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O. Straniero

Publications and source records attributed to O. Straniero.

At least 127 records · Page 7Linked to original sources
arXiv2003

The Chemical Composition of Carbon C(N) stars

A chemical study of normal Galactic C(N) carbon stars is presented. Abundances of Li, CNO isotopes and s-elements are derived. The derived abundances of s-elements nicely agree with theoretical s-process nucleosynthesis predictions during the AGB phase. However, the figures obtained for Li and the 12C/13C ratios might imply the existence of a non-standard mixing process during the AGB phase operating preferentially in low mass stars. The intrinsic or extrinsic nature of C(N) stars is also discussed.

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arXiv2002

Calibration of White Dwarf cooling sequences: theoretical uncertainty

White Dwarf luminosities are powerful age indicators, whose calibration should be based on reliable models. We discuss the uncertainty of some chemical and physical parameters and their influence on the age estimated by means of white dwarf cooling sequences. Models at the beginning of the white dwarf sequence have been obtained on the base of progenitor evolutionary tracks computed starting from the zero age horizontal branch and for a typical halo chemical composition (Z=0.0001, Y=0.23). The uncertainties due to nuclear reaction rates, convection, mass loss and initial chemical composition are discussed. Then, various cooling sequences for a typical white dwarf mass (M=0.6 Mo) have been calculated under different assumptions on some input physics, namely: conductive opacity, contribution of the ion-electron interaction to the free energy and microscopic diffusion. Finally we present the evolution of white dwarfs having mass ranging between 0.5 and 0.9 Mo. Much effort has been spent to extend the equation of state down to the low temperature and high density regime. An analysis of the latest improvement in the physics of white dwarf interiors is presented. We conclude that at the faint end of the cooling sequence (log L/Lo=-5.5) the present overall uncertainty on the age is of the order of 20%, which correspond to about 3 Gyr. We suggest that this uncertainty could be substantially reduced by improving our knowledge of the conductive opacity (especially in the partially degenerate regime) and by fixing the internal stratification of C and O.

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arXiv2002

s-Process Nucleosynthesis in Carbon Stars

We present the first detailed and homogeneous analysis of the s-element content in Galactic carbon stars of N-type. Abundances of Sr,Y, Zr (low-mass s-elements, or ls) and of Ba, La, Nd, Sm and Ce (high-mass s-elements, hs) are derived using the spectral synthesis technique from high-resolution spectra. The N-stars analyzed are of nearly solar metallicity and show moderate s-element enhancements, similar to those found in S stars, but smaller than those found in the only previous similar study (Utsumi 1985), and also smaller than those found in supergiant post-AGB stars. This is in agreement with the present understanding of the envelope s-element enrichment in giant stars, which is increasing along the spectral sequence M-->MS-->S-->SC-->C during the AGB phase. We compare the observational data with recent $s$-process nucleosynthesis models for different metallicities and stellar masses. Good agreement is obtained between low mass AGB star models (M < 3 M_o) and s-elements observations. In low mass AGB stars, the 13C(alpha, n)16O reaction is the main source of neutrons for the s-process; a moderate spread, however, must exist in the abundance of 13C that is burnt in different stars. By combining information deriving from the detection of Tc, the infrared colours and the theoretical relations between stellar mass, metallicity and the final C/O ratio, we conclude that most (or maybe all) of the N-stars studied in this work are intrinsic, thermally-pulsing AGB stars; their abundances are the consequence of the operation of third dredge-up and are not to be ascribed to mass transfer in binary systems.

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arXiv2002

The lithium content of the globular cluster NGC 6397

We make use of high resolution, high signal-to-noise ratio spectra of 12 turn-off stars in the metal-poor globular cluster NGC 6397 to measure its lithium content. We conclude that they all have the same lithium abundance A(Li) = 2.34 with a standard deviation of 0.056 dex. We use this result, together with Monte Carlo simulations, to estimate that the maximum allowed intrinsic scatter is of the order of 0.035 dex. This is a new stringent constraint to be fulfilled by stellar models which predict Li depletion. We argue that although a mild depletion of 0.1 -- 0.2 dex, as predicted by recent models, cannot be ruled out, there is no compelling reason for it. This fact, together with the good agreement with the Li abundance observed in field stars, supports the primordial origin of lithium in metal-poor stars. Taking the above value as the primordial lithium abundance implies a cosmic baryonic density which is either Omega_b h^2 = 0.016 +/- 0.004 or Omega_b h^2 = 0.005 ^{+0.0026}_{-0.0006}, from the predictions of standard big bang nucleosynthesis. The high baryonic density solution is in agreement with recent results on the primordial abundance of deuterium and 3He and on the estimates derived from the fluctuations of the cosmic microwave background.

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arXiv2001

The Kr85 s-process Branching and the Mass of Carbon Stars

We present new spectroscopic observations for a sample of C(N)-type red giants. These objects belong to the class of Asymptotic Giant Branch stars, experiencing thermal instabilities in the He-burning shell (thermal pulses). Mixing episodes called third dredge-up enrich the photosphere with newly synthesized C12 in the He-rich zone, and this is the source of the high observed ratio between carbon and oxygen (C/O > 1 by number). Our spectroscopic abundance estimates confirm that, in agreement with the general understanding of the late evolutionary stages of low and intermediate mass stars, carbon enrichment is accompanied by the appearance of s-process elements in the photosphere. We discuss the details of the observations and of the derived abundances, focusing in particular on rubidium, a neutron-density sensitive element, and on the s-elements Sr, Y and Zr belonging to the first s-peak. The critical reaction branching at Kr85, which determines the relative enrichment of the studied species, is discussed. Subsequently, we compare our data with recent models for s-processing in Thermally Pulsing Asymptotic Giant Branch stars, at metallicities relevant for our sample. A remarkable agreement between model predictions and observations is found. Thanks to the different neutron density prevailing in low and intermediate mass stars, comparison with the models allows us to conclude that most C(N) stars are of low mass (M < 3Mo). We also analyze the C12/C13 ratios measured, showing that most of them cannot be explained by canonical stellar models. We discuss how this fact would require the operation of an ad hoc additional mixing, currently called Cool Bottom Process, operating only in low mass stars during the first ascent of the red giant branch and, perhaps, also during the asymptotic giant branch.

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arXiv2001

The Implications of the New Z=0 Stellar Models and Yields on the Early Metal Pollution of the Intergalactic Medium

Motivated by the recent detection of metals in different components of the high redshift universe and by the abundance ratios measured in the extremely metal-poor stars of our Galaxy, we study the nucleosynthesis constraints that this imposes on an early generation of stars (Population III). To do so we take into account the chemical yields obtained from homogeneous evolutionary calculations of zero metal stars in the mass range $3\la m/M_\odot\la 40$ (Limongi et al. 2000, Chieffi et al. 2001). We also consider the role played by metal-free very massive objects (m$>100$ M$_\odot$). Using both analytical and numerical chemical evolution models, we confront model predictions from the different choices of the mass function proposed for Population III with the observational constraints. We show that low values of star formation efficiency ($<1%$) are required so as not to exceed the minimum metallicity ([C/H]$\approx -2.4$) measured in the high redshift systems for any of the IMFs proposed. We also show that the observational constraints require $Ω_{sr}< 3\times 10^{-3}Ω_b$, confirming previous claims that the possible contribution of the stellar remnants from Population III to the baryonic dark matter is insignificant. At present, however, the scarcity of abundance measurements for high redshift systems does not permit us to put severe limitations on the nature of the initial mass function for Population III. In fact, overabundances of alpha-elements with respect to iron of the order of those measured in damped Lyman-$α$ systems are obtained for any of the IMFs tested. Nevertheless, to account for the very large [C,N/Fe] ratios found in a considerable number of extremely metal-poor stars of our Galaxy, an IMF peaking at the intermediate stellar mass range (4-8 M$_\odot$) is needed.

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arXiv2001

Constraints on the Progenitors of SNeIa & Implications for the Cosmological Equation of State

Detailed calculations for the stellar evolution, explosion and light curves have been performed to quantify the influence of the main sequence mass M(MS) and the metallicity of the progenitor on the structure of the exploding WD which are thought to be the progenitors of SNe Ia. In particular,we study the effects of progenitors on the brightness decline relation M(dM_15) which is a corner stone for the use of SNe Ia as cosmological yard-stick.M(MS) has been identified as the decisive factor to change the energetics of the explosion and dominates the variations in the rise-time to decline relation of light curves. M(MS) has little effect on the color index B-V. For similar decline rates dM_15, the flux at maximum brightness relative to the flux on the radioactive tail decreases systematically with M(MS) by about 0.2mag. This change goes along with a reduc- tion of the photospheric expansion velocity $v_{ph}$ by about 2000 km/sec. A change in the central density has the opposite dependency. The metallicity Z affects mainly the intrinsic color index B-V by up to -0.06mag, and it alters the fluxes in the U band and the UV. B-V is critical if extinction corrections are applied. The spread in the fiducial rise-time to decline relation in local SNe Ia restricts the range of main sequence masses to a factor of 2. The upper limit of 1 day for the difference between the local and distance sample support the need for a positive cosmological constant. The size of evolutionary effects are small (dM abou 0.2mag) but are absolutely critical for the reconstruction of the cosmological equation of state.

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arXiv2000

The O-Na and Mg-Al Anticorrelations in Turn-Off and early Subgiants in Globular Clusters

High dispersion spectra (R>40,000) for a quite large number of stars at the main sequence turn-off and at the base of the subgiant branch in NGC6397 and NGC6752 were obtained with the UVES on Kueyen (VLT UT2). The [Fe/H] values we found are -2.03+/-0.02+/-0.04 and -1.42+/-0.02+/-0.04 for NGC6397 and NGC6752 respectively, where the first error bars refer to internal errors and the second ones to systematic errors. In both clusters the [Fe/H] obtained for TO-stars agree perfectly (within a few per cents) with that obtained for stars at the base of the RGB. The [O/Fe]=0.21+/-0.05 value we obtain for NGC6397 is quite low, but it agrees with previous results obtained for giants in this cluster; also, the star-to-star scatter in both O and Fe is very small, indicating that this small mass cluster is chemically very homogenous. On the other side, our results show clearly and for the first time that the O-Na anticorrelation (up to now seen only for stars on the red giant branches of globular clusters) is present among unevolved stars in the globular cluster NGC6752, a more massive cluster than NGC6397. A similar anticorrelation is present also for Mg and Al, and C and N. It is very difficult to explain the observed Na-O, and Mg-Al anticorrelation in NGC6752 stars by a deep mixing scenario; we think it requires some non internal mechanism.

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arXiv2000

Extreme Type IIp Supernovae as Yardsticks for Cosmology

Evolutionary effects with redshift of core collapse supernovae and their application to cosmology have been studied based on an extensive grid of stellar models between 13 and 25 Mo, and their light curves after the explosion. With decreasing metallicity Z and increasing mass, progenitors tend to explode as compact Blue Supergiants (BSG) and produce subluminous supernovae which are about 1.5 mag. dimmer compared to 'normal' SNeII with Red Supergiant (RSG) progenitors. Progenitors with small masses tend to explode as RSGs even with low Z. The consequences are obvious for probing the chemical evolution. We identify 'extreme SNe IIp' supernovae as a rather homogeneous class which may allow their use as standard candles for distance determination accurate within 30 %. Due to their unique light curves, no spectra need to be taken for their identification, and follow-up observations can be limited to a very small dynamical range in brightness. This means that distance determination by 'extreme SNe IIp" are possible up to redshifts of about 3 using 8-meter class telescopes. SIRTIF may push the limit by another magnitude.

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arXiv2000

Massive Stars in the Range $\rm 13-25 M_\odot$: Evolution and Nucleosynthesis. II. the Solar Metallicity Models

We present the evolutionary properties of a set of massive stellar models (namely 13, 15, 20 and 25 $\rm M_\odot$) from the main sequence phase up to the onset of the iron core collapse. All these models have initial solar chemical composition, i.e. Y=0.285 and Z=0.02. A 179 isotope network, extending from neutron up to $\rm ^{68}Zn$ and fully coupled to the evolutionary code has been adopted from the Carbon burning onward. Our results are compared, whenever possible, to similar computations available in literature.

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arXiv1999

Do zero metal intermediate mass stars experience thermal pulses?

We have studied the evolution of intermediate mass (M.ge.5Mo) zero metal (Z=0) stars with particular attention to the AGB phase. At variance with previous claims we find that these stars experience thermal instability (the so called thermal pulses). The critical quantity which controls the onset of a thermally pulsing phase is the amount of CNO in the envelope during the AGB. For these stars the central He burning starts in the blue side of the HR diagram and the 1^{st} dredge up does not take place. Then the envelope maintains its initial composition up to the beginning of the AGB phase. However, during the early AGB the 2^{nd} dredge-up occurs and fresh He and CNO elements are engulfed in the convective envelope. We find that in stars with M.ge.6Mo the resulting amount of ^{12}C is large enough to sustain a normal CNO burning within the H shell and consequently the star enters the usual thermal pulse phase. In the 5Mo model, owing to the lower ^{12}C enhancement in the envelope after the 2^{nd} dredge-up, the He burning shell suffers weak thermal instabilities. 9 of these thermal oscillation are needed before the He burning luminosity reaches 3x10^5 Lo and a first convective shell develops in between the two burning shells. Later on a second convective shell forms at the base of the H rich envelope. This convective zone cross the H/He discontinuity and partially overlaps the previous one, dredging up fresh ^{12}C. After a huge H flash, a quiescent CNO burning settle on. From this moment a thermal pulse phase starts, which is very similar to the one experienced by the more massive models.

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arXiv1999

Neutron capture in low mass Asymptotic Giant Branch stars: cross sections and abundance signatures

The recently improved information on the stellar (n,gamma) cross sections of neutron-magic nuclei at N = 82, and in particular of 142Nd, turned out to represent a sensitive test for models of s-process nucleosynthesis. While these data were found to be incompatible with the classical approach based on an exponential distribution of neutron exposures, they provide significantly better agreement between the solar abundance distribution of s nuclei and the predictions of models for low mass AGB stars. Particular attention is paid to a consistent description of s-process branchings in the region of the rare earth elements. It is shown that - in certain cases - the nuclear data are sufficiently accurate that the resulting abundance uncertainties can be completely attributed to stellar modelling. Thus, the s process becomes important for testing the role of different stellar masses and metallicities as well as for constraining the assumptions for describing the low neutron density provided by the 13C source.

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arXiv1999

The giant, horizontal and asymptotic branches of galactic globular clusters. I. The catalog, photometric observables and features

A catalog including a set of the most recent Color Magnitude Diagrams (CMDs) is presented for a sample of 61 Galactic Globular Clusters (GGCs). We used this data-base to perform an homogeneous systematic analysis of the evolved sequences (namely, Red Giant Branch (RGB), Horizontal Branch (HB) and Asymptotic Giant Branch (AGB)). Based on this analysis, we present: (1) a new procedure to measure the level of the ZAHB (V_ZAHB) and an homogeneous set of distance moduli obtained adopting the HB as standard candle; (2) an independent estimate for RGB metallicity indicators and new calibrations of these parameters in terms of both spectroscopic ([Fe/H]_CG97) and global metallicity ([M/H], including also the alpha-elements enhancement). The set of equations presented can be used to simultaneously derive a photometric estimate of the metal abundance and the reddening from the morphology and the location of the RGB in the (V,B-V)-CMD. (3) the location of the RGB-Bump (in 47 GGCs) and the AGB-Bump (in 9 GGCs). The dependence of these features on the metallicity is discussed. We find that by using the latest theoretical models and the new metallicity scales the earlier discrepancy between theory and observations (~0.4 mag) completely disappears.

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arXiv1999

The Luminosity Function of M3

We present a high precision, large sample luminosity function (LF) for the Galactic globular cluster M3. With a combination of ground based and Hubble Space Telescope data we cover the entire radial extent of the cluster. The observed LF is well fit by canonical standard stellar models from the red giant branch (RGB) tip to below the main sequence turnoff point. Specifically, neither the RGB LF-bump nor subgiant branch LF indicate any breakdown in the standard models. On the main sequence we find evidence for a flat initial mass function and for mass segregation due to the dynamical evolution of the cluster.

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arXiv1999

Galactic chemical evolution of heavy elements: from Barium to Europium

We follow the chemical evolution of the Galaxy for elements from Ba to Eu, using an evolutionary model suitable to reproduce a large set of Galactic (local and non local) and extragalactic constraints. Input stellar yields for neutron-rich nuclei have been separated into their s-process and r-process components. The production of s-process elements in thermally pulsing asymptotic giant branch stars of low mass proceeds from the combined operation of two neutron sources: the dominant reaction 13C(alpha,n)16O, which releases neutrons in radiative conditions during the interpulse phase, and the reaction 22Ne(alpha,n)25Mg, marginally activated during thermal instabilities. The resulting s-process distribution is strongly dependent on the stellar metallicity. For the standard model discussed in this paper, it shows a sharp production of the Ba-peak elements around Z = Z_sun/4. Concerning the r-process yields, we assume that the production of r-nuclei is a primary process occurring in stars near the lowest mass limit for Type II supernova progenitors. The r-contribution to each nucleus is computed as the difference between its solar abundance and its s-contribution given by the Galactic chemical evolution model at the epoch of the solar system formation. We compare our results with spectroscopic abundances of elements from Ba to Eu at various metallicities (mainly from F and G stars) showing that the observed trends can be understood in the light of the present knowledge of neutron capture nucleosynthesis. Finally, we discuss a number of emerging features that deserve further scrutiny.

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arXiv1998

s-process nucleosynthesis of 142^Nd: crisis of the classical model

The recently improved information on the stellar neutron capture cross sections of neutron magic nuclei at N = 82, and in particular for 142^Nd, turned out to represent a sensitive test for models of s-process nucleosynthesis. While these data were found to be incompatible with the classical approach, they provide significantly better agreement between the observed abundance distribution and the predictions of models for low mass AGB stars.

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arXiv1998

Galactic chemical evolution of Ba-peak elements

The chemical evolution of the Galaxy is followed for the elements affected by neutron capture, in particular for those in the atomic number range 56 to 63 (Ba, La, Ce, Pr, Nd, Sm and Eu). Neutrons by the major 13^C source, released in radiative conditions in the interpulse periods of TP-AGB stars, give rise to an efficient s-processing, making low-mass AGB the major contributors to the chemical evolution of heavy elements. The s-process scenario, characterized is analyzed using AGB stellar evolutionary calculations with the FRANEC Code (FRASCATI Raphson-Newton Evolutionary Code), and are applied over a wide range of stellar masses and metallicities. The presence of r-process elements in low metallicity stars is indicative of a prompt enrichment of the Galaxy by early generation of stars, and low mass SNII appear to be good candidates for primary production of r-nuclei. The chemical evolution model used here is organized over three-zone, halo, thick and thin disk.

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