IMPRS-HD alumni 2017

Alumni 2017

Tanya Edwards (16.1)  -  Peter Zeidler (18.1.)  -  Richard Teague (20.1)  -  Jorge Abreu Vicente (25.1.)  -  Nina Hernitschek (26.1.)

Nina Hernitschek ( Germany )                                                                                                                                        26.01.2017

 Astrophysical Modeling of Time-Domain Surveys   (thesis pdf, 30 MB)

The goal of this work is to develop and apply algorithmic approaches for astrophysical modeling of time- domain surveys. Such approaches are necessary to exploit ongoing and future all-sky time-domain surveys. I focus on quantifying and characterizing source variability based on sparsely and irregularly sampled, non-simultaneous multi-band light curves, with an application to the Pan-STARRS1 (PS1) 3 pi survey: variability amplitudes and timescales are estimated via light curve structure functions. Using PS1 3 pi data on the SDSS "Stripe 82" area whose classification is available, a supervised machine-learning classifier is trained to identify QSOs and RR Lyrae based on their variability and mean colors. This leads to quite complete and pure variability-selected samples of QSO and RR Lyrae (away from the Galactic disk), that are unmatched in their combination of area, depth and fidelity. The sample entails: 4.8 x 10^4 likely RR Lyrae in the Galactic halo, and 3.7 x 10^6 likely QSO. The resulting map of RR Lyrae candidates across 3/4 of the sky reveals targets to 130 kpc, with distances precise to 3%. In particular, the sample leads to an unprecedented map of distance and width of Sagittarius stream, as traced by RR Lyrae. Furthermore, the role of PS1 3 pi as pilot survey for the upcoming LSST survey is discussed.

Supervisor:    Hans-Walter Rix  (MPIA)

Jorge Abreu Vicente ( Spain )                                                                                                                                        25.01.2017

Molecular Cloud Structure at Galactic Scales   (thesis pdf, 60 MB)

Molecular clouds are the sites were stars are born and they play a crucial role in galactic evolution. Despite their main role on star formation and galaxy evolution, physics of molecular clouds are still poorly understood. Particularly, the processes controlling the formation, structure, and evolution of molecular clouds are still a matter of debate and so are the processes that regulate their star–forming activity. Previous to the beginning of this thesis, observational studies of molecular cloud structure and accurate measurements of star–forming activity in molecular clouds existed only for the Solar neighborhood, proving a very limited range of Galactic environments. Extending these studies to larger distances is crucial. This thesis is dedicated to provide the observational assets needed to obtain a Galactic picture of the processes involved in the molecular cloud structure and star– formation. We present the first systematic study of molecular cloud structure and evolution including molecular clouds in nearby spiral arms. We present a census of filamentary–shaped molecular clouds that are thought to be connected to the spiral Galactic structure. Finally, we also develop a new technique that improves the quality of the existing observational data to obtain more accurate observational assets, crucial in the study of molecular cloud structure.

Supervisor: Thomas Henning (MPIA)

Richard Teague ( UK )                                                                                                                                                       20.01.2017

Tracing the Earliest Stages of Planet Formation through Modelling and Sub-mm Observations     (thesis pdf, 20 MB)

This thesis explores the utility of molecular line emission as a tool to unravel the physical structures and processes involved in planet formation.
Observations of molecular ions, HCO+ and DCO+, in the disk of DM Tauri allow for a study of the ionization structure. These constraints are an essential ingredient in modelling the physical and chemical evolution of a disk which directly impact the efficiency of planet formation.
We also present the first spatially resolved direct measurement of turbulence in a protoplanetary disk using CO, CN and CS molecular line emission. Such a measurement is vital in identifying the physical mechanisms driving turbulence. In addition, we perform a thorough analysis of all uncertainties involved when determining turbulent velocities in disks and demonstrate that all measurements of turbulence will be ultimately limited by the precision to which the gas temperature can be derived.
Finally, the CS emission profile from TW Hydrae displays a dip-like feature coincident with features observed in scattered light observations of the disk, suggesting a common origin. extensive modelling demonstrates that this may be the first detection of a surface density perturbation through molecular line emission, potentially the manifestation of an embedded protoplanet or a strong magneto-rotational instability.

Supervisor:   Thomas Henning (MPIA)

Peter Zeidler ( Germany )                                                                                                                                            18.01.2017

Revealing the secrets of Westerlund 2 - A young massive star cluster observed with the Hubble Space Telescope (thesis pdf, 42 MB)

We present a detailed study of the Galactic young massive star cluster Westerlund 2 (Wd2) using an optical and near-infrared high resolution multi-band survey observed with the Hubble Space Telescope. Images obtained in Hα and Paβ filters allowed us to derive a high-resolution pixel-to-pixel E(B − V)_g gas extinction map. This map helped us to individually deredden the stellar photometry, to identify the cluster population, and to determine the properties of Wd2, such as distance (d = 4.16 kpc), total-to-selective extinction (RV = 3.95), and age (1.04 ± 0.72Myr). We identified 240 bona fide pre-main-sequence Hα excess emitters indicating active disk accretion. A careful analysis of the radial dependence of the Hα excess emission shows a 60% lower mass accretion rate in the cluster center, indicating a more rapid disk dispersal in close proximity to the massive OB-stars. We performed state-of-the-art artificial star tests to study the completeness-corrected spatial distribution of the stars. This revealed that Wd2 consists of two subclusters. Additionally, we determined the present-day mass function (PDMF) with a slope of G = −1.53 ± 0.05, which translates to a total stellar cluster mass of (3.6±0.3)·10^4 M_sun. The spatial analysis of the PDMF and the young age of Wd2 indicates that the cluster is, most likely primordial, mass segregated. A spatially uniformly distributed low-mass (< 0.15 M_sun) population, extending into the gas and dust cloud, as well as a confined region of reddened stars, most likely caused by a foreground CO cloud, suggests that cloud-cloud collision might be the origin of the formation of Wd2.

Supervisor: Eva Grebel (ARI)

Tanya Edwards ( UK )                                                                                                                                                    16.01.2017

Separation of gamm-Ray, Electron and Proton induced Air Showers applied to Diffuse Emission Studies with H.E.S.S.     (thesis pdf, 10 MB)

A fundamental issue in ground-based gamma-ray astronomy is the identification of γ-ray events among the overwhelming background of air showers induced by charged cosmic rays. Reconstruction techniques exist to distinguish most of the background of hadrons but an irreducible background of electrons and gamma-like protons still remain. I present here a new technique making use of high-altitude Cherenkov light emitted by the charged primary particle and air shower development properties. This method provides a way to distinguish between electrons and gamma rays on a statistical basis. In addition to this, the remaining proton background can also be identified. The technique was developed, tested and applied to studies using the High Energy Stereoscopic System (H.E.S.S.) located in Namibia. The analysis method is especially important in the detection of diffuse signals and eliminates the necessity of a background region in the field of view. The technique was applied to three scientific studies. The latitude profile of the Galactic diffuse gamma-ray emission was analysed. A width of σ = 0.25±0.05 deg (0.20±0.06 deg) for energies of 380 to 900 GeV (1 to 6 TeV) was determined. The cosmic electron spectrum was measured between 0.38 and 14 TeV and a broken power law was fit to the data. The spectrum steepens from an index of 3.08±0.06 to 3.72±0.12 at a break in energy of 1.11±0.04 TeV. In addition, upper limits on the maximum y-ray contamination from the Isotropic γ-Ray Background was placed at 4 x 10^3 (5 x 10^3) MeV cm^-2 s^-1 sr^-1 for energies of 1 to 6 TeV (380 to 900 GeV).

Supervisor: Werner Hofmann (MPIK)

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