Searcharxiv⌕ Search

arXiv subjects

Alessandro Chieffi

Publications and source records attributed to Alessandro Chieffi.

At least 37 records · Page 2Linked to original sources
arXiv2016

White Paper on Nuclear Astrophysics

This white paper informs the nuclear astrophysics community and funding agencies about the scientific directions and priorities of the field and provides input from this community for the 2015 Nuclear Science Long Range Plan. It summarizes the outcome of the nuclear astrophysics town meeting that was held on August 21-23, 2014 in College Station at the campus of Texas A&M University in preparation of the NSAC Nuclear Science Long Range Plan. It also reflects the outcome of an earlier town meeting of the nuclear astrophysics community organized by the Joint Institute for Nuclear Astrophysics (JINA) on October 9- 10, 2012 Detroit, Michigan, with the purpose of developing a vision for nuclear astrophysics in light of the recent NRC decadal surveys in nuclear physics (NP2010) and astronomy (ASTRO2010). The white paper is furthermore informed by the town meeting of the Association of Research at University Nuclear Accelerators (ARUNA) that took place at the University of Notre Dame on June 12-13, 2014. In summary we find that nuclear astrophysics is a modern and vibrant field addressing fundamental science questions at the intersection of nuclear physics and astrophysics. These questions relate to the origin of the elements, the nuclear engines that drive life and death of stars, and the properties of dense matter. A broad range of nuclear accelerator facilities, astronomical observatories, theory efforts, and computational capabilities are needed. With the developments outlined in this white paper, answers to long standing key questions are well within reach in the coming decade.

astro-ph.SR↗
arXiv2014

Supernova dust formation and the grain growth in the early universe: The critical metallicity for low-mass star formation

We investigate the condition for the formation of low-mass second-generation stars in the early universe. It has been proposed that gas cooling by dust thermal emission can trigger fragmentation of a low-metallicity star-forming gas cloud. In order to determine the critical condition in which dust cooling induces the formation of low-mass stars, we follow the thermal evolution of a collapsing cloud by a one-zone semi-analytic collapse model. Earlier studies assume the dust amount in the local universe, where all refractory elements are depleted onto grains, and/or assume the constant dust amount during gas collapse. In this paper, we employ the models of dust formation and destruction in early supernovae to derive the realistic dust compositions and size distributions for multiple species as the initial conditions of our collapse calculations. We also follow accretion of heavy elements in the gas phase onto dust grains, i.e., grain growth, during gas contraction. We find that grain growth well alters the fragmentation property of the clouds, and that this still does not approach to the value in the local universe. The critical conditions can be written by the gas metallicity Zcr and the initial depletion efficiency fdep,0 of gas-phase metal onto grains, or dust-to-metal mass ratio, as (Zcr/10^{-5.5} Zsun) = (fdep,0/0.18)^{-0.44} with small scatters in the range of Zcr = [0.06--3.2]x10^{-5} Zsun. We also show that the initial dust composition and size distribution are important to determine Zcr.

astro-ph.SR↗
arXiv2014

The origin of the most iron-poor star

We investigate the origin of carbon-enhanced metal-poor (CEMP) stars starting from the recently discovered $\rm [Fe/H]<-7.1$ star SMSS J031300 (Keller et al. 2014). We show that the elemental abundances observed on the surface of SMSS J031300 can be well fit by the yields of faint, metal free, supernovae. Using properly calibrated faint supernova explosion models, we study, for the first time, the formation of dust grains in such carbon-rich, iron-poor supernova ejecta. Calculations are performed assuming both unmixed and uniformly mixed ejecta and taking into account the partial destruction by the supernova reverse shock. We find that, due to the paucity of refractory elements beside carbon, amorphous carbon is the only grain species to form, with carbon condensation efficiencies that range between (0.15-0.84), resulting in dust yields in the range (0.025-2.25)M$_{\odot}$. We follow the collapse and fragmentation of a star forming cloud enriched by the products of these faint supernova explosions and we explore the role played by fine structure line cooling and dust cooling. We show that even if grain growth during the collapse has a minor effect of the dust-to-gas ratio, due to C depletion into CO molecules at an early stage of the collapse, the formation of CEMP low-mass stars, such as SMSS J031300, could be triggered by dust cooling and fragmentation. A comparison between model predictions and observations of a sample of C-normal and C-rich metal-poor stars supports the idea that a single common pathway may be responsible for the formation of the first low-mass stars.

astro-ph.SR↗
arXiv2014

Dust grain growth and the formation of the extremely primitive star SDSS J102915+172927

Dust grains in low-metallicity star-forming regions may be responsible for the formation of the first low-mass stars. The minimal conditions to activate dust-induced fragmentation require the gas to be pre-enriched above a critical dust-to-gas mass ratio Dcr=[2.6--6.3]x10^-9 with the spread reflecting the dependence on the grain properties. The recently discovered Galactic halo star SDSS J102915+172927 has a stellar mass of 0.8 Msun and a metallicity of Z=4.5x10^-5 Zsun and represents an optimal candidate for the dust-induced low-mass star formation. Indeed, for the two most plausible Population III supernova progenitors, with 20 Msun and 35 Msun, the critical dust-to-gas mass ratio can be overcome provided that at least 0.4 Msun of dust condenses in the ejecta, allowing for moderate destruction by the reverse shock. Here we show that even if dust formation in the first supernovae is less efficient or strong dust destruction does occur, grain growth during the collapse of the parent gas cloud is sufficiently rapid to activate dust cooling and likely fragmentation into low-mass and long-lived stars. Silicates and magnetite grains can experience significant grain growth in the density range 10^9 /cc < nH<10^12 /cc by accreting gas-phase species (SiO, SiO2, and Fe) until their gas-phase abundance drops to zero, reaching condensation efficiencies =1. The corresponding increase in the dust-to-gas mass ratio allows dust-induced cooling and fragmentation to be activated at 10^12 /cc < nH < 10^14 /cc, before the collapsing cloud becomes optically thick to continuum radiation. We show that for all the initial conditions that apply to the parent cloud of SDSS J102915+172927, dust-driven fragmentation is able to account for the formation of the star.

astro-ph.GA↗
arXiv2012

The evolution of rotating solar metallicity stars extending in mass from 13 to 120 msun: the hydrostatic evolution and the explosive yields

We present the first set of a new generation of models of massive stars of solar composition extending between 13 and 120 \msun, computed with and without the effects of rotation. We included two instabilities induced by rotation, namely the meridional circulation and the shear instability. We implemented two alternative schemes to treat the transport of the angular momentum: the advection-diffusion formalism and the simpler purely diffusive one. The full evolution from the Pre Main Sequence up to the presupernova stage is followed in detail with a very extended nuclear network. The explosive yields are provided for a variety of possible mass cut and are available at the website \url{http://www.iasf-roma.inaf.it/orfeo/public{\_}html}. We find that both the He and the CO core masses are larger than those of their non rotating counterparts. Also the C abundance left by the He burning is lower than in the non rotating case, especially for stars of initial mass 13-25 \msun, and this affects the final Mass-Radius relation, basically the final binding energy, at the presupernova stage. The elemental yields produced by a generation of stars rotating initially at 300 km/s do not change substantially with respect to those produced by a generation of non rotating massive stars, the main differences being a slight overproduction of the weak s-component and a larger production of F. Since rotation also affects the mass loss rate, either directly and indirectly, we find substantial differences in the lifetimes as O-type and WR-subtypes between rotating and non rotating models. The maximum mass exploding as type IIP supernova ranges between 15 and 20\msun in both sets of models (this value depending basically on the larger mass loss rates in the Red Super Giant phase due to the inclusion of the dust driven wind). This limiting value is in remarkable good agreement with current estimates.

astro-ph.SR↗
arXiv2012

The formation of the extremely primitive star SDSS J102915+172927 relies on dust

The relative importance of metals and dust grains in the formation of the first low-mass stars has been a subject of debate. The recently discovered Galactic halo star SDSS J102915+172927 (Caffau et al. 2011) has a mass less than 0.8 Msun and a metallicity of Z = 4.5 10^{-5} Zsun. We investigate the origin and properties of this star by reconstructing the physical conditions in its birth cloud. We show that the observed elemental abundance trend of SDSS J102915+172927 can be well fitted by the yields of core-collapse supernovae with metal-free progenitors of 20 Msun and 35 Msun. Using these selected supernova explosion models, we compute the corresponding dust yields and the resulting dust depletion factor taking into account the partial destruction by the supernova reverse shock. We then follow the collapse and fragmentation of a star forming cloud enriched by the products of these SN explosions at the observed metallicity of SDSS J102915+172927. We find that [0.05 - 0.1] Msun mass fragments, which then lead to the formation of low-mass stars, can occur provided that the mass fraction of dust grains in the birth cloud exceeds 0.01 of the total mass of metals and dust. This, in turn, requires that at least 0.4 Msun of dust condense in the first supernovae, allowing for moderate destruction by the reverse shock. If dust formation in the first supernovae is less efficient or strong dust destruction does occur, the thermal evolution of the SDSS J102915+172927 birth cloud is dominated by molecular cooling, and only > 8 Msun fragments can form. We conclude that the observed properties of SDSS J102915+172927 support the suggestion that dust must have condensed in the ejecta of the first supernovae and played a fundamental role in the formation of the first low-mass stars.

astro-ph.CO↗
arXiv2011

The Effects of Thermonuclear Reaction-Rate Variations on 26Al Production in Massive Stars: a Sensitivity Study

We investigate the effects of thermonuclear reaction rate variations on 26Al production in massive stars. The dominant production sites in such events were recently investigated by using stellar model calculations: explosive neon-carbon burning, convective shell carbon burning, and convective core hydrogen burning. Post-processing nucleosynthesis calculations are performed for each of these sites by adopting temperature-density-time profiles from recent stellar evolution models. For each profile, we individually multiplied the rates of all relevant reactions by factors of 10, 2, 0.5 and 0.1, and analyzed the resulting abundance changes of 26Al. Our simulations are based on a next-generation nuclear physics library, called STARLIB, which contains a recent evaluation of Monte Carlo reaction rates. Particular attention is paid to quantifying the rate uncertainties of those reactions that most sensitively influence 26Al production. For stellar modelers our results indicate to what degree predictions of 26Al nucleosynthesis depend on currently uncertain nuclear physics input, while for nuclear experimentalists our results represent a guide for future measurements. We tabulate the results of our reaction rate sensitivity study for each of the three distinct massive star sites referred to above. It is found that several current reaction rate uncertainties influence the production of 26Al. Particularly important reactions are 26Al(n,p)26Mg, 25Mg(alpha,n)28Si, 24Mg(n,gamma)25Mg and 23Na(alpha,p)26Mg. These reactions should be prime targets for future measurements. Overall, we estimate that the nuclear physics uncertainty of the 26Al yield predicted by the massive star models explored here amounts to about a factor of 3.

astro-ph.SR↗
arXiv2010

Radioactivities in Low- and Intermediate-Mass Stars

Energy in stars is provided by nuclear reactions, which, in many cases, produce radioactive nuclei. When stable nuclei are irradiated by a flux of protons or neutrons, capture reactions push stable matter out of stability into the regime of unstable species. The ongoing production of radioactive nuclei in the deep interior of the Sun via proton-capture reactions is recorded by neutrinos emitted during radioactive decay and detected on Earth. Radioactive nuclei that have relatively long half lives may also be detected in stars via spectroscopic observations and in stardust recovered from primitive meteorites via laboratory analysis. The vast majority of these stardust grains originated from Asymptotic Giant Branch (AGB) stars. This is the final phase in the evolution of stars initially less massive than ~10 solar masses, during which nuclear energy is produced by alternate hydrogen and helium burning in shells above the core. The long-lived radioactive nucleus 26Al is produced in massive AGB stars (>4:5 solar masses), where the base of the convective envelope reaches high temperatures. Several other long-lived radioactive nuclei, including 60Fe, 87Rb, and 99Tc, are produced in AGB stars when matter is exposed to a significant neutron flux leading to the synthesis of elements heavier than iron. Here, neutron captures occur on a timescale that is typically slower than beta-decay timescales, resulting slow neutron captures (the s-process). However, when radioactive nuclei with half lives greater than a few days are produced they may either decay or capture a neutron, thus branching up the path of neutron captures and defining the final s-process abundance distribution. This nucleosynthesis in AGB stars could produce some long-living radioactive nuclei in relative abundances that resemble those observed in the early solar system.

astro-ph.SR↗
arXiv2009

Why do low-mass stars become red giants?

We revisit the problem of why stars become red giants. We modify the physics of a standard stellar evolution code in order to determine what does and what does not contribute to a star becoming a red giant. In particular, we have run tests to try to separate the effects of changes in the mean molecular weight and in the energy generation. The implications for why stars become red giants are discussed. We find that while a change in the mean molecular weight is necessary (but not sufficient) for a 1 solar mass star to become a red giant, this is not the case in a star of 5 solar masses. It therefore seems that there may be more than one way to make a giant.

astro-ph.SR↗
arXiv2008

Heavy element abundances in giant stars of the globular clusters M4 and M5

We present a comprehensive abundance analysis of 27 heavy elements in bright giant stars of the globular clusters M4 and M5 based on high resolution, high signal-to-noise ratio spectra obtained with the Magellan Clay Telescope. We confirm and expand upon previous results for these clusters by showing that (1) all elements heavier than, and including, Si have constant abundances within each cluster, (2) the elements from Ca to Ni have indistinguishable compositions in M4 and M5, (3) Si, Cu, Zn, and all s-process elements are approximately 0.3 dex overabundant in M4 relative to M5, and (4) the r-process elements Sm, Eu, Gd, and Th are slightly overabundant in M5 relative to M4. The cluster-to-cluster abundance differences for Cu and Zn are intriguing, especially in light of their uncertain nucleosynthetic origins. We confirm that stars other than Type Ia supernovae must produce significant amounts of Cu and Zn at or below the clusters' metallicities. If intermediate-mass AGB stars or massive stars are responsible for the Cu and Zn enhancements in M4, the similar [Rb/Zr] ratios and (preliminary) Mg isotope ratios in both clusters may be problematic for either scenario. For the elements from Ba to Hf, we assume that the s- and r-process contributions are scaled versions of the solar s- and r-process abundances. We quantify the relative fractions of s- and r-process material for each cluster and show that they provide an excellent fit to the observed abundances.

astro-ph↗
arXiv2006

Presupernova evolution and explosion of massive stars with mass loss

We review the main properties of solar metallicity massive stars in the range 11-120 Msun. The influence of the mass loss on the hydrostatic burning stages as well as the final explosion is discussed in some detail. We find that the minimum masses that enter the WNL, WNE and WC stages are 30 Msun, 35 Msun and 40 Msun respectively; the limiting mass between stars exploding as SNII and SNIb/c is between 30 and 35 Msun; the limiting mass between stars forming neutron stars and black holes after the explosion is between 25-30 Msun. We also discuss the properties of the chemical yields integrated over a Salpeter IMF and we find that stars with M>=35 Msun contribute for ~60% to the production of C, N and for ~40% to the production Sc and s-process elements up to Zr, while they do not produce any intermediate mass element because of the large remnant masses

astro-ph↗
arXiv2006

The nucleosynthesis of Al26 and Fe60 in solar metallicity stars extending in mass from 11 to 120 Msun: the hydrostatic and explosive contributions

We present the \nuk{Al}{26} and \nuk{Fe}{60} yields produced by a generation of solar metallicity stars ranging in mass between 11 and 120\msun. We discuss the production sites of these $γ$ ray emitters and quantify the relative contributions of the various components. More specifically we provide the separate contribution of the wind, the C convective shell and the explosive Ne/C burning to the total \nuk{Al}{26} yield per each stellar model in our grid. We provide the contributions of the He convective shell, the C convective shell and the explosive Ne/C burning to the \nuk{Fe}{60} yield as well. From these computations we conclude that, at variance with current beliefs, \nuk{Al}{26} is mainly produced by the explosive C/Ne burning over most of the mass interval presently analyzed while \nuk{Fe}{60} is mainly produced by the C convective shell and the He convective shell.

astro-ph↗
arXiv2004

26Al and 60Fe from massive stars

We discuss the production of 26Al and 60Fe in two massive star models, namely a 25 and a 80 Msun, of initial solar metallicity.

astro-ph↗
arXiv2004

The Distance to NGC 5904 (M 5) via the Subdwarfs Main Sequence Fitting Method

We present a determination of the distance modulus of the globular cluster NGC 5904 (M 5), obtained by means of the subdwarf main-sequence fitting on the (V,V-I) color-magnitude diagram. The subdwarf sample has been selected from the HIPPARCOS catalog in a metallicity range homogeneous with the cluster ([Fe/H] \~= -1.1). Both the cluster and the subdwarfs have been observed with the same telescope+instrument+filters setup (namely, ESO-NTT equipped with the SUSI2 camera), in order to preserve homogeneity and reduce systematic uncertainties. A set of archival HST data has then been used to obtain a deep and precise ridge line. These have been accurately calibrated in the ground photometric system by using the NTT data and used to fit the cluster distance modulus. By adopting the most commonly accepted values for the reddening, E(B-V) = 0.035 and 0.03, we obtain respectively mu_0 = 14.44 +- 0.09 +- 0.07 and mu_0 = 14.41 +- 0.09 +- 0.07, in agreement with recent determinations.

astro-ph↗
arXiv2004

Explosive yields of massive stars from Z=0 to Z=Zsun

We present a new and homogeneous set of explosive yields for masses 13, 15, 20, 25, 30 and 35 Msun and metallicities Z=0, 10^{- 6}, 10^{- 4}, 10^{- 3}, 6x10^{- 3}, 2x10^{-2}. A wide network extending up to Mo has been used in all the computation. We show that at low metallicities (Z<10^{- 4}) the final yields do not depend significantly on the initial chemical composition of the models so that a scaled solar distribution may be safely assumed at all metallicities. Moreover, no elements above Zn are produced by any mass in the grid up to a metallicity ~10^{-3}. These yields are available for any choice of the mass cut upon request.

astro-ph↗
arXiv2003

Expected Changes of Supernovae with Redshift due to Evolution of their Progenitors

We have analyzed the influence of the stellar populations, from which SN progenitors come from, on the observational outcome, including the metal free Pop. III. We use our models to study the evolution of the progenitor, the subsequent explosion and the light curves. For Type Ia, the variation of the main sequence mass of the progenitor of the exploding WD, produces an offset in the maximum-decline relation of 0.2 mag. This effect is critical for the use of high redshift Type Ia SNe as cosmological standard candles. In contrast, the metallicity does not change the above relation (at maximum, Delta M_V < 0.06 mag). For Type II, we find a dependence of the light curve properties with both, main sequence mass and metallicity of the progenitor, and we identify a rather homogeneous subclass, {\it Extreme} II-P, that may be used as a quasi-standard candle. Note that, although not as good as Type Ia for distance determinations, Type II are expected to occur since the first stars were formed.

astro-ph↗
arXiv2003

On the origin of HE0107-5240, the most iron deficient star presently known

We show that the "puzzling" chemical composition observed in the extremely metal poor star HE0107-5240 may be naturally explained by the concurrent pollution of at least two supernovae. In the simplest possible model a supernova of quite low mass (~15 Msun), underwent a "normal" explosion and ejected ~0.06 Msun of 56Ni while a second one was massive enough (~35 Msun) to experience a strong fall back that locked in a compact remnant all the carbon-oxygen core. In a more general scenario, the pristine gas clouds were polluted by one or more supernovae of relatively low mass (less than ~25 Msun). The successive explosion of a quite massive star experiencing an extended fall back would have largely raised the abundances of the light elements in its close neighborhood.

astro-ph↗