I obtained my Master’s degree from Leiden Observatory. As a partial requirement to complete the degree, Master’s students are required to complete two research projects of different nature. My two projects are a search for hypervelocity stars in a Gaia-like astrometric catalogue, and an astrophotometric studies of stars in the Galactic bulge.
Stellar Dynamics in the Galactic Bulge
The role of bulges is often neglected in early studies of galaxy formation, which is understandable since its existence is not readily apparent if we look at an optical map of a galaxy (For example, The Milky Way in the picture on the left). However, if we look past through the optical obscuration caused by the interstellar matter toward the Galactic Centre, a bulge is readily apparent for example in the COBE-DIRBE map of the Milky Way in the Near-Infrared. Understanding the morphology and evolution of bulges is part of answering the question of how galaxies form and evolve.
The bulge of our Galaxy is the nearest bulge to us and the reason to study it is obviously because it permits high-resolution study of bulges. Despite this privileged position, there are still standing questions about our own Bulge. This problem arise because our position within the Galactic Disc: The interstellar matter (ISM) absorption toward the Galactic Centre can reach up to AV ∼ 30 mag and thus limits optical observations to only small “windows” that have low and relatively uniform extinction like, for example, Baade’s Window. Observing toward the direction of this “windows” also poses its own problem as all objects sampled in a line of sight to these windows are a mixture of different populations and thus a method to separate the bulge population from the disc population must be employed.
My project is part of the larger ongoing effort to obtain 3-D kinematics of bulge stars. The project uses images of selected ﬁelds toward the bulge that are publicly available from the Hubble Space Telescope (HST) data archive. The archive has been available for more than a decade and contains enormous amount of images taken during the nineties with the Wide Field Planetary Camera 2 (WFPC2) on board HST. Second epoch images of these ﬁelds has been taken with HST’s Advanced Camera for Survey (ACS). Some results has been reported in Kuijken & Rich (2002), Kuijken (2004) and Soto et al. (2007).
For my thesis, my main task is to decontaminate the bulge stars in one of the selected fields. This field is located ~4 arc minutes away from the center of the globular cluster NGC 6656, and thus is contaminated by the cluster stars which is located in the foreground. The contamination is apparent in the proper motion distribution, which exhibit a bimodality in its distribution. This is an indication of a residual motion in the proper motion measurement, made by assuming that the average motion of stars in the ﬁeld are zero (because of contaminations by a large number of cluster stars, this assumption is no longer true) .
Using the Levenberg-Marquardt Algorithm, I fit the proper motion distribution function to a bimodal-bivariate Gaussian function. Using this function I then decompose the bulge/disk population and correct the proper motion from the residual effect.
Detecting hypervelocity star candidates using astrometric data
The castaway stars, as these stars are dubbed, are created when a close binary closes in to the black hole in the center of our Galaxy (Hills 1988). One star will be captured and orbits the black hole, while another star will gain speed and ejected at enormous speed, more than 1000 km/s! This incredible speed is faster than the escape velocity from the Milky Way, hence they are unbounded from our Galaxy. This event is extremely rare and can happen only once every 1000 years, and there might only be around 1000 of these stars among the hundred billion stars in our Galaxy. Searching for these stars would be like searching for a proverbial needle in a haystack.
My work is to develop a search criteria to find these stars within an astrometric catalogue, I’m working under the supervision of Anthony Brown and Yuri Levin. We develop a kinematical model of a HVS using the basic assumption that HVS move radially away from the Galactic Center. For an arbitrary star, we make a correction of solar motion to the proper motion, and since this correction depends on the distance to the star, we then instead calculate its supposed distance if it is a HVS, and then compare the calculated distance with the observed distance. We also calculate the Galactocentric-rest frame velocity to check whether the star is fast enough to be considered a HVS.
You can read the final draft of the report here (3 MB).