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University of Tokyo – Princeton University Strategic Partnership Initiative Direct Imaging Detection of Planets Motohide Tamura, University of Tokyo, Astrobiology Institute N. Jeremy Kasdin, Princeton University, Department of Mechanical and Aerospace Engineering 1. Abstract We describe a project on the direct imaging of planets to be carried out over the next three years with a new instrument nearing completion, constructed by a team at the University of Tokyo and Princeton University. This joint program provides the opportunity for, and will greatly benefit from, extensive in-person collaborations at the University of Tokyo, at the Japanese telescope Subaru in Hawai‘i and at Princeton. In particular, such a collaboration will provide an entrée into the very-fast-moving field of the discovery of planets around stars other than our Sun for young scientists at both institutions. 2. Introduction The Japanese astronomical community has a long history of working on major cosmological projects with Princeton, beginning over 30 years ago with joint work on theoretical cosmology. Through the years, this work has included the design, construction and operation of the Sloan Digital Sky Survey, scientific participation by Princeton in the HyperSuprimeCam survey on the Subaru telescope, and the current construction of the Prime Focus Spectrograph for Subaru. These projects have led to, and greatly benefited from, extended visits between the two communities by participants from both the Japanese community and from Princeton. At this point, several generations of scientists have participated, often beginning their involvement as graduate students and continuing into their faculty careers, where they have assumed leadership positions in the new projects. In the past six years or so, the scientific scope of the collaboration has broadened into a new area, the detection and study of planets around stars other than our Sun. Beginning twenty years ago with the discovery of a planet-mass body orbiting a pulsar and making its presence known by a small periodic alteration of the pulse arrival time, this field has grown to be a fast-moving, forefront area of astronomy which produces several discoveries a day and drives technological innovation in ever-more ambitious directions, with the eventual goal of imaging an Earth-size planet around another star. The challenge is not the detection of the radiation or reflected light from the planet per se, but doing so against the glare of the star, which outshines the planet by factors of millions to 100 millions (think of everyday situations such as photographing a subject on a sunny day). Most planet-detection techniques are therefore indirect: the planet is not directly seen but its effect on the star which it orbits can be measured. For example, the enormously successful Kepler satellite accurately monitors the light from stars and can detect the periodic slight dimming as a planet crosses in front of the star during its orbit. The most challenging technique is direct detection of the light from the planet, but this carries with it the possibility of measuring the atmospheric composition and the planet’s colors and cloud structures. Doing so requires a technique known as adaptive optics (AO), which greatly increases the brightness 1 2 contrast in an image of a star, allowing faint objects to be detected near the star. SEEDS (Strategic Exploration of Exoplanets and Disks with Subaru) used adaptive optics instrumentation on the Subaru telescope to carry out the first strategic, multi-year program on Subaru. Recently completed, it involved a large international collaboration whose primary partners were the National Astronomical Observatory of Japan and Princeton University. We propose to build on the legacy of SEEDS by carrying out a several-year project using second-generation AO technology currently in an advanced state of construction at the Subaru telescope and at Princeton’s department of Mechanical and Aerospace Engineering. This project depends on international collaboration to optimize the instrumentation, carry out the observations and analyze the results; further, it provides multiple opportunities for participation by both undergraduate and graduate students and access to this fast-moving and important scientific field. The topic of this work, planets around other stars and the possibility of life, has enormous public appeal and visibility, enabling further opportunities for young scientists entering the field. We propose to enhance this effort with multiple long-term professional visits, the development of collaborations involving young scientists, student exchanges, and a series of meetings of the international collaboration partners. We expect the interactions and collaborations so fostered to last well beyond the duration of the proposal period and to lead to many opportunities for future work. 3. Summary of SEEDS Collaborations grow from an interest in common work, and in turn lead to new collaborations. The far-flung SEEDS project is a case in point. SEEDS was proposed in 2008–9 as the first of the Subaru telescope’s strategic projects - multi-year observing campaigns with dedicated observing time, designed to tackle large-scale investigations. Such investigations are open to the entire Japanese astronomical community, and the original SEEDS proposal was submitted by a group of 41 scientists from several Japanese institutions, 3 from Max-Planck-Institute, Munich, one from University of Hawai‘i and 6 from Princeton (postdoctoral fellow Amaya Moro-Martin, now at Space Telescope Science Institute, and Professors Jeremy Kasdin, Gillian Knapp, David Spergel, Edwin Turner and Robert Vanderbei). SEEDS began observing in October 2009 and finished its formal observations in January 2015, although follow-up observations and science analysis continue. The basic goals of SEEDS were to characterize planetary systems around stars other than our own, to map the disks of gas and dust which surround most young stars and in which planets form, to search for signs of planet formation - massive bodies in rotating disks will open gaps and cause wakes in the otherwisesmooth distribution, and to measure the changes in such disks with time – these last, of course, take place on timescales much longer than a human lifetime but trends can be investigated with observations of stars of different ages. SEEDS was very successful, with more than 50 refereed papers to date. These describe observations of structure in several tens of disks, inferences about the masses and locations of the planets forming in these disks, investigations of mechanisms producing planetary orbital alignments, and most relevant for the topic of this proposal, the discovery by imaging of several planets around nearby stars. The discovery of one such object is shown in Figure 1; a “super-Jupiter” planet many times the mass of our solar system’s Jupiter. This planet, one of only a handful of directly-imaged planets, was celebrated by giving its name to a craft beer. Participation in SEEDS by Princeton scientists broadened considerably over the years. Professor Adam Burrows, an expert on the complex atmospheres of gas-giant planets, and postdoctoral 3 Figure 1. Discovery of a many-Jupiter-masses planet orbiting the nearby star κ Andromedae. The adaptive-optics image taken as part of the SEEDS campaign is shown on the left. The center of the image, at the location of the star, is blanked out and some residual light remains. The planet is the faint object in the upper left of the image. Right: Super-Jupiter beer, a grapefruit-flavored IPA, brewed in honor of this planet by the Howe Sound Brewing Company, Squamish, BC. fellow David Spiegel produced physical models to reproduce the data, and Professor Roman Rafikov made several calculations of the dynamics of planet formation and its tell-tale signatures in observated maps of disks. Postdoctoral fellow Michael McElwain and graduate student Timothy Brandt introduced new data-analysis techniques which greatly improved the detectability of planets by improving the contrast between the starlight and the surroundings. Dr. Brandt also analyzed the total sample of planets to gain a statistical understanding of the incidence of massive planets at large distances from their host star, and wrote his doctoral dissertation on the subject. Graduate student Ruobing Dong analyzed the observations of planet-forming disks and constructed theoretical models both to show the effects of planet formation on disk structure (so that one can use observations of structure to infer the presence of planets) and to create images of his models as they would appear when imaged with instrumentation of Subaru’s capability. In addition, Princeton astronomers participated in the observations on Mauna Kea, worked on the choice of stars for which observations should be made, and participated in and hosted SEEDS collaboration meetings. Over the course of this project, the next steps in planet imaging became increasingly well-defined, and point both towards space-based observations and the next developments in ground-based observations. 4. CHARIS Although SEEDS made major contributions to measuring the incidence of massive planets (Jupiter mass and greater) it probed distances from the host stars of 10 AU and greater (an AU, astronomical unit, is the distance between the Earth and the Sun; Jupiter is at a distance of 5 AU). To reach smaller scales, i.e. scales similar to that of the solar system, requires instrumentation 4 with capabilities much enhanced over that of the SEEDS instrumentation. The second-generation AO instrumentation which can access these scales is now in an advanced state of construction: the extreme-AO instrument, built by Olivier Guyon and colleagues, is installed at the Subaru telescope and is currently undergoing commissioning. The light beam produced by this apparatus will be fed into CHARIS, currently in an advanced stage of construction at Princeton’s Department of Mechanical and Aerospace Engineering. CHARIS (Coronagraphic High-AngularResolution Imaging Spectrograph) will provide the close-in high contrast imaging of nearby stars which will enable the discovery of Jupiter-mass planets at distances from the star of a few AU, i.e. at solar system scales. It does this by greatly reducing the effect of “speckles” of starlight, as seen in the central regions of the image in Figure 1. Further, CHARIS will allow for spectroscopic analysis of the light from the planet, which can detect the presence of important atmospheric constituents such as methane, carbon monoxide and water. CHARIS is expected to discover new gas-giant planets at distances from their host stars comparable to those in the solar system, and to provide a robust census of these planets for the nearby stars. Together with the results of similar surveys being carried out by AO instrumentation operating on telescopes in the southern hemisphere, these results will allow the incidence of giant planets on solar-system scales to be measured, with major implications for our understanding of the processes which form planets. These results are of great scientific importance, but the enormous public interest in planets and in our origins will ensure a much more wide-ranging impact of the work across society. To quote the proprietor of the Howe Sound Brewery (see above): “The great things in life should be celebrated”. CHARIS is being built by a small team at Princeton with the financial support of the Japanese science funding agencies. Professors Jeremy Kasdin (MAE, Princeton) and Masahiko Hayashi (NAOJ) are the Principal Investigators, and the CHARIS construction team includes Tyler Groff, Michael Galvin (MAE), MaryAnne Limbach (Limbach Optics), Timothy Brandt (Institute for Advanced Study), Michael Carr, Craig Loomis (Astrophysics), Norman Jarosik (Physics), and Jeffrey Chilcote (University of Toronto), with contributions from graduate student Johnny Greco, Craig Loomis, Robert Lupton. Edwin Turner, James Gunn and Gillian Knapp (Astrophysics). CHARIS is at present in an advanced stage of construction at Princeton. The current schedule calls for it to be shipped to Hawai‘i in the summer of 2016, where it will be installed on the Subaru telescope for commissioning and testing. Once its performance and stability are verified, the CHARIS team will submit an observing request for of order 20 nights of observing over three years (two observing trips per year) to carry out the initial observing program. It is for the support of collaborative work around this astronomical campaign that the present proposal is being put forward. There has been close collaboration among the CHARIS team members since the beginning, which have included several extended visits to Tokyo, the Subaru site and Princeton since the initial decision to pursue this project. This has been critical: the ab initio involvement of instrument builders, scientists, and software builders has optimized the capabilities of the instrumentation. It has also led to further enhancements to the long-term connections between the Princeton and Japanese astronomical communities by bringing young people into the collaboration and, already, beginning to look forward towards further joint work. 5 5. The three-year observing program As CHARIS moves towards its operational phase, the pace of interactions will pick up. There will be a first collaboration meeting in late February 2016 in Japan at which the larger Japanese community will be introduced to CHARIS and which will be attended by several of the US CHARIS team members. This will be followed by two parallel activities for the next several months: CHARIS completion, shipping and commissioning, including the development of the software which will run the device and process the data; and intensive work on the list of stars to be observed and preparation and submission of the observing proposal. Much of this will take place in Japan and Hawai‘i and involve Princeton personnel at every stage. Once the observing is underway, the team will send astronomers to Hawai‘i from both Japan and Princeton, meet in person to finish the software (much of the work will be done beforehand, but much is needed when actual data are encountered), and analyze and publish the science results as they come in. All of this work will be supported by the CHARIS institutions. 6. Activities under the Princeton-Todai Partnership This project provides ample opportunity for strengthening the international collaboration and building upon that collaboration into the future. The specific activities for which we request the support of the Princeton–Todai partnership are: 1. An annual meeting around the discussion of the CHARIS results. We propose to hold the first in Japan in 2017, the second in Princeton in 2018 and the third in Hawai‘i in 2019. The requested support is for local expenses for the host institution (facilities, catering etc); and for travel by participants not at that institution. The participant lists will include young scientists at the undergraduate, graduate and postdoctoral levels, and all interested scientists at the host institution will be welcome. We are interested in opening the third annual meeting, in Hawai‘i, by inviting astronomers from the Southern hemisphere telescopes carrying out planet imaging projects, with the goals of assessing the new science obtained by this work and future directions. This will be a particularly important event for the young astronomers. 2. Travel support for students to participate in the observing sessions at the Subaru telescope and, on occasion, for their mentors. Working space constraints preclude taking parties of students to the telescope, however attractive and desirable that is, and we expect one student per observing trip. 3. Science analysis and publication activities. Although the internet has made far-flung collaborations in science almost ridiculously easy, with paper drafts sent back and forth instantly, something often gets lost, particularly in collaborations of more than a handful of people. That is the day-to-day participation in a science discovery; carrying out the science often defaults to the whole project being done at one institution, where a small group works together and then announces the result to a larger collaboration. Support for extended visits could bring together the participants from different institutions to work together and in the process provide the framework for the development of further collaborative work, This could include the writing of the technical papers, those describing the instrument and the software, as well as those describing the astronomical results. 6 Collaborative work is also likely to develop in additional directions: modeling the planets’ atmospheres to compare with the data; as-yet unanticipated improvements in the software, instrumentation and observing procedures: and the planning for future work that happens whenever scientists sit down and work together. 7. Exchanges and Visits 7.1. Graduate Students. SEEDS involved an increasing number of graduate students as it evolved, including the two from Princeton who did their dissertation research as mentioned above. Astrophysics graduate student Johnny Greco has joined the CHARIS project for his dissertation research, and has already spent a two-week period in Japan with one of his mentors, Tim Brandt. Mr. Greco expects to spend extended periods at the University of Tokyo at least twice a year, and there is a reasonable likelihood that additional graduate students from both Tokyo and Princeton will become involved. 7.2. Undergraduate Students. Both MAE and Astrophysics have large summer undergraduate research programs, and several undergraduates have already worked on technical aspects of CHARIS over the years. CHARIS offers the opportunity for undergraduate students to travel to Japan with their mentors and work on astronomical or technical projects. Likewise, we expect at least one undergraduate student from Tokyo. Both departments are carrying out long-term efforts to increase the number of traditionally underrepresented groups in science, and these include the participation of minority undergraduates. A necessary part of this effort is that it include the opportunity to make connections with the international community. 7.3. Faculty, Staff and Postdoctoral Fellows. We request support for extended visits by the faculty, staff, and postdocs involved in CHARIS between Tokyo and Princeton. As well as working on the CHARIS results, these visits will support consultation and collaboration on theoretical work, such as visits to Princeton by Dr. Satoro Sorahana, postdoctoral fellow at the University of Tokyo, to work on atmospheric modeling. 8. Afterwards As is the case with SEEDS, which led to joint work such as the CHARIS project, the collaborations so formed will lead to further involvement and are indeed already doing so. As one example, there is the possibility of installing a polarizer in the instrument, which will enable the measurement of the detailed structure in the inner regions of planet-forming disks, of which there are many tens accessible to CHARIS. There may well be additional investigations with the existing instrument - following up observations of detected planets for several years so that their orbital motions can be measured, for example. There will be detailed statistical analysis of the results and of their implications for planet and star formation and planet migration (the movement of planets away from the region where they were formed due to gravitational interactions). And there are longer-term developments to which the young participants from both countries will be introduced. These includes the upcoming NASA mission WFIRST (Wide Field InfraRed Space Telescope), which is expected to image several thousand planets and in which Princeton 7 is a key team member, and the science and technology definition teams for a future large space telescope targeted at imaging Earth-like planets around other stars in the next 20 years. 9. Members of Collaboration (Princeton and Tokyo Professor Motohide Tamura, Department of Astronomy and Chair, Program in Astrobiology, University of Tokyo Professor N. Jeremy Kasdin, Department of Mechanical and Aerospace Engineering, Princeton University Mr. Michael Galvin, Department of Mechanical and Aerospace Engineering, Princeton University Dr. Tyler D. Groff, Department of Mechanical and Aerospace Engineering, Princeton University Mr. Johnny Greco, Department of Astrophysical Sciences, Princeton University Professor Masahiko Hayashi, Department of Astronomy, University of Tokyo Professor Emerita Gillian R. Knapp, Department of Astrophysical Sciences, Princeton University Dr. Satoro Sorahana, Department of Astronomy, University of Tokyo