arXiv subjects

Nikos Prantzos

Publications and source records attributed to Nikos Prantzos.

52 records · Page 3Linked to original sources

arXiv2004

Li, Be, B and Cosmic Rays in the Galaxy

A short overview is presented of current issues concerning the production and evolution of Li, Be and B in the Milky Way. It is argued that the currently popular idea that Galactic cosmic rays are accelerated inside metal-rich superbubbles (which leads ``naturally'' to the production of primary Be and B, as observed) encounters the same problems as the previously popular idea of supernovae accelerating their own ejecta. A major challenge to theories of light element production is presented by the recent (and still preliminary) data suggesting a surprisingly high and $\sim$constant abundance of $^6$Li in halo stars; attempts to explain such a ``plateau'' are critically examined.

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arXiv2004

The early phases of Milky Way's chemical evolution

The earliest phases of the chemical evolution of our Galaxy are analysed in the light of the recent VLT results (concerning abundance patterns in the most metal-poor stars of the Galactic halo) and of stellar nucleosynthesis calculations. It is argued that: 1) the unexpected abundance patterns observed in Pop. II stars are not the imprints of an early generation of supermassive Pop. III stars; 2) among the various suggestions made to exlain the observed abundance patterns, nucleosynthesis in asymmetric supernova explosions appears most promising. In the latter case, an indirect correlation between asymmetry and metallicity is suggested by the data. Finally, the VLT data confirm two old ``puzzles'': the existence of primary N early in Galaxy's evolution (which constrains the mixing of protons with He-burning products in massive stars) and the absence of dispersion in abundance ratios, at least up to the Fe peak, in the early Galaxy (which bears on the timescales of homogeneisation of the interstellar medium, but also on yield variations among massive stars).

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arXiv2004

Astrophysical gamma-ray lines: a probe of stellar nucleosynthesis and star formation

Astrophysical gamma-ray spectroscopy is a most valuable tool for studying nuclear astrophysics, as well as recent star formation in the Milky Way. After a short, historical, introduction to the field, I present a brief review of the most important current issues. Emphasis is given to radioactivities produced by massive stars and associated supernova explosions, and in particular, those related to observations presently carried out by INTEGRAL: short-lived Ti-44 from CasA and SN1987A and long-lived Al-26 and Fe-60 from massive stars; various candidate sources of positrons for the 511 keV emission of the Galactic bulge are also critically discussed.

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arXiv2003

On the relative frequencies of core-collapse supernovae sub-types: the role of progenitor metallicity

We show that the observed ratio of the subtypes Ib,c and II core-collapse supernovae depends on the metallicity of the host galaxy, as expected on theoretical grounds. However, the observed relation differs considerably from expectations based on non-rotating models of single stars with mass loss. We argue that the predictions of recent models with rotation offer a much better agreement with observations, at least for progenitor stars with solar metallicity; calculations of models with higher and lower metallicities are required in order to substantiate these conclusions. We also suggest that systematic surveys of core collapse supernovae up to redshift of 1 with the SNAP satellite would allow to probe the effect of metallicity on supernovae properties during the past history of the universe.

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arXiv2003

On the early evolution of the Galactic Halo

It is shown that the low metallicity tail of the stellar metallicity distribution predicted by simple Outflow models for the Milky Way halo depends sensitively on whether Instantaneous Recycling is adopted or relaxed. In both cases, current - and still preliminary - data suggest a ``G-dwarf problem'' for the halo (reminiscent of the local disk). We suggest that the problem can be solved by introducing a (physically motivated) early infall phase. We point out several important implications of such a modification, concerning: the putative Pop. III (super)massive stars, the number of stars expected at very low metallicities, the questions of primary nitrogen and of the dispersion in abundance ratios of halo stars.

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arXiv2003

Jet-Induced Nucleosynthesis in Misaligned Microquasars

The jet axes and the orbital planes of microquasar systems are usually assumed to be approximately perpendicular, eventhough this is not currently an observational requirement. On the contrary, in one of the few systems where the relative orientations are well-constrained, V4641Sgr, the jet axis is known to lie not more than ~36 degrees from the binary plane. Such a jet, lying close to the binary plane, and traveling at a significant fraction of the speed of light may periodically impact the secondary star initiating nuclear reactions on its surface. The integrated yield of such nuclear reactions over the age of the binary system (less the radiative mass loss) will detectably alter the elemental abundances of the companion star. This scenario may explain the anomalously high Li enhancements (roughly ~20-200 times the sun's photospheric value; or, equivalently, 0.1-1 times the average solar system value) seen in the companions of some black-hole X-ray binary systems. (Such enhancements are puzzling since Li nuclei are exceedingly fragile - being easily destroyed in the interiors of stars - and Li would be expected to be depleted rather than enhanced there.) Gamma-ray line signatures of the proposed process could include the 2.22 MeV neutron capture line as well as the 0.478 MeV 7Li* de-excitation line, both of which may be discernable with the INTEGRAL satellite if produced in an optically thin region during a large outburst. For very energetic jets, a relatively narrow neutral pion gamma-decay signature at 67.5 MeV could also be measurable with the GLAST satellite. We argue that about 10-20% of all microquasar systems ought to be sufficiently misaligned as to be undergoing the proposed jet-secondary impacts.

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arXiv2003

Evolution of CNO abundances in the Universe

After summarizing the most important features of current stellar yields of CNO elements (including recent results concerning rotating and mass losing stars) I discuss how these yields may help to interpret relevant observations in the local Galaxy, the Milky Way disk and extragalactic systems (extragalactic HII regions and DLAs).

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arXiv2002

The Milky Way and the evolution of disk galaxies

We present a phenomenological approach to the study of disk galaxy evolution, based on i) a detailed modelling of the Milky Way (used as a prototype disk galaxy) and ii) an extension of the model to other disks through some simple scaling relations, obtained in the framework of Cold Dark matter models. The main conclusion is that, on average, massive disks have formed the bulk of their stars earlier than their lower mass counterparts. It is not yet clear why the ``star formation hierarchy'' has been apparently opposite to the ``dark matter assembly'' hierarchy.

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arXiv2002

Stellar Radioactivities and diffuse gamma-ray line emission in the Milky Way

After a short historical introduction to the field of gamma-ray line astronomy with radioactivities, I present an overview of recent results concerning the massive star yields of those radioactivities. I comment on the implications of those results (concerning long-lived radioactivities, like Al-26 and Fe-60) for gamma-ray line astronomy, in the light of past (COMPTEL and GRIS) and forthcoming (INTEGRAL) observations

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arXiv2001

Deuterium at high redshift: primordial or evolved ?

On the basis of arguments from galactic chemical evolution we suggest that the recent observations of D/H vs. metallicity in several high redshift absorbers are best understood if the primordial D value is in the range 2-3 10$^{-5}$. This range points to a rather high baryonic density ($Ω_Bh^2$=0.019-0.026) compatible to the one obtained by recent estimates based on the Cosmic Microwave Background (CMB) anisotropy measurements. Slightly higher values (D/H=4 10$^{-5}$) are found in Lyman limit systems. Such values are still compatible with CMB estimates but, if taken at face value, they suggest a trend of decreasing D abundance with metallicity. We argue that special assumptions, like differential enrichment, are required to explain the data in that case. A clear test of such a differential enrichment would be an excess of products of low mass stars like C and/or N in those systems, but currently available data of N/Si in DLAs do not favour such a ``non-standard'' senario.

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arXiv2000

Abundance evolution of intermediate mass elements (C to Zn) in the Milky Way halo and disk

We present a comprehensive study of the evolution of the abundances of intermediate mass elements, from C to Zn, in the Milky Way halo and in the local disk. We use a consistent model to describe the evolution of those two galactic subsystems. The halo and the disk are assumed to evolve independently, both starting with gas of primordial composition, and in different ways: strong outflow is assumed to take place during the $\sim$1 Gyr of the halo formation, while the disk is built by slowly infalling gas. This description of the halo+disk evolution can correctly account for the main observational constraints (at least in the framework of simple models of galactic chemical evolution). We utilise then metallicity dependant yields to study the evolution of all elements from C and Zn. Comparing our results to an extensive body of observational data (including very recent ones), we are able to make a critical analysis of the successes and shortcomings of current yields of massive stars. Finally, we discuss qualitatively some possible ways to interpret the recent data on oxygen vs iron, which suggest that oxygen behaves differently from the other alpha-elements.

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arXiv2000

The role of Star Formation in the evolution of spiral galaxies

Spiral galaxies offer a unique opportunity to study the role of star formation in galaxy evolution and to test various theoretical star formation schemes. I review some recent relevant work on the evolution of spiral galaxies. Detailed models are used for the chemical and spectrophotometric evolution, with metallicity dependent stellar yields, tracks and spectra. The models are ``calibrated'' on the Milky Way disk and generalised to other spirals with some simple scaling relations, obtained in the framework of Cold Dark Matter models for galaxy formation. The results compare favourably to the main observables of present day spirals, provided a crucial assumption is made: massive disks form their stars earlier than low mass ones. It is not clear whether this picture is compatible with currently popular hierarchical models of galaxy evolution. The resulting abundance gradients are found to be anticorrelated to the disk scalelength, support radially dependent star formation efficiencies and point to a kind of ``homologuous evolution'' for spirals.

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arXiv1999

Yields of Massive Stars and their Role in Galactic Chemical Evolution Studies

We review the yields of intermediate mass elements (from C to Zn) from massive stars and their associated uncertainties, in the light of recent theoretical results. We consider the role of those yields for our understanding of the chemical evolution of the solar neighbourhood and of the halo of our Galaxy. Current yields reproduce in a satisfactory way the solar system composition, but several problems remain concerning abundance ratios in halo stars.

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arXiv1999

Cosmic radioactivities

Radionuclides with half-lives ranging from some years to billions of years presumably synthesized outside of the solar system are now recorded in ``live'' or ``fossil'' form in various types of materials, like meteorites or the galactic cosmic rays. They bring specific astrophysical messages the deciphering of which is briefly reviewed here, with special emphasis on the contribution of Dave Schramm and his collaborators to this exciting field of research. Short-lived radionuclides are also present in the Universe today, as directly testified by the gamma-ray lines emitted by the de-excitation of their daughter products. A short review of recent developments in this field is also presented.

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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.

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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).

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