New Frontiers in Exoplanet Detection:
High Contrast Imaging with Subaru
Advisers: Edwin L. Turner, Michael W. McElwain
Courtney D. Dressing
“One thing that helped me find a balance was the advice of my adviser, Professor Turner, to start writing early on in the process.”
I have been passionate about astronomy for nearly my entire life, so I entered Princeton knowing that I wanted to major in astronomy and hopefully participate in the accelerating search for extrasolar planets. For my junior papers, I worked first with Professor Jill Knapp to study the magnetic activity of low mass M stars with white dwarf companions and then with postdoctoral researcher Dave Spiegel to model the surface temperatures of theoretical Earth-like planets in a range of orbits around Sun-like stars. My work at Princeton and in summer NASA research programs was not directly related to my senior thesis, but many of the skills I learned during my previous research projects carried over to my senior thesis. For instance, I gained my first exposure to LaTeX when I wrote up my fall junior paper, and my summer internship with Drs. Eldar Noe and Matt Golombek at NASA’s Jet Propulsion Laboratory (JPL) forced me to master IDL, a programming language commonly used by astronomers. I had worked with IDL before, during another internship and for some of my astronomy classes, but my summer at JPL provided me with a much stronger grasp on the language. Consequently, I was able to start writing code for my thesis right away rather than having to devote the first two months to learning a new programming language.
Although I had been planning since freshman year to do “something related to planets” for my senior thesis, my senior thesis topic didn’t transition from a vague notion of studying planets to a more concrete research topic until mid-September of my senior year, when I met with my soon-to-be adviser Professor Edwin Turner and heard about his involvement in the Subaru Strategic Exploration of Exoplanets and Disks (SEEDS) collaboration. SEEDS is an international collaboration led by Dr. Motohide Tamura of the National Astronomical Observatory of Japan to acquire near-infrared images of massive planets around nearby stars using the Subaru telescope in Hawaii. While more than 300 planets had been detected by fall of my senior year, only 11 had been directly imaged. The vast majority of the planets had been detected indirectly due to the gravitational influence of the planet on its host star. Due to the much brighter luminosity of a star compared to a planet and the close proximity of planets to their much brighter host stars, acquiring images of planets is a challenging task and is only just becoming a reality. I was excited by the opportunity to get involved with a team working to expand the frontiers of planet detection and I eagerly agreed to work with Professor Turner on the SEEDS project. Professor Turner introduced me to Dr. Michael McElwain, a postdoctoral researcher working on SEEDS, and invited me to begin attending lunch meetings with the Princeton SEEDS group to learn more about the project.
The SEEDS team uses several different high-contrast imaging techniques to acquire images of exoplanets and disks. One of those methods is angular differential imaging (ADI). Because the Earth rotates during a telescope observation, the telescope normally rotates about its axis so that the orientation of the target in the field of view remains constant. In ADI mode, however, the telescope rotates at a slightly different rate so that the telescope point spread function is fixed with respect to the science camera detector, and the noise speckles introduced by the telescope optics and the residual atmospheric distortions appear stationary while possible planetary companions rotate freely. For my thesis, I worked closely with Dr. McElwain and Professor Turner to study the relationship between increased field rotation and the sensitivity of an observation to planetary companions. We knew that increasing field rotation should increase sensitivity, but we wanted to better characterize the relationship so that the SEEDS team could make more informed decisions about the timing and duration of target observations. Specifically, my role was to divide existing SEEDS data sets into smaller subsets, reduce those subsets, and then compare the signal to noise maps and final images produced for each sub-reduction of the data set.
While I had anticipated that I would learn valuable scientific skills from my senior thesis, I had not realized how educational it would be to participate in an international research partnership. The SEEDS team consists of researchers from around the globe, but there are approximately a dozen SEEDS collaborators at Princeton, so I was able to experience both small, in-person meetings and large, all-team teleconferences. Fortunately, our colleagues in Germany were willing to wake up in the middle of the night so that those of us in New Jersey, Japan, and Hawaii could meet at more reasonable hours! As SEEDS is still in its early stages (the project started scientific observations in fall 2009), I was able to participate in discussions of how to divide observing time between the different target categories, choose which targets to observe, handle possible planet candidates, organize SEEDS papers, deal with bad weather or noisy data, incorporate new researchers, and a whole slew of other questions that already would have been long settled in an older research collaboration. By joining SEEDS so early in its inception, I was able to witness part of a collaboration that most undergraduates never see, and I’m very grateful for my glimpse into how scientific collaborations actually operate.
In addition to participating in the target selection process, I also was granted the opportunity to travel to the Subaru Telescope in Mauna Kea, Hawaii, to participate firsthand in SEEDS observations. At the summit, I met a large fraction of the SEEDS team and worked with them to acquire and reduce our observations. I thoroughly appreciated the chance to take part in an observing run, and I returned to Princeton with a much deeper understanding of how professional astronomical observations are planned, acquired, and analyzed. That knowledge allowed me to add a section to my thesis that addressed how to optimize SEEDS observations in order to extract the maximum amount of scientific value from a limited amount of observing time.
Writing my senior thesis was both the hardest and the most rewarding aspect of my time at Princeton. While classes had required me to devote hours to papers and problem sets, my independent work forced me to familiarize myself with scientific literature and analyze real data in pursuit of answers that weren’t listed in the final pages of a course textbook. Additionally, while most problem sets are completed in one or two weeks, the senior thesis is an in-depth project that spans a full academic year. As such, it can be easy to justify putting aside the thesis—which isn’t due for seven months anyway—in favor of tasks with short-term deadlines such as problem sets, papers, and applications to jobs or graduate programs. Although that strategy might work well in October, it will quickly lead to disaster as the thesis deadline looms closer. At the same time, seniors are still expected to take classes and plan for life after college while working on their theses, so they cannot completely neglect their coursework in favor of their theses. In my opinion, one of the most challenging aspects of senior year is striking a balance between working toward short-term goals, slowly sculpting a thesis, and spending time with friends before everyone spreads out around the globe.
One thing that helped me find a balance was the advice of my adviser, Professor Turner, to start writing early on in the process. Professor Turner told me that there was no reason not to begin drafting the introduction or writing up the background section before the actual research was completed and suggested that I try to have at least something written by winter break. I didn’t quite manage to have a full chapter written before break, but I had a decent stab at an introduction/background section by the end of January. While January might not be considered “early” for writing-intensive departments with early deadlines such as history and English, I was definitely ahead of the game compared to my fellow science major friends. Even though I knew that my first few pages would likely be unrecognizable by the time they appeared in my finished thesis, it was reassuring to know that I actually had a “thesis.tex” document on my computer and that I was no longer working on a purely theoretical “thesis” that had yet to materialize. Every individual has a different way of approaching large-scale writing assignments, but I would like to second Professor Turner’s advice to start writing early. In my personal experience, waiting to start writing until you know exactly what you want to say can make the writing process much more intimidating than it needs to be. Yes, it’s a senior thesis, but it’s your senior thesis. Even though it may end up being longer than the sum of all of the other papers you’ve written in college, the thesis still begins with a single sentence. That sentence will probably change dozens of times between when you first write it and when you submit the thesis, so it doesn’t matter if your thesis initially begins with the worst sentence you’ve ever written. As long as you choose a topic that inspires you and devote a reasonable amount of time to your thesis on a regular basis, you will eventually end up with a finished thesis and a respectable first sentence. Good luck!
New Frontiers in Exoplanet Detection:
High Contrast Imaging with Subaru
Courtney D. Dressing
Edwin L. Turner
Professor of Astrophysical Sciences
“One of the many excellent features of the University’s senior thesis system is that it allows some of our undergraduates to experience the challenges and joys of real fieldwork ...”
The two major components of a faculty member’s professional and intellectual life at Princeton are, of course, teaching and scholarship. For those of us in the sciences, the latter normally means technical research investigations of one sort or another. The wonderful thing about the Princeton senior thesis, from my perspective, is that it combines both of these components in a single, and thus doubly rewarding, activity. Over my now 32-plus years on the Princeton faculty, I have only very rarely found supervising a senior thesis project to be anything other than a real joy and privilege. Courtney Dressing’s exceptionally ambitious and successful thesis definitely fits that pattern. During the course of the work, both her technical expertise and her scientific style, or one might even say “identity,” grew impressively quickly and vigorously. As a teacher, being a part of such a process is about as good as it gets. And from my perspective as a research scientist, it also was a great pleasure to witness and participate secondhand in the analysis of important data and subsequent development of new understanding of an emerging mode of astronomical observations.
One aspect of Courtney’s thesis project was novel in my personal experience and hence somewhat challenging. Namely, her work focused on analyzing the properties of a relatively new mode for acquiring astronomical high-contrast images that are needed to study planetary systems orbiting nearby stars beyond the solar system. This technique, called angular differential imaging (ADI), was emerging during the course of Courtney’s work as the primary observational tool for a large international research project called the Subaru Strategic Exploration of Exoplanets and Disks (SEEDS). The SEEDS project will eventually utilize 100 to 120 nights of observing time on the 8.2-meter optical/infrared telescope operated on the summit of the extinct volcano Mauna Kea (in Hawaii) by the National Astronomical Observatory of Japan (NAOJ). The SEEDS project is a collaboration of just over 100 Ph.D.-level astronomers distributed among 28 separate research universities and institutions located in eight different countries. Although it is hard to do a precise accounting, the total financial cost of the project is in the range of tens of millions of dollars. Moreover, the SEEDS project itself is but the smaller of two major projects currently (the number may increase in the future) being undertaken as a part of a major and formal institutional collaboration (called N-PAC) between Princeton’s Department of Astrophysical Sciences and NAOJ. In other words, Courtney’s thesis project had to work effectively in two rather different roles or contexts; it needed to be, like any Princeton student’s senior thesis, an important culminating element of her undergraduate education, and at the same time it was obliged to be an effective contribution to a very much larger, more complex, and longer-term research effort of great importance to many scientists and multiple research organizations. The fact that Courtney was the first (but will not be the last, I trust) Princeton undergraduate to be involved in SEEDS further increased the significance of her undertaking.
Perhaps needless to say, I would never have suggested such a thesis project to any student in whom I did not already have high confidence, based in this case on having gotten to know her in classes (notably GEO/AST/EEB/CHM 255, “Life in the Universe”) earlier in her time at Princeton and via following closely her work on a junior paper she did with David Spiegel (an astrophysical sciences postdoctoral fellow) during the previous academic year. Other, more informal, but no less important, encounters with her at department seminars, scientific discussions at coffees and teas, and casual hallway conversations strongly reinforced my opinion that she was up to the challenge of a quite ambitious thesis project.
In addition to those considerations I was much encouraged by the willingness and enthusiasm of Mike McElwain, another astrophysical sciences postdoctoral fellow, to participate in the support and guidance of Courtney’s project. Because he is one of the world’s leading young scientists working in the field of high-contrast imaging instrumentation and observations, as well as one of the key members of the SEEDS collaboration’s development of the ADI technique, it seemed to me that his involvement would be a huge advantage.
As it turned out, Courtney’s thesis work and resulting significant technical contributions to the SEEDS project well exceeded my already high expectations. She not only produced a superb piece of work, one of the best few senior theses with which I have been associated during my decades on the Princeton faculty, but also established her own presence and identity within the larger international SEEDS research group. Her success reflects well among our colleagues elsewhere, not only on her but also on Princeton astrophysics generally. It was a great treat to witness (and occasionally influence, positively I hope) her virtuoso performance “from a front row seat!”
There is one other aspect of Courtney’s thesis experience that deserves special mention. One of a typical research astronomer’s most intense and influential experiences is working at an observatory, using a professional telescope and its instrumentation to acquire the all-important empirical data that drives scientific understanding in the direction of the truth. The Subaru (the Japanese name for the Pleiades) Telescope is one of the world’s most powerful, simply the most powerful for some applications, and is located at one of the world’s highest-quality observing sites. I was delighted that Courtney was able to be present for one of the SEEDS observing “runs” at Subaru in January 2009 and thus to participate in the acquisition of some of the data used in her thesis analysis. I suspect that this was a quite valuable experience for her in multiple ways. This travel was made possible by an internal Princeton grant from the Council for International Teaching and Research’s Global Collaborative Networks Fund; the grant supports international research connections in the area of exoplanet studies. One of the many excellent features of the University’s senior thesis system is that it allows some of our undergraduates to experience the challenges and joys of real fieldwork, an activity that is typically only available to advanced graduate students in most institutions.
New Frontiers in Exoplanet Detection:
High Contrast Imaging with Subaru
Courtney D. Dressing
Michael W. McElwain
Postdoctoral Research Fellow,
Department of Astrophysical Sciences
“These formidable observations are a fundamental step
in resolving the age-old question,
‘Are we alone in the universe?’ ”
The senior thesis is an important part of the Princeton experience, and as such, the first question I ask Princeton alumni is about their senior thesis work. The vigor and pride with which alumni describe their theses is almost as remarkable as the quality and timelessness of these projects. As a postdoctoral researcher who was curious about this Princeton capstone, I was excited to be a co-adviser of Courtney Dressing’s thesis to help shape this study and be a part of this academic highlight.
Princeton recently formed a partnership with the National Astronomical Observatory of Japan to embark on the preeminent survey of exoplanetary systems through direct imaging. This survey uses a suite of highly specialized instrumentation onboard the Subaru Telescope to attenuate the light from the host star while preserving the light of planets in orbit around them. This allows us to discover and characterize these objects for the first time in history. New technologies in detectors and adaptive optics have enabled these observations about our place in space that in the past were only philosophical questions. This nascent era of exoplanetary science is ripe for new methodologies in observations and analysis techniques for these new datasets. Professor Edwin Turner and I proposed to Courtney a broad task related to this exoplanet survey—using instrument commissioning data, produce a quantitative analysis that can be used to optimize the observing strategy. We suggested a few key parameters that were likely to produce meaningful results.
Like many good projects, the boundaries were not well defined. Courtney was able to get a quick start due to her prior research experiences both within the University and at NASA. She was enthusiastic about the survey and quickly became expert in the subject by reading all of the seminal journal articles on the subject and the instrument design documents for the telescope. Courtney learned the data reduction software package by parsing through the routines and subroutines. However, she internalized the pipeline and was crafting her own routines and adding functionality within just a few months time. Courtney took full ownership over the reduction of the data sets and the development of routines to make empirical measurements of several key parameters. By the end of her thesis, she completed an analysis of the datasets and came to specific conclusions about the observation timing and amount of time per target that should be used for survey scheduling.
Furthermore, Courtney wrote a wonderful exposition on the current status of exoplanets and how her work fits in with the current set of techniques. She confidently presented her findings to a core group of our international colleagues that came to a workshop we hosted in Jadwin Hall’s Center for Theoretical Science. We plan to complete a predictive model for the sensitivity of the data sets given various parameters such as stellar brightness, stellar distance, age of the system, position on the sky, and so forth. While Courtney has moved forward to pursue other transiting exoplanets in graduate school, she will continue to collaborate on this project with Subaru and her senior thesis will have a lasting effect on this large-scale survey.
These formidable observations are a fundamental step in resolving the age-old question, “Are we alone in the universe?” One day in Courtney’s lifetime, we will have a good idea about the answer to this question, and when asked her opinion, she will be able to add anecdotes from her senior thesis in Peyton Hall.