Struktur- und Staubentwicklung in zirkumstellaren Scheiben um TTauri-Sterne - Analyse und Modellierung hochauflösender Beobachtungen in verschiedenen Wellenlängenbereichen

Im Zentrum dieser Doktorarbeit steht die Untersuchung der inneren Strukturen zirkumstellarer Scheiben um TTauri-Sterne sowie die Analyse zirkumstellarer Staub- und Eisteilchen und ihres Einflusses auf die Scheibenstruktur. Unter Zuhilfenahme von theoretisch berechneten Vergleichsspektren gibt der Verlauf der 10um-Emissionsbande in den Spektren junger stellarer Objekte Hinweise auf den Entwicklungsgrad von Silikatstaub. Die Silikatbanden von 27 TTauri-Objekten werden analysiert, um nach potentiell vorliegenden Korrelationen zwischen der Silikatstaubzusammensetzung und den stellaren Eigenschaften zu suchen. Analog erlaubt das Absorptionsband bei 3um, das dem Wassereis zugeschrieben wird, eine Untersuchung der Entwicklung von Eiskörnern in jungen stellaren Objekten. Erstmals ist es gelungen, kristallines Wassereis im Spektrum eines TTauri-Objektes nachzuweisen. Unser wichtigstes Hilfsmittel zur Analyse der Temperatur- und Dichtestrukturen zirkumstellarer Scheiben ist ein Monte-Carlo-Strahlungstransportprogramm. In einem erweiterten Modellansatz wird die Dichtestruktur der Scheibe in Abhängigkeit von deren Temperaturverteilung selbstkonsistent, mit Hilfe der hydrostatischen Gleichgewichtsbeziehung ermittelt. Die korrekte numerische Umsetzung wird in einer Vergleichsstudie verifiziert. Interferometrische Beobachtungen von 14 TTauri-Objekten im mittleren und nahen infraroten Wellenlängenbereich liefern Informationen über die räumliche Verteilung von warmem und heißem Staub. Ebenfalls sind Aussagen über die radiale Abhängigkeit des Entwicklungsgrades von Silikatstaub in den zirkumstellaren Scheiben um TTauri-Sterne möglich. Zur Modellierung interferometrischer Messungen und gleichzeitig der spektralen Energieverteilungen der beobachteten Objekte wird der Ansatz der passiven Scheibe bzw. der aktiven Scheibe mit oder ohne zirkumstellarer Hülle herangezogen. Mit Hilfe der interferometrischen Messungen kann die Anzahl der Modelle, die allein die spektrale Energieverteilung reproduzieren können, verringert werden

Shadows, gaps, and ring-like structures in protoplanetary disks

We study the structure of passively heated disks around T Tauri and Herbig Ae stars, and present a vectorized Monte Carlo dust radiative transfer model of protoplanetary disks. The vectorization provides a speed up factor of 100 when compared to a scalar version of the code. Disks are composed of either fluffy carbon and silicate grains of various sizes or dust of the diffuse ISM. The IR emission and the midplane temperature derived by the MC method differ from models where the radiative transfer is solved in slab geometry of small ring segments. In the MC treatment, dusty halos above the disks are considered. Halos lead to an enhanced IR emission and warmer midplane temperature than do pure disks. Under the assumption of hydrostatic equilibrium we find that the disk in the inner rim puffs up, followed by a shadowed region. The shadow reduces the temperature of the midplane and decreases the height of the extinction layer of the disk. It can be seen as a gap in the disk unless the surface is again exposed to direct stellar radiation. There the disk puffs up a second time, a third time and so forth. Therefore several gaps and ring-like structures are present in the disk surface and appear in emission images. They result from shadows in the disks and are present without the need to postulate the existence of any companion or planet. As compared to Herbig Ae stars, such gaps and ring-like structures are more pronounced in regions of terrestrial planets around T Tauri stars.Comment: Accepted for publication in A&A. 11 pages, 7 figures, 1 tabl

Mid-infrared observations of the transitional disks around DH Tau, DM Tau, and GM Aur

Aims: We present mid-infrared observations and photometry of the transitional disks around the young stellar objects DH Tau, DM Tau, and GM Aur, obtained with VISIR/VLT in N band. Our aim is to resolve the inner region and the large-scale structures of these transitional disks, carrying potential signatures of intermediate or later stages of disk evolution and ongoing planet formation. Methods: We use the simultaneously observed standard-stars as PSF reference to constrain the radial flux profiles of our target objects. Subtracting the obtained standard-star profile from the corresponding science object profile yields the flux residuals produced by the star-disk system. A detection threshold takes into account the background standard deviation and also the seeing variations during the observations to evaluate the significance of these flux residuals. On the basis of a simple model for the dust re-emission, we derive constraints on the inner radius of the dust disk. Results: We spatially resolve the transitional disk around GM Aur and determine an inner-disk hole radius of 20.5(+1.0,-0.5) AU. The circumstellar disks around DH Tau and DM Tau are not spatially resolved but we are able to constrain the inner-disk hole radius to <15.5(+9.0,-2.0) AU and <15.5(+0.5,-0.5) AU, respectively. The performed photometry yields fluxes of 178+-31 mJy for DH Tau, 56+-6 mJy for DM Tau, and 229+-14 mJy for GM Aur.Comment: Accepted for publication in Astronomy & Astrophysics. (6 pages, including 7 figures and 5 tables

Circumstellar disks and planets. Science cases for next-generation optical/infrared long-baseline interferometers

We present a review of the interplay between the evolution of circumstellar disks and the formation of planets, both from the perspective of theoretical models and dedicated observations. Based on this, we identify and discuss fundamental questions concerning the formation and evolution of circumstellar disks and planets which can be addressed in the near future with optical and infrared long-baseline interferometers. Furthermore, the importance of complementary observations with long-baseline (sub)millimeter interferometers and high-sensitivity infrared observatories is outlined.Comment: 83 pages; Accepted for publication in "Astronomy and Astrophysics Review"; The final publication is available at http://www.springerlink.co

Proposed DRLs for Mammography in Switzerland.

&#xD;The aim of this study is to propose Diagnostic Reference Levels (DRLs) values for mammography in Switzerland.&#xD;Methods:&#xD;For the data collection, a survey was conducted among a sufficient number of centres, including 5 University hospitals, several cantonal hospitals, and large private clinics, covering all linguistic regions of Switzerland to be representative of the clinical practice. The data gathered contained the mean glandular dose (MGD), the compressed breast thickness (CBT), the mammography model and the examination parameters for each acquisition. The data collected was sorted into the following categories: 2D or digital breast tomosynthesis (DBT) examination, craniocaudal (CC) or mediolateral oblique (MLO) projection, and 8 categories of compressed breast thickness (CBT) ranging from 20mm to 100mm in 10mm intervals.&#xD;Results:&#xD;24762 acquisitions acquired in 31 centres on 36 mammography units from 6 different manufacturers were collected. The analysis showed that the data reflects the practice in Switzerland.The results revealed that the MGD is larger for DBT than for 2D acquisitions for the same CBT. From 20-30mm to 90-100mm of CBT, the 75th percentile of the MGD values obtained increased from 0.81mGy to 2.55mGy for 2D CC acquisitions, from 0.83mGy to 2.96mGy for 2D MLO acquisitions, from 1.22mGy to 3.66mGy for DBT CC acquisitions and from 1.33mGy to 4.04mGy for DBT MLO acquisitions.&#xD;Conclusion:&#xD;The results of the survey allow us to propose Swiss DRLs for mammography according to the examination type (2D/DBT), projection (CC/MLO) and CBT. The proposed values are very satisfactory in comparison with other studies.&#xD