Are the most massive stars concentrated towards the Galactic plane?
Supervisor: Prof. Lex Kaper
Stars are formed in dense molecular clouds that make up the thin disk of the Milky Way, a giant spiral galaxy. The thin disk has a scale height of about 50 pc as shown by, e.g., the spatial distribution of molecular gas and massive OB-type stars. With the advent of Gaia DR3 the spatial distribution of O stars can be accurately measured. It turns out that a significant fraction of these most massive stars are so-called runaways, i.e. they have left their parent OB association with a high velocity. Two scenarios have been proposed to explain the existence of runaway stars: 1) Dynamical ejection from a young cluster due to gravitational interactions, and 2) The supernova explosion of the (initially) most massive star in a binary. Recent developments, such as the observation that magnetars (strongly magnetic neutron stars) have a scale height of only 30 pc, suggest that the most massive stars are even more concentrated to the Galactic plane. This could be due to the merging of massive binaries that sink to the core of a young cluster. The goal of this project is to measure the distances and Galactic height of O stars in the Gaia DR3 database and compare this to the space distribution of magnetars in the Milky Way.
Learning numerical methods for black hole accretion
Supervisor: Prof. Sera Markoff
The Event Horizon Telescope data are modelled using complex numerical simulations that solve the equations of magnetohydrodynamics on a grid, including general relativity. In this project the student will learn to run a public GRMHD code HARMPI, and work through some tutorials on setting up numerical problems and studying what happens when magnetised plasma interacts with a black hole. The project will involve using a Jupyter notebook and python interface as well as learning to change parameter files and code setup, so is a good opportunity for students who want to improve their coding skills as well.
Visualization and analysis of binary neutron-star merger simulations
Supervisor: dr. Philipp Moesta
This project will involve developing and running python-based analysis and 3D visualization tools to analyze output data from a set of binary neutron-star merger simulations. The simulation data consists of large datasets with full 3D fields for the state variables of the simulations (density, temperature, magnetic fields, etc). The student will work on calculating additional quantities like the total mass ejected in outflows from these simulations, perform 3D ray-casting visualizations, and learn how to postprocess the simulation data with nuclear reaction networks to analysis the elements formed in these events. These outputs will then be used to interface with observational data for gravitational waves, EM counterparts, and nucleosynthesis from these events.
Supervisor: dr. Philipp Moesta
This project will involve porting and profiling individual computation kernels for our GPU-GRMHD code
GRHydro-X. GRHydro-X is a dynamical-spacetime general-relativistic magnetohydrodynamics code for simulation binary neutron-star mergers and supernova explosions. The project is flexible and can be focused on performance testing and optimization of existing modules of the code for specific GPU architectures or developing new physics modules (e.g. neutrinos, equation of state, magnetic fields). The former will give the student experience working with state-of-the-art GPU systems and gain insights into modern GPU programming while the latter also involves algorithm development for computational physics in astrophysical magnetohydrodynamic simulations.
Test particle methods in simulations of astrophysical plasma.
Supervisor: Sebastiaan Selvi, dr. Oliver Porth
High energy astrophysical phenomena are shaped by the properties of relativistic particles, giving rise to non-thermal emission and cosmic rays. For a full description of the particle spectrum and dynamics, it is necessary to solve the set of (kinetic) equations which is quite costly. It has been argued however that much of the particle acceleration process can be understood with much more affordable "test-particle" simulations.
In this project, you will study how charged test particles embedded in an astrophysical flow can be accelerated into high energy cosmic rays. We are keenly interested to explore the advantages and drawbacks of this method to build better models for a variety of sources, observed e.g. by the EventHorizonTelescope, the GRAVITY interferometer or the NICER X-ray satellite.
Inverse Compton verliezen: stochastische effecten
Supervisor: dr. Jacco Vink
Highly relativistic electrons lose energy mainly through two radiative process: synchrotron radiation and inverse Compton scattering (ICS). For magnetic fields around 5 microGauss these losses are comparable, whereas for higher magnetic fields synchrotron radiation wins. These losses are the astronomers gain: we can detect these electrons in the radio to X-rays through synchrotron radiation, and in gamma-rays through ICS. Often the two processes are lumped tighter. However, ICS works from an electron colliding with a photon. This means it is a discrete process: very high energy electrons either have or do not have had a collision. What does this discretization do with the spectrum of the electrons, in particular for relatively young sources like supernova remnants with relatively low magnetic fields?
This will be theoretical work that can be done semi-analytically and with Montecarlo simulations.
Imaging X-ray polarization spectroscopy
Supervisor: dr. Jacco Vink
I am involved with the new NASA X-ray mission IXPE (Imaging X-ray Polarization Explorer), launched in December 2021. Sofar three supernova remnants have been observed: Cas A, Tycho’s SNR and SN1006. The analysis of these data sofar has used only the imaging capabalities. In this project we are going to explore the data with spectral fitting with integrated polarization capabilities. This will be done with the official NASA software.
Collisionless shocks in space plasmas
Supervisor: dr. Jacco Vink
Supernova remnants shocks are collisionless shocks. This means that the shocks do not occur through atom-atom collisions, but due to selfgenerated magnetic- and electric field fluctuations. As a result the outcome of these shocks are a bit unusual: electrons and ions may not have the same temperature, and some particles are accelerated to very high temperatures.
How these shocks actually work in supernova remnants is difficult to study as they are far away. However, I am also involved in a collaboration with people measuring spacecrafts moving through shocks in the space between Earth and the Sun, and at the Earth bow shock. In this project we will work together with researchers at Uppsala University to study electron and ion heating, and particle acceleration in those shocks using a new database of measured shocks.
Predicting the rate of radio flares from NS mergers
Supervisor: Prof. Ralph Wijers
Recent theoretical work by A. Cooper et al. has indicated that we may expect coherent radio flares from mergers of binary neutron stars, or NS-BH mergers. Using these predictions and the best current estimates of merger rates, we will predict the number of detectable radio flares of this kind with various current and future radio telescopes, both if triggered by gravitational-wave event and in blind surveys.
The skills required for this project are: basic understanding of radiation, elementary python programming.
An unsupervised investigation of fast radio bursts from CHIME
Supervisor: dr. Dany Vohl, Prof. Jason Hessels
Fast radio bursts (FRBs) are topical as their nature remains mysterious. FRBs are short (faster than the blink of an eye) and energetic (as much energy in a few milliseconds as the Sun outputs in a day) transient events detected by radio telescope. A key observable for FRBs is their integrated profile where we combine the signal received at all our observed frequencies into a one-dimensional light curve. To date, the most complete and self-consistent FRB sample has been produced by the CHIME radio telescope located in Canada. The CHIME/FRB collaboration is at its second public data release which constitutes a self-consistent sample of FRB observations and derived parameters. The progenitor source emitting these bursts is still being debated, but theory points towards a neutron star origin. In this project, you will get familiar with FRBs and its phenomenology—investigating the variety of profiles with our graph theory-based unsupervised machine learning, and try to connect these to profiles from pulsars and magnetars. You will mainly work with Python and Jupyter notebooks, and get familiar with virtual observatory tools.
TESS light-curve analysis of OB(e) stars
Supervisor: Prof. dr. H.F. Henrichs (UvA/VU)
The Transiting Exoplanet Survey Satellite (TESS), launched in 2018, is performing a near all-sky survey to search for planets transiting nearby stars, see https://heasarc.gsfc.nasa.gov/docs/tess/objectives.html.
The satellite delivers extremely high-precision photometry with typically a two-minute cadence for all objects in the field of view, covering up to 85% of the sky.
Earlier studies based on space photometry of incidental O, B, and Be stars have revealed unexplained periodicities on timescales of hours, most likely associated with the (unknown) rotation rate, and/or to predicted but unexplored magnetic or pulsational surface phenomena.
The direct goal of the proposed research is to characterize the photometric periodicities of a dozen well-chosen bright OB stars, with the prime emphasis on the role of rotation. An earlier case study of a particular Be star showed that the many prominent photometric periods are not stable, and the question remains from where the strong lightvariations originate.
The recently proposed “Impulsive Magnetic Rotator Model” will be tested against the outcome of the data analysis.
This research is a systematic investigation to the origin of the well-documented obiquitous spectroscopic variability, in order to further understand the dominant role of surface phenomena in causing light variations.
The method of analysis will be primarily be a sophisticated fourier analysis of the data, to be downloaded from the archives, https://mast.stsci.edu/tesscut/, and https://mast.stsci.edu/portal/Mashup/Clients/Mast/Portal.html.
Ample tools (python) are available, https://docs.lightkurve.org/.
In a second stage the found periodicities will be linked to available spectroscopic periodicities.
“Celestial Cinematography” - a movie of the nightsky
Supervisor; dr. Jan van Roestel, dr. Silvia Toenen
While the night sky might look the same each night, this is far from the truth. Many stars change brightness; some periodically, others show irregular outburst, and some stars are only seen to explode once never to be seen again. By studying variable stars and explosions (‘transients’) we can learn about the evolution of stars, mass-accretion processes, magnetic fields, the structure of extreme stars, and many other types of excotic physics.
For my research I use robotic survey telescopes; for example the Zwicky Transient Facility (ZTF, youtube), the Dutch MEERlicht/BlackGEM telescopes, but also the Gaia and TESS satellites. In addition, I also use large telescopes (e.g. Keck) to study stars in detail. The available projects either involve searching the telescope data for weird and interesting objects or focus on single objects that were recently discovered and detailed followup observations are available.
A few specific example projects are the study of white dwarf binary stars, highly magnetized white dwarfs (100 MG), mass-transfering white dwarf systems (Cataclysmic Variables), quadruple star systems, and stellar mergers. All these projects are observationally oriented, and some python and Linux experience is useful. If you are interested, some of these projects can also include machine learning.
Mapping neutron stars
Supervisor: Prof. Anna Watts
Using NICER, an X-ray telescope on the International Space Station, we are able make surface maps of pulsars and to measure their mass and radius. To do this we use models that combine physics and astrophysics as well as computational and statistical inference techniques. Our work to date has started to place limits on the types of quark matter that can exist in the neutron star core, and revealed complex magnetic field structures that challenge our views of neutron star evolution. And of course, we have more questions than when we started!
We are able to offer two positions in the group to work on some of the things that are puzzling us at present. These run from the very astrophysical to the more computational, so we can accommodate a wide range of interests. Examples of aspects that projects could focus on include neutron star atmosphere models, how we might benefit from X-ray polarimetry, the role of the neutron star spin, and comparisons of different statistical sampling packages.
Measuring magnetic fields on the sun with the APO Heliostat
Supervisor: Stephanie Heikamp, Rasjied Sloot
The Sun is currently 3.5 years into Solar Cycle 25 and its activity is ramping up. Increased activity is measured by the number of sunspots which are concentrated regions of strong magnetic fields. The magnetic field causes the Zeeman effect: the splitting of spectral lines which we can measure in the solar spectrum.
This project aims at observing the Zeeman effect and measuring the magnetic field strength that is associated with (groups of) sunspot using a high-resolution echelle fiber spectrograph.
During this project the student will learn to observe independently with the APO heliostat and spectrograph, reduce data using pyhton and estimate the magnetic field strength using atomic physics. The project has a strong programming component.
Time resolved PSF photometry at the Anton Pannekoek Observatory
Supervisor: Rasjied Sloot
There are many variable objects in star clusters which can be observed with the equipment at the APO. Conventional aperture photometry is a proven method of measuring stellar flux if stellar light can be isolated. However, in crowded stellar regions like globular clusters this method fails. Another way of measuring stellar flux is to fit the stars with a Point Spread Function (PSF) model of the telescope's optics. This method should work on overlapping sources making photometry in crowded stellar regions feasible. This project aims at developing the PSF photometry technique at APO to detect variable sources or transients. This project has a strong programming component so experience using python is a plus.
During this project the student will learn to observe independently at APO for 3-5 nights, write code to compare measurements between aperture photometry and PSF photometry and identify variable sources in the observed star clusters (or even in Galaxy clusters).
Searching for transient and variable sources with Swift UV Observations
Supervisor: Prof. Rudy Wijnands
The sky is very variable at all electromagnetic wavelengths and has been intensively studied at all time scales. However, one understudied regime is the ultraviolet (UV), mostly because UV radiation is nearly completely absorbed by the Earth's atmosphere and space-based instrumentation is prohibitively expensive (i.e., for instruments that have the large field of views, FOV, to make serendipitous sources for UV variables and transients viable). However, small FOV instruments are available: e.g., the UV and optical telescope (UVOT) aboard the Swift satellite. Over the last few years, we have developed an extensive pipeline (in Python) to search newly acquired data as well as archival observations obtained with the UVOT to search for serendipitous variable objects as well as transients in the UV regime. This project could place several BSc students that can use this pipeline to search for such objects. Prime targets are Galactic globular clusters (to search for outbursting accreting white dwarfs) as well as other densely populated objects (like the center of our Galaxy or other galaxies).
Additional information: several BSc students can work on this project but all will work independently on their own data sets although with close collaboration with each other and other group members. The student(s) will have weekly meetings with the supervisors as well as participate in the weekly group meetings. One of the projects (the globular cluster one) also entails data obtained using the ground-based INT facility (based on La Palma) and the student could work on these data as well.
Software packages used: Knowledge of Python would be very useful.