arXiv subjects

S. Cassisi

Publications and source records attributed to S. Cassisi.

At least 253 records · Page 14Linked to original sources

arXiv2000

Hydrogen-Accreting Carbon-Oxygen White Dwarfs of Low Mass: Thermal and Chemical Behavior of Burning Shells

Numerical experiments have been performed to investigate the thermal behavior of a cooled down white dwarf of initial mass $M_{\rm WD} = 0.516 M_{\sun}$ which accretes hydrogen-rich matter with Z = 0.02 at the rate $\dot{M}=10^{-8}$ \msun \yrm1, typical for a recurrent hydrogen shell flash regime. The evolution of the main physical quantities of a model during a pulse cycle is examined in detail. From selected models in the mass range $M_{\rm WD} = 0.52÷0.68$ \msunend, we derive the borders in the $M_{\rm WD}$ - $\dot{M}$ plane of the steady state accretion regime when hydrogen is burned at a constant rate as rapidly as it is accreted. The physical properties during a hydrogen shell flash in white dwarfs accreting hydrogen-rich matter with metallicities Z = 0.001 and Z = 0.0001 are also studied. For a fixed accretion rate, a decrease in the metallicity of the accreted matter leads to an increase in the thickness of the hydrogen-rich layer at outburst and a decrease in the hydrogen-burning shell efficiency. In the $M_{\rm WD}$-$\dot{M}$ plane, the borders of the steady state accretion band are critically dependent on the metallicity of the accreted matter: on decreasing the metallicity, the band is shifted to lower accretion rates and its width in $\dot{M}$ is reduced.

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arXiv2000

Galactic Globular Clusters as a test for Very Low-Mass stellar models

We make use of the Next Generation model atmospheres by Allard et al. (1997) and Hauschildt, Allard & Baron (1999) to compute theoretical models for low and very low-mass stars for selected metallicities in the range Z= 0.0002 to 0.002. On this basis, we present theoretical predictions covering the sequence of H-burning stars as observed in galactic globulars from the faint end of the Main Sequence up to, and beyond, the cluster Turn Off. The role played by the new model atmospheres is discussed, showing that present models appear in excellent agreement with models by Baraffe et al. (1997) as computed on quite similar physical basis. One finds that the theoretical mass-luminosity relations based on this updated set of models, are in good agreement with the empirical data provided by Henry & McCarthy (1993). Comparison with HST observation discloses that the location in the Color-Magnitude diagram of the lower Main Sequence in galactic globular clusters appears again in good agreement with the predicted sensitive dependence of these sequences on the cluster metallicity.

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arXiv2000

On the helium content of Galactic globular clusters via the R parameter

We estimate the empirical R parameter in 26 Galactic Globular Clusters covering a wide metallicity range, imaged by WFPC2 on board the HST. The improved spatial resolution permits a large fraction of the evolved stars to be measured and permits accurate assessment of radial populaton gradients and completeness corrections. In order to evaluate both the He abundance and the He to metal enrichment ratio, we construct a large set of evolutionary models by adopting similar metallicities and different He contents. We find an absolute He abundance which is lower than that estimated from spectroscopic measurements in HII regions and from primordial nucleosynthesis models. This discrepancy could be removed by adopting a C12O16 nuclear cross section about a factor of two smaller than the canonical value, although also different assumptions for mixing processes can introduce systematical effects. The trend in the R parameter toward solar metallicity is consistent with an upper limit to the He to metal enrichment ratio of the order of 2.5.

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arXiv2000

Stellar evolution and large extra dimensions

We discuss in detail the information on large extra dimensions which can be derived in the framework of stellar evolution theory and observation. The main effect of large extra dimensions arises from the production of the Kaluza-Klein (KK) excitations of the graviton. The KK-graviton and matter interactions are of gravitational strength, so the KK states never become thermalized and always freely escape. In this paper we first pay attention to the sun. Production of KK gravitons is incompatible with helioseismic constraints unless the 4+n dimensional Planck mass M_s exceeds 300 Gev/c^2. Next we show that stellar structures in their advanced phase of H burning evolution put much more severe constraints, M_s > 3-4 TeV/c^2, improving on current laboratory lower limits.

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arXiv1999

Synthetic Stellar Clusters for Pop III

We present preliminary results of an incoming theoretical work concerning the integrated properties of the Population III clusters of stars. On the basis of synthetic Color-Magnitude Diagrams, we provide a grid of optical and near-IR colors of Simple Stellar Populations with very low metallicity (Z=10$^{-10}$ and Z=10$^{-6}$) and age which spans from 10 Myr to 15 Gyr. A comparison with higher metallicities up to 0.006 is also shown, disclosing sizable differences in the CMD morphology, integrated colors and Spectral Energy Distribution (SED).

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arXiv1999

Evolution of low-mass metal-free stars including effects of diffusion and external pollution

We investigate the evolution of low-mass metal-free Population III stars. Emphasis is laid upon the question of internal and external sources for CNO-elements, which - if present in sufficient amounts in the hydrogen-burning regions - lead to a strong modification of the stars' evolutionary behavior. For the production of carbon due to nuclear processes inside the stars, we use an extended nuclear network, demonstrating that hot pp-chains do not suffice to produce enough carbon or are less effective than the triple3-alpha-process. As an external source of CNO-elements we test the efficiency of pollution by a nearby massive star combined with particle diffusion. For all cases investigated, the additional metals fail to reach nuclear burning regions before deep convection on the Red Giant Branch obliterates the previous evolution. The surface abundance history of the polluted Pop III stars is presented. The possibilities to discriminate between a Pop II and a polluted Pop III field star are also discussed.

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arXiv1999

A new investigation on the Antlia Dwarf Galaxy

We present deep (I,V-I) and (I,B-I) color-magnitude diagrams (CMDs) of the Antlia dwarf galaxy, based on Science Verification (SV) data collected with the FORS I camera on the ESO Very Large Telescope (VLT). The CMDs present two key features: a well-defined RGB, and a sample of bright blue stars, belonging to a young stellar component. The comparison between theory and observations confirms that this sample of bright stars is consistent with the occurrence of a star formation episode $\approx0.1$ Gyr ago. In agreement with previous investigations (Sarajedini et al. 1997; Aparicio et al. 1997), we also find that this young stellar population is more centrally concentrated than the old one. By adopting the new calibration of the Tip of the RGB (TRGB) provided by Salaris & Cassisi (1998) we estimated that the Antlia distance modulus is $(m-M)_0=25.89\pm0.10$ mag, and therefore a distance $D=1.51\pm 0.07$ Mpc. This distance determination is $\approx13%$ larger than the values suggested in previous investigations. By adopting the calibration of the RGB $(V-I)$ color index as a function of the metallicity, we estimated that the mean metallicity of Antlia stellar population is of the order of $[Fe/H]\approx-1.3$. The differential RGB luminosity function shows an excess in the observed counts -at the $2σ$ level- when compared with theoretical predictions. Plain arguments on the dependence of the TRGB luminosity on stellar age suggest that this discrepancy might be due to a stellar component with an age approximately equal to 0.7 Gyr.

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arXiv1999

The Initial Mass Function of the Galactic Bulge Down to ~0.15 Msun

We present a luminosity function (LF) for lower main sequence stars in the Galactic bulge near (l,b)=(0,-6) to J=24, corresponding to M_J~9.3. This LF is derived from HST + NICMOS observations with the F110W and F160W filters. The main sequence locus in the infrared shows a strong change in slope at J~20.5 (M_J~5.75) which is well fit by new low mass models that include water and molecular hydrogen opacity. Our derived mass function (which is not corrected for binary companions) is the deepest measured to date in the bulge, and extends to 0.15 $M_\odot$ with a power law slope of $α=-1.33\pm0.07$; a Salpeter mass function would have $α=-2.35$. We also combine our J band LF with previously published data for the evolved stars to produce a bulge LF spanning ~15 magnitudes. We show that this mass function has negligible dependence on the adopted bulge metallicity and distance modulus. Although shallower than the Salpeter slope, the slope of the bulge IMF is steeper than that recently found for the Galactic disk ($α=-0.8$ and $α=-0.54$ from the data of Reid & Gizis, 1997, and Gould et al. 1997, respectively, in the same mass interval), but is virtually identical to the disk IMF derived by Kroupa et al. (1993). The bulge IMF is also quite similar to the mass functions derived for those globular clusters which are believed to have experienced little or no dynamical evolution. Finally, we derive the $M/L_J$ ratio of the bulge to be $\sim 0.9\pm0.1$ $M_\odot/L_\odot$, and briefly discuss the implications of this bulge IMF for the interpretation of the microlensing events observed in the direction of the Galactic bulge.

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arXiv1999

Nonradial modes in RR Lyrae stars

We present a survey of nonradial mode properties in evolutionary sequences of RR Lyrae star models. Attention is focused on the modes that may be driven by the opacity mechanism and on those that may be excited as a consequence of the 1:1 resonance with the radial pulsation. Qualitatively, all the models share the same properties of the nonradial modes. At the quantitative level, the properties are to a large extent determined by the radial mode periods. There is only weak dependence on the star metallicity and no apparent dependence on the evolutionary status, that is on the helium exhaustion in the convective core. In the whole range of RRab and RRc star parameters we find unstable nonradial modes driven by the opacity mechanism. An instability of radial pulsation to a resonant excitation of nonradial oscillations is also a common phenomenon in both types. We discuss a possible role of nonradial modes in amplitude modulation observed in certain RR Lyrae stars.

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arXiv1999

On the very long term evolutionary behavior of hydrogen-accreting Low-Mass CO white dwarfs

Hydrogen-rich matter has been added to a CO white dwarf of initial mass 0.516 \msun at the rates $10^{-8}$ and $2\times 10^{-8}$ \msun \yrm1, and results are compared with those for a white dwarf of the same initial mass which accretes pure helium at the same rates. For the chosen accretion rates, hydrogen burns in a series of recurrent mild flashes and the ashes of hydrogen burning build up a helium layer at the base of which a He flash eventually occurs. In previous studies involving accretion at higher rates and including initially more massive WDs, the diffusion of energy inward from the H shell-flashing region contributes to the increase in the temperature at the base of the helium layer, and the mass of the helium layer when the He flash begins is significantly smaller than in a comparison model accreting pure helium; the He shell flash is not strong enough to develop into a supernova explosion. In contrast, for the conditions adopted here, the temperature at the base of the He layer becomes gradually independent of the deposition of energy by H shell flashes, and the mass of the He layer when the He flash occurs is a function only of the accretion rate, independent of the hydrogen content of the accreted matter. When the He flash takes place, due to the high degeneracy at the base of the He layer, temperatures in the flashing zone will rise without a corresponding increase in pressure, nuclear burning will continue until nuclear statistical equilibrium is achieved; the model will become a supernova, but not of the classical type Ia variety.

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arXiv1999

Comparison between predicted and empirical DV(Bump -HB) in Galactic Globular Clusters

We present observational estimates of DV(Bump-HB) in a sample of 28 Galactic Globular Clusters (GGCs) observed by HST. The photometric accuracy and the sizable number of stars measured in each cluster allowed us to single out the RGB Bump both in metal-poor and in metal-rich GGCs. Empirical values are compared with homogeneous theoretical predictions which account for both H and He burning phases over a wide range of metal abundances (0.0001 < Z < 0.02). We found that, within current observational uncertainties on both iron and $α$-element abundances, theory and observations are in very good agreement, provided that the metallicity scale by Carretta & Gratton (1997) as extended by Cohen et al. (1999) is adopted. Interesting enough, we also found that both theoretical and observed values show a change in the slope of the DV(Bump-HB)-[M/H] relation toward higher metal contents.

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arXiv1998

A pulsational approach to the luminosity of Horizontal Branch stellar structures

We discuss an alternative approach to constrain the absolute bolometric luminosity of Zero Age Horizontal Branch structures by using the observational pulsational properties of ab type RR Lyrae stars and theoretical expectations concerning both the relation connecting the pulsational properties of these variables to their evolutionary ones, as luminosity, mass and effective temperature and, also the location in the H-R diagram for the fundamental pulsators instability strip boundaries. Since the main goal of this work is to obtain an evaluation of the ZAHB bolometric luminosity as much as possible independent on stellar evolution theory, we have minimized the use of evolutionary prescriptions, being the only adopted evolutionary input the allowed mass range for fundamental pulsators. Nevertheless, the effects on our final results related to the use of these evolutionary prescriptions have been carefully checked. The reliability of the suggested method to obtain the ZAHB luminosity is shown by applying it to a selected sample of globular clusters (GCs), whose heavy elements abundance covers almost all the complete GCs metallicity range. The results obtained for the ZAHB bolometric luminosities have been compared with evolutionary prescriptions on such quite important quantity as given by recent evolutionary computations has been also performed. The existence of evident mismatches between current results and some evolutionary models has been verified and discussed.

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arXiv1998

Stellar populations in the dwarf spheroidal galaxy Leo I

We present a detailed study of the color magnitude diagram (CMD) of the dwarf spheroidal galaxy Leo I, based on archival Hubble Space Telescope data. Our photometric analysis, confirming previous results on the brighter portion of the CMD, allow us to obtain an accurate sampling of the stellar populations also at the faint magnitudes corresponding to the Main Sequence. By adopting a homogeneous and consistent theoretical scenario for both hydrogen and central helium-burning evolutionary phases, the various features observed in the CMD are interpreted and reliable estimations for both the distance modulus and the age(s) for the main stellar components of Leo I are derived. More in details, from the upper luminosity of the Red Giant Branch and the lower luminosity of the Subgiant Branch we simultaneously constrain the galaxy distance and the age of the oldest stellar population in Leo I. In this way we obtain a distance modulus (m-M)_V=22.00$\pm$0.15 mag and an age of 10--15 Gyr or 9--13 Gyr, adopting a metallicity Z=0.0001 and 0.0004, respectively. The reliability of this distance modulus has been tested by comparing the observed distribution of the Leo I anomalous Cepheids in the period-magnitude diagram with the predicted boundaries of the instability strip, as given by convective pulsating models.

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arXiv1998

Galactic Globular Cluster Stars: from Theory to Observation

We use evolutionary calculations presented in a recent paper (Cassisi et al. 1998) to predict B,V,I magnitudes for stars in galactic globulars. The effect of the adopted mixing length on stellar magnitudes and colors is discussed, showing that the uncertainty on such a theoretical parameter prevents the use of MS stars as bona fide theoretical standard candles. However, comparison with Hipparcos data for field subdwarfs discloses a substantial agreement between theory and observation. Present predictions concerning the magnitude of TO and of HB stars are compared with similar results appeared in the recent literature. We present and discuss a theoretical calibration of the difference in magnitude between HB and TO as evaluated with or without element sedimentation. Finally we use theoretical HB magnitudes to best fit the CM diagram of M68 and M5, taken as representative of metal poor and intermediate metallicity galactic globulars, deriving an age of 11$\pm$1.0 Gyr and 10$\pm$1.0 Gyr, respectively, for the adopted chemical compositions, plus an additional uncertainty of $\pm$1.4 Gyr if the uncertainty on the chemical composition is taken into account. This result is discussed on the basis of current evaluations concerning cluster ages and distance moduli.

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arXiv1998

On the Blue Tail of Horizontal Branch Stars

We discuss theoretical predictions for ZAHB models constructed either by including or neglecting the He extra-mixing effect recently suggested by Sweigart (1997). The comparison in the $log T_e -log g$ plane suggests that within current observational uncertainties canonical hot HB models based on new input physics agree with empirical data in Galactic globular clusters and in the field. We also briefly discuss the impact of the new class of variable stars EC14026 on constraining the parameters of blue HB stars and their distribution along the tail.

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arXiv1998

A new analysis of the Red Giant Branch "Tip" distance scale and the value of the Hubble constant

The theoretical evaluations of the Red Giant Branch Tip (TRGB) luminosity presented in Salaris & Cassisi (1997) are extended to higher metallicities, and compared with analogous independent results recently published. The comparison between our ZAHB and TRGB distance scales for galactic globular clusters presented in Salaris & Cassisi (1997) is now supplemented with a comparison with the HIPPARCOS distance scale set by local subdwarfs with accurate parallax determinations. The overall agreement between ZAHB and HIPPARCOS distances is quite good. The ZAHB and TRGB distances to resolved galaxies are in good agreement, whereas the comparison between TRGB and Cepheid distances, computed by using the calibration suggested by Madore & Freedman (1991), reveals a systematic discrepancy of the order of 0.12 mag. The TRGB distances are systematically longer in comparison with the Cepheid ones. This result supports the case for a revision of the zero point of the Cepheid distance scale, as already suggested by other authors on the basis of HIPPARCOS parallaxes. The application of our TRGB distance scale to NGC3379, provides a new distance to the Leo I group, which coupled with recent independent determinations of the distance Coma cluster-Leo I, provides a determination of H_0 at the Coma cluster in the range: H_0= 60+-11 Km s^{-1} Mpc^{-1}.

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arXiv1997

The "Tip" of the Red Giant Branch as a distance indicator: theoretical calibration and the value of H_0

Updated theoretical relations for the run of the bolometric and I magnitude of the Tip of the Red Giant Branch (TRGB) with respect to the metallicity are provided. An analogous relation for the V magnitude of the Zero Age Horizontal Branch at the RR Lyrae instability strip is also provided. A comparison has been performed among our ZAHB and TRGB distances, the Cepheid distance scale by Madore & Freedman (1991) and the HIPPARCOS distances set by local subdwarfs with accurate parallax determinations. The application of our TRGB distance scale to NGC3379 provides a distance to the Leo I group that is about 8% higher than the one obtained by Sakai et al. (1997) adopting the TRGB brightness calibration by Da Costa & Armandroff (1990). Our distance to the Leo I group, coupled with the relative distance Coma cluster-Leo I, provides a determination of H_0 at the Coma cluster: H_0=64_{-9}^{+10} Km s^{-1} Mpc^{-1}.

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arXiv1997

Stellar models for very low mass main sequence stars: the role of model atmospheres

We present Very Low Mass stellar models as computed including non-grey model atmospheres for selected assumptions about the star metallicities. The role of atmospheres is discussed and the models are compared with models based on the Eddington approximation and with similar models appeared in the recent literature. Theoretical predictions concerning both the HR diagram location and the mass-luminosity relation are presented and discussed in terms of expectations in selected photometric bands. Comparison with available observational data concerning both galactic globular clusters and dwarfs in the solar neighborhood reveals a satisfactory agreement together with the existence of some residual mismatches.

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