on three eclipsing binaries. The available evidence indicates that this approach can provide distances to globulars hosting RRLs with a precision better than 2--3\%.

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arXiv2014

Kinematics of classical Cepheids in the Nuclear Stellar Disk

Classical Cepheids are useful tracers of the Galactic young stellar population because their distances and ages can be determined from their period-luminosity and period-age relations. In addition, the radial velocities and chemical abundance of the Cepheids can be derived from spectroscopic observations, providing further insights into the structure and evolution of the Galaxy. Here, we report the radial velocities of classical Cepheids near the Galactic Center, three of which were reported in 2011, the other reported for the first time. The velocities of these Cepheids suggest that the stars orbit within the Nuclear Stellar Disk, a group of stars and interstellar matter occupying a region of 200 pc around the Center, although the three-dimensional velocities cannot be determined until the proper motions are known. According to our simulation, these four Cepheids formed within the Nuclear Stellar Disk like younger stars and stellar clusters therein.

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arXiv2014

The Carina Project VII: Towards the breaking of the age-metallicity degeneracy of red giant branch stars using the c_UBI index

We present an analysis of photometric and spectroscopic data of the Carina dSph galaxy, testing a new approach similar to that used to disentangle multiple populations in Galactic globular clusters (GCs). We show that a proper colour combination is able to separate a significant fraction of the red giant branch (RGB) of the two main Carina populations (the old one, \sim 12 Gyr, and the intermediate-age one, 4-8 Gyr). In particular, the c_UBI=(U-B)-(B-I) pseudo-colour allows us to follow the RGB of both populations along a relevant portion of the RGB. We find that the oldest stars have more negative c_UBI pseudo-colour than intermediate-age ones. We correlate the pseudo-colour of RGB stars with their chemical properties, finding a significant trend between the iron content and the c_UBI. Stars belonging to the old population are systematically more metal poor ([Fe/H]=-2.32\pm0.08 dex) than the intermediate-age ones ([Fe/H]=-1.82\pm0.03 dex). This gives solid evidence on the chemical evolution history of this galaxy, and we have a new diagnostic that can allow us to break the age-metallicity degeneracy of H-burning advanced evolutionary phases. We compared the distribution of stars in the c_UBI plane with theoretical isochrones, finding that no satisfactory agreement can be reached with models developed in a theoretical framework based on standard heavy element distributions. Finally, we discuss possible systematic differences when compared with multiple populations in GCs.

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arXiv2014

A state-of-the-art analysis of the dwarf irregular galaxy NGC 6822

We present a detailed photometric study of the dwarf irregular galaxy NGC 6822 aimed at investigating the properties of its stellar populations and, in particular, the presence of stellar radial gradients. Our goal is to analyse the stellar populations in six fields, which cover the whole bar of this dwarf galaxy. We derived the quantitative star formation history (SFH) of the six fields using the IAC method, involving IAC-pop/MinnIAC codes. The solutions we derived show an enhanced star formation rate (SFR) in Fields 1 and 3 during the past 500 Myr. The SFRs of the other fields are almost extinguished at very recent epochs and. We study the radial gradients of the SFR and consider the total mass converted into stars in two time intervals (between 0 and 0.5 Gyr ago and between 0.5 and 13.5 Gyr ago). We find that the scale lengths of the young and intermediate-to-old populations are perfectly compatible, with the exception of the young populations in Fields 1 and 3. The recent SF in these two fields is greater than in the other ones. This might be an indication that in these two fields we are sampling incipient spiral arms. Further evidence and new observations are required to prove this hypothesis. In addition, we derived the age-metallicity relations. As expected, the metallicity increases with time for all of the fields. We do not observe any radial gradient in the metallicity.

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arXiv2014

STREGA: STRucture and Evolution of the GAlaxy. I. Survey Overview and First Results

STREGA (STRucture and Evolution of the GAlaxy) is a Guaranteed Time survey being performed at the VST (the ESO VLT Survey Telescope) to map about 150 square degrees in the Galactic halo, in order to constrain the mechanisms of galactic formation and evolution. The survey is built as a five-year project, organized in two parts: a core program to explore the surrounding regions of selected stellar systems and a second complementary part to map the southern portion of the Fornax orbit and extend the observations of the core program. The adopted stellar tracers are mainly variable stars (RR~Lyraes and Long Period Variables) and Main Sequence Turn-off stars for which observations in the g,r,i bands are obtained. We present an overview of the survey and some preliminary results for three observing runs that have been completed. For the region centered on $ω$~Cen (37 deg^2), covering about three tidal radii, we also discuss the detected stellar density radial profile and angular distribution, leading to the identification of extratidal cluster stars. We also conclude that the cluster tidal radius is about 1.2 deg, in agreement with values in the literature based on the Wilson model.

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arXiv2014

Spectroscopic Signatures of Extra-Tidal Stars Around the Globular Clusters NGC 6656 (M22), NGC 3201 and NGC 1851 from RAVE

Stellar population studies of globular clusters have suggested that the brightest clusters in the Galaxy might actually be the remnant nuclei of dwarf spheroidal galaxies. If the present Galactic globular clusters formed within larger stellar systems, they are likely surrounded by extra-tidal halos and/or tails made up of stars that were tidally stripped from their parent systems. The stellar surroundings around globular clusters are therefore one of the best places to look for the remnants of an ancient dwarf galaxy. Here an attempt is made to search for tidal debris around the supernovae enriched globular clusters M22 and NGC 1851 as well as the kinematically unique cluster NGC 3201. The stellar parameters from the Radial Velocity Experiment (RAVE) are used to identify stars with RAVE metallicities, radial velocities and elemental-abundances consistent with the abundance patterns and properties of the stars in M22, NGC 1851 and NGC 3201. The discovery of RAVE stars that may be associated with M22 and NGC 1851 are reported, some of which are at projected distances of ~10 degrees away from the core of these clusters. Numerous RAVE stars associated with NGC 3201 suggest that either the tidal radius of this cluster is underestimated, or that there are some unbound stars extending a few arc minutes from the edge of the cluster's radius. No further extra-tidal stars associated with NGC 3201 could be identified. The bright magnitudes of the RAVE stars make them easy targets for high resolution follow-up observations, allowing an eventual further chemical tagging to solidify (or exclude) stars outside the tidal radius of the cluster as tidal debris. In both our radial velocity histograms of the regions surrounding NGC 1851 and NGC 3201, a peak of stars at 230 km/s is seen, consistent with extended tidal debris from omega Centauri.

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arXiv2014

Optical and Near-Infrared UBVRIJHK Photometry for the RR Lyrae stars in the Nearby Globular Cluster M4 (NGC 6121)

We present optical and near-infrared UBVRIJHK photometry of stars in the Galactic globular cluster M4 (NGC 6121) based upon a large corpus of observations obtained mainly from public astronomical archives. We concentrate on the RR Lyrae variable stars in the cluster, and make a particular effort to accurately reidentify the previously discovered variables. We have also discovered two new probable RR Lyrae variables in the M4 field: one of them by its position on the sky and its photometric properties is a probable member of the cluster, and the second is a probable background (bulge?) object. We provide accurate equatorial coordinates for all 47 stars identified as RR Lyraes, new photometric measurements for 46 of them, and new period estimates for 45. We have also derived accurate positions and mean photometry for 34 more stars previously identified as variable stars of other types, and for an additional five non-RR Lyrae variable stars identified for the first time here. We present optical and near-infrared color-magnitude diagrams for the cluster and show the locations of the variable stars in them. We present the Bailey (period-amplitude) diagrams and the period-frequency histogram for the RR Lyrae stars in M4 and compare them to the corresponding diagrams for M5 (NGC 5904). We conclude that the RR Lyrae populations in the two clusters are quite similar in all the relevant properties that we have considered. The mean periods, pulsation-mode ratios, and Bailey diagrams of these two clusters show support for the recently proposed "Oosterhoff-neutral" classification.

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arXiv2014

On the fine structure of the Cepheid metallicity gradient in the Galactic thin disk

We present homogeneous and accurate iron abundances for 42 Galactic Cepheids based on high-spectral resolution (R~38,000) high signal-to-noise ratio (SNR>100) optical spectra collected with UVES at VLT (128 spectra). The above abundances were complemented with high-quality iron abundances provided either by our group (86) or available in the literature. We paid attention in deriving a common metallicity scale and ended up with a sample of 450 Cepheids. We also estimated for the entire sample accurate individual distances by using homogeneous near-infrared photometry and the reddening free Period-Wesenheit relations. The new metallicity gradient is linear over a broad range of Galactocentric distances (Rg~5-19 kpc) and agrees quite well with similar estimates available in the literature (-0.060+/-0.002 dex/kpc). We also uncover evidence which suggests that the residuals of the metallicity gradient are tightly correlated with candidate Cepheid Groups (CGs). The candidate CGs have been identified as spatial overdensities of Cepheids located across the thin disk. They account for a significant fraction of the residual fluctuations, and in turn for the large intrinsic dispersion of the metallicity gradient. We performed a detailed comparison with metallicity gradients based on different tracers: OB stars and open clusters. We found very similar metallicity gradients for ages younger than 3 Gyrs, while for older ages we found a shallower slope and an increase in the intrinsic spread. The above findings rely on homogeneous age, metallicity and distance scales. Finally we found, by using a large sample of Galactic and Magellanic Cepheids for which are available accurate iron abundances, that the dependence of the luminosity amplitude on metallicity is vanishing.

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arXiv2014

The Araucaria Project. OGLE-LMC-CEP-1718: An exotic eclipsing binary system composed of two classical overtone Cepheids in a 413-day orbit

We have obtained extensive high-quality spectroscopic observations of the OGLE-LMC-CEP-1718 eclipsing binary system in the Large Magellanic Cloud which Soszynski et al. (2008) had identified as a candidate system for containing two classical Cepheids in orbit. Our spectroscopic data clearly demonstrate binary motion of the Cepheids in a 413-day eccentric orbit, rendering this eclipsing binary system the first ever known to consist of two classical Cepheid variables. After disentangling the four different radial velocity variations in the system we present the orbital solution and the individual pulsational radial velocity curves of the Cepheids. We show that both Cepheids are extremely likely to be first overtone pulsators and determine their respective dynamical masses, which turn out to be equal to within 1.5 %. Since the secondary eclipse is not observed in the orbital light curve we cannot derive the individual radii of the Cepheids, but the sum of their radii derived from the photometry is consistent with overtone pulsation for both variables. The existence of two equal-mass Cepheids in a binary system having different pulsation periods (1.96 and 2.48 days, respectively) may pose an interesting challenge to stellar evolution and pulsation theories, and a more detailed study of this system using additional datasets should yield deeper insight about the physics of stellar evolution of Cepheid variables. Future analysis of the system using additional near-infrared photometry might also lead to a better understanding of the systematic uncertainties in current Baade-Wesselink techniques of distance determinations to Cepheid variables.

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arXiv2014

On the kinematic separation of field and cluster stars across the Bulge globular NGC 6528

We present deep and precise multi-band photometry of the Galactic Bulge globular cluster NGC6528. The current dataset includes optical and near-infrared images collected with ACS/WFC, WFC3/UVIS, and WFC3/IR on board the Hubble Space Telescope. The images cover a time interval of almost ten years and we have been able to carry out a proper-motion separation between cluster and field stars. We performed a detailed comparison in the m_F814W, m_F606W - m_F814W Color-Magnitude Diagram with two empirical calibrators observed in the same bands. We found that NGC6528 is coeval with and more metal-rich than 47Tuc. Moreover, it appears older and more metal-poor than the super-metal-rich open cluster NGC6791. The current evidence is supported by several diagnostics (red horizontal branch, red giant branch bump, shape of the sub-giant branch, slope of the main sequence) that are minimally affected by uncertainties in reddening and distance. We fit the optical observations with theoretical isochrones based on a scaled-solar chemical mixture and found an age of 11 +- 1 Gyr and an iron abundance slightly above solar ([Fe/H = +0.20). The iron abundance and the old cluster age further support the recent spectroscopic findings suggesting a rapid chemical enrichment of the Galactic Bulge.

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arXiv2014

The globular cluster NGC 6528 the ferrous side of the Galactic Bulge

We present new and accurate optical photometry of the Bulge globular cluster NGC 6528. The images were collected with ACS at HST and together with WFC3 (UVIS, IR) allowed us to measure the proper motion to separate cluster and field stars. We adopted two empirical calibrators and we found that NGC 6528 is coeval with and more metal-rich than 47 Tuc. Moreover, it appears older and more metal-poor than the super-metal-rich old open cluster NGC 6791. We also performed a preliminary analysis of field stellar populations located around NGC 6528 and NGC 6522 by using ground-based near-infrared photometry collected with SOFI at NTT. The comparison of evolved stellar components (red giant branch, red horizontal branch, red clump stars) indicates that they share similar properties in this region of the Baade's Window.

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arXiv2014

Stellar populations in the ELT perspective

We discuss the impact that the next generation of Extremely Large Telescopes will have on the open astrophysical problems of resolved stellar populations. In particular, we address the interplay between multiband photometry and spectroscopy.

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arXiv2013

Blue Straggler masses from pulsation properties. I. The case of NGC6541

We used high-spatial resolution images acquired with the WFC3 on board HST to probe the population of variable blue straggler stars in the central region of the poorly studied Galactic globular cluster NGC 6541. The time sampling of the acquired multi wavelength (F390W, F555W and F814W) data allowed us to discover three WUma stars and nine SX Phoenicis. Periods, mean magnitudes and pulsation modes have been derived for the nine SX Phoenicis and their masses have been estimated by using pulsation equations obtained from linear non adiabatic models. We found masses in the range 1.0-1.1Mo, with an average value of 1.06+-0.09 Mo (sigma = 0.04), significantly in excess of the cluster Main Sequence Turn Off mass (~ 0.75Mo). A mild trend between mass and luminosity seems also to be present. The computed pulsation masses turn out to be in very good agreement with the predictions of evolutionary tracks for single stars, indicating values in the range ~ 1.0-1.2 Mo for most of the BSS population, in agreement with what discussed in a number of previous studies.

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arXiv2013

The Carina Project. VI. The helium burning variable stars

We present new optical (BVI) time-series data for the evolved variable stars in the Carina dwarf spheroidal galaxy. The quality of the data and the observing strategy allowed us to identify 14 new variable stars. Eight out of the 14 are RR Lyrae (RRL) stars, four are Anomalous Cepheids (ACs) and two are geometrical variables. Comparison of the period distribution for the entire sample of RRLs with similar distributions in nearby dSphs and in the Large Magellanic Cloud indicates that the old stellar populations in these systems share similar properties. This finding is also supported by the RRL distribution in the Bailey diagram. On the other hand, the period distribution and the Bailey diagram of ACs display significant differences among the above stellar systems. This evidence suggests that the properties of intermediate-age stellar populations might be affected both by environmental effects and structural parameters. We use the BV Period--Wesenheit (PW) relation of RRLs together with evolutionary prescriptions and find a true distance modulus of 20.09+/-0.07(intrinsic)+/-0.1(statistical) mag that agrees quite well with similar estimates available in the literature. We identified four peculiar variables. Taking into account their position in the Bailey diagram and in the BV PW relation, two of them (V14, V149) appear to be candidate ACs, while two (V158, V182) might be peculiar RRLs. In particular, the variable V158 has a period and a V-band amplitude very similar to the low-mass RRL ---RRLR-02792---recently identified by Pietrzynski at al. (2012) in the Galactic bulge.

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