Publications (28)24.38 Total impact
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Article: An extended main sequence turn-off in the Small Magellanic Cloud star cluster NGC411
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ABSTRACT: Based on new observations with the Wide Field Camera 3 onboard the Hubble Space Telescope, we report the discovery of an extended main sequence turn-off (eMSTO) in the intermediate-age star cluster NGC411. This is the second case of an eMSTO being identified in a star cluster belonging to the Small Magellanic Cloud (SMC), after NGC419. Despite the present masses of these two SMC clusters differ by a factor of 4, the comparison between their colour--magnitude diagrams (CMD) shows striking similarities, especially regarding the shape of their eMSTOs. The loci of main CMD features are so similar that they can be well described, in a first approximation, by the same mean metallicity, distance and extinction. NGC411, however, presents merely a trace of secondary red clump as opposed to its prominent manifestation in NGC419. This could be due either to the small number statistics in NGC411, or by the star formation in NGC419 having continued for 60 Myr longer than in NGC411. Under the assumption that the eMSTOs are caused by different generations of stars at increasing age, both clusters are nearly coeval in their first episodes of star formation. The initial period of star formation, however, is slightly more marked in NGC419 than in NGC411. We discuss these findings in the context of possible scenarios for the origin of eMSTOs.03/2013; -
Article: Red Giant evolution and specific problems
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ABSTRACT: In spite of the great effort made in the last decades to improve our understanding of stellar evolution, significant uncertainties still remain due to our poor knowledge of some complex physical processes that still require an empirical calibration, such as the efficiency of convective heat transport and interior mixing. Here we will review the impact of these uncertainties on the evolution of red giant stars.01/2013; -
Article: The star formation history of the Large Magellanic Cloud star clusters NGC1846 and NGC1783
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ABSTRACT: NGC1846 and NGC1783 are two massive star clusters in the Large Magellanic Cloud, hosting both an extended main sequence turn-off and a dual clump of red giants. They present similar masses but differ mainly in angular size. Starting from their high-quality ACS data in the F435W, F555W and F814W filters, and updated sets of stellar evolutionary tracks, we derive their star formation rates as a function of age, SFR(t), by means of the classical method of CMD reconstruction which is usually applied to nearby galaxies. The method confirms the extended periods of star formation derived from previous analysis of the same data. When the analysis is performed for a finer resolution in age, we find clear evidence for a 50-Myr long hiatus between the oldest peak in the SFR(t), and a second prolonged period of star formation, in both clusters. For the more compact cluster NGC1846, there seems to be no significant difference between the SFR(t) in the cluster centre and in an annulus with radii between 20 and 60 arcsec (from 4.8 to 15.4 pc). The same does not occur in the more extended NGC1783 cluster, where the outer ring (between 33 and 107 arcsec, from 8.0 to 25.9 pc) is found to be slightly younger than the centre. We also explore the best-fitting slope of the present-day mass function and binary fraction for the different cluster regions, finding hints of a varying mass function between centre and outer ring in NGC1783. These findings are discussed within the present scenarios for the formation of clusters with multiple turn-offs.01/2013; -
Article: Evolution of Thermally Pulsing Asymptotic Giant Branch Stars I. The COLIBRI Code
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ABSTRACT: We present the COLIBRI code for computing the evolution of stars along the TP-AGB phase. Compared to purely synthetic TP-AGB codes, COLIBRI relaxes a significant part of their analytic formalism in favour of a detailed physics applied to a complete envelope model, in which the stellar structure equations are integrated from the atmosphere down to the bottom of the hydrogen-burning shell. This allows to predict self-consistently: (i) the effective temperature, and more generally the convective envelope and atmosphere structures, correctly coupled to the changes in the surface chemical abundances and gas opacities; (ii) sphericity effects in the atmospheres; (iii) the core mass-luminosity relation and its break-down due to hot bottom burning (HBB) in the most massive AGB stars, (iv) the HBB nucleosynthesis via the solution of a complete nuclear network (pp chains, and the CNO, NeNa, MgAl cycles), including also the production of 7Li via the Cameron-Fowler beryllium transport mechanism; (v) the chemical composition of the pulse-driven convective zone; (vi) the onset and quenching of the third dredge-up, with a suitable temperature criterion. At the same time COLIBRI pioneers new techniques in the treatment of the physics of stellar interiors. It is the first evolutionary code ever to use accurate on-the-fly computation of the equation of state for roughly 800 atoms, ions, molecules, and of the Rosseland mean opacities throughout the deep envelope. Another distinguishing aspect of COLIBRI is its high computational speed. This feature is necessary for calibrating the uncertain parameters and processes that characterize the TP-AGB phase, a step of paramount importance for producing reliable stellar population synthesis models of galaxies up to high redshift. (abridged)01/2013; -
Article: Resolved Near-infrared Stellar Populations in Nearby Galaxies
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ABSTRACT: We present near-infrared (NIR) color-magnitude diagrams (CMDs) for the resolved stellar populations within 26 fields of 23 nearby galaxies ( 4 Mpc), based on images in the F110W and F160W filters taken with the Wide-Field Camera 3 (WFC3) on the Hubble Space Telescope (HST). The CMDs are measured in regions spanning a wide range of star formation histories, including both old dormant and young star-forming populations. We match key NIR CMD features with their counterparts in more familiar optical CMDs, and identify the red core helium-burning (RHeB) sequence as a significant contributor to the NIR flux in stellar populations younger than a few 100 Myr old. The strength of this feature suggests that the NIR mass-to-light ratio can vary significantly on short timescales in star-forming systems. The NIR luminosity of star-forming galaxies is therefore not necessarily proportional to the stellar mass. We note that these individual RHeB stars may also be misidentified as old stellar clusters in images of nearby galaxies. For older stellar populations, we discuss the CMD location of asymptotic giant branch (AGB) stars in the HST filter set and explore the separation of AGB subpopulations using a combination of optical and NIR colors. We empirically calibrate the magnitude of the NIR tip of the red giant branch in F160W as a function of color, allowing future observations in this widely adopted filter set to be used for distance measurements. We also analyze the properties of the NIR red giant branch (RGB) as a function of metallicity, showing a clear trend between NIR RGB color and metallicity. However, based on the current study, it appears unlikely that the slope of the NIR RGB can be used as an effective metallicity indicator in extragalactic systems with comparable data. Finally, we highlight issues with scattered light in the WFC3, which becomes significant for exposures taken close to a bright Earth limb.The Astrophysical Journal Supplement Series 12/2011; 198(1):6. · 13.46 Impact Factor -
Article: Catalogues of hot white dwarfs in the Milky Way from GALEX’s ultraviolet sky surveys: constraining stellar evolution
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ABSTRACT: We present comprehensive catalogues of hot star candidates in the Milky Way (MW), selected from Galaxy Evolution Explorer (GALEX) far-UV (FUV; 1344–1786 Å) and near-UV (NUV; 1771–2831 Å) imaging. The FUV and NUV photometry allows us to extract the hottest stellar objects, in particular hot white dwarfs (WD), which are elusive at other wavelengths because of their high temperatures and faint optical luminosities. We generated catalogues of UV sources from two GALEX’s surveys: All-Sky Imaging Survey (AIS; depth AB magnitude ∼19.9/20.8 in FUV/NUV) and Medium-depth Imaging Survey (MIS; depth ∼22.6/22.7 mag). The two catalogues (from GALEX fifth data release) contain 65.3/12.6 million (AIS/MIS) unique UV sources with errorNUV≤ 0.5 mag, over 21 435/1579 deg2. We also constructed subcatalogues of the UV sources with matched optical photometry from Sloan Digital Sky Survey (SDSS; seventh data release): these contain 0.6/0.9 million (AIS/MIS) sources with errors ≤0.3 mag in both FUV and NUV, excluding sources with multiple optical counterparts, over an area of 7325/1103 deg2. All catalogues are available online. We then selected 28 319 (AIS)/9028 (MIS) matched sources with FUV − NUV < −0.13; this colour cut corresponds to stellar Teff hotter than ∼18 000 K (the exact value varying with gravity). An additional colour cut of NUV−r > 0.1 isolates binaries with largely differing Teffs, and some intruding quasi-stellar objects (QSOs; more numerous at faint magnitudes). Available spectroscopy for a subsample indicates that hot-star candidates with NUV−r < 0.1 (mostly ‘single’ hot stars) have negligible contamination by non-stellar objects. We discuss the distribution of sources in the catalogues, and the effects of error and colour cuts on the samples. The density of hot-star candidates increases from high to low Galactic latitudes, but drops on the MW plane due to dust extinction. Our hot-star counts at all latitudes are better matched by MW models computed with an initial–final mass relation (IFMR) that favours lower final masses. The model analysis indicates that the brightest sample is likely composed of WDs located in the thin disc, at typical distances between 0.15 and 1 kpc, while the fainter sample comprises also a fraction of thick disc and halo stars. Proper motion distributions, available only for the bright sample (NUV < 18 mag), are consistent with the kinematics of a thin-disc population.Monthly Notices of the Royal Astronomical Society 03/2011; 411(4):2770 - 2791. · 4.90 Impact Factor -
Article: The ACS Nearby Galaxy Survey Treasury. IX. Constraining Asymptotic Giant Branch Evolution with Old Metal-poor Galaxies
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ABSTRACT: In an attempt to constrain evolutionary models of the asymptotic giant branch (AGB) phase at the limit of low masses and low metallicities, we have examined the luminosity functions and number ratios between AGB and red giant branch (RGB) stars from a sample of resolved galaxies from the ACS Nearby Galaxy Survey Treasury. This database provides Hubble Space Telescope optical photometry together with maps of completeness, photometric errors, and star formation histories for dozens of galaxies within 4 Mpc. We select 12 galaxies characterized by predominantly metal-poor populations as indicated by a very steep and blue RGB, and which do not present any indication of recent star formation in their color-magnitude diagrams. Thousands of AGB stars brighter than the tip of the RGB (TRGB) are present in the sample (between 60 and 400 per galaxy), hence, the Poisson noise has little impact in our measurements of the AGB/RGB ratio. We model the photometric data with a few sets of thermally pulsing AGB (TP-AGB) evolutionary models with different prescriptions for the mass loss. This technique allows us to set stringent constraints on the TP-AGB models of low-mass, metal-poor stars (with M < 1.5 M ☉, ). Indeed, those which satisfactorily reproduce the observed AGB/RGB ratios have TP-AGB lifetimes between 1.2 and 1.8 Myr, and finish their nuclear burning lives with masses between 0.51 and 0.55 M ☉. This is also in good agreement with recent observations of white dwarf masses in the M4 old globular cluster. These constraints can be added to those already derived from Magellanic Cloud star clusters as important mileposts in the arduous process of calibrating AGB evolutionary models.The Astrophysical Journal 11/2010; 724(2):1030. · 6.02 Impact Factor -
Article: Catalogs of Hot White Dwarfs in the Milky Way from GALEX's Ultraviolet Sky Surveys. Constraining Stellar Evolution
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ABSTRACT: We present comprehensive catalogs of hot star candidates in the Milky Way, selected from GALEX far-UV (FUV, 1344-1786 AA) and near-UV (NUV, 1771-2831 AA) imaging. The FUV and NUV photometry allows us to extract the hottest stellar objects, in particular hot white dwarfs (WD), which are elusive at other wavelengths because of their high temperatures and faint optical luminosities. We generated catalogs of UV sources from two GALEX's surveys: AIS (All-Sky Imaging Survey, depth ABmag~19.9/20.8 in FUV/NUV) and MIS (Medium-depth Imaging Survey, depth ~22.6/22.7mag). The two catalogs (from GALEX fifth data release) contain 65.3/12.6 million (AIS/MIS) unique UV sources with error(NUV)<0.5mag, over 21,435/1,579 square degrees. We also constructed subcatalogs of the UV sources with matched optical photometry from SDSS (7th data release): these contain 0.6/0.9million (AIS/MIS) sources with errors <0.3mag in both FUV and NUV, excluding sources with multiple optical counterparts, over an area of 7,325/1,103 square degrees. All catalogs are available online. We then selected 28,319(AIS)/9,028(MIS) matched sources with FUV-NUV<-0.13; this color cut corresponds to stellar Teff hotter than ~18,000 K. An additional color cut of NUV-r>0.1 isolates binaries with largely differing Teff's, and some intruding QSOs. Available spectroscopy for a subsample indicates that hot-star candidates with NUV-r<0.1 have negligible contamination by non-stellar objects. We discuss the distribution of sources in the catalogs, and the effects of error and color cuts on the samples. The density of hot-star candidates increases from high to low Galactic latitudes, but drops on the MW plane due to dust extinction. Our hot-star counts at all latitudes are better matched by Milky Way models computed with an initial-final mass relation that favours lower final masses. (ABRIDGED) Comment: To appear in MNRAS. Better printed in colour11/2010; -
Article: TP-AGB stars in population synthesis models
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ABSTRACT: In spite of its relevance, the Thermally Pulsing Asymptotic Giant Branch (TP-AGB) phase is one of the most uncertain phases of stellar evolution, and a major source of disagreement between the results of different population synthesis models of galaxies. I will briefly review the existing literature on the subject, and recall the basic prescriptions that have been used to fix the contribution of TP-AGB stars to the integrated light of stellar populations. The simplicity of these prescriptions greatly contrasts with the richness of details provided by present-day databases of AGB stars in the Magellanic Clouds, which are now being extended to other nearby galaxies. I will present the first results of an ongoing study aimed at simulating photometry, chemistry, pulsation, mass loss, dust properties of AGB star populations in resolved and un-resolved galaxies. We test our predictions against observations from various surveys of the Magellanic Clouds (DENIS, 2MASS, OGLE, MACHO, Spitzer, and AKARI). I will discuss the implications and outline the plan of future developments.Proceedings of the International Astronomical Union 07/2009; 5:36 - 43. -
Article: Thermally-pulsing asymptotic giant branch stars in the Magellanic Clouds
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ABSTRACT: We present the latest results of a theoretical project aimed at investigating the properties of thermally-pulsing asymptotic giant branch (TP-AGB) stars in different host systems. For this purpose, we have recently calculated calibrated synthetic TP-AGB tracks — covering a wide range of metallicities (0.0001 ≤ Z ≤ 0.03) up to the complete ejection of the envelope by stellar winds (Marigo & Girardi 2007) — and used them to generate new sets of stellar isochrones (Marigo et al. 2008). The latter are converted to about 25 different photometric systems, including the mid-infrared filters of Spitzer and AKARI as the effect of circumstellar dust from AGB stars is taken into account. First comparisons with AGB data in the MC field and stellar clusters are discussed.Proceedings of the International Astronomical Union 06/2008; 4:385 - 390. -
Article: Evolution of asymptotic giant branch stars II. Optical to far-infrared isochrones with improved TP-AGB models
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ABSTRACT: We present a large set of theoretical isochrones, whose distinctive features mostly reside on the greatly improved treatment of the thermally pulsing asymptotic giant branch (TP-AGB) phase. Essentially, we have coupled the TP-AGB tracks described in Paper I, at their stages of pre-flash quiescent H-shell burning, with the evolutionary tracks for the previous evolutionary phases from Girardi et al. (2000). Theoretical isochrones for any intermediate value of age and metallicity are then derived by interpolation in the grids. We take care that the isochrones keep, to a good level of detail, the several peculiarities present in these TP-AGB tracks. Theoretical isochrones are then converted to about 20 different photometric systems -- including traditional ground-based systems, and those of recent major wide-field surveys such as SDSS, OGLE, DENIS, 2MASS, UKIDSS, etc., -- by means of synthetic photometry applied to an updated library of stellar spectra, suitably extended to include C-type stars. Finally, we correct the predicted photometry by the effect of circumstellar dust during the mass-losing stages of the AGB evolution, which allows us to improve the results for the optical-to-infrared systems, and to simulate mid- and far-IR systems such as those of Spitzer and AKARI. Access to the data is provided both via a web repository of static tables (http://stev.oapd.inaf.it/dustyAGB07 and CDS), and via an interactive web interface (http://stev.oapd.inaf.it/cmd) that provides tables for any intermediate value of age and metallicity, for several photometric systems, and for different choices of dust properties. Comment: 25 pages, accepted for publication in A&A, revised according to the latest referee's indications, isochrones are available at http://stev.oapd.inaf.it/cmd11/2007; -
Article: Evolution of asymptotic giant branch stars I. Updated synthetic TP-AGB models and their basic calibration
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ABSTRACT: We present new synthetic models of the TP-AGB evolution. They are computed for 7 values of initial metal content (Z from 0.0001 to 0.03) and for initial masses between 0.5 and 5.0 Msun, thus extending the low- and intermediate-mass tracks of Girardi et al. (2000) until the beginning of the post-AGB phase. The calculations are performed by means of a synthetic code that incorporates many recent improvements, among which we mention: (1) the use of detailed and revised analytical relations to describe the evolution of quiescent luminosity, inter-pulse period, third dredge-up, hot bottom burning, pulse cycle luminosity variations, etc.; (2) the use of variable molecular opacities -- i.e. opacities consistent with the changing photospheric chemical composition -- in the integration of a complete envelope model, instead of the standard choice of scaled-solar opacities; (3) the use of formalisms for the mass-loss rates derived from pulsating dust-driven wind models of C- and O-rich AGB stars; and (4) the switching of pulsation modes between the first overtone and the fundamental one along the evolution, which has consequences in terms of the history of mass loss. It follows that, in addition to the time evolution on the HR diagram, the new models predict in a consistent fashion also variations in surface chemical compositions, pulsation modes and periods, and mass-loss rates. The onset and efficiency of the third dredge-up process are calibrated in order to reproduce basic observables like the carbon star luminosity functions in the Magellanic Clouds, and TP-AGB lifetimes (star counts) in Magellanic Cloud clusters. Forthcoming papers will present the theoretical isochrones and chemical yields derived from these tracks, and additional tests performed with the aid of a complete population synthesis code.04/2007; -
Article: Can the third dredge-up extinguish hot-bottom burning in massive AGB stars?
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ABSTRACT: Marigo (2002) has highlighted the crucial importance of molecular opacities in modelling the evolution of AGB stars at varying surface C/O ratio. In particular, it has been shown the large inadequacy of solar-scaled opacities when applied to models of carbon stars, and hence the need for correctly coupling the molecular opacities to the current surface chemical composition of AGB stars. The aim of the present follow-up study is to investigate the effects of variable molecular opacities on the evolutionary properties of luminous AGB stars with massive envelopes, i.e. with initial masses from ~3.5 Msun up to 5-8 Msun, which are predicted to experience both the third dredge-up and hot-bottom burning. It is found that if the dredge-up of carbon is efficient enough to lead to an early transition from C/O<1 to C/O>1, then hot-bottom burning may be weakened, extinguished, or even prevented. The physical conditions for this occurrence are analysed and a few theoretical and observational implications are discussed. Importantly, it is found that the inclusion of variable molecular opacities could significantly change the current predictions for the chemical yields contributed by intermediate-mass AGB stars, with M~3.5 - 4.0 Msun that make as much as ~ 30-50 % of all stars expected to undergo hot-bottom burning. Comment: 10 pages, 5 figures, accepted for publication in A&A02/2007; -
Article: Towards simulating the photometry, chemistry, mass loss and pulsational properties of AGB star populations in resolved galaxies
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ABSTRACT: Extended and updated grids of TP-AGB tracks have been implemented in the TRILEGAL population synthesis code, which generates mock stellar catalogues for a galaxy given its mass, distance, star formation history and age-metallicity relation, including also the Milky Way foreground population. Among the stellar parameters that are simulated, we now include the surface chemistry, mass-loss rates, pulsation modes and periods of LPVs. This allows us to perform a series of consistency checks between AGB model predictions and observations, that we are just starting to explore. We present a few examples of model--data comparisons, mostly regarding the near-infrared and variability data for AGB stars in the Magellanic Clouds.02/2007; -
Article: The TP-AGB phase. Lifetimes from C and M star counts in Magellanic Cloud clusters
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ABSTRACT: Using available data for C and M giants with Mbol<-3.6 in Magellanic Cloud clusters, we derive limits to the lifetimes of the corresponding evolutionary phases, as a function of stellar mass. The C-star phase is found to have a duration between 2 and 3 Myr for stars in the mass range from 1.5 to 2.8 Msun. There is also an indication that the peak of C-star lifetime shifts to lower masses (from slightly above to slightly below 2 Msun) as we move from LMC to SMC metallicities. The M-giant lifetimes also peak at 2 Msun in the LMC, with a maximum value of about 4 Myr, whereas in the SMC their lifetimes appear much shorter but, actually, they are poorly constrained by the data. These numbers constitute useful constraints to theoretical models of the TP-AGB phase. We show that several models in the literature underestimate the duration of the C-star phase at LMC metallicities. Comment: 8 pages, to appear in Astronomy & Astrophysics09/2006; -
Chapter: Towards Understanding the Planetary Nebula Luminosity Function
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ABSTRACT: We report on a theoretical work in progress about the planetary nebula luminosity function. A new synthetic model is developed to follow the evolution of a planetary nebula and its central star, starting from the onset of AGB phase up to the white dwarf cooling sequence. Both H- and He-burning Post-AGB tracks are used. Basic properties of the circumstellar nebula are predicted, namely: the dynamics and chemical composition of the stellar ejecta, the photoionisation of H and He by the central star, the formation of a few relevant optical emission lines (e.g. H b\beta ;HeII l\lambda 4686; [OIII] l\lambda 5007). As a first application, the planetary nebula luminosity function in the LMC is analysed adopting a population synthesis approach. It turns out that a reasonable reproduction of the observed bright end in the [OIII] l\lambda 5007 luminosity cannot be obtained assuming that all central stars are H-burners, but rather considering that the observed population of PNe is excited by a mixture of H- and He-burner central stars.02/2006: pages 248-251; -
Article: Youn open clusters in the Carina region
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ABSTRACT: We present results of a new investigation (Carraro et al. 2002) aimed at clarifying the mutual relationship between the most prominent young open clusters close to eta Carinae, namely Trumpler 16, Trumpler 14 and Collinder 232.09/2002; -
Article: Asymptotic Giant Branch evolution at varying surface C/O ratio: effects of changes in molecular opacities
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ABSTRACT: We investigate the effects of molecular opacities on the evolution of TP-AGB stars that experience the third dredge-up, i.e. with surface abundances of carbon and oxygen varying with time. To this aim, a routine is constructed to derive the molecular concentrations through dissociation equilibrium calculations, and estimate the opacities due to H2, H2O, OH, C2, CN, and CO for any given density, temperature, and chemical composition of the gas. Then, synthetic TP-AGB models with dredge-up are calculated by either 1) adopting the newly developed routine, or 2) interpolating between fixed opacity tables for solar chemical composition, which is the usual prescription in AGB evolution calculations. The comparison between the two cases shows that the change in the dominant opacity sources, as the C/O ratio grows from below to above unity, crucially affects the evolution of the effective temperature, i.e. causing a notable cooling of the carbon-rich models (with C/O>1). From the comparison with observational data, it turns out that TP-AGB models with variable molecular opacities are able to reproduce the observed range of effective temperatures, mass-loss rates, and wind expansion velocities of C-type giants in the solar neighbourhood, otherwise failed if assuming fixed molecular opacities for solar-scaled mixtures. Finally, we mention other important evolutionary and observational effects that result from the adoption of the variable opacities, such as: i) significant shortening of the C-star phase due to the earlier onset of the super-wind; ii) consequent reduction of the carbon yields; iii) reproduction of the observed range of near-infrared colours of C-stars. Comment: 13 pages, 10 postscript figures, to appear in A&A03/2002; -
Article: The intermediate-age open cluster NGC 2158
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ABSTRACT: We report on $UBVRI$ CCD photometry of two overlapping fields in the region of the intermediate-age open cluster NGC 2158 down to V=21. By analyzing Colour-Colour (CC) and Colour-Magnitude Diagrams (CMD) we infer a reddening $E_{B-V}= 0.55\pm0.10$, a distance of $3600 \pm 400$ pc, and an age of about 2 Gyr. Synthetic CMDs performed with these parameters (but fixing $E_{B-V}=0.60$ and $[{\rm Fe/H}]=-0.60$), and including binaries, field contamination, and photometric errors, allow a good description of the observed CMD. The elongated shape of the clump of red giants in the CMD is interpreted as resulting from a differential reddening of about $\Delta E_{B-V}=0.06$ across the cluster, in the direction perpendicular to the Galactic plane. NGC 2158 turns out to be an intermediate-age open cluster with an anomalously low metal content. The combination of these parameters together with the analysis of the cluster orbit, suggests that the cluster belongs to the old thin disk population. Comment: 11 pages, 12 figures, in press in MNRAS02/2002; -
Article: Coupling emitted light and chemical yields from stars: a basic constraint to population synthesis models of galaxies
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ABSTRACT: In this paper we emphasize the close connection between the chemical and spectrophotometric evolution of stellar systems: Chemical yields from stars correspond to a precise fraction of their emitted light. We translate this concept quantitatively. Starting from simple stellar populations, we derive useful analytical relations to calculate the stellar fuel consumption (emitted light) as a function of basic quantities predicted by stellar models, i.e. the mass of the core and the chemical composition of the envelope. The final formulas explicate the relation between integrated light contribution (total or limited to particular evolutionary phases), chemical yields and stellar remnants. We test their accuracy in the case of low- and intermediate-mass stars, and indicate the way to extend the analysis to massive stars. This formalism provides an easy tool to check the internal consistency between the different stellar inputs adopted in galaxy models: The fuel computed by means of the analytical formulas (corresponding to a given set of chemical yields) should be compared to the exact values given by the luminosity integration along the stellar evolutionary tracks or isochrones (corresponding to a given set of spectrophotometric models). Only if both estimates of the fuel are similar, the stellar inputs can be considered self-consistent in terms of their energetics. This sets an important requirement to galaxy models, also in consideration of the fact that different sources of input stellar data are frequently used to model their spectro-photometric and chemical evolution. Comment: 17 pages, 7 figures, 3 tables, accepted for publication in A&A07/2001;
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Institutions
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2008–2013
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Università degli Studi di Padova
- Department of Physics and Astronomy "Galileo Galilei"
Padova, Veneto, Italy
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2000–2009
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University-Hospital of Padova
Padova, Veneto, Italy
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