Searcharxiv⌕ Search

arXiv subjects

Elisabeth Vangioni-Flam

Publications and source records attributed to Elisabeth Vangioni-Flam.

34 records · Page 2Linked to original sources
arXiv2000

Big Bang Nucleosynthesis updated with the NACRE Compilation

We update the Big Bang Nucleosynthesis calculations on the basis of the recent NACRE compilation. The average values of the calculated abundances of light nuclei do not differ significantly from that obtained using the previous Fowler's compilation. ${^7}Li$ is slightly depressed at high baryon to photon ratio $η$. The main uncertainty concerns the $D(p,γ){^3}He$ reaction rate affecting the synthesis of ${^7}Li$ (via the ${^3}He(α,γ){^7}Be(eν){^7}Li$) at rather high baryonic density. On the left part of the lithium valley the uncertainty is strongly reduced due to the improvement of the measurement of the $T(α,γ)^{7}Li$ reaction rate. We use lithium-7 as the main baryometer, since, though much efforts have been devoted to the determination of Deuterium in absorbing clouds in the line of sight of remote quasars, the statistics is very poor compared to the long series of lithium measurements. Taking into account the lithium constraints, two possible baryonic density ranges emerge, $η_{10}= 1.4 - 1.9$ and $η_{10} = 3.3 - 5.1$. The Be and B abundances produced in the big bang are orders of magnitudes lower, and spallation of fast carbon and oxygen is probably their unique source, in the early Galaxy.

astro-ph↗
arXiv2000

LiBeB Production and Associated Astrophysical Sites

The various modes of spallative LiBeB production are summarized, and classified according to their dependence or independence on the abundance of medium heavy elements (CNO) illustrated by that of oxygen in the interstellar medium. The predictions of the models are confronted to the available observational correlations (Be, B vs O). Clearly, a primary mechanism should lead to a slope one in the lg(Be/H) vs [O/H] plot and a secondary mechanism to a slope two. Due to the ambiguity of the O data, another criterion, based on energetics, can help us to select an adequate model. A purely secondary origin in the very early Galaxy is much more energy demanding than a primary one. Indeed, magnesium seems to be a possible surrogate of oxygen and iron since i) it is spectroscopically more easy to cope with and ii) its nucleosynthetic yield is independent of the mass cut and does not depend on metallicity.

astro-ph↗
arXiv1999

Testing Spallation Processes With Beryllium and Boron

The nucleosynthesis of Be and B by spallation processes provides unique insight into the origin of cosmic rays. Namely, different spallation schemes predict sharply different trends for the growth of LiBeB abundances with respect to oxygen. ``Primary'' mechanisms predict BeB $\propto$ O, and are well motivated by the data if O/Fe is constant at low metallicity. In contrast, ``secondary'' mechanisms predict BeB $\propto$ O$^2$ and are consistent with the data if O/Fe increases towards low metallicity as some recent data suggest. Clearly, any primary mechanism, if operative, will dominate early in the history of the Galaxy. In this paper, we fit the BeB data to a two-component scheme which includes both primary and secondary trends. In this way, the data can be used to probe the period in which primary mechanisms are effective. We analyze the data using consistent stellar atmospheric parameters based on Balmer line data and the continuum infrared flux. Results depend sensitively on Pop II O abundances and, unfortunately, on the choice of stellar parameters. When using recent results which show O/Fe increasing toward lower metallicity, a two-component Be-O fits indicates that primary and secondary components contribute equally at [O/H]$_{eq}$ = -1.8 for Balmer line data; and [O/H]$_{eq}$ = -1.4 to -1.8 for IRFM. We apply these constraints to recent models for LiBeB origin. The Balmer line data does not show any evidence for primary production. On the other hand, the IRFM data does indicate a preference for a two-component model, such as a combination of standard GCR and metal-enriched particles accelerated in superbubbles. These conclusions rely on a detailed understanding of the abundance data including systematic effects which may alter the derived O-Fe and BeB-Fe relations.

astro-ph↗
arXiv1999

Lithium-Beryllium-Boron : Origin and Evolution

The origin and evolution of Lithium-Beryllium-Boron is a crossing point between different astrophysical fields : optical and gamma spectroscopy, non thermal nucleosynthesis, Big Bang and stellar nucleosynthesis and finally galactic evolution. We describe the production and the evolution of Lithium-Beryllium-Boron from Big Bang up to now through the interaction of the Standard Galactic Cosmic Rays with the interstellar medium, supernova neutrino spallation and a low energy component related to supernova explosions in galactic superbubbles.

astro-ph↗
arXiv1999

Nucleosynthesis and Gamma Ray-Line Astronomy

The most energetic part of the electromagnetic spectrum bears the purest clues to the synthesis of atomic nuclei in the universe. The decay of radioactive species, synthesized in stellar environments and ejected into the interstellar medium, gives rise to specific gamma ray lines. The observations gathered up to now show evidence for radioactivities throughout the galactic disk, in young supernova remnants (Cas A, Vela), and in nearby extragalactic supernovae (SN 1987A, SN 1991T and SN1998bu), in the form of specific gamma ray lines resulting, respectively, from the radioactive decay of 26Al, 44Ti and 56Co. The various astrophysical sites of thermal nucleosynthesis of the radioactive nuclei were discussed: AGB and Wolf-Rayet stars, novae, and type Ia and type II supernovae. Nuclear excitations by fast particles also produce gamma ray lines which have been observed in great detail from solar flares, and more hypothetically from active star forming regions where massive supernovae and WR stars abound. This non thermal process and its nucleosynthetic consequences was reviewed. The 511 keV line arising from e+ + e- annihilation also provides important information on explosive nucleosynthesis, as well as on the nature of the interstellar medium where the positrons annihilate. INTEGRAL, the main mission devoted to high resolution nuclear spectroscopy, should lead to important progress in this field.

astro-ph↗
arXiv1999

Cosmic Lithium-Beryllium-Boron Story

Light element nucleosynthesis is an important chapter of nuclear astrophysics. Specifically, the rare and fragile light nuclei Lithium, Beryllium and Boron (LiBeB) are not generated in the normal course of stellar nucleosynthesis (except Li7) and are, in fact, destroyed in stellar interiors. This characteristic is reflected in the low abundance of these simple species. Optical measurements of the beryllium and boron abundances in halo stars have been achieved by the 10 meter KECK telescope and the Hubble Space Telescope.These observations indicate a quasi linear correlation between Be and B vs Fe, at least at low metallicity. Aside GCRs, which are accelerated in the general interstellar medium (ISM) and create LiBeB through the break up of CNO by fast protons and alphas, Wolf-Rayet stars (WR) and core collapse supernovae (SNII) grouped in superbubbles could produce copious amounts of light elements via the fragmentation in flight of rapid carbon and oxygen nuclei colliding with H and He in the ISM.

astro-ph↗
arXiv1998

Lithium-6 : Evolution from Big Bang to Present

The primordial abundances of Deuterium, he4, and li7 are crucial to determination of the baryon density of the Universe in the framework of standard Big Bang nucleosynthesis (BBN). li6 which is only produced in tiny quantities and it is generally not considered to be a cosmological probe. However, recent major observational advances have produced an estimate of the li6/li7 ratio in a few very old stars in the galactic halo which impacts the question whether or not the lithium isotopes are depleted in the outer layers of halo stars, through proton induced reactions at the base of (or below) the convective zone. li6 is a pure product of spallation through the major production reactions, fast oxygen and alphas interacting on interstellar H, He (especially in the early Galaxy). The rapid nuclei are both synthesized and accelerated by SN II. In this context, the \li6 evolution should go in step with that of beryllium and boron, recently observed by the Keck and HST telescopes. Li6 adds a new constraint on the early spallation in the Galaxy. In particular, if confirmed, the Li6/Be9 ratio observed in two halo stars (HD 84937, BD +263578) gives strong boundary conditions on the composition and the spectrum of the rapid particles involved. We show that Li6 is essentially intact in halo stars, and a fortiori \li7. We can define a range of the Li6 abundance in the very early Galaxy consistent with Big Bang nucleosynthesis (5.6 10(-14) to 3. 10(-13) . Following the evolution at increasing metallicity, we explain the abundance in the solar system within a factor of about 2.

astro-ph↗
arXiv1998

Testing the Primary Origin of Be and B in the Early Galaxy

Two types of models have been proposed to explain the linear rise of the Be and B abundances as a function of iron observed in metal poor halo stars. In both cases, this linearity indicates that freshly synthesized C and O are accelerated by Type II supernovae and subsequently fragmented into Be and B. One mechanism advocates shock acceleration in the gaseous phase of superbubbles excavated by collective SNII explosions. Because of their short lifetimes, only the most massive stars (with an initial mass greater than 60\msun) do not drift out of superbubbles, and participate in BeB production. The second mechanism is based on the acceleration of the debris of grains formed in the ejecta of all SNIIs (originating from stars with initial mass greater than 8\msun). Here again, fresh C and O are sped up to cosmic ray energies by shocks. We propose a possible test to discriminate between the two scenarios. If supernovae of all masses are involved in BeB production, the Be/Fe ratio is constant, since both elements are produced in the same events. Alternatively, when only the most massive stars are involved in Be production, Be/Fe is enhanced at very early times because of the shorter lifetimes of these stars. This predicted difference in the behavior of Be/Fe could be tested by high quality observations at [Fe/H] $\lsim -3$. We also note that the solution invoking only the most massive supernovae mimics a flat evolution of both Be/H and B/H as a function of Fe/H at low metallicity, and could thus resemble a "plateau" for these elements despite a lack of a primordial Big Bang nucleosynthesis origin. Consequently, there may be no need to invoke inhomogeneous Big Bang models to explain the initial production of BeB should a plateau be discovered.

astro-ph↗
arXiv1998

Star Formation Histories versus Redshift : Consequences for Overall Metallicity and Deuterium Destruction

The flood of new data on deep surveys, and above all the CFRS (Canada-France-Redshift-Survey), has had a great impact on studies of galactic evolution. On the basis of cosmological models consistent with the improved values of the Hubble parameter, different star formation histories are tested against the observed UV, B and IR broad band comoving luminosity densities. Using these spectrophotometric results, we analyze the global metal enrichment with the help of chemical evolutionary models and we discuss the pertinence of different metallicity tracers (quasar absorption systems and clusters of galaxies) as representative of the bulk chemical evolution of the Universe. Moreover, as deuterium is very fragile, this isotope is destroyed in all stars and its evolution is particularly sensitive to the history of star formation. Relying on models constrained to fit the solar vicinity, it is shown that models with high D destruction corresponding to a large decrease of the star formation rate (SFR) from $z=1.5$ to 0 are in good agreement with spectrophotometric data. In contrast, low D destruction models which require only a moderate variation of the SFR in the same redshift range seem to encounter difficulties in matching the evolution of the luminosity densities (UV, B and IR) versus redshift. The sensitivity of the results with the cosmological models of the universe is discussed.

astro-ph↗
arXiv1996

The Effects of an Early Galactic Wind on the Evolution of D, \he3 and Z

The predictions of the abundances of D and \he3 from Big Bang Nucleosynthesis (BBN) and recent observations of these two isotopes suggest the need to develop new chemical evolution models. In particular, we examine the role of an early episode of massive star formation that would induce a strong destruction of D and a galactic wind. We discuss the ability of these models to match the observed local properties of the solar neighborhood such as the gas mass fraction, oxygen abundance, the age-metallicity relation, and the present-day mass function (PDMF). We also examine in detail the ability of the chemical evolution models discussed to reproduce the apparent lack of low mass, low metallicity stars in the solar neighborhood, namely the G-dwarf distribution. Indeed, we find models which satisfy the above constraints while at the same time allowing for a large primordial D/H ratio as is reportedly measured in some quasar absorption systems at high $z$, without the overproduction of heavy elements. The latter constraint is achieved by employing a simple dynamical model for a galactic wind.

astro-ph↗
arXiv1996

Nuclear and Gamma-ray Production by Supernova Ejecta

The fastest ejecta of supernova explosions propagate as a precursor to the main supernova shock wave, and can be quite energetic. The spectrum of such fast ejecta is estimated based on recent analytic and numerical supernova models, and found to be a power law having a cutoff at an energy of order 10 MeV/nucl, the precise value of which depends on the supernova mass and energetics. With cutoffs in this range there can be significant flux with energies above the thresholds for $γ$-ray and Li, Be, and B production. These nuclear interactions should inevitably accompany the passage of prompt supernova ejecta through the surrounding medium. The fast particle composition is that of the outermost layers of the progenitor; if the progenitor experienced significant loss of its envelope to a companion or by mass loss, then the composition is nonsolar, and in particular, metal-rich. Such a composition and spectrum of fast particles is required to explain the recent COMPTEL observations of $γ$-ray emission in Orion. Supernova ejecta from one progenitor having lost a large amount of mass are shown to quantitatively account for the Orion $γ$ rays, and to imply the presence of other, weaker lines and of a supernova explosion in Orion in the last $\la 10^4$ years. Implications for this mechanism as a mode of nucleosynthesis, and as a potential supernova diagnostic, are discussed. Model dependences and uncertainties are noted, and the need is shown for accurate measurements of $γ$-ray and spallation production cross sections at energies near threshold.

astro-ph↗
arXiv1995

LiBeB Production by Nuclei and Neutrinos

The production of LiBeB isotopes by nuclear and neutrino spallation are compared in the framework of galactic evolutionary models. As motivated by $γ$-ray observations of Orion, different possible sources of low-energy C and O nuclei are considered, such as supernovae of various masses and WC stars. We confirm that the low energy nuclei (LEN), injected in molecular clouds by stellar winds and type II supernovae originating from the most massive progenitors, can very naturally reproduce the observed Be and B evolution in the early galaxy (halo phase). Assuming the global importance of the LEN component, we compute upper and lower bounds to the neutrino process contribution corresponding to limiting cases of LEN particle spectra. A consistent solution is found with a spectrum of the kind proposed by Ramaty \etal (1995a,b), e.g. flat up to $E_c=30$ MeV/n and decreasing abruptly above. This solution fulfills the challenge of explaining at the same time the general Be and B evolution, and their solar system abundances without overproducing \li7 at very low metallicities, and the meteoritic \b11/\b10 ratio. In this case, neutrino spallation is constrained to play a limited role in the genesis of the solar system \b11. Galactic cosmic rays (GCR) become operative late in the evolution of the disk ([Fe/H]$>$-1), but their contribution to the solar abundances of \be9, \b10 and \b11 is not dominant (35\%, 30\% and 20\% respectively). Thus, with this LEN spectrum, GCR are {\it not}\ the main source of \be9 and B in the Galaxy. The most favorable case for neutrinos, (adopting the same kind of spectrum) has $E_c=20$ MeV/n. Even in this case, the neutrino yields of Woosley and Weaver (1995) must to be reduced by a factor of 5 to avoid \b11 overproduction. Furthermore, this solution leads to a high B/Be

astro-ph↗
arXiv1994

What's The Problem With ^3He?

We consider the galactic evolutionary history of \he3 in models which deplete deuterium by as much as a factor of 2 to $\sim$ 15 from its primordial value to its present day observed value in the ISM. We show that when \he3 production in low mass stars (1 -- 3 $M_\odot$) is included over the history of the galaxy, \he3 is greatly over-produced and exceeds the inferred solar values and the abundances determined in galactic \hii regions. Furthermore, the ISM abundances show a disturbing dispersion which is difficult to understand from the point of view of standard chemical evolution models. In principle, resolution of the problem may lie in either 1) the calculated \he3 production in low mass stars; 2) the observations of the \he3 abundance; or 3) an observational bias towards regions of depleted \he3. Since \he3 observations in planetary nebula support the calculated \he3 production in low mass stars, option (1) is unlikely. We will argue for option (3) since the \he3 interstellar observations are indeed made in regions dominated by massive stars in which \he3 is destroyed. In conclusion, we note that the problem with \he3 seems to be galactic and not cosmological.

astro-ph↗
arXiv1993

On the Galactic Evolution of $D$ and $^3He$

The determined abundances of primordial $^4He$ and $^7Li$ provide a basis with which to test the standard model of big bang nucleosynthesis in conjunction with the other two light element isotopes $D$ and $^3He$, also produced in the big bang. Overall, consistency in the standard big bang nucleosynthesis model is best achieved for a baryon-to-photon ratio of typically $3 \times 10^{-10}$ for which the primordial value of $D$ is five times greater than the present observed abundance and about three times greater than the pre-solar value. We consider various models for the chemical evolution of the Galaxy to test the feasibility for the destruction of D without the overproduction of $^3He$ and overall metallicity. Models which are capable of achieving this goal include ones with a star formation rate proportional to the gas mass fraction or an exponentially decreasing star formation rate. We discuss the effect of parameters that govern the initial mass function and of surviving fractions of $^3He$ in stars between one and three solar masses.

astro-ph↗
arXiv1993

Neutrino Process Nucleosynthesis and the $^{11}$B/$^{10}$B Ratio

We consider the evolution of the light elements ($Li, Be$ and $B$) incorporating the effects of their production by both neutrino process and cosmic-ray nucleosynthesis. We test the viability of the neutrino process to resolve the long standing problem of the $^{11}$B/$^{10}$B isotopic ratio which amounts to 4 at the time of the formation of the solar system. This hypothesis may be ultimately constrained by the $B/Be$ ratio observed in halo stars. Though we are able to obtain a solar isotopic ratio $^{11}B/^{10}B \simeq 4$, the current paucity of data at low metallicity prevents us from making a definitive conclusion regarding the resolution of this problem. We show however, that neutrino process nucleosynthesis leads to a relatively model independent prediction that the $B/Be$ elemental ratio is large ($>$ 50) at low metallicities ($[Fe/H] < -3.0$), if $Be$ is produced as a secondary element (as is the case in the conventional scenario of galactic cosmic-ray nucleosynthesis).

astro-ph↗