Nikos Prantzos

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

Aims. Galactic globular cluster (GC) stars exhibit abundance patterns that are not shared by their field counterparts, e.g. the welldocumented O-Na and Mg-Al anticorrelations. Recent spectroscopic observations of GC turnoff stars have provided compelling evidence that these abundance anomalies were already present in the gas from which the observed stars formed. A widely held hypothesis is that the gas was “polluted” by stars that were more massive (and evolving faster) than the presently observed low-mass stars. In the framework of this “self-enrichment” scenario for GCs, we present a new method of deriving the initial mass function (IMF) of the polluters, by using the O/Na abundance distribution. Methods. We focus on NGC 2808, a GC for which the largest sample of O and Na abundance determinations is presently available. We use the abundance distribution of [O/Na] to derive the amount of polluted material with respect to the original composition. We explore two scenarios in detail for the self-enrichment of the cluster, which differ by the assumptions made on the composition of the polluter ejecta. In each case we consider two classes of possible “culprits”: massive asymptotic giant branch (AGB) stars (4−9 M� ) and winds of massive stars (WMS) in the mass range 10−100 M� . Results. 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 in NGC 2808. Conclusions. We find that the polluter IMF had to be much flatter than the presently observed IMFs in stellar clusters, which agrees with the results of two other GC IMF determination methods, which we also discuss. Likewise, we find that the present mass of the GC should be totally dominated by stellar remnants if the polluters were AGB stars, which is not the case if the culprits are WMS. We critically analyse the advantages and shortcomings of each potential polluter class and find the WMS scenario more attractive.

The star formation rate in disc galaxies: thresholds and dependence on gas amount

We reassess the applicability of the Toomre criterion in galactic discs and we study the local star formation law in 16 disc galaxies for which abundance gradients are published. The data we use consist of stellar light profiles, atomic and molecular gas (deduced from CO with a metallicity-dependent conversion factor), star formation rates (from Ha emissivities), metal-licities, dispersion velocities and rotation curves. We show that the Toomre criterion applies successfully to the case of the Milky Way disc, but it has limited success with the data of our sample; depending on whether or not the stellar component is included in the stability analysis, we find average values for the threshold ratio of the gas surface density to the critical surface density in the range 0.5-0.7. We also test various star formation laws proposed in the literature, i.e. either the simple Schmidt law or modifications of it, that take into account dynamical factors. We find only small differences among them as far as the overall fit to our data is concerned; in particular, we find that all three star formation laws (with parameters derived from the fits to our data) match observations in the Milky Way disc particularly well. In all cases we find that the exponent n of our best-fitting star formation rate has slightly higher values than in other recent works and we suggest several reasons that may cause that discrepancy.

Radioactive 26Al in the galaxy: observations versus theory

Abstract 26 Al is the first cosmic radioactivity ever detected, more than ten years ago, through its characteristic 1.8 MeV gamma-ray line. Its ≈10 6 yr lifetime, much shorter than the ≈10 10 yr of galactic evolution, convincingly demonstrates that nucleosynthesis is currently active in our Galaxy. Current models of nucleosynthesis are still too uncertain to allow identification of the sites of that nucleosynthetic activity, despite their continuous improvement in the past ten years. The recent results of the Compton Gamma-Ray Observatory shed, for the first time, some light on the origin of galactic 26 Al, favoring massive stars as the main sources. The various measurements of 1.8 MeV emission and the theoretical models of 26 Al sources are presented in this review, along with the implications of the latest results for nuclear, stellar and galactic astrophysics.

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

Aims. 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 NGC 6752. Methods. 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. Results. Although simplistic, our analysis provides original results and new constraints on the self-enrichment scenario for GCs. Our parametric calculations allow us to determine a narrow range of temperature where the observed extreme abundances of all light elements and isotopes in NGC 6752 are nicely reproduced simultaneously. This agreement is obtained after mixing of the H-processed material with ∼30% of unprocessed gas. We show that 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 various dilution factors. Li production by the stars that build up the other abundance anomalies is not mandatory in the case of NGC 6752. Conclusions. 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. Furthermore, observations require dilution of H-burning processed material with pristine one. They provide no clue, however, as to the nature of the unprocessed material required for mixing. The complementary observations of the fragile Li and F clearly point to ISM origin for the mixed material.

Star Formation and Chemical Evolution in the Milky Way: Cosmological Implications

We propose an expression for the star formation rate in spiral galaxies and a model of chemical evolution with a minimal number of adjustable parameters. Our model accounts for most of the relevant data in the Milky Way. By adopting our Galaxy as a prototype, we are able to derive cosmological implications for the comoving star formation rate, gas amount, gaseous abundances, and supernova rates as a function of redshift.

Production and evolution of LiBeB isotopes in the Galaxy

We reassess the problem of the production of the light elements lithium, beryllium, and boron, by energetic collisions between Galactic cosmic rays (GCR) and interstellar gas nuclei, in the framework of a consistent model for the chemical evolution of our Galaxy. We propose a new scenario for the propagation of GCR in the early Galaxy suggesting that, during its halo phase, GCR were more efficiently confined than today and had flatter spectra at low energy

Relative frequencies of supernovae types: dependence on host galaxy magnitude, galactocentric radius, and local metallicity

Context. Stellar evolution theory suggests that the relationship between number ratios of supernova (SN) types and metallicity can provide important information about the physical properties of the progenitor stars (e.g., mass, metallicity, rotation, binarity). Aims. We investigate the metallicity dependence of the number ratios of various SN types, using a large sample of SN along with information about their radial position within, and magnitude of, their host galaxy. Methods. We derive global galaxy gas-phase metallicities (using the well-known metallicity-luminosity relation) and local metallicities, i.e., at the position of the SN. In the latter case, we use the empirical fact that the metallicity gradients in disk galaxies are ∼constant when expressed in dex/R25. Results. We confirm a dependence of the N(Ibc)/N(II) ratio on metallicity. Single star models with rotation and binary star models with no rotation appear to reproduce equally well the metallicity dependence. The size of our sample does not allow significant conclusions about the N(Ic)/N(Ib) ratio. Finally, we find an unexpected metallicity dependence of the ratio of thermonuclear to core collapse supernovae, which we interpret in terms of the star formation properties of the host galaxies.

The signature of 44Ti in Cassiopeia a Revealed by IBIS/ISGRI on INTEGRAL

We report the detection of both the 67.9 and 78.4 keV 44 Sc g-ray lines in Cassiopeia A with the INTEGRAL IBIS/ISGRI instrument. Besides the robustness provided by spectroimaging observations, the main improvements compared to previous measurements are a clear separation of the two 44 Sc lines together with an improved significance of the detection of the hard X-ray continuum up to 100 keV. These allow us to refine the determination of the 44 Ti yield and to constrain the nature of the nonthermal continuum emission. By combining COMPTEL, BeppoSAX PDS and ISGRI measurements, we find a line flux of

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. [...]

Production and evolution of Li, Be, and B isotopes in the Galaxy

Context. We reassess the problem of the production and evolution of the light elements Li, Be and B and of their isotopes in the Milky Way in the light of new observational and theoretical developments. Aims. The main novelty is the introduction of a new scheme for the origin of Galactic cosmic rays (GCR), which for the first time enables a self-consistent calculation of their composition during galactic evolution. Methods. The scheme accounts for key features of the present-day GCR source composition, it is based on the wind yields of the Geneva models of rotating, mass-losing stars and it is fully coupled to a detailed galactic chemical evolution code. Results. We find that the adopted GCR source composition accounts naturally for the observations of primary Be and helps understanding why Be follows Fe more closely than O. We find that GCR produce ∼70% of the solar 11 B/ 10 B isotopic ratio; the remaining 30% of 11 B presumably result from ν-nucleosynthesis in massive star explosions. We find that GCR and primordial nucleosynthesis can produce at most ∼30% of solar Li. At least half of the solar Li has to originate in low-mass stellar sources (red giants, asymptotic giant branch stars, or novae), but the required average yields of those sources are found to be much higher than obtained in current models of stellar nucleosynthesis. We also present radial profiles of LiBeB elemental and isotopic abundances in the Milky Way disc. We argue that the shape of those profiles – and the late evolution of LiBeB in general – reveals important features of the production of those light elements through primary and secondary processes.