Bulletin of the American Physical Society
2006 37th Meeting of the Division of Atomic, Molecular and Optical Physics
Tuesday–Saturday, May 16–20, 2006; Knoxville, TN
Session C2: Astrophysics |
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Chair: Kate Kirby, Harvard Center for Astrophysics Room: Knoxville Convention Center Ballroom EFG |
Wednesday, May 17, 2006 10:30AM - 11:06AM |
C2.00001: Millimeter/Sub-millimeter Spectroscopy: Revealing the Building Blocks of Complex Molecules in Space Invited Speaker: At present, over 125 different chemical compounds have been detected in the interstellar medium. The majority of these molecules have been identified on the basis of their pure rotational spectrum, which can be measured using radio/millimeter astronomy. The success of these astronomical observations relies on the availability of high resolution laboratory data. Hence, our knowledge of the ``molecular universe'' has its basis in the laboratory. Over the past twenty years, studies of interstellar molecules have revealed that there is far more molecular material than previously thought in the Galaxy. Unusual species are often detected, such as the metal cyanide species MgCN and AlNC. Other molecules observed are routinely used in the organic lab, such as acetone and even a simple sugar, glycolaldehyde, CH$_{2}$OHCHO. The common appearance of organic molecules and simple species with a metal center suggest that the building blocks of life may have actually originated in interstellar space. Evaluating the limits of chemical synthesis in interstellar gas requires extensive laboratory measurements. Such measurements, however, often involve studying transient radicals and ions that are difficult to create. Such molecules can exhibit complex patterns arising from fine and hyperfine structure. Even ``simple'' organic species have complicated spectra due to internal rotation and multiple conformers. Using a combination of millimeter/sub-millimeter direct absorption techniques, velocity modulation spectroscopy, and Fourier transform microwave methods, the Ziurys group have been actively measuring the pure rotational spectra of potential interstellar molecules. Among the species studies are those containing cosmically-abundant metals, in particular ions such as FeCO$^{+}$ and TiF$^{+}$. Other molecules of interest are ``prebiotic'' compounds such as HOCH$_{2}$COCH$_{3}$. The challenges of these types of measurements will be presented , as well as their implications for astrophysics. [Preview Abstract] |
Wednesday, May 17, 2006 11:06AM - 11:42AM |
C2.00002: X-Ray Emission Cross Sections following Charge Exchange by Multiply-Charged Ions of Astrophysical Interest Invited Speaker: State selective \textit{nl}-electron capture cross sections are presented for highly charged ions with Z = 6-10 colliding with molecules. The energy range investigated was from 1 eV/amu (v = 1.4x10$^{6}$ cm/s) to 100 keV/amu (v = 4.4x10$^{8}$ cm/s). The K-shell x-ray emission cross sections are determined by using the calculated state-selective electron capture results as input and then applying the branching and cascading values for the photon emission. A major shift in the line emission from being almost solely Lyman-alpha transitions at the highest collisions energies to strong high-$n$ to 1s transitions at the lowest energies is observed. The calculated emission cross sections are in reasonable accord with measurements made by Greenwood \textit{et} \textit{al}$^{1}$ for O$^{8+}$ and Ne$^{10+}$ on various targets at 3 keV/amu. The calculations are also in accord with x-ray emission cross section data obtained on the EBIT machine at LLNL where O$^{8+}$ and Ne$^{10+}$ high resolution measurements were made at a temperature of 10 eV/amu for a series of targets with varying ionization potentials. The Ne$^{10+}$ data clearly show the contribution from multiple capture followed by Auger autoionization in the line emission spectra. Our calculated line emission cross sections are used to provide an \textit{ab initio} determination of the soft x-ray spectrum of comet C/Linear 1999 S4 that was observed on the Chandra X-ray Observatory. The calculations reproduce the measured spectrum and show that it is due to charge exchange of the neutral gases in the comet's coma with the ions of the slow solar wind. Details of the calculations are presented in a recent paper by Otranto \textit{et a}l$^{2}$. 1. J. B. Greenwood, I. D. Williams, S. J. Smith and A. Chutjian, \textit{Phys. Rev. A} \textbf{63}, 062707 (2001). 2. S. Otranto, R. E. Olson, and P. Beiersdorfer, \textit{Phys. Rev. A} (in press). [Preview Abstract] |
Wednesday, May 17, 2006 11:42AM - 12:18PM |
C2.00003: Electron-impact ionization measurements for use in astrophysics Invited Speaker: Spectroscopic data produced by an array of past, present, and future satellite and space observatories are being used to address many fundamental questions in astrophysics. Analyzing the collected spectra from electron-ionized plasmas, however, requires accurate ionization balance calculations for plasmas in collisional ionization equilibrium (CIE) as well as plasmas under conditions of non-equilibrium ionization (NEI). Therefore accurate electron-impact ionization (EII) data are needed for ions found in many cosmic plasmas where EII is the dominant ionization mechanism, such as supernova remnants, stellar coronae, the interstellar medium (ISM), the intracluster medium in clusters of galaxies, and elliptical galaxies. Here we report on a program of laboratory measurements of EII for ions in the berylliumlike isoelectronic sequence at the ORNL Multicharged Ion Research Facility. These results are compared with state-of-the-art non-perturbative calculations. Our studies also focus on measurements of EII for heliumlike and oxygenlike ions. For these three isoelectronic sequences there appear to be significant errors and uncertainties in the EII data used in astrophysics, primarily due to uncertainty in the fraction of metastable ions encountered in prior measurements. By determining the metastable ion fractions independently of the ionization experiments, our measurements will provide new EII data with total uncertainties of 15\% or less. \\ \\ This material is based upon work supported by NASA under Award Nos. NNH04AA151 and NNH04AA72I and by the U.S. Department of Energy under Contract No. DE-AC05-00OR22725 with UT-Battelle, LLC. [Preview Abstract] |
Wednesday, May 17, 2006 12:18PM - 12:54PM |
C2.00004: Pressure broadened alkali-metal resonance lines in brown dwarf and extrasolar planet spectra Invited Speaker: In the last ten years many brown dwarfs and many extrasolar planets have been discovered creating excitement in the astronomical sciences. Brown dwarfs are substellar objects and they can be classified by various parameters. The cooler dwarfs, class L and class T, exhibit prominent, broad spectral features thought to originate from the resonance lines of Na and K pressure broadened via He and molecular hydrogen in their atmospheres. In addition, astrophysicists believe that for certain specific cases (hot Jupiters) extrasolar giant planets irradiated by their host star will exhibit similar spectral features. We are modeling the pressure broadening using a combination of experiment and theory. Our application of atomic and molecular physics to this frontier of astrophysics will be discussed. [Preview Abstract] |
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