The Explosive Yields Produced by the First Generation of Core Collapse Supernovae and the Chemical Composition of Extremely Metal Poor Stars

We present a detailed comparison between an extended set of elemental abundances observed in some of the most metal poor stars presently known and the ejecta produced by a generation of primordial core collapse supernovae. We used five stars which form our initial database and define a "template" ultra metal poor star which is then compared to the theoretical predictions. Our main findings are as follows: a) the fit to [Si/Mg] and [Ca/Mg] of these very metal poor stars seems to favor the presence of a rather large C abundance at the end of the central He burning; in a classical scenario in which the border of the convective core is strictly determined by the Schwarzschild criterion, such a large C abundance would imply a rather low C12(alpha,gamma)O16 reaction rate; b) a low C abundance left by the central He burning would imply a low [Al/Mg] (<-1.2 dex) independently on the initial mass of the exploding star while a rather large C abundance would produce such a low [Al/Mg] only for the most massive stellar model; c) at variance with current beliefs that it is difficult to interpret the observed overabundance of [Co/Fe], we find that a mildly large C abundance in the He exhausted core (well within the present range of uncertainty) easily and naturally allows a very good fit to [Co/Fe]; d) our yields allow a reasonable fit to 8 out of the 11 available elemental abundances; e) within the present grid of models it is not possible to find a good match of the remaining three elements, Ti, Cr and Ni (even for an arbitrary choice of the mass cut); f) the adoption of other yields available in the literature does not improve the fit; g) since no mass in our grid provides a satisfactory fit to these three elements, even an arbitrary choice of the initial mass function would not improve their fit.Comment: 30 pages, 8 figures, 8 tables. Accepted for publication on Ap

Inhomogeneous chemical evolution of the galactic halo

I present a stochastic chemical evolution model to investigate the enrichment of the interstellar medium (ISM) during Galaxy formation. Contrary to classical chemical evolution models, it is able to resolve local chemical inhomogeneities in the ISM caused by single core-collapse supernovae. These inhomogeneities lead to different element abundance patterns in very metal-poor stars which can be seen as scatter in the abundances of halo stars with metallicities [Fe/H] . The early chemical evolution of the halo proceeds in different enrichment phases: At [Fe/H] , the halo ISM is unmixed and dominated by local inhomogeneities caused by individual core-collapse supernova events. For metallicities [Fe/H] the halo ISM is well mixed, showing an element abundance pattern integrated over the initial mass function. In the range [Fe/H] a continuous transition from the unmixed to the well mixed ISM occurs. For some elements (Si, Ca), the scatter in element-to-iron ratios of metal-poor halo stars can be reproduced. Stellar yields of other elements, however, predict a scatter which, compared to observations, is too large (O, Mg) or too small (Ni). This shows, that inhomogeneous chemical evolution models are heavily dependent on theoretical nucleosynthesis yields of core-collapse supernovae. Hence inhomogeneous chemical evolution models present themselves as a test for stellar nucleosynthesis calculations. One problem revealed by the model is the predicted scatter in [O/Fe] and [Mg/Fe] which is too large compared to the one observed in metal-poor halo stars. This can be either due to the oxygen or magnesium yields or due to the iron yields (or both). However, oxygen and magnesium are -elements that are produced mainly during hydrostatic burning and thus are not affected by the theoretical uncertainties afflicting the collapse and explosion of a massive star. Stellar iron yields, on the other hand, depend heavily on the choice of the mass-cut between ejecta and proto-neutron star and are therefore very uncertain. In this work, iron yield distributions as function of progenitor mass are derived which are consistent with the abundance distribution of metal-poor halo stars and are in agreement with observed yields of core-collapse supernovae with known progenitor masses. The iron yields of lower-mass Type II supernovae (in ) are well constrained by these observations. Present observations, however, do not allow the range us to determine a unique solution for higher-mass Type II supernovae. Nevertheless, the main dependence of the stellar iron yields as function of progenitor mass can be derived and may be used as a constraint for future core-collapse supernova/hypernova models. A prediction of hypernova models which can be tested by future observations is the existence of ultra -element enhanced stars at metallicities [Fe/H] . The results are of importance for the earliest stages of galaxy formation when the ISM is dominated by local chemical inhomogeneities and the instantaneous mixing approximation is not valid. The astrophysical nature of r-process sites is a long standing mystery and many probable sources were suggested in the past, among them lower-mass core-collapse supernovae (in the range ), higher-mass core-collapse supernovae (with masses ) and neutron star mergers. In this work, I present a detailed inhomogeneous chemical evolution study that considers for the first time neutron star mergers as major rprocess sources, and compare this scenario to the ones in which core-collapse supernovae act as dominant r-process sites. Furthermore, the enrichment of the interstellar medium with neutron-capture elements during Galaxy formation by r- and s-process sources is investigated. I conclude that, due to the lack of reliable iron and r-process yields as function of progenitor mass, it is not possible to date to distinguish between the lower-mass and higher-mass supernovae scenario within the framework of inhomogeneous chemical evolution. However, neutron-star mergers seem to be ruled out as dominant r-process source, since their low coalescence rates are not consistent with observations of r-process elements at very low metallicities. Furthermore, the considerable injection of r-process material by a single neutron-star merger leads to a scatter in r-process abundances at later times which is much too large compared to observations. Finally, a low star-formation efficiency is required during halo formation to be consistent with the appearance of s-process elements at very low metallicities

Data Literacy projects in Canada: Field notes from the Open Data Institute, Toronto node

Open data is flourishing in Canada, but there are few formalized data literacy initiatives. Civic technology organizations such as the Toronto Node of the Open Data Institute (ODI Toronto), in partnership with public institutions and advocacy groups, are helping to fill the gap in data literacy through workshops and accessible hackathons. These organizations are collaboratively pursuing the goal of ensuring that open data benefits more than just a minority of technologically privileged Canadians

Isolated Star Formation: A Compact HII Region in the Virgo Cluster

We report on the discovery of an isolated, compact HII region in the Virgo cluster. The object is located in the diffuse outer halo of NGC 4388, or could possibly be in intracluster space. Star formation can thus take place far outside the main star forming regions of galaxies. This object is powered by a small starburst with an estimated mass of \sim 400\msun and age of \sim 3\myr. From a total sample of 17 HII region candidates, the present rate of isolated star formation estimated in our Virgo field is small, \sim 10^{-6} Msun arcmin}^{-2} yr^{-1}. However, this mode of star formation might have been more important at higher redshifts and be responsible for a fraction of the observed intracluster stars and total cluster metal production. This object is relevant also for distance determinations with the planetary nebula luminosity function from emission line surveys, for high-velocity clouds and the in situ origin of B stars in the Galactic halo, and for local enrichment of the intracluster gas by Type II supernovae.Comment: 5 pages, LaTeX, 1 figure. ApJ Letters, in press (scheduled Dec 1, 2002

Constraints on the Astrophysical Nature of r-Process Nucleosynthesis Sites from Inhomogeneous Chemical Evolution Models

We present a detailed inhomogeneous chemical evolution study that considers for the first-time neutron star mergers as major r-process sources, and compare this scenario with the ones in which lower-mass (in the range 8-10 M⊙) or higher-mass core-collapse supernovae (with masses ≥20 M⊙) act as dominant r-process sites. We conclude that it is not possible at present to distinguish between the lower-mass and higher-mass supernovae scenarios within the framework of inhomogeneous chemical evolution. However, neutron-star mergers seem to be ruled out as the dominant r-process source, since their low rates of occurrence would lead to r-process enrichment that is not consistent with observation

Stochastic chemical enrichment in metal-poor systems II. Abundance ratios and scatter

A stochastic model of the chemical enrichment of metal-poor systems by core-collapse supernovae is used to study the scatter in stellar abundance ratios. The resulting scatter in abundance ratios, e.g. as functions of the overall metallicity, is demonstrated to be crucially dependent on the as yet uncertain supernovae yields. The observed abundance ratios and their scatters therefore have diagnostic power as regards the yields. The relatively small star-to-star scatter observed in many chemical abundance ratios, e.g. by Cayrel et al. (2004) for stars down to [Fe/H] = -4, is tentatively explained by the averaging of a large number of contributing supernovae and by the cosmic selection effects favoring contributions from supernovae in a certain mass range for the most metal-poor stars. The scatter in observed abundances of alpha-elements is understood in terms of observational errors only, while additional spread in yields or sites of nucleosynthesis may affect the odd-even elements Na and Al. For the iron-group elements we find systematically too high predicted Cr/Fe and Cr/Mg ratios, as well as differences between the different sets of yields, both in terms of predicted abundance ratios and scatter. The semi-empirical yields recently suggested by Francois et al. (2004) are found to lead to scatter in abundance ratios significantly greater than observed, when applied in the inhomogeneous models. "Spurs", very narrow sequences in abundance-ratio diagrams, may disclose a single-supernova origin of the elements of the stars on the sequence. Verification of the existence of such features, called single supernova sequences (SSSs), is challenging. This will require samples of several hundred stars with abundance ratios observed to accuracies of 0.05 dex or better.Comment: 19 pages, 20 figures, accepted for publication in Astronomy & Astrophysic

Deriving the Metallicity Distribution Function of Galactic Systems

The chemical evolution of the Milky Way is investigated using a dual-phase metal-enriched infall model in which primordial gas fuels the earliest epoch of star formation, followed by the ongoing formation of stars from newly accreted gas. The latest metallicity distribution of local K-dwarfs is reproduced by this model, which allows the Galactic thin disk to form from slightly metal-enriched gas with alpha-element enhancement. Our model predicts ages for the stellar halo and thin disk of 12.5 and 7.4 Gyr, respectively, in agreement with empirically determined values. The model presented in this paper is compared with a similar dual-phase infall model from Chiappini et al. (2001). We discuss a degeneracy that enables both models to recover the K-dwarf metallicity distribution while yielding different star formation histories. The metallicity distribution function (MDF) of K-dwarfs is proposed to be more directly comparable to chemical evolution model results than the G-dwarf distribution because lower mass K-dwarfs are less susceptible to stellar evolutionary effects. The K-dwarf MDF should consequently be a better probe of star formation history and provide a stronger constraint to chemical evolution models than the widely used G-dwarf MDF. The corrections that should be applied to a G-dwarf MDF are quantified for the case of the outer halo of NGC 5128.Comment: 16 pages, 5 figures, accepted for publication in PASA (Publications of the Astronomical Society of Australia

Eine unheilige Allianz: das Schweizer Bürgerrecht zwischen kommunaler Rechtstradition, bundesstaatlichem Laisser-faire und ethnisch-nationaler Fremdenabwehr 1848-1933

"Der Beitrag zeichnet die Entwicklung des föderalistischen Schweizer Bürgerrechts zwischen der Bundesstaatsgründung im Jahr 1848 und der Verankerung einer restriktiven Einbürgerungs- und Niederlassungspolitik in der Zwischenkriegszeit nach. Ausgehend von der schwierigen staatsbürgerlichen Integration der ausländischen Wohnbevölkerung in Geschichte und Gegenwart der Schweiz fragt die Autorin nach den Ursachen für die Gewährung und Verweigerung des Schweizer Bürgerrechts. Dabei wird deutlich, dass sich die Entwicklung und Ausprägung des Zugangs zum Schweizer Bürgerrecht nicht allein auf Vorstellungen von der schweizerischen Nation oder auf staatliche Interessen reduzieren lassen. Vielmehr waren sie Ergebnis eines permanenten politischen Aushandlungs- und Koordinationsprozesses zwischen Bund, Kantonen und Gemeinden: Das Gemeindebürgerrecht bildete aufgrund seiner armenrechtlichen Bedeutung bis weit ins 20. Jahrhundert hinein das Nadelöhr für Einbürgerungen. Im Gegensatz dazu versuchten der Bund und einzelne Kantone wie Zürich, Basel und Genf die seit den 1880er Jahren stark angestiegene Zahl der ausländischen Wohnbevölkerung durch die Liberalisierung der Einbürgerung zu verringern. Der Ausbruch des Ersten Weltkriegs setzte diesen Bestrebungen ein Ende. Im Zuge des Aufstiegs einer 'neuen Rechten' seit 1900, der Gründung der Eidgenössischen Fremdenpolizei im Jahr 1917 und der Institutionalisierung der behördlichen 'Überfremdungsbekämpfung' wurde das schweizerische Staatsangehörigkeitsrecht nachträglich ethnisiert. Die kulturelle 'Assimilation' an die 'schweizerische Eigenart' galt nun als Voraussetzung dafür, um Schweizer Bürger zu werden. Dabei verband sich die neue bundesstaatliche Fremdenabwehr mit der traditionell restriktiven Politik der Gemeinden, eine unheilige Allianz, die erst in den 1980er Jahren aufzubrechen begann." (Autorenreferat)"The article traces the development of the federal structure of Swiss citizenship between the founding of the federal state in 1948 and the entrenchment of a restrictive naturalisation and establishment policy in the interwar period. Considering the difficult integration of the foreign residents through naturalisation in the past and present in Switzerland, the author examines the causes for the granting and refusal of Swiss citizenship. She shows that the development of and arrangements for access to Swiss citizenship cannot be reduced only to notions about the Swiss nation or national interests. They are the result of a permanent process of political negotiation and coordination between the federation, cantons, and local authorities: owing to its importance in social assistance matters, local citizenship constituted an impediment to naturalisation until well into the 20th century. In contrast, the federation and certain cantons like Zurich, Basle, and Geneva had sought since the 1880s to reduce the strongly increasing number of foreign residents by liberalising naturalisation. The outbreak of the Second World War put an end to these endeavours. With the rise of a 'new right' since 1900, the setting up of the Central Office of the Police for Foreigners in 1917, and the institutionalisation of the authorities' 'fight against foreign infiltration,' Swiss nationality law became ethnicized. Cultural 'assimilation' into the 'particularity of Swiss society' was now regarded as a precondition for becoming a Swiss citizen. The new federal rejection of foreigners thus joined with the traditionally restrictive policy of local authorities in an unholy alliance that began to breach only in the 1980s." (author's abstract

nu-Process Nucleosynthesis in Population III Core-Collapse Supernovae

We investigate the effects of neutrino-nucleus interactions (the nu-process) on the production of iron-peak elements in Population III core-collapse supernovae. The nu-process and the following proton and neutron capture reactions produce odd-Z iron-peak elements in complete and incomplete Si burning region. This reaction sequence enhances the abundances of Sc, Mn, and Co in the supernova ejecta. The supernova explosion models of 15 M_sol and 25 M_sol stars with the nu-process well reproduce the averaged Mn/Fe ratio observed in extremely metal-poor halo stars. In order to reproduce the observed Mn/Fe ratio, the total neutrino energy in the supernovae should be 3 - 9 x 10^{53} ergs. Stronger neutrino irradiation and other production sites are necessary to reproduce the observed Sc/Fe and Co/Fe ratios, although these ratios increase by the nu-process.Comment: 10 pages, 12 figures, Accepted for publication in The Astrophysical Journa