Ricky Leon Murphy Project
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Ricky Leon Murphy The Appendices: Project – HET606 Appendix 1 – Known Exoplanets Semester 2 – 2004 Appendix 2 – Location of Tau Bootis Appendix 3 – Location of HD 209458 Appendix 4 – Image Reduction Search for Other Worlds Introduction As of September 2004, there are 136 known planets outside our solar system (http://exoplanets.org). These extra-solar planets, or exoplanets, are one of the most current and highly studied subjects in Astronomy today and it is one of the very few subjects that involve both amateur and professional astronomers. The huge telescopes perched atop Mauna Kea in Hawaii are pointed at these objects, as are 8” telescopes purchased from the local shopping mall – and many others around the world. Why are we finding these planets now if only 8” telescopes can detect them? Simple; we know what we are looking for and we have better tools to get the job done. While telescope size does not seem to matter with the search and study of exoplanets, it’s what you do not hear about that really matters: improved sensitivity in CCD cameras, improved resolution in spectroscopy, and fast computers to perform the mathematics. The goal of gathering repeatable data is very important when studying exoplanets. The rewards of such study carry implications across the board in Astronomy: we can learn about our own solar system and test the theories of solar system formation and evolution, improve the sensitivity to detect small Earth-like planets, and possibly provide targets for the spaced based telescopes and SETI projects; however, the most important implication is that perhaps for the first time in history, amateurs and professionals from around the world are engaged in this subject and working together to share the data. The greatest reason for this collaboration is telescope time: there are only a finite number of professional telescopes with tightly guarded schedules that limit prolonged data collection. Amateurs have all the time they can spend and the equipment to help. There are several methods of detecting these exoplanets; however, amateur astronomers are only capable of performing only two – the measurement of radial velocity and the transit method, both will be discussed in detail. The Formation of a Solar System 1 The foundation is the base in which ideas are built upon. In this case, the foundation of this project will be to briefly visit the current theory of how a solar system is formed. This is important because this gives clues for us to know what we are looking for when we study other star systems. The formation of our Solar System can be traced all the way back to the Big Bang. With hydrogen being the most abundant element in the Universe, clouds of hydrogen began to form under its own gravity. By gravity and rotation, these clouds compressed to form the first stars (and galaxies) of the Universe – Population II stars. Population II is the designation for stars that do not contain heavy elements – that is heavier than helium. The natural cycle of these stars resulted in supernova explosions that introduced the heavier elements into the interstellar spaces due to intense heat and pressure of the explosion. While hydrogen is still the most abundant element, other heavier elements are now present – elements like iron, carbon, silicone and many others. Supernova stimulates nearby hydrogen clouds and introduces the heavy elements. As the stimulated clouds collapse, they form something a little different, a proto-planetary disk, as well as the central proto-star (figure 1). When these metal-rich stars – called Population I – are formed, they may host a ring of molecular material that begins to collide with one another resulting in the sweeping up of material. Figure 1: The molecular cloud begins to collapse, eventually forming a dense area at the heart of the cloud. The continual contraction raises the temperature of the cloud causing rotation. Eventually, there will be enough heat and density that a T Tauri star will form at the heart of this cloud with the remaining disk material possibly forming planets. A: the slowly rotating molecular cloud B: Faster rotation with denser and hotter central region C: Faster rotation with T Tauri star at the center of rotation By retaining orbital momentum, the shape of the disk is formed. The consequence of this is the formation of larger objects that also contain gravity and also spin as a result of the orbiting momentum and the further collection of material as it “sweeps” through the ring. 2 These objects are called planetesimals, and continue to collect more material as they continue to orbit the host star. Proof to support this Solar Nebula theory has been discovered by the study of ancient meteorites - called chondrites - found on Earth (Beatty et al, 1999). The material makeup of the chondrites, which originate from space, is found to contain the same elements found on Earth – one of which is an isotope of hydrogen called deuterium. Additional proof to this theory comes from remarkable images taken by the Hubble Space Telescope. Looking deep into the heart of the Great Orion Nebula (figure 2), the Hubble Telescope was able to spy tiny solar systems in the making (figure 3). These images show the T Tauri star in the center of a protoplanetary disks, or proplyds. This offers us a remarkable look into the very earliest history into the formation of a stellar system (Figures 4, 5, and 6). Figure 2: This beautiful image of the Orion nebula was captured using special filters by amateur astronomer Russell Crowman using a 14.5 inch telescope and a Santa Barbara Instrument Group ST-11000 CCD camera. Figure 3: This close-up image of the center of Orion nebula – care of the Hubble Space Telescope – shows several knotted looking objects. These are the proplyds. 3 While these proplyds look very impressive, the host T Tauri stars are stars that have not initiated hydrogen fusing – and must be observed in the near infrared. Much of the material that surrounds the proto-star will be blown away by the shockwave resulting from the initiation of hydrogen fusion at the heart of the star (Ostlie and Carroll, 1996). Figures 4, 5, and 6 (clockwise): These are Hubble Telescope close-up images of three of these knots. They show disks of dusty material surrounding their host T-Tauri stars. Because of the density of the dust, these images are photographed in the near-infrared so the host star is visible. Evidence of possible planetary formation Have you ever looked into a telescope at a star, only to find that the star does not appear any larger than with the unaided eye? This same phenomenon is familiar with even the largest telescopes. As a matter of fact, the only star that has been able to be resolved into a disk on a consistent basis is Betelgeuse - the red giant star forming the upper left shoulder of the constellation of Orion (Burnham, 1978). This red giant star is only 520 light years away, and with a diameter between 550 to 920 times our Sun (Betelgeuse is a variable star, so its size fluctuates) it can be easily resolved by our largest optical telescopes (figure 7). With this exception, the majority of stars cannot be resolved as a disk. 4 Figure 7: This image of Betelgeuse was taken by the 50cm COAST telescope. It shows the surface of Betelgeuse, which from Earth is only 0.1 arc-seconds across. For comparison, the planet Pluto is also only 0.1 arc-seconds across. On average, the planet Mars is around 3 arc-seconds across. With this fact alone, it seems impossible to detect a small planet orbiting a star – after all, if we cannot even resolve the star, how can we possibly detect a planet which is much smaller and does not give off any light? There is one method of imaging that will allow us a view of the end stage of the protoplanetary disk. By masking out the bright central star, it is possible to image the residual disk material called the circumstellar disk (figure 8). Figure 8: By masking out this central star (star designation HD 141569A), imaging of the circumstellar disk is possible. All the stars in the field are overexposed, so look much larger than they would be under normal imaging circumstances. Masking of the image is important as the host object is a main-sequence star – that is, a star that has already initiated hydrogen fusing. While much of the protoplanetary nebula can be swept away during the initiation of hydrogen fusion, these images of the circumstellar disks around a main-sequence star tells us that planetary formation is even more likely than with evidence of the protoplanetary disks. 5 While the proplyds and circumstellar disks offer evidence to material that can result in planetary formation, another type of object can also be used to look for evidence of circumstellar disk formation. Herbig-Haro objects (figure 9) are T Tauri stars with an active circumstellar accretion disk (Ostlie and Carroll, 1996). The rotation of this disk is shown by massive lobes of gas that appear perpendicular to the rotating disk. These tell us that the protoplanetary material does in fact rotate about their host star, which can result in planet formation. Figure 9: These are four Herbig-Haro objects photographed by the Hubble Space Telescope. The green jets are the expulsion of gas from the perpendicular circumstellar disk. These jets are present as a result of circumstellar disk rotation. Perhaps the most remarkable image of a circumstellar disk is that of Beta Pictoris.