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Nikos Prantzos

Publications and source records attributed to Nikos Prantzos.

At least 37 records · Page 2Linked to original sources
arXiv2013

Superbubble dynamics in globular cluster infancy II. Consequences for secondary star formation in the context of self-enrichment via fast rotating massive stars

The self-enrichment scenario for globular clusters (GC) requires large amounts of residual gas after the initial formation of the first stellar generation. Recently, Krause et al. (2012) found that supernovae may not be able to expel that gas, as required to explain their present day gas-free state, and suggested that a sudden accretion on to the dark remnants, at a stage when type II supernovae have ceased, may plausibly lead to fast gas expulsion. Here, we explore the consequences of these results for the self-enrichment scenario via fast rotating massive stars (FRMS). We analyse the interaction of FRMS with the intra-cluster medium (ICM), in particular where, when and how the second generation of stars may form. From the results, we develop a timeline of the first approximately 40 Myr of GC evolution. The results of Paper I imply three phases during which the ICM is in a fundamentally different state, namely the wind bubble phase (lasting 3.5 to 8.8 Myr), the supernova phase (lasting 26.2 to 31.5 Myr), and the dark remnant accretion phase (lasting 0.1 to 4 Myr): (i) Quickly after the first generation massive stars have formed, stellar wind bubbles compress the ICM into thin filaments. No stars may form in the normal way during this phase, due to the high Lyman-Werner flux density. If the first generation massive stars have however equatorial ejections, as we proposed in the FRMS scenario, accretion may resume in the shadow of the equatorial ejecta. The second generation stars may then form due to gravitational instability in these discs that are fed by both the FRMS ejecta and pristine gas. [...]

astro-ph.GA↗
arXiv2012

Superbubble dynamics in globular cluster infancy I. How do globular clusters first lose their cold gas?

The picture of the early evolution of globular clusters has been significantly revised in recent years. Current scenarios require at least two generations of stars of which the first generation (1G), and therefore also the protocluster cloud, has been much more massive than the currently predominating second generation (2G). Fast gas expulsion is thought to unbind the majority of the 1G stars. Gas expulsion is also mandatory to remove metal-enriched supernova ejecta, which are not found in the 2G stars. It has long been thought that the supernovae themselves are the agent of the gas expulsion, based on crude energetics arguments. Here, we assume that gas expulsion happens via the formation of a superbubble, and describe the kinematics by a thin-shell model. We find that supernova- driven shells are destroyed by the Rayleigh-Taylor instability before they reach escape speed for all but perhaps the least massive and most extended clusters. More power is required to expel the gas, which might plausibly be provided by a coherent onset of accretion onto the stellar remnants. The resulting kpc-sized bubbles might be observable in Faraday rotation maps with the planned Square Kilometre Array radio telescope against polarised background radio lobes if a globular cluster would happen to form in front of such a radio lobe.

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arXiv2011

Topics on Galactic Chemical Evolution

I discuss three different topics in Galactic chemical evolution:the "puzzling" absence of any observational signature of secondary elements ; the building of the Galactic halo in the framework of hierarchical galaxy formation, as evidenced from its metallicity distribution ; and the potentially important role that radial migration may play in the evolution of galactic disks, according to recent studies.

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arXiv2011

Nucleosynthesis and gamma-ray lines

Astrophysical gamma-ray spectroscopy is an invaluable tool for studying nuclear astrophysics, supernova structure, recent star formation in the Milky Way and mixing of nucleosynthesis products in the interstellar medium. 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 carried out by INTEGRAL: short-lived Ti-44 from CasA and long-lived Al-26 and Fe-60 from massive stars. The observed 511 keV emission from positron annihilation in the Galaxy and the role of stellar radioactivity and other potential positron sources are also discussed.

astro-ph.HE↗
arXiv2010

Origin of cosmic rays and evolution of spallogenic nuclides Li, Be and B

A short overview is presented of current issues concerning the production and evolution of Li, Be and B in the Milky Way. In particular, the observed "primary-like" evolution of Be is re-assessed in the light of a novel idea: it is argued that Galactic Cosmic Rays are accelerated from the wind material of rotating massive stars, hit by the forward shock of the subsequent supernova explosions. The pre-galactic levels of both Li isotopes remain controversial at present, making it difficult to predict their Galactic evolution. A quantitative estimate is provided of the contributions of various candidate sources to the solar abundance of Li.

astro-ph.GA↗
arXiv2008

The metallicity distribution of the halo and the satellites of the Milky way in the hierarchical merging paradigm

To account for the observed differential metallicity distribution (DMD) of the Milky Way halo, a semi-analytical model is presented in the framework of the hierarchical merging paradigm for structure formation. It is assumed that the Milky Way halo is composed of a number of sub-haloes with properties either as observed in the dwarf satellite galaxies of the Local group (shape of metallicity distribution, effective yield) or derived from calculations of structure formation (sub-halo distribution function). With reasonable assumptions for the parameters involved, we find that the overall shape and effective yield of the Galactic halo DMD can be reproduced in the framework of such a simple model. The low metallicity tail of the DMD presents a defficiency of stars with respect to the simple model predictions (akin to the G-dwarf problem in the solar neighborhood); it is suggested that an early infall phase can account for that problem, as well as for the observed DMDs of dwarf satellite galaxies.Accretion of galaxies similar (but not identical) to the progenitors of present day dwarf satellites of the Milky Way may well have formed the Galactic halo.

astro-ph↗
arXiv2007

An Introduction to Galactic Chemical Evolution

The formalism of the simple model of galactic chemical evolution (GCE) and its main ingredients (stellar properties, initial mass function, star formation rate and gas flows) are presented in this tutorial. It is stressed that GCE is not (yet) an astrophysical theory, but it merely provides a framework in which the large body of data concerning the chemical composition of stars and gas in galaxies may be intrepreted. A few examples illustrating those concepts are provided, through studies of the Solar neighborhood and of the Milky Way's halo.

astro-ph↗
arXiv2007

Light nuclei in galactic globular clusters : constraints on the self-enrichment scenario from nucleosynthesis

Hydrogen-burning is the root cause of the star-to-star abundance variations of light nuclei in Galactic globular clusters (GC). In the present work we constrain the physical conditions that gave rise to the observed abundance patterns of Li, C, N, O, Na, Mg, Al, as well as Mg isotopes in the typical case of NGC6752. We perform nucleosynthesis calculations at constant temperature, adopting realistic initial abundances for the proto-cluster gas. We use a detailed nuclear reaction network and state-of-the-art nuclear reaction rates. Although simplistic, our analysis provides original results and new constraints on the self-enrichment scenario for GCs. Our parametrized calculations allow us to determine a narrow range of temperature where the observed extreme abundances of all light elements and isotopes in NGC6752 are nicely reproduced simultaneously. This agreement is obtained after mixing of the H-processed material with 30 % of unprocessed gas. The observed C-N, O-Na, Mg-Al, Li-Na and F-Na anticorrelations, as well as the behaviour of the Mg isotopes can be recovered by assuming mixing with even larger dilution factors. Li production by the stars that build up the other abundance anomalies is not mandatory in the case of NGC 6752. Observations of O, Na, Mg and Al constrain the temperature range for H-burning; such temperatures are encountered in the two main candidate ``polluters'' proposed for GCs, namely massive AGBs and the most massive main-sequence stars. (Abridged).

astro-ph↗
arXiv2007

Origin and evolution of the light nuclides

After a short historical (and highly subjective) introduction to the field, I discuss our current understanding of the origin and evolution of the light nuclides D, He-3, He-4, Li-6, Li-7, Be-9, B-10 and B-11. Despite considerable observational and theoretical progress, important uncertainties still persist for each and every one of those nuclides. The present-day abundance of D in the local interstellar medium is currently uncertain, making it difficult to infer the recent chemical evolution of the solar neighborhood. To account for the observed quasi-constancy of He-3 abundance from the Big Bang to our days, the stellar production of that nuclide must be negligible; however, the scarce observations of its abundance in planetary nebulae seem to contradict this idea. The observed Be and B evolution as primaries suggests that the source composition of cosmic rays has remained quasi-constant since the early days of the Galaxy, a suggestion with far reaching implications for the origin of cosmic rays; however, the main idea proposed to account for that constancy, namely that superbubbles are at the source of cosmic rays, encounters some serious difficulties. The best explanation for the mismatch between primordial Li and the observed "Spite-plateau" in halo stars appears to be depletion of Li in stellar envelopes, by some yet poorly understood mechanism. But this explanation impacts on the level of the recently discovered early ``Li-6 plateau'', which (if confirmed), seriously challenges current ideas of cosmic ray nucleosynthesis.

astro-ph↗
arXiv2006

On the early chemical evolution of the Milky Way

A few topics concerning the early chemical evolution of the Milky Way are critically discussed. In particular, it is argued that: 1) Observed abundance patterns of extremely metal poor stars (of Pop. II) do not constrain the mass range of the first generation (Pop. III) stars; the latter may well be normal massive stars (10-50 Msun) or very massive ones (140-1000 Msun) or a combination of the two classes. 2) The discrepancy between primordial Li abundance (after WMAP) and the observed ``Spite plateau'' cannot be due to astration by a generation of massive Pop. III stars, as recently suggested, unless if such stars eject negligible amounts of metals. 3) The observed halo metallicity disribution may well be understood in the framework of hierarchical galaxy formation, as shown here with a simple semi-analytical model. 4) Formation of the Milky Way's halo from a myriad of smaller sub-haloes may have important implications for our understanding of the abundance patterns of r-elements, the origin of which remains still unclear.

astro-ph↗
arXiv2006

On the "Galactic Habitable Zone"

The concept of Galactic Habitable Zone (GHZ) was introduced a few years ago as an extension of the much older concept of Circumstellar Habitable Zone. However, the physical processes underlying the former concept are hard to identify and even harder to quantify. That difficulty does not allow us, at present, to draw any significant conclusions about the extent of the GHZ: it may well be that the entire Milky Way disk is suitable for complex life.

astro-ph↗
arXiv2006

Abundance Anomalies in Galactic Globular Clusters - Looking for the Stellar Culprits

Galactic globular cluster stars exhibit abundance patterns which are not shared by their field counterparts. It is clear from recent spectroscopic observations of GC turnoff stars that these abundance anomalies were already present in the gas from which the observed stars formed. This provides undisputed support to the so-called self-enrichment scenario according to which a large fraction of GC low-mass stars have formed from material processed through hydrogen-burning at high temperatures and then lost by more massive and faster evolving stars (and perhaps mixed with some original gas). Within this framework we present a new method to derive the Initial Mass Function of the polluter stars.

astro-ph↗
arXiv2006

On the self-enrichment scenario of galactic globular clusters: Constraints on the IMF

Galactic globular cluster (GC) stars exhibit abundance patterns which are not shared by their field counterparts, In the framework of the widely accepted "self-enrichment" scenario for GCs, we present a new method to derive the Initial Mass Function (IMF) of the polluter stars, by using the observed O/Na abundance distribution. We focus on NGC 2808, a GC for which the largest sample of O and Na abundance determinations is presently available. We consider two classes of possible "culprits" : massive Asymptotic Giant Branch (AGB) stars (4-9 Msun) and winds of massive stars (WMS) in the mass range 10-100 Msun. We obtain upper limits for the slope of the IMF (assumed to be given by a power-law) of the stars initially more massive than the present turnoff mass. We also derive lower limits for the amount of stellar residues. We find that the polluter IMF had to be much flatter than presently observed IMFs in stellar clusters, in agreement with the results of two other methods for GC IMF determination. Additionaly, we find that the present mass of the GC should be totally dominated by stellar remnants if the polluters were AGB stars, but not so in the case of WMS. We critically analyse the advantages and shortcomings of each potential polluter class, and we find the WMS scenario more attractive.

astro-ph↗
arXiv2006

Microquasar Jet Irradiation of the Proto-Solar Nebula?

We explore the possibility that a now-extinct microquasar may have irradiated the proto-solar neighborhood, causing the 'anomalously' high local 11B/10B isotopic ratio. Given the population and typical lifetimes of radio-emitting X-ray binaries, we find the probability of such an event having occurred is not unreasonable. We comment on some tests of the scenario that could be carried out by observing the elemental abundances in the vicinity of microquasars, in particular SS433.

astro-ph↗
arXiv2005

On the intensity and spatial morphology of the 511 keV emission in the Milky Way

The positron emissivity of the Galactic bulge and disk, resulting from radioactivity of SNIa, is reassessed in the light of a recent evaluation of the SNIa rate. It is found that the disk may supply more positrons than required by recent SPI/INTEGRAL observations, but the bulge (where the characteristic positron annihilation line at 511 keV is in fact observed) only about 10%. It is argued that a large fraction of the disk positrons may be transported via the regular magnetic field of the Galaxy into the bulge, where they annihilate. This would increase both the bulge emissivity and the bulge/disk ratio, alleviating considerably the constraints imposed by INTEGRAL data analysis. We argue that the bulge/disk positron emissivity ratio can be considerably smaller than the values derived by the recent analysis of Knoedlseder et al. (2005), if the disk positrons diffuse sufficiently away from their sources, as required by our model; this possibility could be tested in the future, as data are accumulated in the SPI detectors. The success of the proposed scenario depends critically upon the, very poorly known at present, properties of the galactic magnetic field and of the propagation of low energy positrons in it.

astro-ph↗
arXiv2005

The energetics, evolution, and stellar depletion of Li6 in the early Galaxy

Motivated by the recent report of a high Li6 ``plateau'' extending to low metallicities in Galactic halo stars, we study the energetics of an early production of Li6 through the interaction of energetic particles with the interstellar medium. We then explore the potential of various candidate sources of pre-galactic energetic particles and show that, in general, they fail to satisfy the observational and theoretical requirements (especially if the Li6 plateau is at a considerably higher level than observed). Succesful candidates appear to be: supernova explosions with abnormally low metal yield; the massive black hole in the Galactic center (provided that it was formed early on and that it was then radiating much more efficiently than today); and, perhaps, an early accretion phase of supermassive black holes in galaxies. Assuming that Li6 is indeed pre-galactic, we study the galactic evolution of the light isotopes Li7, Li6, and Be in a self-consistent way. We find that the existence of a Li6 plateau is hard to justify, unless a fine-tuned and metallicity-dependent depletion mechanism of Li6 in stellar envelopes is invoked. The depletion of Li6 should be different, both in magnitude and in its metallicity dependence, than the depletion required to explain current observations of Li (mostly Li7) in halo stars. If the recently reported Li6 ``plateau'' is confirmed, our analysis suggests important implications for our understanding of the production, evolution, and stellar depletion of the Li isotopes.

astro-ph↗
arXiv2005

Astrophysical constraints from gamma-ray spectroscopy

Gamma-ray lines from cosmic sources provide unique isotopic information, since they originate from energy level transitions in the atomic nucleus. Gamma-ray telescopes explored this astronomical window in the past three decades, detecting radioactive isotopes that have been ejected in interstellar space by cosmic nucleosynthesis events and nuclei that have been excited through collisions with energetic particles. Short-lived radioactivities have been detected in a couple of supernovae (56Co and 57Co in SN1987A, 44Ti in Cas A), the diffuse glow of long-lived 26Al has been mapped along the entire plane of the Galaxy, several excited nuclei have been detected in solar flares, and, last but not least, positron annihilation has been observed in the inner Galaxy since the 70ies. Recent imaging and line shape measurements of e-/e+ annihilation emission from the Galactic bulge can hardly be accounted for by conventional sources of positrons; recent 26Al emission and line width measurement from the inner Galaxy and from the Cygnus region can constrain the properties of the interstellar medium; a diffuse 60Fe gamma-ray line emission appears rather weak, in view of current theoretical predictions. Recent Galactic core-collapse supernovae are studied through 44Ti radioactivity, but, apart from Cas A, no other source has been found. The characteristic signature of 22Na-line emission from a nearby O-Ne-Mg novae is expected to be measured during INTEGRAL's lifetime.

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