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DETAILED CHEMICAL ABUNDANCES in the Sgr and Cma DWARF GALAXIES

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DETAILED CHEMICAL ABUNDANCES in the Sgr and Cma DWARF GALAXIES Universit`adegli studi di Roma “Tor Vergata” Facolt`adi Scienze Matematiche, Fisiche e Naturali Dottorato di Ricerca in Astronomia XVII ciclo (A. A. 2001/2002–2003/2004) DETAILED CHEMICAL ABUNDANCES IN THE Sgr AND CMa DWARF GALAXIES Luca Sbordone Coordinatore Relatori esterni Prof. Roberto Buonanno Dott. Piercarlo Bonifacio Dott. Gianni Marconi Relatore interno Prof. Roberto Buonanno 2 Abstract Detailed chemical abundances for up to 23 elements from Oxygen to Eu- ropium have been obtained from VLT-UVES spectra for 12 giants in the Sagittarius Dwarf Spheroidal galaxy (Sgr dSph), 5 in the associated globular cluster Terzan 7, and 3 in the direction of the newly discovered and still con- troversial CMa overdensity. The analysis has been accomplished by means of a GNU-Linux ported version of R. L. Kurucz’s codes for atmosphere mod- eling (ATLAS), abundance determination (WIDTH) and spectral synthesis (SYNTHE). The porting details are discussed in the thesis, the ported codes are freely available on line. Sgr dSph is the nearest confirmed dwarf spheroidal galaxy. Moving along a short period, quasi-polar orbit around the Milky Way (MW) it is undergoing tidal disruption inside the Halo. It is, consequently, an ideal test case for studying the role of tidal merging in shaping our Galaxy. In this study we evidence the presence of a metal rich (from [Fe/H]∼ −0.8 up to slightly over-solar) young (about 1 GYr) population in Sgr dSph, also displaying a very characteristics overall chemical signature, including sub-solar [α/Fe] ratio, low Na, Al, Sc, Ni, Cu and Zn ratios over iron, and strongly enhanced La, Ce and Nd. These highly characteristic abundances are also shared by the MW globular cluster Palomar 12, thus demonstrating definitively that it originated inside the Sgr dSph system, and was consequently stripped from it. We also report some results from an ongoing FLAMES study aiming to obtaining spectroscopic metallicities for a large sample of Sgr dSph stars, indicating that a very metal poor population also exist in the galaxy, down to [Fe/H]s∼-3. All together, these findings suggest a very long and slow star formation history for Sgr dSph (the metallicity spread being comparable with the one encountered inside the MW), thus leading to infer that a large quantity of gas should have been present in the galaxy. Such gas is also necessary to explain the formation of at least five globulars. We also looked for interstellar absorption systems at high radial velocities (compatible with −1 the one of Sgr dSph stars, Vrad ∼140 kms in the spectra of our targets, finding them superimposed to two stars only. This enforces the previous results (from radio surveys) indicating that very little gas survived inside Sgr dSph, adding an hint that it should likely have a spotty distribution. 4 We also observed 5 cool, low gravity giants in Terzan 7, which showed a mean metallicity of [Fe/H]=-0.57, and an abundance pattern coherent with the one identified in the Sgr dSph main body stars. On three of these stars, we were able to accomplish the first Sulphur abundance measurement in an extragalactic star. Finally, we present the results of a first FLAMES exploratory study of the CMa overdensity, suspected to be the residual of an in-plane accretion event, associated with the Galactic Anticenter Stellar Structure (GASS). We concentrate on the analysis of the three (out of seven) UVES targets which radial velocity is compatible with the one of the overdensity. One of them is likely a MW interloper, one appears of uncertain origin, the third one appears to be of extragalactic origin due to its peculiar chemical abundances. It is a subgiant (log g 3.5, Teff 5337 K) showing over-solar iron content, α elements depletion, La, Ce and Nd enhancement and a significant Cu enhancement ([Cu/Fe]=+0.25). It is worth noticing that, while some of these anomalies are coherent with the ones found in Sgr dSph and in other dSph (the α elements depletion and the Lanthanides enhancement), others are significantly at odd with them. We also describe the results of the first metallicity and radial velocity analysis in the GIRAFFE sample, showing that an overdensity appears to exist in phase space, with respect to the previsions of the Besan¸conMW model, in the observed area. All in all, our findings support the claimed detection of a tidal accretion remain in CMa, but the size of this first sample is way too small to provide conclusive results. Keywords:Stars: abundances, Stars: atmospheres, ISM: general, Galax- ies: abundances, Galaxies: dwarf, Galaxies: evolution, Galaxies: stellar con- tent, Galaxies: individual: Sgr dSph, Galaxy: globular clusters: individual: Terzan 7 Sommario Abbondanze chimiche dettagliate (fino a 23 elementi, dall’Ossigeno all’Eu- ropio) sono state ottenute, da spettri VLT-UVES, per 12 stelle giganti nella Galassia Nana Sferoidale del Sagittario (Sgr dSph), 5 nell’amasso globulare Terzan 7, ad essa associato, e 3 nella direzione della recentemente scoperta (ed ancora controversa) Sovradensit`adel Cane Maggiore. L’analisi `estata compiuta per mezzo di una versione da noi portata sotto GNU-Linux dei codici di R. L. Kurucz per il calcolo di modelli di atmosfera (ATLAS), la determinazione delle abbondanze (WIDTH) e la spettrosintesi (SYNTHE). I dettagli del porting sono discussi nella tesi, i codici sono scaricabili gratuita- mente dalla rete. Sgr dSph `ela pi`uvicina nana sferoidale accertata. In moto lungo un’orbita quasi polare di corto periodo attorno alla Via Lattea (MW), sta venendo dissolta nell’Alone a causa dell’interazione mareale con la Galassia. Di con- seguenza, costituisce un test ideale per studiare il ruolo rivestito dall’accre- scimento mareale nella formazione della Galassia. In questa tesi eviden-ziamo la presenza di una popolazione metal rich (da [Fe/H]∼ −0.8 fino a valori lieve- mente sovrasolari) e giovane (circa 1 GYr) all’interno della Sgr dSph. Tale popolazione inoltre presenta una “firma” chimica complessiva molto caratter- istica, che comprende [α/Fe] sub-solare, basse abbondanze di Na, Al, Sc, Ni, Cu e Zn, e forti sovrabbondanze di La, Ce e Nd. Simili peculiarit`achimiche sono state osservate anche nell’ammasso globulare galattico Palomar 12, il che dimostra definitivamente come esso abbia avuto origine entro la Sgr dSph, e sia stato successivamente acquisito dalla Via Lattea. Inoltre, riportiamo i primi risultati di uno studio FLAMES (attualmente in corso) mirato ad ottenere velocit`aradiali e metallicit`aper un vasto campione di stelle della Sgr dSph, indicanti la presenza di una popolazione estremamente povera di metalli, fino a [Fe/H]∼-3. Presi congiuntamente, questi risultati suggeriscono che la formazione stellare in Sgr dSph debba essere stata molto prolungata (l’intervallo di metallicit`aosservate essendo comparabile con quello caratter- istico della Via Lattea) e lenta, il che permette di inferire che una grande quantit`adi gas dovesse essere presente nella galassia. Ci`o`eanche necessario per giustificare la formazione di almeno 5 ammassi globulari. Abbiamo anche 6 compiuto una ricerca di sistemi di assorbimento interstellari ad alta velocit`a −1 radiale (compatibili con una appartenenza alla Sgr dSph, Vrad ∼140 kms ) negli spettri delle stelle da noi osservate, trovandone negli spettri di sole due stelle. Ci`oconferma i precedenti risultati (da survey radio) indicanti che ben poco gas deve essere sopravvissuto entro Sgr dSph, e suggerisce inoltre che esso sia distribuito sotto forma di piccole nubi locali. Abbiamo anche osservato 5 giganti di bassa temperatura e gravit`anell’am- masso globulare Terzan 7, che ha mostrato una metallicit`amedia di [Fe/H]= -0.57, ed abbondanze coerenti con quelle osservate nel corpo principale della Sgr dSph. Per tre di queste stelle `estato possibile (per la prima volta in oggetti al di fuori della Via Lattea) misurare l’abbondanza di Zolfo. Infine, presentiamo i risutati di un primo studio esplorativo FLAMES della Sovradensit`adel Cane Maggiore (CMa), sospettata essere il residuo di una galassia nana, distrutta in seguito ad un’interazione lungo il piano della Via Lattea, ed associata alla cosiddetta Galactic Anticenter Stellar Struc- ture (GASS). In questa tesi ci concentriamo sull’analisi delle tre (di sette totali) stelle osservate con UVES le cui velocit`aradiali sono compatibili con un’appartenenza alla Sovradensit`a. Una di esse appartiene probabil- mente alla Via Lattea, una appare avere origine incerta, la terza `equasi sicuramente di origine extragalattica a causa delle sue peculiari abbondanze chimiche. Si tratta di una subgigante (log g 3.5, Teff 5337 K) caratterizzata da un’abbondanza di ferro sovrasolare, povert`adi elementi α, sovrabbon- danza in La, Ce e Nd, e una significativa sovrabbondanza di Cu ([Cu/Fe]=+0.25). E` interessante notare che, mentre alcune di tali anomalie sono coerenti con quanto riscontrato in Sgr dSph ed in altre nane sferoidali (il basso contenuto di elementi α e la sovrabbondanza di Lantanoidi), altre sono significativa- mente in contrasto con esse. Descriviamo anche i risultati delle prime analisi di velocit`aradiali e metallicit`adel campione di stelle GIRAFFE, indicanti che, rispetto alle previsioni del modello di Besan¸condella Via Lattea, una sovradensit`anello spazio delle fasi appare effettivamente esistere nell’area osservata. Complessivamente, i nostri risultati supportano la teoria che la Sovradensit`asia il residuo di un accrescimento mareale, ma la dimensione di questo primo campione `edecisamente troppo limitata per permettere di affermare alcunch´edi conclusivo. Contents 1 Introduction 21 1.1 Abundance analysis in dwarf galaxies with high resolution spectroscopy . 22 1.2 Production channels for different elements . 23 1.2.1 Light odd elements . 23 1.2.2 α elements . 24 1.2.3 Iron peak elements . 25 1.2.4 Heavy neutron capture elements . 25 1.3 Observed abundance ratios in Dwarf Galaxies .
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