Flares and variability from Sagittarius A(star): five nights of simultaneous multi-wavelength observations

Aims. We report on simultaneous observations and modeling of mid-infrared (MIR), near-infrared (NIR), and submillimeter (sub-mm) emission of the source Sgr A * associated with the supermassive black hole at the center of our Galaxy. Our goal was to monitor the activity of Sgr A* at different wavelengths in order to constrain the emitting processes and gain insight into the nature of the close environment of Sgr A*. Methods. We used the MIR instrument VISIR in the BURST imaging mode, the adaptive optics assisted NIR camera NACO, and the sub-mm antenna APEX to monitor Sgr A* over several nights in July 2007. Results. The observations reveal remarkable variability in the NIR and sub-mm during the five nights of observation. No source was detected in the MIR, but we derived the lowest upper limit for a flare at 8.59 mu m (22.4 mJy with A(8.59 mu m) = 1.6 +/- 0.5). This observational constraint makes us discard the observed NIR emission as coming from a thermal component emitting at sub-mm frequencies. Moreover, comparison of the sub-mm and NIR variability shows that the highest NIR fluxes (flares) are coincident with the lowest sub-mm levels of our five-night campaign involving three flares. We explain this behavior by a loss of electrons to the system and/or by a decrease in the magnetic field, as might conceivably occur in scenarios involving fast outflows and/or magnetic reconnection.

OXYGEN ABUNDANCES IN LOW- AND HIGH-alpha FIELD HALO STARS AND THE DISCOVERY OF TWO FIELD STARS BORN IN GLOBULAR CLUSTERS

Oxygen abundances of 67 dwarf stars in the metallicity range -1.6 < [Fe/H] < -0.4 are derived from a non-LTE analysis of the 777 nm O I triplet lines. These stars have precise atmospheric parameters measured by Nissen and Schuster, who find that they separate into three groups based on their kinematics and alpha-element (Mg, Si, Ca, Ti) abundances: thick disk, high-alpha halo, and low-alpha halo. We find the oxygen abundance trends of thick-disk and high-alpha halo stars very similar. The low-alpha stars show a larger star-to-star scatter in [O/Fe] at a given [Fe/H] and have systematically lower oxygen abundances compared to the other two groups. Thus, we find the behavior of oxygen abundances in these groups of stars similar to that of the a elements. We use previously published oxygen abundance data of disk and very metal-poor halo stars to present an overall view (-2.3 < [Fe/H] < +0.3) of oxygen abundance trends of stars in the solar neighborhood. Two field halo dwarf stars stand out in their O and Na abundances. Both G53-41 and G150-40 have very low oxygen and very high sodium abundances, which are key signatures of the abundance anomalies observed in globular cluster (GC) stars. Therefore, they are likely field halo stars born in GCs. If true, we estimate that at least 3% +/- 2% of the local field metal-poor star population was born in GCs.

Homogeneous metallicities and radial velocities for Galactic globular clusters

Well determined radial velocities and abundances are essential for analyzing the properties of the globular cluster system of the Milky Way. However more than 50% of these clusters have no spectroscopic measure of their metallicity. In this context, this work provides new radial velocities and abundances for twenty Milky Way globular clusters which lack or have poorly known values for these quantities. The radial velocities and abundances are derived from spectra obtained at the Ca II triplet using the FORS2 imager and spectrograph at the VLT, calibrated with spectra of red giants in a number of clusters with well determined abundances. For about half of the clusters in our sample we present significant revisions of the existing velocities or abundances, or both. We also confirm the existence of a sizable abundance spread in the globular cluster M 54, which lies at the center of the Sagittarius dwarf galaxy. In addition evidence is provided for the existence of a small intrinsic internal abundance spread (sigma[Fe/H](int) approximate to 0.11-0.14 dex, similar to that of M 54) in the luminous distant globular cluster NGC 5824. This cluster thus joins the small number of Galactic globular clusters known to possess internal metallicity ([Fe/H]) spreads.

Stochastic gravito-inertial modes discovered by CoRoT in the hot Be star HD51452

Context. Be stars are rapidly rotating stars with a circumstellar decretion disk. They usually undergo pressure and/or gravity pulsation modes excited by the kappa-mechanism, i.e. an effect of the opacity of iron-peak elements in the envelope of the star. In the Milky Way, p-modes are observed in stars that are hotter than or equal to the B3 spectral type, while g-modes are observed at the B2 spectral type and cooler. Aims. We observed a B0IVe star, HD51452, with the high-precision, high-cadence photometric CoRoT satellite and high-resolution, ground-based HARPS and SOPHIE spectrographs to study its pulsations in great detail. We also used the lower resolution spectra available in the BeSS database. Methods. We analyzed the CoRoT and spectroscopic data with several methods: CLEAN-NG, FREQFIND, and a sliding window method. We also analyzed spectral quantities, such as the violet over red (V/R) emission variations, to obtain information about the variation in the circumstellar environment. We calculated a stellar structure model with the ESTER code to test the various interpretation of the results. Results. We detect 189 frequencies of variations in the CoRoT light curve in the range between 0 and 4.5 c d(-1). The main frequencies are also recovered in the spectroscopic data. In particular we find that HD51452 undergoes gravito-inertial modes that are not in the domain of those excited by the kappa-mechanism. We propose that these are stochastic modes excited in the convective zones and that at least some of them are a multiplet of r-modes (i.e. subinertial modes mainly driven by the Coriolis acceleration). Stochastically excited gravito-inertial modes had never been observed in any star, and theory predicted that their very low amplitudes would be undetectable even with CoRoT. We suggest that the amplitudes are enhanced in HD51452 because of the very rapid stellar rotation. In addition, we find that the amplitude variations of these modes are related to the occurrence of minor outbursts. Conclusions. Thanks to CoRoT data, we have detected a new kind of pulsations in HD51452, which are stochastically excited gravito-inertial modes, probably due to its very rapid rotation. These modes are probably also present in other rapidly rotating hot Be stars.

The nature of extremely red galaxies in the local universe

We investigate the nature of extremely red galaxies (ERGs), objects whose colours are redder than those found in the red sequence present in colour–magnitude diagrams of galaxies. We selected from the Sloan Digital Sky Survey Data Release 7 a volume-limited sample of such galaxies in the redshift interval 0.010 < z < 0.030, brighter than Mr = −17.8 (magnitudes dereddened, corrected for the Milky Way extinction) and with (g − r) colours larger than those of galaxies in the red sequence. This sample contains 416 ERGs, which were classified visually. Our classification was cross-checked with other classifications available in the literature. We found from our visual classification that the majority of objects in our sample are edge-on spirals (73 per cent). Other spirals correspond to 13 per cent, whereas elliptical galaxies comprise only 11 per cent of the objects. After comparing the morphological mix and the distributions of Hα/Hβ and axial ratios of ERGs and objects in the red sequence, we suggest that dust, more than stellar population effects, is the driver of the red colours found in these extremely red galaxies.

Stochastic gravito-inertial modes discovered by CoRoT in the hot Be star HD51452

Context. Be stars are rapidly rotating stars with a circumstellar decretion disk. They usually undergo pressure and/or gravity pulsation modes excited by the κ-mechanism, i.e. an effect of the opacity of iron-peak elements in the envelope of the star. In the Milky Way, p-modes are observed in stars that are hotter than or equal to the B3 spectral type, while g-modes are observed at the B2 spectral type and cooler. Aims. We observed a B0IVe star, HD51452, with the high-precision, high-cadence photometric CoRoT satellite and high-resolution, ground-based HARPS and SOPHIE spectrographs to study its pulsations in great detail. We also used the lower resolution spectra available in the BeSS database. Methods. We analyzed the CoRoT and spectroscopic data with several methods: Clean-NG, FreqFind, and a sliding window method. We also analyzed spectral quantities, such as the violet over red (V/R) emission variations, to obtain information about the variation in the circumstellar environment. We calculated a stellar structure model with the ESTER code to test the various interpretation of the results. Results. We detect 189 frequencies of variations in the CoRoT light curve in the range between 0 and 4.5 c d−1. The main frequencies are also recovered in the spectroscopic data. In particular we find that HD51452 undergoes gravito-inertial modes that are not in the domain of those excited by the κ-mechanism. We propose that these are stochastic modes excited in the convective zones and that at least some of them are a multiplet of r-modes (i.e. subinertial modes mainly driven by the Coriolis acceleration). Stochastically excited gravito-inertial modes had never been observed in any star, and theory predicted that their very low amplitudes would be undetectable even with CoRoT. We suggest that the amplitudes are enhanced in HD51452 because of the very rapid stellar rotation. In addition, we find that the amplitude variations of these modes are related to the occurrence of minor outbursts. Conclusions. Thanks to CoRoT data, we have detected a new kind of pulsations in HD51452, which are stochastically excited gravito-inertial modes, probably due to its very rapid rotation. These modes are probably also present in other rapidly rotating hot Be stars.

Echtzeitsuche nach Neutrinoausbrüchen von Supernovae mit dem AMANDA-II-Detektor

Der AMANDA-II Detektor ist primär für den richtungsaufgelösten Nachweis hochenergetischer Neutrinos konzipiert. Trotzdem können auch niederenergetische Neutrinoausbrüche, wie sie von Supernovae erwartet werden, mit hoher Signifikanz nachgewiesen werden, sofern sie innerhalb der Milchstraße stattfinden. Die experimentelle Signatur im Detektor ist ein kollektiver Anstieg der Rauschraten aller optischen Module. Zur Abschätzung der Stärke des erwarteten Signals wurden theoretische Modelle und Simulationen zu Supernovae und experimentelle Daten der Supernova SN1987A studiert. Außerdem wurden die Sensitivitäten der optischen Module neu bestimmt. Dazu mussten für den Fall des südpolaren Eises die Energieverluste geladener Teilchen untersucht und eine Simulation der Propagation von Photonen entwickelt werden. Schließlich konnte das im Kamiokande-II Detektor gemessene Signal auf die Verhältnisse des AMANDA-II Detektors skaliert werden. Im Rahmen dieser Arbeit wurde ein Algorithmus zur Echtzeit-Suche nach Signalen von Supernovae als Teilmodul der Datennahme implementiert. Dieser beinhaltet diverse Verbesserungen gegenüber der zuvor von der AMANDA-Kollaboration verwendeten Version. Aufgrund einer Optimierung auf Rechengeschwindigkeit können nun mehrere Echtzeit-Suchen mit verschiedenen Analyse-Zeitbasen im Rahmen der Datennahme simultan laufen. Die Disqualifikation optischer Module mit ungeeignetem Verhalten geschieht in Echtzeit. Allerdings muss das Verhalten der Module zu diesem Zweck anhand von gepufferten Daten beurteilt werden. Dadurch kann die Analyse der Daten der qualifizierten Module nicht ohne eine Verzögerung von etwa 5 Minuten geschehen. Im Falle einer erkannten Supernova werden die Daten für die Zeitdauer mehrerer Minuten zur späteren Auswertung in 10 Millisekunden-Intervallen archiviert. Da die Daten des Rauschverhaltens der optischen Module ansonsten in Intervallen von 500 ms zur Verfgung stehen, ist die Zeitbasis der Analyse in Einheiten von 500 ms frei wählbar. Im Rahmen dieser Arbeit wurden drei Analysen dieser Art am Südpol aktiviert: Eine mit der Zeitbasis der Datennahme von 500 ms, eine mit der Zeitbasis 4 s und eine mit der Zeitbasis 10 s. Dadurch wird die Sensitivität für Signale maximiert, die eine charakteristische exponentielle Zerfallszeit von 3 s aufweisen und gleichzeitig eine gute Sensitivität über einen weiten Bereich exponentieller Zerfallszeiten gewahrt. Anhand von Daten der Jahre 2000 bis 2003 wurden diese Analysen ausführlich untersucht. Während die Ergebnisse der Analyse mit t = 500 ms nicht vollständig nachvollziehbare Ergebnisse produzierte, konnten die Resultate der beiden Analysen mit den längeren Zeitbasen durch Simulationen reproduziert und entsprechend gut verstanden werden. Auf der Grundlage der gemessenen Daten wurden die erwarteten Signale von Supernovae simuliert. Aus einem Vergleich zwischen dieser Simulation den gemessenen Daten der Jahre 2000 bis 2003 und der Simulation des erwarteten statistischen Untergrunds kann mit einem Konfidenz-Niveau von mindestens 90 % gefolgert werden, dass in der Milchstraße nicht mehr als 3.2 Supernovae pro Jahr stattfinden. Zur Identifikation einer Supernova wird ein Ratenanstieg mit einer Signifikanz von mindestens 7.4 Standardabweichungen verlangt. Die Anzahl erwarteter Ereignisse aus dem statistischen Untergrund beträgt auf diesem Niveau weniger als ein Millionstel. Dennoch wurde ein solches Ereignis gemessen. Mit der gewählten Signifikanzschwelle werden 74 % aller möglichen Vorläufer-Sterne von Supernovae in der Galaxis überwacht. In Kombination mit dem letzten von der AMANDA-Kollaboration veröffentlicheten Ergebnis ergibt sich sogar eine obere Grenze von nur 2.6 Supernovae pro Jahr. Im Rahmen der Echtzeit-Analyse wird für die kollektive Ratenüberhöhung eine Signifikanz von mindestens 5.5 Standardabweichungen verlangt, bevor eine Meldung über die Detektion eines Supernova-Kandidaten verschickt wird. Damit liegt der überwachte Anteil Sterne der Galaxis bei 81 %, aber auch die Frequenz falscher Alarme steigt auf bei etwa 2 Ereignissen pro Woche. Die Alarm-Meldungen werden über ein Iridium-Modem in die nördliche Hemisphäre übertragen, und sollen schon bald zu SNEWS beitragen, dem weltweiten Netzwerk zur Früherkennung von Supernovae.

Dynamics of the halo gas in disc galaxies

This Thesis studies the dynamics of hot and cold gas outside the plane in galaxies like the Milky-Way (extra-planar gas) and focuses on the interaction between disc and halo material. Stationary models for the cold phase of the extra-planar gas are presented. They show that the kinematics of this phase must be influenced by the interaction with an ambient medium that we identify as the hot cosmological corona that surrounds disc galaxies. To study this interaction a novel hydrodynamical code has been implemented and a series of hydrodynamical simulations has been run to investigate the mass and momentum exchange between the cold extra-planar gas clouds and the hot corona. These simulations show that the coronal gas can condense efficiently in the turbulent wakes that form behind the cold clouds and it can be accreted by the disc to sustain star formation. They also predict that the corona cannot be a static structure but it must rotate and lag by approximately 80-120 km/s with respect to the disc. Implications of the results of this Thesis for the evolution of star-forming galaxies and for the large-scale dynamics of galactic coronae are also briefly discussed.

Supernova neutrinos in AMANDA and IceCube - Monte Carlo development and data analysis

Supernovae are among the most energetic events occurring in the universe and are so far the only verified extrasolar source of neutrinos. As the explosion mechanism is still not well understood, recording a burst of neutrinos from such a stellar explosion would be an important benchmark for particle physics as well as for the core collapse models. The neutrino telescope IceCube is located at the Geographic South Pole and monitors the antarctic glacier for Cherenkov photons. Even though it was conceived for the detection of high energy neutrinos, it is capable of identifying a burst of low energy neutrinos ejected from a supernova in the Milky Way by exploiting the low photomultiplier noise in the antarctic ice and extracting a collective rate increase. A signal Monte Carlo specifically developed for water Cherenkov telescopes is presented. With its help, we will investigate how well IceCube can distinguish between core collapse models and oscillation scenarios. In the second part, nine years of data taken with the IceCube precursor AMANDA will be analyzed. Intensive data cleaning methods will be presented along with a background simulation. From the result, an upper limit on the expected occurrence of supernovae within the Milky Way will be determined.

Homogeneous analysis of a sample of Open Clusters in the context of the BOCCE project and the Gaia-ESO Survey

The open clusters (OC) are gravitationally bound systems of a few tens or hundreds of stars. In our Galaxy, the Milky Way, we know about 3000 open clusters, of very different ages in the range of a few millions years to about 9 Gyr. OCs are mainly located in the Galactic thin disc, with distances from the Galactic centre in the range 4-22 kpc and a height scale on the disc of about 200 pc. Their chemical properties trace those of the environment in which they formed and the metallicity is in the range -0.5<[Fe/H]<+0.5 dex. Through photometry and spectroscopy it is possible to study relatively easily the properties of the OCs and estimate their age, distance, and chemistry. For these reasons they are considered primary tracers of the chemical properties and chemical evolution of the Galactic disc. The main subject of this thesis is the comprehensive study of several OCs. The research embraces two different projects: the Bologna Open Cluster Chemical Evolution project (BOCCE) and the Gaia-ESO Survey. The first is a long-term programme, aiming at studying the chemical evolution of the Milky Way disc by means of a homogeneous sample of OCs. The latter is a large public spectroscopy survey, conducted with the high-resolution spectrograph FLAMES@VLT and targeting about 10^5 stars in different part of the Galaxy and 10^4 stars in about 100 OCs. The common ground between the two projects is the study of the properties of the OCs as tracers of the disc's characteristics. The impressive scientific outcome of the Gaia-ESO Survey and the unique framework of homogeneity of the BOCCE project can propose, especially once combined together, a much more accurate description of the properties of the OCs. In turn, this will give fundamental constraints for the interpretation of the properties of the Galactic disc.

Accretion of Mass and Angular Momentum onto The Discs of Spiral Galaxies

In this Thesis, we study the accretion of mass and angular momentum onto the disc of spiral galaxies from a global and a local perspective and comparing theory predictions with several observational data. First, we propose a method to measure the specific mass and radial growth rates of stellar discs, based on their star formation rate density profiles and we apply it to a sample of nearby spiral galaxies. We find a positive radial growth rate for almost all galaxies in our sample. Our galaxies grow in size, on average, at one third of the rate at which they grow in mass. Our results are in agreement with theoretical expectations if known scaling relations of disc galaxies are not evolving with time. We also propose a novel method to reconstruct accretion profiles and the local angular momentum of the accreting material from the observed structural and chemical properties of spiral galaxies. Applied to the Milky Way and to one external galaxy, our analysis indicates that accretion occurs at relatively large radii and has a local deficit of angular momentum with respect to the disc. Finally, we show how structure and kinematics of hot gaseous coronae, which are believed to be the source of mass and angular momentum of massive spiral galaxies, can be reconstructed from their angular momentum and entropy distributions. We find that isothermal models with cosmologically motivated angular momentum distributions are compatible with several independent observational constraints. We also consider more complex baroclinic equilibria: we describe a new parametrization for these states, a new self-similar family of solution and a method for reconstructing structure and kinematics from the joint angular momentum/entropy distribution.

Galactic fountain flows in high-resolution hydrodynamical simulations of galaxy disks

Feedback from the most massive components of a young stellar cluster deeply affects the surrounding ISM driving an expanding over-pressured hot gas cavity in it. In spiral galaxies these structures may have sufficient energy to break the disk and eject large amount of material into the halo. The cycling of this gas, which eventually will fall back onto the disk, is known as galactic fountains. We aim at better understanding the dynamics of such fountain flow in a Galactic context, frame the problem in a more dynamic environment possibly learning about its connection and regulation to the local driving mechanism and understand its role as a metal diffusion channel. The interaction of the fountain with a hot corona is hereby analyzed, trying to understand the properties and evolution of the extraplanar material. We perform high resolution hydrodynamical simulations with the moving-mesh code AREPO to model the multi-phase ISM of a Milky Way type galaxy. A non-equilibrium chemical network is included to self consistently follow the evolution of the main coolants of the ISM. Spiral arm perturbations in the potential are considered so that large molecular gas structures are able to dynamically form here, self shielded from the interstellar radiation field. We model the effect of SN feedback from a new-born stellar cluster inside such a giant molecular cloud, as the driving force of the fountain. Passive Lagrangian tracer particles are used in conjunction to the SN energy deposition to model and study diffusion of freshly synthesized metals. We find that both interactions with hot coronal gas and local ISM properties and motions are equally important in shaping the fountain. We notice a bimodal morphology where most of the ejected gas is in a cold $10^4$ K clumpy state while the majority of the affected volume is occupied by a hot diffuse medium. While only about 20\% of the produced metals stay local, most of them quickly diffuse through this hot regime to great scales.

Neutrinos

Neutrinos represent a new “window” to the Universe, spanning a large range of energy. We discuss the science of neutrino astrophysics and focus on two energy regimes. At “lower” energies (≈1 MeV), studies of neutrinos born inside the sun, or produced in interactions of cosmic rays with the atmosphere, have allowed the first incontrovertible evidence that neutrinos have mass. At energies typically one thousand to one million times higher, sources further than the sun (both within the Milky Way and beyond) are expected to produce a flux of particles that can be detected only through neutrinos.

A massive association around the obscured open cluster RSGC3

Context. Four clusters of red supergiants have been discovered in a region of the Milky Way close to base of the Scutum-Crux Arm and the tip of the Long Bar. Population synthesis models indicate that they must be very massive to harbour so many supergiants. If the clusters are physically connected, this Scutum Complex would be the largest and most massive star-forming region ever identified in the Milky Way. Aims. The spatial extent of one of these clusters, RSGC3, has not been investigated. In this paper we explore the possibility that a population of red supergiants could be located in its vicinity. Methods. We utilised 2MASS JHKS photometry to identify candidate obscured luminous red stars in the vicinity of RSGC3. We observed a sample of candidates with the TWIN spectrograph on the 3.5-m telescope at Calar Alto, obtaining intermediate-resolution spectroscopy in the 8000−9000 Å range. We re-evaluated a number of classification criteria proposed in the literature for this spectral range and found that we could use our spectra to derive spectral types and luminosity classes. Results. We measured the radial velocity of five members of RSGC3, finding velocities similar to the average for members of Stephenson 2. Among the candidates observed outside the cluster, our spectra revealed eight M-type supergiants at distances <18′ from the centre of RSGC3, distributed in two clumps. The southern clump is most likely another cluster of red supergiants, with reddening and age identical to RSGC3. From 2MASS photometry, we identified four likely supergiant members of the cluster in addition to the five spectroscopically observed. The northern clump may be a small cluster with similar parameters. Photometric analysis of the area around RSGC3 suggests the presence of a large (>30) population of red supergiants with similar colours. Conclusions. Our data suggest that the massive cluster RSGC3 is surrounded by an extended association, which may be very massive ( ≳ 105 M⊙). We also show that supergiants in the Scutum Complex may be characterised via a combination of 2MASS photometry and intermediate-to-high-resolution spectroscopy in the Z band.

Red supergiants around the obscured open cluster Stephenson 2

Context. Several clusters of red supergiants have been discovered in a small region of the Milky Way close to the base of the Scutum-Crux Arm and the tip of the Long Bar. Population synthesis models indicate that they must be very massive to harbour so many supergiants. Amongst these clusters, Stephenson 2, with a core grouping of 26 red supergiants, is a strong candidate to be the most massive young cluster in the Galaxy. Aims. Stephenson 2 is located close to a region where a strong over-density of red supergiants had been found. We explore the actual cluster size and its possible connection to this over-density. Methods. Taking advantage of Virtual Observatory tools, we have performed a cross-match between the DENIS, USNO-B1 and 2MASS catalogues to identify candidate obscured luminous red stars around Stephenson 2, and in a control nearby region. More than 600 infrared bright stars fulfill our colour criteria, with the vast majority having a counterpart in the I band and >400 being sufficiently bright in I to allow observation with a 4-m class telescope. We observed a subsample of ~250 stars, using the multi-object, wide-field, fibre spectrograph AF2 on the WHT telescope in La Palma, obtaining intermediate-resolution spectroscopy in the 7500–9000 Å range. We derived spectral types and luminosity classes for all these objects and measured their radial velocities. Results. Our targets turned out to be G and K supergiants, late (≥ M4) M giants, and M-type bright giants (luminosity class II) and supergiants. We found ~35 red supergiants with radial velocities similar to Stephenson 2 members, spread over the two areas surveyed. In addition, we found ~40 red supergiants with radial velocities incompatible in principle with a physical association. Conclusions. Our results show that Stephenson 2 is not an isolated cluster, but part of a huge structure likely containing hundreds of red supergiants, with radial velocities compatible with the terminal velocity at this Galactic longitude (and a distance ~6 kpc). In addition, we found evidence of several populations of massive stars at different distances along this line of sight.