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Where Are the Hot Ion Lines in Classical T Tauri Stars Formed?

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Where Are the Hot Ion Lines in Classical T Tauri Stars Formed? Accretion, winds and jets: High-energy emission from young stellar objects Dissertation zur Erlangung des Doktorgrades des Departments Physik der Universit¨at Hamburg vorgelegt von Hans Moritz G¨unther aus Hamburg Hamburg 2009 ii Gutachter der Dissertation: Prof. Dr. J. H. M. M. Schmitt Prof. Dr. E. Feigelson Gutachter der Disputation: Prof. Dr. P. H. Hauschildt Prof. Dr. G. Wiedemann Datum der Disputation: 13.03.2009 Vorsitzender des Pr¨ufungsausschusses: Dr. R. Baade Vorsitzender des Promotionsausschusses: Prof. Dr. R. Klanner Dekan der MIN Fakult¨at: Prof. Dr. A. Fr¨uhwald (bis 28.02.2009) Prof. Dr. H. Graener (ab 01.03.2009) iii Zusammenfassung Sterne entstehen durch Gravitationsinstabilit¨aten in molekularen Wolken. Wegen der Erhaltung des Drehimpulses geschieht der Kollaps nicht sph¨arisch, sondern das Material f¨allt zun¨achst auf eine Akkretionsscheibe zusammen. In dieser Doktorarbeit wird hochenergetische Strahlung von Sternen untersucht, die noch aktiv Material von ihrer Scheibe akkretieren, aber nicht mehr von einer Staub- und Gash¨ulle verdeckt sind. In dieser Phase nennt man Sterne der Spektraltypen A und B Herbig Ae/Be (HAeBe) Sterne, alle sp¨ateren Sterne heißen klassische T Tauri Sterne (CTTS); eigentlich werden beide Typen ¨uber spektroskopische Merkmale definiert, aber diese fallen mit den hier genannten Entwicklungsstadien zusammen. In dieser Arbeit werden CTTS und HAeBes mit hochaufl¨osender Spektroskopie im R¨ontgen- und UV-Bereich untersucht und Simulationen f¨ur diese Stadien gezeigt. F¨ur zwei Sterne werden R¨ontgenspektren reduziert und vorgestellt: Der CTTS V4046 Sgr wurde mit Chandra f¨ur 100 ks beobachtet. Die Lichtkurve dieser Beobachtung zeigt einen Flare und die Triplets der He Isosequenz (Si xiii, Ne ix und O vii) deuten auf hohe Dichten im emittierenden Plasma hin. Die zweite Beobachtung enth¨alt Daten f¨ur den HAeBe Stern HD 163296, die in 130 ks mit XMM-Newton gewonnen wurden. Diese Lichtkurve zeigt nur wenig Aktivit¨at und die Elementh¨aufigkeiten entsprechen dem f¨ur aktive Sterne ¨ublichem Muster. Das Triplet von O vii ist vergleichbar mit koronalen Quellen, also beeinflussen weder hohe Dichten noch ein starkes UV Feld die Emission. Aus diesen und ¨ahnlichen Beobachtungen, l¨aßt sich schließen, dass mindestens drei Prozesse zur hochenergetischen Emission vom CTTS beitragen: Zum Einen haben diese Sterne ak- tive Koronen ¨ahnlich denen normaler Hauptreihensterne, zum Zweiten bildet sich ein starker Akkretionsschock auf der Sternoberfl¨ache, der das von der Scheibe einfallende Material auf mehrere Millionen Kelvin heizt und zum Dritten haben manche CTTS energiereiche Jets. Schocks in diesen Jets k¨onnen das Material so weit erw¨armen, dass es im R¨ontgenbereich leuchtet. Koronen sind bereits gut studiert; f¨ur die anderen beiden Mechanismen werden in dieser Arbeit Modelle vorgestellt. Der Akkretionsschock wird in einem ein-dimensionalen Modell behandelt, welches aber kein Ionisationsgleichgewicht voraussetzt. Die Vereinfachung auf nur eine Dimension ist zul¨assig, weil das Magnetfeld Bewegungen senkrecht zur Feldrichtung unterdr¨uckt. Die Strahlungsverluste des als optisch d¨unn angenommenen Gases werden mit CHIANTI berechnet. Eine Kombination aus diesem Modell und koronalem Gas wird an die Beobachtungen von TW Hya und V4046 Sgr angepasst. F¨ur beide Sterne reichen relativ −10 geringe Massenakkretionsraten, um die R¨ontgenleuchtkraft zu erkl¨aren (2 × 10 M⊙/yr bzw. −11 3 × 10 M⊙/yr). In dieser Arbeit wird ein Modell vorgestellt, das die weiche Emission in der Umgebung von DG Tau durch Schockwellen entlang des Jets erkl¨art. Schockgeschwindigkeiten zwischen 400 und 500 km s−1 sind notwendig, um das beobachteten Spektrum zu erzeugen. Bei Dichten > 105 cm−3 ist die Ausdehnung des Schocks so klein, dass erkl¨art ist, warum er in optischen Beobachtungen nicht entdeckt wird. Da im R¨ontgenbereich die spektrale Aufl¨osung nicht f¨ur eine Linienprofilanalyse ausreicht, wer- den UV Daten daf¨ur herangezogen. Die Linienprofile im UV Bereich der mit FUSE beobachteten CTTS sind bis zu 500 km s−1 breit. Vermutlich tragen Akkretion und Winde zu der Emission bei, aber im Moment k¨onnen die Akkretionsmodelle die Linienprofile, besonders die Breite, noch nicht erkl¨aren. HAeBe Sterne besitzen heißes, koronales Plasma, genau wie CTTS. Akkretionsschocks tragen kaum zur Emission bei; das Triplet von O vii weist auf einen Ursprung in Jets ¨ahnlich wie bei DG Tau hin. Ein vergleichbares Modell kann auch hier die R¨ontgenemission erkl¨aren. iv Abstract Stars form by gravitational collapse from giant molecular clouds. Due to the conservation of angular momentum this collapse does not happen radially, but the matter forms circumstellar disk first and is consequently accreted from the disk onto the star. This thesis deals with the high-energy emission from young stellar objects, which are on the one hand still actively accreting from their disk, and on the other hand are no longer deeply obscured by their natal cloud. Stars of spectral type B and A are called Herbig Ae/Be (HAeBe) stars in this stage, all stars of later spectral type are termed classical T Tauri stars (CTTS); strictly speaking both types are defined by spectroscopic signatures, which are equivalent to the evolutionary stage described above. In this thesis CTTS and HAeBes are studied through high-resolution X-ray and UV spectroscopy and through detailed physical simulations. Spectroscopic X-ray data is reduced and presented for two targets: The CTTS V4046 Sgr was observed with Chandra for 100 ks, using a high-resolution grating spectrometer. The lightcurve contains one flare and the He-like triplets of Si xiii, Ne ix and O vii indicate high densities in the X-ray emitting regions. The second target is the HAeBe HD 163296, which was observed with XMM-Newton for 130 ks. The lightcurve shows only moderate variability, the elemental abundance follows a pattern, that is usual for active stars. The He-like triplet of O vii exhibits line ratios similar to coronal sources, indicating that neither a high density nor a strong UV-field is present in the region of the X-ray emission. Using these and similar observations, it can be concluded that at least three mechanisms con- tribute to the observed high-energy emission from CTTS: First, those stars have active coronae similar to main-sequence stars, second, the accreted material passes through a strong accretion shock at the stellar surface, which heats it to a few MK, and, third, some CTTS drive powerful outflows. Shocks within these jets can heat the matter to X-ray emitting temperatures. The first is already well characterised; for the latter two scenarios models are presented in this the- sis. The accretion shock is treated in a stationary 1D model, taking non-equilibrium ionisations explicitly into account. The magnetic field is strong enough to suppress motion perpendicular to the field lines, so the use of a 1D geometry is justified. The radiative loss is calculated as optically thin emission with the CHIANTI database. A combination of simulated post-shock cooling zone spectra and coronal gas is fitted to the observations of the CTTS TW Hya and V4046 Sgr. Both stars require only small mass accretion rates to power the X-ray emission −10 −11 (2 × 10 M⊙/yr and 3 × 10 M⊙/yr, respectively). The CTTS DG Tau is heavily absorbed and the observed soft X-ray emission originates spatially offset from the star. In this thesis a physical model is presented which explains the emission by a shock front travelling along the ejected jet. Shock velocities between 400 and 500 km s−1 are required to explain the observed spectrum. For a electron density > 105 cm−3 all shock dimensions are so small that they remain undetectable in optical observations as observed. The spectral resolution in X-rays is not sufficient to analyse the line profiles, so UV data is used for this purpose. Line profiles extend up to 500 km s−1 in sample of CTTS observed with FUSE. Likely contribution from both, infalling and outflowing gas, contributes to the observed emission. The current models do not explain the observed line profiles in detail, especially the line width causes problems. HAeBe stars have hot plasma, which can only be explained as an active corona, similar to the CTTS. Accretion does not contribute significantly to the X-ray emission, instead the line ratios in the He-like triplets point to an origin in the outflows, similar to the CTTS jets. A model comparable to DG Tau reproduces the observed emission. CONTENTS v Contents 1 Introduction 1 1.1 Fromcloudstostars ............................... ... 1 1.2 Themain-sequence................................. 4 1.3 Thelastphasesinstellarlife . ....... 4 2 Young stars 5 2.1 ClassicalTTauristars . ..... 5 2.1.1 Observations .................................. 5 2.1.2 The magnetically funnelled infall model . ........ 8 2.1.3 Properties and driving of jets and outflows . ....... 9 2.2 HerbigAe/Bestars................................ 10 2.3 Openquestions ................................... 11 2.3.1 What are the X-ray properties of CTTS? . 11 2.3.2 Do the accretion shocks significantly contribute to the observed X-ray emissionfromCTTS? ............................. 11 2.3.3 Do accretion shocks significantly contribute to the observed UV emission fromCTTS? .................................. 12 2.3.4 What is the nature of the CTTS winds and outflows? . ..... 12 2.3.5 What powers the X-ray emission of jets from CTTS? . ...... 12 2.3.6 HowdoHAeBesdifferfromCTTS? . 12 2.4 Observational methods of X-ray astronomy . ........ 12 3 X-ray accretion signatures in the binary V4046 Sgr 19 4 X-ray emission from CTTS: Accretion and coronae? 25 5 Modelling the X-rays of CTTS 37 6 Formation of hot ion lines in CTTS 41 7 Revealing the fastest component of the DG Tau jet 53 8 X-rays from HD 163296: Jet, accretion or corona? 61 9 Summary and outlook 75 9.1 Summary ........................................ 75 9.2 The importance of high-energy observations .
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