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Research Highlights |
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 Sun-Kun King: | | Trans-neptunian objects (TNOs) are probably small comets beyond the orbit of Neptune. Study of TNOs might be able to explain the origin of short-period comets and to help our understanding of the process of planets formation and the early history of the solar system. Occultation survey is currently the only way available to detect these objects down to the size of a few kilometers. Instead of detecting the directly reflected light of a TNO, diffraction and the apparent size of background stars during an occultation are two major constraints. Both geometric shadow and physical diffraction can be considered in an occultation. They show some interesting mathematical and physical features. Though, with such a large distance from the earth, Fresnel diffraction is needed here if the shadow of a kilometer size object at around 30 to 50 AU is observed in optical wavelength. According to current estimate of the size distribution of TNOs, the small objects close to the diffraction limit will contribute to most of the occultation events in a dedicated occultation survey like TAOS. It is of crucial importance to understand the properties of diffraction shadow under certain sampling frequency.
Using Kirchhoff diffraction theory, we can simulate the diffraction pattern generated by these objects of various kinds of size and shape. Various statistics can be applied to study these light curves. For example, the effect of shot noise and sampling rate can be analyzed either theoretically or through a simulation. It is also interesting to know the capability of a TAOS like experiment. Can we tell the distance or general shape of occulting objects in addition to a simple number count and size distribution? How far can we probe into the deep space such as the inner Oort cloud? These kinds of questions should be well answered before we take the next step beyond TAOS. They would be also helpful while doing data reduction for TAOS in the near future. |
| Yi-Jehng Kuan: | | Besides analyzing the meteorites on Earth, ASIAA astronomers have been pointing our telescopes, SMA and TAOS, to study comets in distant space. Cometary aterials appear to retain a significant interstellar signature and are thought to be pristine remnants of the interstellar material that collapsed to form the Solar Nebula. Hence study of comets can provide a record of the physical and chemical
conditions in the Solar Nebula. The growing inventory of organic molecules found in interstellar clouds, similar to the Solar Nebula when compared to the composition of comets, suggests that much of the organic biomolecules required to start the Earth's prebiotic chemistry may be delivered by cometary impacts, and one could in principle trace the Earth's prebiotic chemistry back to the parent molecular cloud. We have thus carried out SMA observations of Comet C/2002 T7 (LINEAR) during its 2004 apparition at 1.3 millimeter. With an angular resolution of 3".2¡Ñ1".8, which corresponds to a linear resolution of 1500¡Ñ900 km when the comet was at a distance of 0.61 AU from the Earth, we successfully detected and imaged the 234-GHz thermal dust continuum, as well as the spectral emission of CS and methanol emission (Figure 2) from the inner coma. This was the first comet
observation ever conducted by the SMA. | | |
| Pin-Gao Gu: | | Observations of extrasolar planets implied by radial-velocity curves of nearby solar type stars have shown that the orbits of Jovian planets within 0.07 AU from their parent stars are almost circularized, albeit eccentric orbits are rather common on average beyond 0.07 AU. Similar to circular orbits of close binary stars, circular orbits of close-in Jovian planets are possibly caused by the tides exerted by their parent stars on these planets. Working with Peter Bodenheimer and Doug Lin of UCO/Lick observatory (Gu, Lin, & Bodenheimer 2003, ApJ, 588, 509), I showed that for young Jupiter-mass planets with an orbital period less than 3 days, and initial radius about 2 Jupiter radii, and an orbital eccentricity greater than 0.2, the energy dissipated during the circularization of their orbits is sufficiently intense to inflate their sizes up to their Roche radii. Consequently the inflated planet loses mass through its inner Lagrangian point. Due to the conservation of total angular momentum, these mass-losing planets migrate outwards such that their semi-major axes and Roche radii increase while their mass, eccentricity and tidal dissipation rate decrease until the mass loss in quenched. The thermal expansion of a giant planet in this case is driven by the tidal inflation instability: the growth rate of radiative cooling decreases as the planet expands and therefore the planet is thermally unstable to the internal tidal dissipation. We showed that the instability is caused by the transition of the equation of state of the planet interior from a stiff state to a rather soft state (Gu, Bodenheimer, & Lin 2004, ApJ, 608, 1076). Our model of outward migration of Roche-lobe overflowing Jovian planets in eccentric orbits might be of some significance to be responsible for the lack of extrasolar planets with an orbital period less than 3 days. One of the most important but uncertain assumptions made in the above tidal-inflation model is to hypothesize a uniformly-distributed volume heating rate in a tidally inflated planet. This makes planet inflation with great efficiency since the entire planet, which is almost convective, undergoes expansion. Tidal dissipation is still an extremely murky area in astrophysics. However, if convective viscosity and radiative damping are the sources of energy dissipation, the tidal dissipation would not be uniformly distributed but would be concentrated in the outer part of the planet where the Brunt-Vaisala frequency is comparable to the tidal forcing frequency. Thus, lacking the heating source in the interior leads to a failure of expansion up to Roche radii. The question of whether or not the tidal dissipation is able to occur in the deep interior of a planet definitely deserves more investigation. We investigated a number of models for various locations of tidal dissipation inside a Jovian planet. While the expansion rate of the planet is reduced, the inner part of the planet without any entropy input can still be inflated as a result of hydrostatic adjustment to the thermal inflation of the outer part of the planet (Gu, Bodenheimer, & Lin 2004, ApJ, 608, 1076) |
| Typhoon Lee: | | High sensitivity is the most important requirement because astronomical observation shows that interstellar dusts are typically smaller than 1 micrometer in size (i.e. consisting of a few billion atoms). Therefore, the less abundant elements would be difficult to measure without high ion yield. The second requirement is to measure as many elements as possible for a single grain in order to better constrain the physical conditions of its source. For these reasons we have initiated collaboration with M. Pellin's group at Argonne National Lab who provided the design of their Secondary neutral mass spectrometer as the baseline configuration of Dust Buster. Ar or Ga primary ion sputtering or 213 nm 5X-Nd-YAG laser desorption is used to evaporate the neutral atoms from the sample surface. Then a 157 nm UV beam from a F2 excimer laser ionizes the secondary neutral. An overall yield of one detected ion for every three atoms consumed should be achievable. A time of flight analyzer with reflectron compensation is used to obtain the entire mass spectrum. We have set up procedures for the analysis of two dozens of isotope ratios from Mg to Mo (A=100 amu). These elements should be within reach for a one micrometer dust with chondritic composition. |
| Hsien Shang: | | By modeling the abundance of radioactive elements found in calcium-aluminum-rich inclusions (CAIs) of meteorites, the origin and the evolution of the solar nebula is gradually being unraveled. Two explanations exist for the short-lived radionuclides (half-life T1/2 < 5 Myr) present in the solar system when CAIs first formed. They originated either from the ejecta of a supernova or by the in situ irradiation of nebular dust by energetic particles. With a half-life of only 53 days, 7Be is the key discriminant, since it can only be made by irradiation. Using the same irradiation model developed earlier, the yield of 7Be can be calculated (see Figure 1). Within model uncertainties associated mainly with nuclear cross sections, the results agree with experimental values. Moreover, if 7Be and 10Be have the same origin, the irradiation time must be short (a few to tens of years), and the proton flux can be constrained as well. The X-wind model provides a natural astrophysical setting that gives the requisite conditions. In the same irradiation environment, 26Al, 36Cl, and 53Mn are also generated at the measured levels within model uncertainties, provided that irradiation occurs under conditions reminiscent of solar impulsive events (steep energy spectra and high 3He abundance). The decoupling of the 26Al and 10Be observed in some rare CAIs receives a quantitative explanation when rare gradual events (shallow energy spectra and low 3He abundance) are considered. The yields of 41Ca are compatible with an initial solar system value inferred from the measured initial 41Ca/40Ca ratio and an estimate of the thermal metamorphism time, alleviating the need for two-layer proto-CAIs. | | |
| Jen-Hung Wang: | | The TAOS project aims to detect comets of regular size (a few km in diameter) beyond the range of perturbation by Jovian planets (>60 AU) through their blockage of the light from background stars. Such an occultation event is expected to be deep (>>10%) and short (<<1sec) yet extremely rare, probably around one part in a trillion per year per star.
So robotic telescopes are needed in order to perform the billions of photometry required for the detection of one true occultation even if we monitor 1000 stars simultaneously. Moreover, we have to use the so called zipper mode of observing (i.e. hopping the image electronically across the CCD camera) to achieve sub-second time resolution without shutter.
Finally, the false alarm rate for each telescope must be low enough so that only a small array of them working in coincidence will be sufficient to keep the combined false alarm rate much lower than the real event rate.
We have exposures of our star field no. 23, each lasting 0.25 sec., for the night of 11/25/2003. Among a total of 615 stars identified, the brighter 336 (limiting magnitude = 13.6) satisfy our criterion that for each light curve the standard deviation is less than 1/5 of the mean intensity. We then filtered the data with four additional criteria and found only three sudden (single reading of 0.2 sec) and large (>30% and >5 standard deviations decrease) intensity drops left. This corresponds to a false alarm rate of 0.5 ppm that seems to be remarkably close to the >5 sigma probability of 0.3 ppm for a normal distribution. Therefore, maybe we only need two telescopes in coincidence since the combined false alarm rate would already be below the 1 ppt nominal target. In that case, the two remaining telescopes can be set up in the N-S direction to help determine the size of the comet. |
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