Evolution of Massive Stars Up to the End of Central Oxygen Burning

We present a detailed study of the evolution of massive stars of masses 15, 20, 25 and 30 \msun assuming solar-like initial chemical composition. The stellar sequences were evolved through the advanced burning phases up to the end of core oxygen burning. We present a careful analysis of the physical characteristics of the stellar models. In particular, we investigate the effect of the still unsettled reaction $^{12}$C($\alpha$,$\gamma$)$^{16}$O on the advanced evolution by using recent compilations of this rate. We find that this rate has a significant impact on the evolution not only during the core helium burning phase, but also during the late burning phases, especially the shell carbon-burning. We have also considered the effect of different treatment of convective instability based on the Ledoux criterion in regions of varying molecular weight gradient during the hydrogen and helium burning phases. We compare our results with other investigations whenever available. Finally, our present study constitutes the basis of analyzing the nucleosynthesis processes in massive stars. In particular we will present a detail analysis of the {\it s}-process in a forthcoming paper.Comment: 46 pages, 15 figures. To be published in ApJ vol 611, August 10, 200

The origin of short-lived radionuclides and the astrophysical environment of solar system formation

Based on early solar system abundances of short-lived radionuclides (SRs), such as $^{26}$Al (T$_{1/2} = 0.74$ Myr) and $^{60}$Fe (T$_{1/2} = 1.5$ Myr), it is often asserted that the Sun was born in a large stellar cluster, where a massive star contaminated the protoplanetary disk with freshly nucleosynthesized isotopes from its supernova (SN) explosion. To account for the inferred initial solar system abundances of short-lived radionuclides, this supernova had to be close ($\sim$ 0.3 pc) to the young ($\leqslant$ 1 Myr) protoplanetary disk. Here we show that massive star evolution timescales are too long, compared to typical timescales of star formation in embedded clusters, for them to explode as supernovae within the lifetimes of nearby disks. This is especially true in an Orion Nebular Cluster (ONC)-type of setting, where the most massive star will explode as a supernova $\sim$ 5 Myr after the onset of star formation, when nearby disks will have already suffered substantial photoevaporation and/or formed large planetesimals. We quantify the probability for {\it any} protoplanetary disk to receive SRs from a nearby supernova at the level observed in the early solar system. Key constraints on our estimate are: (1) SRs have to be injected into a newly formed ($\leqslant$ 1 Myr) disk, (2) the disk has to survive UV photoevaporation, and (3) the protoplanetary disk must be situated in an enrichment zone permitting SR injection at the solar system level without disk disruption. The probability of protoplanetary disk contamination by a supernova ejecta is, in the most favorable case, 3 $\times$ 10$^{-3}$

On the origin and composition of Galactic cosmic rays

The composition of Galactic Cosmic Rays (GCR) presents strong similarities to the standard (cosmic) composition, but also noticeable differences, the most important being the high isotopic ratio of Ne22/Ne20 which is about 5 times higher in GCR than in the Sun. This ratio provides key information on the GCR origin. We investigate the idea that GCR are accelerated by the forward shocks of supernova explosions, as they run through the presupernova winds of the massive stars and through the interstellar medium. We use detailed wind and core yields of rotating and non-rotating models of massive stars with mass loss, as well as simple models for the properties of the forward shock and of the circumstellar medium. We find that the observed GCR Ne22/Ne20 ratio can be explained if GCR are accelerated only during the early Sedov phase, for shock velocities >1500-1900 km/s. The acceleration efficiency is found to be of the order of 1.e-6 - 1.e-5, i.e. a few particles out of a million encountered by the shock escape the SN at GCR energies. We also show quantitatively that the widely publicized idea that GCR are accelerated in superbubbles fails to account for the high Ne22/Ne20 ratio in GCRComment: 13 pages, 9 figures, accepted in Astronomy and Astrophysic

Young Star Cluster Complexes in NGC 4038/39: Integral field spectroscopy using VIMOS-VLT

We present the first results of a survey to obtain Integral Field Spectroscopy of merging galaxies along the Toomre Sequence. In the present work, we concentrate on the star cluster complexes in the Antennae galaxies (NGC 4038/39) in the overlap region as well as the nuclear region of NGC 4038. Using optical spectroscopy we derive the extinction age, metallicity, velocity, velocity dispersion of the gas and star formation rate for each of the eight complexes detected. We supplement this study with archival HST-WFPC2 U, B, V, H$\alpha$, and I band imaging. Correcting the observed colours of the star clusters within the complexes for extinction, measured through our optical spectra, we compare the clusters with simple stellar population models, with which we find an excellent agreement. In five of the complexes we detect strong Wolf-Rayet emission features, indicating young ages (3-5 Myr). The ionized gas surrounding the complexes is expanding at speeds of 20-40 km/s. This slow expansion can be understood as a bubble, caused by the stellar winds and supernovae within the complexes, expanding into the remnant of the progenitor giant molecular cloud. We also find that the complexes themselves are grouped, at about the largest scale of which young star clusters are correlated, representing the largest coherent star forming region. We show that the area normalized star formation rates of these complexes clearly place them in the regime of star forming regions in starburst galaxies, thereby justifying the label of localized starbursts. Finally, we estimate the stability of the complexes, and find that they will probably loose a large fraction of their mass to the surrounding environment, although the central regions may merge into a single large star cluster.Comment: 14 pages, accepted A&A. A high-resolution pdf version of the paper can be found at http://www.star.ucl.ac.uk/~bastian/bastian-vimos.pd

The Interstellar Environment of our Galaxy

We review the current knowledge and understanding of the interstellar medium of our galaxy. We first present each of the three basic constituents - ordinary matter, cosmic rays, and magnetic fields - of the interstellar medium, laying emphasis on their physical and chemical properties inferred from a broad range of observations. We then position the different interstellar constituents, both with respect to each other and with respect to stars, within the general galactic ecosystem.Comment: 39 pages, 12 figures (including 3 figures in 2 parts

Al-26 and the formation of the Solar System from a molecular cloud contaminated by Wolf-Rayet winds

In agreement with previous work, we show that the presence of the short-lived radionuclide Al-26 in the early Solar System was unlikely (<2% a priori probability) to be the result of direct introduction of supernova ejecta into the gaseous disk during the Class II stage of protosolar evolution. We also show that any Bondi-Hoyle accretion of contaminated residual gas from the natal star cluster made a negligible contribution to the primordial Al-26 inventory of the Solar System. These results are consistent with the absence of the oxygen isotopic signature expected with any late introduction of supernova ejecta into the protoplanetary disk. Instead, the presence of Al-26 in the oldest Solar System solids (calcium-aluminum-rich inclusions or CAIs) and its apparent uniform distribution with the inferred canonical Al-26/Al-27 ratio of (4.5-5)E-5 support the inheritance of Al-26 from the parent giant molecular cloud. We propose that this radionuclide originated in a prior generation of massive stars that formed in the same host molecular cloud as the Sun and contaminated that cloud by Wolf-Rayet winds. We calculated the Galactic distribution of Al-26/Al-27 ratios that arise from such contamination using the established embedded cluster mass and stellar initial mass functions, published nucleosynthetic yields from the winds of massive stars, and by assuming rapid and uniform mixing into the cloud. Although our model predicts that the majority of stellar systems contain no Al-26 from massive stars, and that the a priori probability that the Al-26/Al-27 ratio will reach or exceed the canonical Solar System value is only ~6%, the maximum in the distribution of non-zero values is close to the canonical ratio.Comment: accepted to the Astrophysical Journa

Standalone vertex finding in the ATLAS muon spectrometer

A dedicated reconstruction algorithm to find decay vertices in the ATLAS muon spectrometer is presented. The algorithm searches the region just upstream of or inside the muon spectrometer volume for multi-particle vertices that originate from the decay of particles with long decay paths. The performance of the algorithm is evaluated using both a sample of simulated Higgs boson events, in which the Higgs boson decays to long-lived neutral particles that in turn decay to bbar b final states, and pp collision data at √s = 7 TeV collected with the ATLAS detector at the LHC during 2011

Measurements of Higgs boson production and couplings in diboson final states with the ATLAS detector at the LHC

Measurements are presented of production properties and couplings of the recently discovered Higgs boson using the decays into boson pairs, H →γ γ, H → Z Z∗ →4l and H →W W∗ →lνlν. The results are based on the complete pp collision data sample recorded by the ATLAS experiment at the CERN Large Hadron Collider at centre-of-mass energies of √s = 7 TeV and √s = 8 TeV, corresponding to an integrated luminosity of about 25 fb−1. Evidence for Higgs boson production through vector-boson fusion is reported. Results of combined fits probing Higgs boson couplings to fermions and bosons, as well as anomalous contributions to loop-induced production and decay modes, are presented. All measurements are consistent with expectations for the Standard Model Higgs boson

Measurement of the top quark-pair production cross section with ATLAS in pp collisions at \sqrt{s}=7\TeV

A measurement of the production cross-section for top quark pairs(\ttbar) in $pp$ collisions at \sqrt{s}=7 \TeV is presented using data recorded with the ATLAS detector at the Large Hadron Collider. Events are selected in two different topologies: single lepton (electron $e$ or muon $\mu$) with large missing transverse energy and at least four jets, and dilepton ($ee$, $\mu\mu$ or $e\mu$) with large missing transverse energy and at least two jets. In a data sample of 2.9 pb-1, 37 candidate events are observed in the single-lepton topology and 9 events in the dilepton topology. The corresponding expected backgrounds from non-\ttbar Standard Model processes are estimated using data-driven methods and determined to be $12.2 \pm 3.9$ events and $2.5 \pm 0.6$ events, respectively. The kinematic properties of the selected events are consistent with SM \ttbar production. The inclusive top quark pair production cross-section is measured to be \sigmattbar=145 \pm 31 ^{+42}_{-27} pb where the first uncertainty is statistical and the second systematic. The measurement agrees with perturbative QCD calculations.Comment: 30 pages plus author list (50 pages total), 9 figures, 11 tables, CERN-PH number and final journal adde

Measurement of the top quark pair cross section with ATLAS in pp collisions at √s=7 TeV using final states with an electron or a muon and a hadronically decaying τ lepton

A measurement of the cross section of top quark pair production in proton-proton collisions recorded with the ATLAS detector at the Large Hadron Collider at a centre-of-mass energy of 7 TeV is reported. The data sample used corresponds to an integrated luminosity of 2.05 fb -1. Events with an isolated electron or muon and a τ lepton decaying hadronically are used. In addition, a large missing transverse momentum and two or more energetic jets are required. At least one of the jets must be identified as originating from a b quark. The measured cross section, σtt-=186±13(stat.)±20(syst.)±7(lumi.) pb, is in good agreement with the Standard Model prediction