Studying Hydrogen Emission Lines from Classical T Tauri Stars: Telluric Line Removal, Physical Conditions in the Emitting Gas, and Reddening

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Studying Hydrogen Emission Lines from Classical T Tauri Stars: Telluric Line Removal, Physical Conditions in the Emitting Gas, and Reddening Studying Hydrogen Emission Lines from Classical T Tauri Stars: Telluric Line Removal, Physical Conditions in the Emitting Gas, and Reddening Dissertation zur Erlangung des Doktorgrades des Fachbereichs Physik der Universit¨atHamburg vorgelegt von Natascha Rudolf aus Hamburg Hamburg 2014 ii Gutachter der Dissertation: Prof. Dr. J¨urgenH. M. M. Schmitt Prof. Dr. Ralph Neuh¨auser Gutachter der Disputation: Prof. Dr. Robi Banerjee Dr. Frederic V. Hessman Datum der Disputation: 26. Juni 2014 Vorsitzender des Pr¨ufungsausschusses: Dr. Robert Baade Vorsitzende des Promotionsausschusses: Prof. Dr. Daniela Pfannkuche Dekan der MIN Fakult¨at: Prof. Dr. Heinrich Graener iii Zusammenfassung W¨ahrendihrer Entstehung durchlaufen Sterne verschiedene Entwicklungsphasen, bevor sie die Hauptreihe erreichen. Sie bilden sich durch den gravitativen Kollaps von molekularen Wolken. Die Erhaltung des Drehimpulses erfordert es, dass die Massenakkretion nicht radial, sondern von einer Scheibe aus stattfindet. Sterne mit niedriger Masse, die sich in einer Entwicklungsphase befinden, in der sich das meiste Material in der Scheibe befindet und der Stern nicht mehr in der Wolke verborgen ist, werden klassische T Tauri Sterne (CTTS) genannt. Diese Arbeit befasst sich mit der zirkumstellaren Umgebung dieser Sterne. Die Untersuchungen basieren auf VLT/X-Shooter-Spektren von 20 Sternen mit unterschiedlichen Spektraltypen und Massenakkretionsraten. Die X-Shooter-Spektren decken zeitgleich einen Spektralbereich von etwa 3000 A˚ bis etwa 25 000 A˚ bei einer mittleren Aufl¨osung(R ∼ 10 000) ab. Ein wichtiger Schritt der Datenreduktion ist die Entfernung der tellurischen Linien, die die Erdatmosph¨aredem Spektrum hinzuf¨ugt.An Stelle der Nutzung beobachteter tellurischer Standardsterne verwende ich Modelle der atmosph¨arischen Transmission, die an die Wetterbe- dingungen zum Zeitpunkt der Beobachtung angepasst sind, um diese Kontamination zu entfer- nen. F¨urdie Analyse von CTTS-Spektren ist es sehr wichtig die St¨arke der R¨otungzu kennen, da- her nutze ich eine neue Methode um die R¨otungzu messen. Das Verh¨altnis von emittierten Wasserstofflinien mit dem selben oberen Niveau ist unabh¨angigvon den Bedingungen im Gas und gegeben durch Konstanten, die aus der Atomphysik bekannt sind. Somit kann eine Differenz zwischen den beobachteten und den theoretischen Linienverh¨altnissennur durch die R¨otung verursacht werden. Die Messung der Extinktion f¨urmehrere Linienpaare mit dem selben oberen Niveau auf diese Weise erlaubt es mir, neue Extinktionswerte AV f¨ursechs Objekte zu bestim- men. Obwohl die Form des R¨otungsgesetzesmit dieser neuen Methode ebenfalls untersucht wer- den kann, finde ich aufgrund relativ hoher Fehler der Daten keine Abweichung vom ¨ublicherweise f¨urden Infrarotbereich genutzten R¨otungsgesetz, A ∼ λ−1:84. Das aktuelle Modell der Akkretion auf CTTS ist das Modell des magnetisch kontrollierten Einfalls. In diesem Modell f¨alltdas Material entlang der Magnetfeldlinien in Akkretionstrichtern auf den Stern. Dieses Modell erkl¨artdie wesentlichen Merkmale, die wir bei CTTS beobachten, jedoch sind die physikalischen Eigenschaften des emittierende Gases noch unklar. Die Tempe- ratur T und die Elektronendichte ne im Gas k¨onnenaus einer Analyse von Wasserstoffemissi- onslinien bestimmt werden. Ich messe die Fl¨usseder Wasserstoffemissionslinien der Paschen- und Brackett-Serie und vergleiche die Linienverh¨altnisseder h¨oherenPaschen-Linien zu Paβ, Brγ zu den h¨oherenPaschen-Linien und der h¨oherenBrackett-Linien zu Brγ mit den Vorher- sagen der sogenannten Fall-B-Modelle f¨urrekombinierenden Wasserstoff. Die Fall-B-Modelle nehmen an, dass das Gas in der Lyman-Serie optisch dick ist und optisch d¨unnin allen an- deren Uberg¨angen.¨ Dies ergibt Temperaturen zwischen 500 K und 5000 K und Dichten zwis- chen 109 cm−3 und 1010 cm−3. Eine große Quelle f¨urUnsicherheiten in dieser Analyse ist die Entr¨otungder Linienfl¨usse.Allerdings ergeben die Fall-B-Modelle selbst bei Objekten, bei denen die Entr¨otungplausibel erscheint, oft keine statistisch gute Ubereinstimmung¨ mit den Daten. Dies wirft die Frage auf, ob die Annahmen der Fall-B-Modelle die angemessene Beschreibung sind f¨urdie Bedingungen in der Region von CTTS, in der die Wasserstofflinien entstehen. Es scheint, dass allgemeinere Modelle der Linienanregung und -rekombination n¨otigsind um die Beobachtungen zu erkl¨aren. Wenn die gefundenen physikalischen Eigenschaften zutreffend sind, stimmen sie mit den Vor- hersagen von magnetosph¨arischen Akkretionsmodellen bezogen auf die Dichten ¨uberein, die gefundenen Temperaturen sind allerdings niedriger. Die niedrigen Temperaturen des Gases im- plizieren sehr kurze Abk¨uhlzeitenvon nur einigen Minuten. Dies deutet auf die Notwendigkeit hin, das Modell des magnetisch kontrollierten Einfalls weiterzuentwickeln um weitere Prozesse und Beitr¨ageeinzuschließen, die f¨urdie Entstehung von Emissionslinien wichtig sind. iv Abstract During their formation stars go through different evolutionary stages before reaching the main sequence. They form from a molecular cloud via gravitational collapse. The conservation of an- gular momentum requires the mass accretion to proceed not radially but from a disk. Low-mass stars at an evolutionary stage, where most material is in the disk and the star is not hidden inside a cloud any more, are called classical T Tauri stars (CTTS). This thesis deals with the circumstellar environment of these stars. The study is based on VLT/X-Shooter spectra of 20 stars with different spectral types and mass accretion rates. The X-Shooter spectra simultaneously cover the spectral range from about 3000 A˚ to about 25 000 A˚ at medium resolution (R ∼ 10 000). An important data reduction step is the removal of telluric lines imprinted on the spectra by the atmosphere of the Earth. Instead of using observed telluric standards, I employ models of the atmospheric transmission adapted to the weather conditions at the time of observation to remove the contamination. For the analysis of CTTS spectra, it is very important to know the amount of reddening of the light. Therefore, I apply a new method to measure the reddening. The ratio of emitted hydrogen lines with a common upper level is independent of the conditions in the gas and given by constants known from atomic physics. Thus, a difference between the observed and the theoretical line ratios can only be caused by reddening. Measuring the extinction for several common upper level line pairs in this way allows me to determine new extinction values AV for six objects. Although the shape of the reddening law can also be tested with this new method, I do not find a deviation from the standard reddening law in the near-infrared, A ∼ λ−1:84, due to relatively high uncertainties of the data. The current model of accretion onto CTTS is the magnetically funnelled infall model. In this model, the material falls onto the star along the magnetic field lines in accretion funnels. This model explains the main features we observe from CTTS, but the physical conditions in the emitting gas are still uncertain. The temperature T and the electron density ne in the gas can be deduced from an analysis of hydrogen emission lines. I measure the hydrogen emission line fluxes in the Paschen and Brackett lines and compare the line ratios of the higher order Paschen lines to Paβ, Brγ to the higher order Paschen lines, and the higher order Brackett lines to Brγ to the predictions of the so-called case B models for recombining hydrogen. The case B models assume that the gas is optically thick in the Lyman series and optically thin in all other transitions. This yields temperatures between 500 K and 5000 K and densities between 109 cm−3 and 1010 cm−3. A major source of uncertainty for this analysis is the dereddening of the line fluxes. However, even for objects where the dereddening seems reasonable, the case B models often do not provide a statistically good fit to the data. This raises the question whether the assumptions of the case B model are the appropriate description of the conditions in the hydrogen line formation region of CTTS. It seems that more general models of line excitation and recombination are required to explain the observations. If applicable, the physical conditions found agree with predictions by magnetospheric accretion models in terms of density, but the temperatures found here are lower. The low temperatures of the gas imply very short cooling times of only a few minutes. This points towards a necessity to refine the magnetically funnelled infall model to include further processes and contributions that are important for the emission line formation. CONTENTS v Contents 1 Introduction 1 1.1 From clouds to stars..................................1 1.2 The main sequence...................................3 1.3 The final phases of a stellar life............................4 1.4 Remarks on the structure of this work........................5 2 T Tauri Stars and Their Environment7 2.1 Observational characteristics.............................7 2.1.1 Disk and outflows...............................8 2.1.2 Accretion....................................8 2.2 The magnetically funnelled infall model....................... 10 2.3 Reddening and dust.................................. 12 3 Observations and Data Reduction 17 3.1 The instrument: VLT/X-Shooter........................... 17 3.2 The sample....................................... 18 3.3 Data reduction using the ESO pipeline........................ 18 3.4 Flux calibration....................................
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