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The Flaring Activity of Pre-Main Sequence Stars in Very Young Open Clusters

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The Flaring Activity of Pre-Main Sequence Stars in Very Young Open Clusters http://lib.ulg.ac.be http://matheo.ulg.ac.be The flaring activity of pre-main sequence stars in very young open clusters Auteur : Nelissen, Marie Promoteur(s) : Rauw, Gregor Faculté : Faculté des Sciences Diplôme : Master en sciences spatiales, à finalité approfondie Année académique : 2016-2017 URI/URL : http://hdl.handle.net/2268.2/2504 Avertissement à l'attention des usagers : Tous les documents placés en accès ouvert sur le site le site MatheO sont protégés par le droit d'auteur. Conformément aux principes énoncés par la "Budapest Open Access Initiative"(BOAI, 2002), l'utilisateur du site peut lire, télécharger, copier, transmettre, imprimer, chercher ou faire un lien vers le texte intégral de ces documents, les disséquer pour les indexer, s'en servir de données pour un logiciel, ou s'en servir à toute autre fin légale (ou prévue par la réglementation relative au droit d'auteur). 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The flaring activity of pre-main sequence stars in very young open clusters Nelissen Marie Second year of the Space Sciences Master’s degree at the University of Liege` Master’s thesis director: Rauw Gregor Academic year 2016-2017 Contents 1 Introduction 3 1.1 Pre-main sequence stars.....................................3 1.2 The open cluster NGC 6530..................................6 1.3 The XMM-Newton satellite...................................7 1.4 Summary............................................8 2 Processing 10 2.1 Lists of events......................................... 10 2.2 EPIC cameras.......................................... 11 2.3 SAS processing......................................... 13 3 Source detection 17 3.1 Detection on combined images................................. 17 3.2 Detection verification...................................... 21 3.3 Detection on individual observations.............................. 21 3.4 Three colors image....................................... 22 4 Correlations 24 4.1 Catalogs............................................. 24 4.2 Correlation radius........................................ 25 5 Rotation rates 29 6 Hertzsprung-Russell diagram 31 6.1 Color-magnitude diagram.................................... 31 6.2 Hertzsprung-Russell diagram.................................. 32 7 Inter-pointing variability 36 7.1 Correlation........................................... 36 7.2 Chi squared........................................... 37 7.3 Probability............................................ 37 8 Lightcurves 40 8.1 Extraction............................................ 40 8.2 Analysis............................................. 41 1 8.3 Intra-pointing variability.................................... 45 8.4 Comparison with other studies................................. 48 8.4.1 Loop sizes....................................... 48 8.4.2 Flaring frequency.................................... 48 9 Spectra 51 9.1 Extraction............................................ 51 9.2 Spectral fits........................................... 51 9.3 Comments on flaring sources.................................. 56 10 Conclusions 57 2 Chapter 1 Introduction 1.1 Pre-main sequence stars For stars of mass similar to our Sun, their formation begins with a molecular cloud. When a portion of this cloud collapses - due to its own gravity1 - into several pieces, these pieces then form protostars surrounded by accretion disks. The establishment of a hydrostatic equilibrium in the core of the future star corresponds to the transition from the protostar phase to the pre-main sequence phase. Initially, the core of the pre-main sequence star (or PMS) is entirely hidden by the portion of molecular cloud from which it was formed. The gas surrounding the celestial body then dissipates or is accreted by the PMS object and the star is revealed (Dupret 2015-2016; Schulz 2012). Figure 1.1: Star formation (”Stars, Supernovas and Neutron Stars - Black Holes and Wormholes - The Physics of the Universe”, 2017). 1Molecular clouds have a high mass and therefore a significant auto-gravity. This means that they are in an unstable equilibrium. Forces due to the interstellar magnetic field, internal pressure gradients and centrifugal effects due to rotation work to maintain this equilibrium. Instability factors are a transit in a high-density zone or a shock wave due to a supernova explosion. 3 1.1. PRE-MAIN SEQUENCE STARS CHAPTER 1. INTRODUCTION Before reaching the main sequence, pre-main sequence stars move along the Hayashi tracks, corre- sponding to models of entirely convective stars in hydrostatic equilibrium (but not necessarily in thermal equilibrium, unlike main sequence stars) of constant mass and chemical composition. They are represented in the Hertzsprung-Russell diagram as quasi vertical curves, each track corresponding to a specific stellar mass (see Fig. 1.2)(Dupret 2015-2016). Figure 1.2: The Hayashi tracks (Dupret 2015-2016). Protostars and PMS can be classified into different categories, based on their IR excess2 and Hα emis- sion3 (see Fig. 1.3)(Feigelson & Montmerle 1999; Rauw 2016-2017; Schulz 2012): The Class 0 sources or infalling protostars are young protostars deeply embedded in their nascent molec- ular cloud (which is collapsing toward the central regions, accreting onto the protostar and forming a disk). They are invisible in the optical domain and rarely detected in the X-ray domain due to heavy circumstellar extinction. The Class I sources or evolved protostars correspond to protostars surrounded by a thick disk. They present a strong IR excess, are visible in the optical domain and emit weakly in the X-ray domain. Class II sources or classical T Tauri stars (or cTTs) are PMS surrounded by a thick disk that is believed to be slowly dissipating. They present a weak IR excess, are visible in the optical domain, are strong Hα and X-ray emitters. Class III sources or weak-lined T Tauri stars (or wTTs) are PMS surrounded by a very thin or non- existent disk (and mostly do not accrete matter from it). They present no IR excess, are visible in the optical domain, present weak (if any) Hα emission and very strong X-ray emission. 2This refers to the level of IR excess compared to the emission of a stellar photosphere. It is believed that thermal emission from the dust present in circumstellar envelopes is responsible for the IR excess. 3The Hα emission line is produced when the matter that migrates from a circumstellar disk toward the surface of the star (within 3 or 4 stellar radii) is forced by the stellar magnetosphere to flow into accretion streams and columns. 4 1.1. PRE-MAIN SEQUENCE STARS CHAPTER 1. INTRODUCTION Figure 1.3: Types of protostars and PMS (Feigelson & Montmerle 1999). T Tauri stars probably emit X-rays thanks to a scaled-up version of solar-like magnetic activity. Indeed, they are thought to contain a dynamo, called the α – Ω dynamo, that is generated through the interplay of convection and differential rotation at the interface between the radiative core and the convective envelope. This dynamo in turn produces an external magnetic field. These stars then generate fairly strong4 and fre- quently variable X-ray emissions (Feigelson & Montmerle 1999; Rauw 2016-2017; Schulz 2012). While both ends of magnetic loops can be connected to the stellar photospheres, T Tauri stars may also emit X-rays because of magnetic interactions with their circumstellar disks5 (Feigelson & Montmerle 1999; Rauw 2016-2017; Schulz 2012). Younger T Tauri stars, on the other hand, are fully convective (and the α – Ω dynamo cannot work). This means that they probably contain a dynamo linked to convective motion (Rauw 2016-2017; Schulz 2012). The PMS X-ray emissions frequently take the form of flares, i.e. sudden increases (in minutes or hours) of the X-ray luminosity6 (by up to a factor 100) followed by a decline (of several hours) ruled by the size of the loops (Gudel¨ & Naze´ 2009; Schulz 2012). 4The X-ray luminosity of PMS is typically 100 to 1 000 times the X-ray luminosity of the Sun. 5One could also consider the possibility of magnetic field lines with both feet in the disks. In this case, a dynamo would be generated thanks to convective motions and differential rotation. 6Flares emit not only in the X-ray domain, but all over the electromagnetic spectrum. However, their contrast is usually higher at higher photon energies. 5 1.2. THE OPEN CLUSTER NGC 6530 CHAPTER 1. INTRODUCTION Flares are produced by the reconnections of unstable magnetic field lines. In the reconnection zone, particles are heated and accelerated. When they reach the stellar surface (after travelling along the ”new” field lines), they heat the plasma to temperatures of the order of 107 K. The induced pressure forces the hot plasma into loops and flares appear (Feigelson & Montmerle 1999;G udel¨ & Naze´ 2009; Schulz 2012). The observation of PMS flaring activity can provide information about whether
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