Hypervelocity Planets and Transits Around Hypervelocity Stars. Neukom Prize for
Outstanding Graduate Research in Computational Science. Idan Ginsburg ...
Hypervelocity Planets and Transits Around Hypervelocity Stars Neukom Prize for Outstanding Graduate Research in Computational Science Idan Ginsburg, 2012 Introduction: At the center of the Milky Way galaxy lies a black hole with mass ∼ 4 × 106 that of the Sun. When a binary star system (two stars gravitationally bound to one another) approaches this massive black hole (MBH), the system can be tidally disrupted. This disruption can lead to the ejection of one star from the Milky Way at velocities of ∼ 1.5 million miles per hour. This is known as a hypervelocity star (HVS). This class of objects has been computationally studied for over 20 years (Hills 1988), but only recently have they been observed (Brown et al. 2005, 2009). In our paper (Ginsburg et al. 2012) which was recently accepted for publication in Monthly Notices of the Royal Astronomical Society, we consider what would happen if binary star systems near the MBH harbor planets. Until our work, this important possibility had never been explored. Using direct N-body code, we simulated 104 orbits of stars and planets around the MBH. Our simulations show that planets can be ejected from the Milky Way in much the same manner that HVSs are ejected, however the planets can reach significantly higher velocities. We dub these objects “hypervelocity planets (HVPs)”. Our simulations also show that HVSs can be produced with planets still in orbit. If a HVS does have a planet in orbit, there is a strong possibility that the planet would be detectable as it transits around the star. The detection of even one such planet, would shed light on planetary formation and evolution at the Galactic Center. These results received considerable attention, including a press release 1 , and were featured in over 80 news channels including Time Magazine and National Geographic 2 . Computational Methods: A hypervelocity star (HVS) can be produced via a three-body interaction between the massive black hole (MBH), such as the one at the center of our Galaxy, and a binary star system (see Figure 1). As the binary is tidally disrupted by the MBH, one star falls into an elliptical orbit around the black hole, while the other star gets a “kick” that can impart it with a velocity of over 1000 kms−1 (around 1.5 million mph), and thus send it careening out of the Milky Way (Ginsburg & Loeb, 2006, MNRAS). Until our work, no one had studied what happens to planets around stars that are disrupted by the MBH (see Figure 2). The study of three or more bodies under mutual gravitational attraction, as is the case with our models, has no analytical solution to the equations of motions, and thus requires the use of computational techniques. Our N-body code is a well known code of Aarseth (1999, PASP) which uses a direct N-body integrator where each particle is followed with its own integration step. We ran around 104 simulations of stars and planets in orbit around the MBH. We slightly varied the distance between the stars as well as orientation in order to minimize any bias that may occur. Results: We found that there is a high (∼ 50%) probability to produce a “hypervelocity planet” (HVP). Furthermore, such planets are typically 1.5-4 times faster than their HVS counterparts, with some HVPs reaching over 4% the speed of light (see Figure 3). Such velocities were unexpected, but theoretically very exciting. In fact, other than subatomic particles, HVPs are the fastest (theoretical) objects in the galaxy. Just as exciting was the fact that you can have HVSs produced with 1 http://www.cfa.harvard.edu/news/2012/pr201206.html 2 http://www.time.com/time/health/article/0,8599,2109932,00.html
http://news.nationalgeographic.com/news/2012/03/120323-runaway-planets-hyper-speed-loeb-harvard-space-science/
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planets still in orbit. The probability of producing a HVS with one or more planets in orbit is not more than 10%, however such a fraction is still significant. Furthermore, if a HVS does have a planet in orbit, then the probability of being able to detect the planet is considerable, at over 50% (see Figure 4). The detection of such a planet around a HVS will be of great importance in the study of the Galactic Center. It is currently not known whether planets can form and survive near the MBH. The detection of a just a single planet around a HVS will indicate that planets can form near the Galactic Center, and this has far-reaching consequences for our understanding of planet formation and evolution. Hence, in our paper, we note the importance of such observations, and also note that these observations are doable with today’s ground-based instruments. It is our hope that this paper will inspire astronomers to start an observing program looking for planets around HVSs3 . Future Work: Currently, we do not have the technology to directly observe lone HVPs, however this certainly does not mean that in the future HVPs will not be directly observable. Furthermore, we do possess the technology to detect a planet around a HVS from ground-based telescopes. Although HVSs are difficult to detect (only 21 have currently been confirmed), a recent paper by Palladino et al. (2012) lists 677 new HVS candidates. This significantly raises the chances that some of these stars are HVSs and thus may also have planets in orbit. Thanks to the computational work done here at Dartmouth, astronomers may soon discover the first transit around a HVS. Furthermore, there are still numerous unanswered questions, many of which must be tested computationally. For example, we are combining results from our N-body simulations with code we wrote for an adaptive stepsize control for the Runge-Kutta method of integration. This allows us to extend our results and study the Galactic potential which in turn tells us about the dark matter distribution of the Milky Way. Such simulations are vital if we hope to eventually understand the nature of dark matter. Thus, our simulations have far reaching consequences and are on the forefront of current astrophysical research. Acknowledgments: This work would not have been possible without the help of Professor Gary Wegner here at Dartmouth College, and Professor Avi Loeb at Harvard University. I’m fortunate to be able to continue my research with both of them, and I look forward to seeing further exciting results from our simulations. References: Aarseth S.J., 1999, PASP, 111, 1333 Brown, W.R., Geller, M.J., Kenyon, S.J., Kurtz, M.J., 2005, ApJL, 622, L33 Brown, W.R., Geller, M.J., Kenyon, S.J., Bromley B., 2009, ApJL, 690, L69 Ginsburg, I., Loeb, A., 2006, MNRAS, 368, 221 Ginsburg, I., Loeb, A., Wegner, G.A., 2012, accepted for publication in MNRAS Hills, J.G., 1988, Nature, 331, 687 Kenyon, S.J., Bromley, B.C., 2008 ApJ, 680, 312 Palladino, L.E., Holley-Bockelmann, K., Morrison, H., Durrell, P.R., Ciardullo, R., Feldmeier, J., Wade, R.A., Kirkpatrick, D., Lowrance, P., 2012, AJ, 1433, 128 3I
have spoken with a few colleagues at various institutions that have expressed interest in searching for transits around HVSs.
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Figure 1: This illustrations shows the production of a hypervelocity star (HVS). A binary star system (one star is denoted in red, the other in blue) approaches the massive black hole, and the tidal forces disrupt the system so that one star (blue) loses energy and falls into the gravitational well and remains in orbit around the black hole. The other star (red) gains the lost energy. Sometimes this energy is enough to “kick” the star completely out of the Milky Way. In a similar manner, a planet around one of these stars can receive a “kick” and be ejected as a hypervelocity planet (HVP). It is also possible for a star to be ejected with a planet still in orbit. In such cases, the planet may be detected as it transits around the star.
Figure 2: This illustration shows the initial setup for our simulations. We have two stars separated by a distance a? which we varied but kept within known bounds. Next to each star, we placed one or two planets, at constant distance. We let the system approach the massive black hole (see Figure 1) and then studied the results. In total, we ran 104 such simulations.
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Figure 3: Velocity distribution of HVSs and HVPs. This sample comes from 1000 simulations. These runs are strictly for systems with two planets, however we get similar distributions for systems with four planets. The lowest HVP velocities are ∼ 700 km s−1 , which corresponds with the lowest velocities for HVSs. The average HVP velocity is ∼ 3000 km s−1 , and the average HVS velocity is ∼ 1500 km s−1 . The HVPs are denoted in light blue, and the HVSs in the darker color. The overall shape of the distribution is similar in both cases, and in both cases there are outliers. Note that there are ∼ 3 HVPs for each HVS produced, and that the extreme outliers are all HVPs.
Figure 4: This illustrations shows how it may be possible to detect a planet around a HVS. As the planet orbits the star, the planet periodically blocks light from reaching us. This can be observed as a “dip” in the intensity of the light vs time (a so-called “lightcurve”). Such periodic dips are indicative of a planet orbiting a star, which is called a “transit”. Depending on the size of the star and the distance from the star, the dip in the lightcurve can be as much as 1%. However, even small dips in the vicinity of 0.1% are detectable with today’s ground-based telescopes.
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