# List of Bachelor Projects

In their third year bachelor students can enlist in a 15 ECTS research project between February and August. If you are interested or if you want to know more, please contact the project supervisor. For general questions please contact project coordinator Dr. Anna Watts.

#### 1.  Verification and early science with the LOFAR Multifrequency Snapshot Sky Survey (MSSS)

Supervisors: Dr Jess Broderick and Dr Joe Callingham (ASTRON)

The Multifrequency Snapshot Sky Survey (MSSS; Heald et al. 2015, A&A, 582, A123) is the first northern sky survey carried out at low radio frequencies (<200 MHz) with the Low-Frequency Array (LOFAR). With its competitive combination of bandwidth (8 x 2 MHz), sensitivity (~5 mJy/beam), and angular resolution (45 arcsec, equivalent to the widely used 1.4 GHz NRAO VLA Sky Survey), MSSS will facilitate novel science in areas such as supernova remnants and HII regions, nearby galaxies, pulsars, radio transients, and extended objects such as giant radio galaxies, clusters and relics. It will also have significant legacy value in the scientific literature.

In anticipation of the first public data release later this year, the MSSS team is finalising the data products and conducting a variety of quality control checks. The student will play an important role in these efforts by analysing a variety of large-area (~200 deg^2), multi-band (119-158 MHz) mosaics that cover the entire northern sky. Not only will key metrics be assessed, but given that each mosaic is expected to contain up to 1000 radio sources, there will be the exciting opportunity to carry out early science on a range of interesting, and indeed sometimes unusual, objects (e.g. see Stewart et al. 2016, MNRAS, 456, 2321; Clarke et al. 2017, A&A, 601, A25).

### Planet formation and exoplanets

#### 2.  The location of water vapour in the HD100546 planet-forming disk

Planet-forming disks contain large amounts of water ice, but it is unknown how much and where in the disk this water ice resides. Some of this ice sublimates close to the star. Another small amount of ice sublimates near the disk surface under the influence of the star’s ultraviolet radiation. Water vapour is very difficult to detect from the ground. Using the Herschel Space Observatory we have detected emission lines from water vapour in the planter-forming disk around the star HD100546. These lines are spectrally resolved, which means that we can use the line profile and the assumption of Kepler rotation to find the regions in the disks that have vapour present. In this project, you will run detailed physical models of the disk around HD100546 and the line-formation of water emission to infer the location where the emission originates, and deduce which physical processes are responsible for the release of water vapour in this disk. All necessary codes are already available, but the student needs to have some basic familiarity with python.

Supervisors:

• #### prof. dr. M.R. (Michiel) Hogerheijde

M.R.Hogerheijde@uva.nl |

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#### 3.  Meet the Neighbours

The hunt for Earth 2.0 is well underway and the last two years have revealed three very nearby systems that all have potential to host a life-bearing planet. Our very nearest neighbouring rocky worlds are the inspiration for the next great space race. While surveys are searching for planets, we will ask a slightly different question: Of all the nearby stars (<10pcs), if we assume they all host a habitable planet, which one is the easiest for us to observe from our remote vantage point on Earth, and determine whether or not it’s atmosphere shows signals of life i.e. biosignatures. This project will teach you about the different methods of exoplanet atmosphere characterization, and you will learn about instrument capabilities and how to optimise an observing strategy. You will use detailed databases on the very nearest stars to understand their flux output and activity and ultimately predict which star gives us the best possible chance for observing an inhabited world that we may one day visit.

Supervisor:

• #### dr. J.L. (Jayne) Birkby

J.L.Birkby@uva.nl | T: 0205257366

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#### 4.  Peek-a-boo: is Proxima b hiding in the flares?

Our nearest exoplanet, Proxima b, orbits a small cool M-dwarf star at a distance where liquid water could exist on its surface. It makes it a prime target in the search for life beyond our Solar system. However, its host star is very active, with strong flares occurring every few days and with the potential to complete obscure the signature of the exoplanet. One of the most promising techniques for studying the atmosphere of Proxima b relies on high-resolution spectroscopy of the star and planet. In this project, you will assess how the strong flares from Proxima affect the lines in its spectrum, and determine how important their impact is in our search for biomarkers in its companion exoplanet.

Supervisor:

• #### dr. J.L. (Jayne) Birkby

J.L.Birkby@uva.nl | T: 0205257366

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#### 5.  Simulation of emission spectra of exoplanets with JWST

The James Webb Space Telescope will be launched in 2019. It is thus timely to prepare the next set of observations of exoplanets with this new capability. The goal of this project is to use exoplanet atmospheric models to create template of atmospheres, then ingest these templates in the JWST simulator in order to produce realistic data. The student will then take these synthetic data and apply atmospheric retrieval models to extract the temperature structure and composition of these modeled atmospheres. Ultimately, this project will teach us on the best observing strategy to adopt with JWST for each templates and draw common properties for families of exoplanets.

Supervisor:

• #### dr. J.M.L.B. (Jean-Michel) Desert

J.M.L.B.Desert@uva.nl | T: 0205257466

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### Multi-wavelength accretion studies

#### 6.  Direct detection a sub-stellar object in a white dwarf binary

Accreting white dwarf binaries are the most numerous interacting systems with compact objects in the Galaxy. These systems contain a low-mass star donating mass to the white dwarf, often creating an accretion disc around it. Despite their well-understood evolution, several discrepancies and questions remain.  One such disagreement is the lack of short-orbital period systems (1-2 hour orbit) harbouring a sub-stellar object as donors. Until recently, only a handful of candidates had been discovered and only one direct detection has been performed. The project involves the data reduction and analysis of new optical and near-infrared spectroscopy at the Very Large Telescope of a promising candidate, PHL 1445. This multi-wavelength dataset will allow us to dissect the different light-components in the system and search for evidence of a Jupiter-sized object being consumed by the central compact object. The physical properties inferred from this analysis (such as mass and temperature) will confirm the sub-stellar nature of the donor.

Supervisors:  Dr Juan Hernandez-Santisteban, Dr Nathalie Degenaar and Drs. Jakob van den Eijnden

• #### dr. J.V. (Juan) Hernandez Santisteban

J.V.HernandezSantisteban@uva.nl |

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#### 7.  Understanding accretion onto compact objects with X-ray and infrared observations

Accretion is a very common process in the universe, wherein an object attracts surrounding material with its gravity. For instance, in binary systems with containing both a neutron star or black hole and a normal star, gas can be stripped from the orbiting star and accreted onto the compact object. Such systems are very interesting: we can study the stream of material as it accretes, test general relativity as the gas gets close to the accretor, and probe the poorly understood interior of the neutron star when the gas hits the surface. This project will focus on understanding the accretion process itself, by comparing the radiation such systems emit in X-rays with the infrared emission coming from the entire binary system.

The project will be an observational one, meaning that you will directly work with astronomical data. You will perform the analysis of archival X-ray spectra, from a number of different space telescopes, of a large sample of different neutron stars and black holes. Around the same time, your supervisors (i.e. not you) will use the 6.5-m Magellan telescope in Chile to obtain infrared observations of these same systems. By combining your X-ray results with these new infrared observations, you will test the influence of the accretor on the other components of the binary such as the accretion flow and donor star

Supervisors:.Drs.  Jakob van den Eijnden, Dr Nathalie Degenaar, Dr Juan Hernandez- Santisteban

• #### Mr A.J. (Jakob) van den Eijnden MSc

A.J.vandenEijnden@uva.nl |

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#### 9.  A new type of transient X-ray event (note: only one of projects 8 or 9 will be offered)

Ever since 2006, the Swift satellite has been taking ~20-min X-ray snapshot images of the center of our Galaxy nearly every day. The Galactic center is a very rich environment to study accretion onto black holes and neutron stars. Not only does it harbor the supermassive black hole Sgr A*, the Swift campaign also covers about 15 X-ray binaries. In these X-ray binaries, a stellar-mass black hole or a neutron star is swallowing gas from a companion star. This usually happens in a transient fashion; the black holes and neutron stars typically feast on their companion for a few weeks at a time and then remain dormant for years before exhibiting another outburst of accretion. During accretion episodes, an X-ray binary brightens by orders of magnitude; they can thus be seen to switch on and off. In addition, Sgr A* occasionally shows a brief burp of X-ray emission, which typically lasts a few hours, of which the origin is not known.

By now, more than a 1000 X-ray images of the Galactic center have been obtained by Swift. We recently noticed a new type of transient X-ray event in two of these images; brief flashes of X-rays that are not associated with any known X-ray source. While the brightness of these flashes is similar to the accretion outbursts of X-ray binaries, their duration seems to be much shorter. In this project, you will have to think of clever ways to mine the rich Swift data of the Galactic center to search for more of these brief transient events. By characterising their basic properties (e.g. duration, X-ray spectral shape) and occurrence rate, the aim is to understand the nature of the brief X-ray flashes.

Supervisors: Dr Nathalie Degenaar , dr. Juan Hernandez-Santisteban, Drs. Jakob van den Eijnden.

• #### dr. N.D. (Nathalie) Degenaar

N.D.Degenaar@uva.nl |

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#### 8.  Exploring the UV properties of X-ray binaries (note: only one of projects 8 or 9 will be offered)

Neutron stars and black holes are extreme objects that are involved in the most explosive and energetic phenomena observed in the universe. When located in binary star systems, black holes and neutron stars can pull off and swallow gas from their companion star. This process, called accretion, converts enormous amounts of gravitational energy into electromagnetic radiation. As a result, these so-called X-ray binaries can be observed at radio, infrared, optical, ultra-violet (UV), X-ray, and sometimes even gamma-ray, wavelengths.

Whereas X-ray binaries have been known and extensively studied for over 5 decades, little is still known about their UV properties. However, in recent years several X-ray binaries have been observed at UV wavelengths with the Hubble Space Telescope and the Swift satellite. This observational project will focus on characterising the UV properties of X-ray binaries, and linking this to their X-ray properties, using satellite data.

Supervisors: Dr Nathalie Degenaar , dr. Juan Hernandez-Santisteban, Drs. Jakob van den Eijnden.

• #### dr. N.D. (Nathalie) Degenaar

N.D.Degenaar@uva.nl |

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### Massive stars, supernovae, neutron stars and black holes

#### 10.  Massive stars running through space

OB runaways are massive (OB) stars that travel through interstellar space with anomalously high velocities. The space velocity of these stars can be as high as 100 km/s, which is about ten times the average velocity of normal'' OB stars in the Milky Way. Many of them can be traced back to a nearby OB association where they seem to have originated from. But how did these massive stars obtain such a high velocity?

The two most popular scenarios for the formation of runaway stars are the binary supernova model (Blaauw 1961) and the cluster ejection mechanism (Poveda et al. 1967). Blaauw suggested that when an OB star is bound to another OB star in a binary system, the supernova explosion of one of the stars (i.e. the initially most massive one) causes the disruption of the binary system since more than half of the total mass of the system would be lost after the supernova explosion of the primary. As a consequence, the remaining massive star escapes preserving its (relatively high) orbital velocity. The modern version
of this scenario includes a phase of mass transfer inverting the original mass ratio, so that the resulting runaway star has a large probability to remain bound to the compact remnant (a neutron star or a black hole) produced by the supernova. The mass transfer from the
evolved star to the future runaway star could increase its atmospheric helium abundance. Furthermore, the angular momentum associated with the accreted material would result in a higher rotation rate of the future runaway. The binary supernova model predicts that many OB runaways should have a compact companion. Searches for compact stars around OB runaways have, however, up to now not been successful (e.g. Philp et al. 1996). The detection of a wind bow shock around the high-mass X-ray  binary system Vela X-1 (Kaper et al. 1997) demonstrated that Blaauw's scenario works. And that Vela X-1 likely originates from the OB association Vel OB2

An alternative explanation for the existence of OB-runaway stars is the cluster ejection model: the dynamical interaction in a compact cluster of stars results in the ejection of one or more of the members. This model also seems to work: a beautiful example is the OB runaway pair AE Aur and mu Col originating from the Orion Nebular Cluster.

In April 2018 the second release of Gaia observations will provide a unique opportunity to test the two scenarios for the formation of OB runaway stars, and to provide stringent constraints on massive binary evolution.

Supervisor:

• #### prof. dr. L. (Lex) Kaper

L.Kaper@uva.nl | T: 0205257474

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#### 11. A multiwavelength analysis of the peculiar supernova remnant VRO 42.05.01

VRO 42.05.01 is a supernova remnant with a peculiar morphology; its shape is reminiscent of an UFO, with a wide low half and a small round top-half!  The remnant belongs to the class of mixed-morphology remnants, characterised by a shell-like appearance at radio wavelength, and a center-filled morphology in X-rays. The reason for this peculiar property of mixed-morphology remnants is not clear, but may have to do something with the type of stars that exploded, and the immediate surroundings in which the supernova remnant shock propagates.

The aim of this bachelor project is to use a study of the X-ray data of VRO 42.05.01 (XMM-Newton satellite) to learn more about the star that exploded and to try to understand what caused its peculiar shape. There is also optical data of this remnant available, which can be part of the project.

The project will be supervised by Jacco Vink and his PhD students Maria Arias and Vladimir Domcek. Maria Arias is also working on LOFAR radio observations of this object, so the bachelor project may be incorporated in a scientific publication. The project is part of a bigger effort to understand the class of mixed-morphology remnants in general.

Supervisors: Dr Jacco Vink, Drs. Maria Arias de Saavedra Benitez, Drs. Vladimir Domcek

• #### dr. J. (Jacco) Vink

J.Vink@uva.nl | T: 0205257491

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#### 12.  Unraveling the heating processes in accreting neutron stars: is deep crustal heating necessary to explain the cooling of accretion-heated crusts?

Neutron stars are the compact remnants of high-mass main-sequence stars formed in supernova explosions. Many neutron stars are part of a binary system in which they (episodically) accrete matter from a companion star. These systems can be observed in X-rays and are therefore referred to as X-ray binaries. During the accretion process, matter accumulates on the surface of the neutron star which induces various processes that release heat in the crust of the neutron star (~outer 1 km). As a consequence, the crust of the neutron star is heated up and becomes hotter than the core. When accretion stops, the crust will cool down in order to restore thermal equilibrium with the core. Crust cooling can be observed and by comparing the cooling rates with theoretical models, properties of the crust can be constrained and thus the behaviour and physics of matter at very high densities can be probed.

The neutron star crust is often assumed to be heated up by two types of processes which operate at different depths in the crust: deep crustal heating (~800-900 meters in the crust) and shallow heating (~150 meters deep). However, this assumption has not really been tested and there are indications that only shallow heating might be necessary to explain the observations. In this project the student will investigate whether both of these processes are truly necessary to explain the observed cooling curves of various neutron stars, or that shallow heating alone is sufficient. The student will use our crust cooling code NSCool to model the thermal evolution of neutron stars during and after an accretion outburst and fit observed cooling curves with and without assuming that the deep crustal heating processes are active. The student will learn about dense matter physics, nuclear reactions in neutron star crusts, accretion, and modelling the thermal evolution of neutron stars.

Supervisors:  Prof. Rudy Wijnands, Drs. Laura Ootes

• #### prof. dr. R.A.D. (Rudy) Wijnands

R.A.D.Wijnands@uva.nl | T: 0205257206

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#### 13.  Launching relativistic jets from black holes

Work with one of the world's fastest simulation codes (now in full 3D  and incorporating general relativity) and dive in to the world of black  holes and jets, which form some of the most energetic systems in the  cosmos. The student will make use of Python (with possible use of C) in order to analyze data from one of the largest simulations of tilted  accretion discs with a variety of black hole spins and try to decipher the intricate relationship between the disc and the jet. Given enough motivation, the student could work towards publishing a research paper in a peer-reviewed journal.

Supervisor:

• #### prof. dr. S.B. (Sera) Markoff

S.B.Markoff@uva.nl | T: 0205257478

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