arXiv subjects

Santi Cassisi

Publications and source records attributed to Santi Cassisi.

At least 91 records · Page 5Linked to original sources

arXiv2013

Photometric properties of stellar populations in Galactic globular clusters: the role of the Mg-Al anticorrelation

We have computed low-mass stellar models and synthetic spectra for an initial chemical composition that includes the full C-N, O-Na, and Mg-Al abundance anticorrelations observed in second generation stars belonging to a number of massive Galactic globular clusters. This investigation extends a previous study that has addressed the effect of only the C-N and O-Na anticorrelations, seen in all globulars observed to date. We find that the impact of Mg-Al abundance variations at fixed [Fe/H] and Helium abundance is negligible on stellar models and isochrones (from the main sequence to the tip of the red giant branch) and bolometric corrections, when compared to the effect of C-N and O-Na variations. We identify a spectral feature at 490-520 nm, for low-mass main sequence stars, caused by MgH molecular bands. This feature has a vanishingly small effect on bolometric corrections for Johnson and Stroemgren filters that cover that spectral range. However, specific narrow-band filters able to target this wavelength region can be powerful tools to investigate the Mg-poor unevolved stars and highlight possible splittings of the MS due to variations of Mg abundances.

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arXiv2013

Theoretical Modeling of the RR Lyrae Variables in NGC 1851

The RR Lyrae instability strip (IS) in NGC 1851 is investigated, and a model is presented which reproduces the pulsational properties of the RR Lyrae population. In our model, a stellar component within the IS that displays minor helium variations (Y~0.248-0.270) is able to reproduce the observed periods and amplitudes of the RR Lyrae variables, as well as the frequency of fundamental and first-overtone RR Lyrae variables. The RR Lyrae variables therefore may belong to an He-enriched second generation of stars. The RR Lyrae variables with a slightly enhanced helium (Y~0.270-0.280) have longer periods at a given amplitude, as is seen with Oosterhoff II (OoII) RR Lyrae variables, whereas the RR Lyrae variables with Y~0.248-0.270 have shorter periods, exhibiting properties of Oosterhoff I (OoI) variables.

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arXiv2012

The Horizontal Branch in the UV Colour Magnitude Diagrams. II. The case of M3, M13 and M79

We present a detailed comparison between far-UV/optical colour Magnitude Diagrams obtained with high-resolution Hubble Space Telescope data and suitable theoretical models for three Galactic Globular Clusters: M3, M13 and M79. These systems represents a classical example of clusters in the intermediate metallicity regime that, even sharing similar metal content and age, show remarkably different Horizontal Branch morphologies. As a consequence, the observed differences in the colour distributions of Horizontal Branch stars cannot be interpreted in terms of either first (metallicity) or a second parameter such as age. We investigate here the possible role of variations of initial Helium abundance (Y). Thanks to the use of a proper setup of far-UV filters, we are able to put strong constraints on the maximum Y (Y_{max}) values compatible with the data. We find differences Delta Y_{max} ~ 0.02-0.04 between the clusters with M13 showing the largest value (Y_{max} ~ 0.30) and M3 the smallest (Y_{max} ~ 0.27). In general we observe that these values are correlated with the colour extensions of their Horizontal Branches and with the range of the observed Na-O anti-correlations.

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arXiv2012

The Horizontal Branch of NGC 1851: Constraints from its RR Lyrae Variables

We use the pulsational properties of the RR Lyrae variables in the globular cluster NGC 1851 to obtain detailed constraints of the various sub-stellar populations present along its horizontal branch. On the basis of detailed synthetic horizontal branch modeling, we find that minor helium variations (Y~0.248-0.280) are able to reproduce the observed periods and amplitudes of the RR Lyrae variables, as well as the frequency of fundamental and first-overtone RR Lyrae stars. Comparison of number ratios amongst the blue and red horizontal branch components and the two observed subgiant branches also suggest that the RR Lyrae variables originated from the progeny of the bright subgiant branch. The RR Lyrae variables with a slightly enhanced helium (Y~0.270-0.280) have longer periods at a given amplitude, as is seen with Oosterhoff II (OoII) RR Lyrae variables, whereas the RR Lyrae variables with Y~0.248-0.270 have shorter periods, exhibiting properties of Oosterhoff I (OoI) variables. This correlation does suggest that the pulsational properties of RR Lyrae stars can be very useful for tracing the various sub-populations and can provide suitable constraints on the multiple population phenomenon. It appears of great interest to explore whether this conclusion can be generalized to other globular clusters hosting multiple populations.

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arXiv2011

Very Low-Mass Stars: structural and evolutionary properties

We briefly review the main physical and structural properties of Very Low-Mass stars. The most important improvements in the physical inputs required for the stellar models computations are also discussed. We show some comparisons with observational measurements concerning both the Color-Magnitude diagrams, mass-luminosity relations and mass-radius one, in order to disclose the level of agreement between the present theoretical framework and observations.

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arXiv2011

Uncertainties and systematics in stellar evolution models of Red Giant Stars

In this last decade, our knowledge of evolutionary and structural properties of stars of different mass and chemical composition has significantly improved. This notwithstanding, updated stellar models are still affected by significant and, usually, not negligible uncertainties. These uncertainties are related to our poor knowledge of some physical processes occurring in the real stars such as the efficiency of mixing processes. These drawbacks of stellar models have to be properly taken into account when comparing theory with observations. In this paper we briefly review current uncertainties affecting low-mass stellar models, i.e. those structures with mass in the range between 0.6 Msun and ~1.4 Msun, during the Red Giant Branch stage.

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arXiv2011

New constraints on the chemical evolution of the solar neighbourhood and Galactic disc(s). Improved astrophysical parameters for the Geneva-Copenhagen Survey

We present a re-analysis of the Geneva-Copenhagen survey, which benefits from the infrared flux method to improve the accuracy of the derived stellar effective temperatures and uses the latter to build a consistent and improved metallicity scale. Metallicities are calibrated on high-resolution spectroscopy and checked against four open clusters and a moving group, showing excellent consistency. The new temperature and metallicity scales provide a better match to theoretical isochrones, which are used for a Bayesian analysis of stellar ages. With respect to previous analyses, our stars are on average 100 K hotter and 0.1 dex more metal rich, which shift the peak of the metallicity distribution function around the solar value. From Stromgren photometry we are able to derive for the first time a proxy for alpha elements, which enables us to perform a tentative dissection of the chemical thin and thick disc. We find evidence for the latter being composed of an old, mildly but systematically alpha-enhanced population that extends to super solar metallicities, in agreement with spectroscopic studies. Our revision offers the largest existing kinematically unbiased sample of the solar neighbourhood that contains full information on kinematics, metallicities, and ages and thus provides better constraints on the physical processes relevant in the build-up of the Milky Way disc, enabling a better understanding of the Sun in a Galactic context.

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arXiv2011

The ACS LCID project. V. The Star Formation History of the Dwarf Galaxy \objectname[]{LGS-3}: Clues for Cosmic Reionization and Feedback

We present an analysis of the star formation history (SFH) of the transition-type (dIrr/dSph) Local Group galaxy \objectname[]{LGS-3} (Pisces) based on deep photometry obtained with the {\it Advanced Camera for Surveys} onboard the {\it Hubble Space Telescope}. Our analysis shows that the SFH of \objectname[]{LGS-3} is dominated by a main episode $\sim 11.7$ Gyr ago with a duration of $\sim$ 1.4 Gyr which formed $\sim 90%$ of the stars. Subsequently, \objectname[]{LGS-3} continued forming stars until the present, although at a much lower rate. The lack of early chemical enrichment is in contrast to that observed in the isolated dSph galaxies of comparable luminosity, implying that the dSphs were more massive and subjected to more tidal stripping. We compare the SFH of \objectname[]{LGS-3} with expectations from cosmological models. Most or all the star formation was produced in \objectname[]{LGS-3} after the reionization epoch, assumed to be completed at $z\sim6$ or $\sim 12.7$ Gyr ago. The total mass of the galaxy is estimated to be between 2 and $4\times 10^8$ M$_\odot$, corresponding to circular velocities between 28 km\ s$^{-1}$ to 36 km\ s$^{-1}$. These values are close to but somewhat above the limit of 30 km\ s$^{-1}$ below which the UV background is expected to prevent any star formation after reionization. Feedback from SNe associated with the initial episode of star formation (mechanical luminosity from SNe $L_w=5.3\times 10^{38}$ erg s$^{-1}$) is probably inadequate to completely blow away the gas. However, the combined effects of SN feedback and UV background heating might be expected to completely halt star formation at the reionization epoch for the low mass of \objectname[]{LGS-3}; this suggests that self-shielding is important to the early evolution of galaxies in this mass range.

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arXiv2011

A Classical Cepheid in a LMC eclipsing binary: evidence of shortcomings in current stellar evolutionary models?

The recent discovery and analysis of a classical Cepheid in the well detached, double-lined, eclipsing binary OGLE-LMC-CEP0227, has provided the first determination of the dynamical mass of a classical Cepheid variable to an unprecedented 1% accuracy. We show here that modern stellar evolution models widely employed to study Galactic and extragalactic stellar systems, are able to match simultaneously mass and radius (and effective temperature) of the two components with a single value for the age of the system, without any specific fine-tuning. Our conclusion is that there is no discrepancy between dynamical and evolutionary masses for the Cepheid star in this system, contrary to previous claims of an overestimate of the Cepheid mass by stellar evolution theory.

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arXiv2010

On the magnitude difference between the main sequence turn off and the red giant branch bump in Galactic globular clusters

We present new measurements of the magnitude of the main sequence turn off and the red giant branch bump in the luminosity function of a sample of Galactic globular clusters with updated estimates of [Fe/H] and [$α$/Fe], employing photometric data collected with the Advanced Camera for Survey on board the HST. We compare measured and predicted values of the magnitude difference between these two features, a rarely employed diagnostic of the internal structure of low-mass stars at the beginning of their red giant evolution. Our analysis discloses a clear discrepancy between theory and observations, the theoretical red giant branch bump magnitudes being too bright by on average $\sim0.2$~mag. This corroborates results from the more widely studied magnitude difference between horizontal branch and red giant bump, avoiding the well known problems associated to the determination of the horizontal branch level from colour magnitude diagrams, and to uncertainties in the luminosity of horizontal branch stellar models. We briefly discuss several potential solutions of this discrepancy.

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arXiv2010

Period Change Similarities among the RR Lyrae Variables in Oosterhoff I and Oosterhoff II Globular Systems

We present period change rates (dP/dt) for 42 RR Lyrae variables in the globular cluster IC$\,$4499. Despite clear evidence of these period increases or decreases, the observed period change rates are an order of magnitude larger than predicted from theoretical models of this cluster. We find there is a preference for increasing periods, a phenomenon observed in most RR Lyrae stars in Milky Way globular clusters. The period-change rates as a function of position in the period-amplitude plane are used to examine possible evolutionary effects in OoI clusters, OoII clusters, field RR Lyrae stars and the mixed-population cluster $ω$~ Centauri. It is found that there is no correlation between the period change rate and the typical definition of Oosterhoff groups. If the RR Lyrae period changes correspond with evolutionary effects, this would be in contrast to the hypothesis that RR Lyrae variables in OoII systems are evolved HB stars that spent their ZAHB phase on the blue side of the instability strip. This may suggest that age may not be the primary explanation for the Oosterhoff types.

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arXiv2010

The impact of an updated $^{14}N(p,γ)^{15}O$ reaction rate on advanced evolutionary stages of low-mass stellar models

We have investigated the impact of the $^{14}N(p,γ)^{15}O$ reaction rate recently redetermined by the LUNA experiment, on the shell H-burning and core He-burning phases of low-mass, metal poor stellar models. The new reaction rate has small but noticeable effects, the largest one being a $\sim$7-8\% reduction of the red giant branch lifetimes. To different degrees, the lifetimes and luminosities of horizontal branch models, the mass of the stellar models evolving within the RR Lyrae instability strip, the luminosity of the red giant branch luminosity function bump, the theoretical calibrations of the R-parameter and tip of the red giant branch luminosity are also affected. Predictions for the tip of the red giant branch luminosity, in particular, are in very good agreement with the currently available empirical constraints.

astro-ph.SR↗

arXiv2010

Helium in first and second-generation stars in Globular Clusters from spectroscopy of red giants

(abridged) Recent spectroscopic and photometric observations show the existence of various generations of stars in GCs, differing in the abundances of products of H-burning at high temperatures (the main final product being He). It is important to study the connections between stars properties and He content. We consider here the about 1400 stars on the Red Giant Branch (RGB) observed with FLAMES@VLT in 19 Galactic GCs, part of out Na-O anticorrelation projet. Stars with different He are expected to have different temperatures (i.e. colours), slightly different [Fe/H], and different luminosity levels of the RGB bump. All these differences are small, but our study has the necessary precision, good statistics, and homogeneity to detect them. We also computed suitable sets of stellar models (BaSTI) for various assumptions about the initial helium content. Differences in observable quantities that can be attributed to variations in He content are generally detectable between stars of the Primordial (P, first-generation) and Extreme (E, second-generation) populations, but not between the Primordial and Intermediate ones (I). The only exception (differences are significant also between P and I populations) is NGC2808, where three populations are clearly separated also on the Main Sequence and the Horizontal Branch. The average enhancement in the He mass fraction Y between P and E stars is about 0.05-0.11, depending on the assumptions. The differences in Y, for NGC2808 alone, are about 0.11-0.14 between P and I stars, and about 0.15-0.19 between P and E stars, again depending on the assumptions. The RGB bump luminosity of first and second-generation stars has different levels; the implied Y difference is more difficult to quantify, but is in agreement with the other determinations.

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arXiv2010

The impact of enhanced He and CNONa abundances on globular cluster relative age-dating methods

The impact that unrecognised differences in the chemical patterns of Galactic globular clusters have on their relative age determinations is studied. The two most widely used relative age-dating methods, horizontal and vertical, together with the more recent relative MS-fitting method, were carefully analyzed on a purely theoretical basis. The BaSTI library was adopted to perform the present analysis. We find that relative ages derived using the horizontal and vertical methods are largely dependent on the initial He content and heavy element distribution. Unrecognized cluster-to-cluster chemical abundance differences can lead to an error in the derived relative ages as large as ~0.5 (or ~6 Gyr if an age of 12.8 Gyr is adopted for normalization), and even larger for some extreme cases. It is shown that the relative MS-fitting method is by far the age-dating technique for which undetected cluster-to-cluster differences in the He abundance have less impact. Present results are used in order to pose constraints on the maximum possible spread in the He and CNONa elements abundances on the basis of the estimates - taken from the literature - of the Galactic globular clusters relative age dispersion obtained with the various relative age-dating techniques. Finally, it is shown that the age-metallicity relation found for young Galactic globular clusters by the GC Treasury program is a real age sequence and cannot be produced by variations in the He and/or heavy element distribution.

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arXiv2010

Properties of stellar generations in Globular Clusters and relations with global parameters

ABRIDGED) We revise the formation of Galactic GCs by adding the detailed chemical composition of their different stellar generations (from 1200 giants in 19 GCs) to their global parameters. We propose to identify as GCs those showing the Na-O anticorrelation, and we classify the GCs according to kinematics and location in the Galaxy in disk/bulge, inner, and outer halo. We find that the LF of GCs is fairly independent of their population, suggesting that it is imprinted by the formation mechanism, and only marginally affected by the ensuing evolution. We show that a large fraction of the primordial population should have been lost by the proto-GCs. The extremely low Al abundances found for the primordial population of massive GCs indicate a very fast enrichment process before the formation of the primordial population. We suggest a scenario for the formation of GCs including at least 3 main phases: i) the formation of a precursor population (likely due to the interaction of cosmological structures similar to those leading to dwarf spheroidals, but residing at smaller Rgc, with the early Galaxy or with other structures), ii) which triggers a large episode of star formation (the primordial population), and iii) the formation of the current GC, mainly within a cooling flow formed by the slow winds of a fraction of the primordial population. The precursor population is very effective in raising the metal content in massive and/or metal poor (mainly halo) clusters, while its role is minor in small and/or metal rich (mainly disk) ones. Finally, we use PCA and multivariate relations to study the phase of metal-enrichment from 1st to 2nd generation. Most of the chemical signatures of GCs may be ascribed to a few parameters, the most important being [Fe/H], mass, and age of the cluster, with the location within the Galaxy also playing some role.

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arXiv2010

The ACS LCID Project. II. Faint Variable Stars in the Isolated Dwarf Irregular Galaxy IC1613

(Abridged) We present the results of a new search for variable stars in the Local Group (LG) isolated dwarf galaxy IC1613, based on 24 orbits of F475W and F814W photometry from the ACS camera onboard the HST. We detected 259 candidate variables in this field, of which only 13 (all of them bright Cepheids) were previously known. Out of the confirmed variables, we found 90 RR Lyrae stars, 49 classical Cepheids (including 36 new discoveries), and 38 eclipsing binary stars for which we could determine a period. The RR Lyrae include 61 fundamental (RRab) and 24 first-overtone (RRc) pulsators, and 5 pulsating in both modes simultaneously (RRd). As for the majority of LG dwarfs, the mean periods of the RRab and RRc (0.611 and 0.334 day, respectively) as well as the fraction of overtone pulsators (f_c=0.28) place this galaxy in the intermediate regime between the Oosterhoff types. From their position on the period-luminosity diagram and light-curve morphology, we can unambiguously classify 25 and 14 Cepheids as fundamental and first-overtone mode pulsators, respectively. Another two are clearly second-overtone Cepheids, the first ones to be discovered beyond the Magellanic Clouds. We estimate the distance to IC1613 using various methods based on the photometric and pulsational properties of the Cepheids and RR Lyrae stars. The values we find are in very good agreement with each other and with previous estimates based on independent methods. When corrected to a common reddening of E(B-V)=0.025 and true LMC distance modulus of (m-M)_{LMC,0}=18.515+-0.085, we find that all the distance determinations from the literature converge to a common value of (m-M)_0=24.400+-0.014 (statistical), or 760 kpc.

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arXiv2009

Stellar models: firm evidence, open questions and future developments

During this last decade our knowledge of the evolutionary properties of stars has significantly improved. This result has been achieved thanks to our improved understanding of the physical behavior of stellar matter in the thermal regimes characteristic of the different stellar mass ranges and/or evolutionary stages. This notwithstanding, the current generation of stellar models is still affected by several, not negligible, uncertainties related to our poor knowledge of some thermodynamical processes and nuclear reaction rates, as well as the efficiency of mixing processes. These drawbacks have to be properly taken into account when comparing theory with observations, to derive evolutionary properties of both resolved and unresolved stellar populations. In this paper we review the major sources of uncertainty along the main evolutionary stages, and emphasize their impact on population synthesis techniques.

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arXiv2009

The ACS LCID Project. I. Short-Period Variables in the Isolated Dwarf Spheroidal Galaxies Cetus & Tucana

(abridged) We present the first study of the variable star populations in the isolated dwarf spheroidal galaxies (dSph) Cetus and Tucana. Based on Hubble Space Telescope images obtained with the Advanced Camera for Surveys in the F475W and F814W bands, we identified 180 and 371 variables in Cetus and Tucana, respectively. The vast majority are RR Lyrae stars. In Cetus we also found three anomalous Cepheids, four candidate binaries and one candidate long-period variable (LPV), while six anomalous Cepheids and seven LPV candidates were found in Tucana. Of the RR Lyrae stars, 147 were identified as fundamental mode (RRab) and only eight as first-overtone mode (RRc) in Cetus, with mean periods of 0.614 and 0.363 day, respectively. In Tucana we found 216 RRab and 82 RRc giving mean periods of 0.604 and 0.353 day. These values place both galaxies in the so-called Oosterhoff Gap, as is generally the case for dSph. We calculated the distance modulus to both galaxies using different approaches based on the properties of RRab and RRc, namely the luminosity-metallicity and period-luminosity-metallicity relations, and found values in excellent agreement with previous estimates using independent methods: (m-M)_{0,Cet}=24.46+-0.12 and (m-M)_{0,Tuc}=24.74+-0.12, corresponding to 780+-40 kpc and 890+-50 kpc. We also found numerous RR Lyrae variables pulsating in both modes simultaneously (RRd): 17 in Cetus and 60 in Tucana. Tucana is, after Fornax, the second dSph in which such a large fraction of RRd (~17%) has been observed. We provide the photometry and pulsation parameters for all the variables, and compare the latter with values from the literature for well-studied dSph of the Local Group and Galactic globular clusters.

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