M. Koehler

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Publications (3)0 Total impact

  • Source
    Article: Anomalous Silicate Dust Emission in the Type 1 LINER Nucleus of M81
    Howard A. Smith, Aigen Li, M. P. Li, M. Koehler, M. L. N. Ashby, G. Fazio, J S Huang, M. Marengo, Z Wang, S. Willner, A. Zezas, L. Spinoglio, Y L Wu
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    ABSTRACT: We report the detection and successful modeling of the unusual 9.7\mum Si--O stretching silicate emission feature in the type 1 (i.e. face-on) LINER nucleus of M81. Using the Infrared Spectrograph (IRS) instrument on Spitzer, we determine the feature in the central 230 pc of M81 to be in strong emission, with a peak at ~10.5\mum. This feature is strikingly different in character from the absorption feature of the galactic interstellar medium, and from the silicate absorption or weak emission features typical of galaxies with active star formation. We successfully model the high signal-to-noise ratio IRS spectra with porous silicate dust using laboratory-acquired mineral spectra. We find that the most probable fit uses micron-sized, porous grains of amorphous silicate and graphite. In addition to silicate dust, there is weak PAH emission present (particularly at 11.3\mum, arising from the C--H out-of-plane bending vibration of relatively large PAHs of ~500--1000 C atoms) whose character reflects the low-excitation AGN environment, with some evidence that small PAHs of ~100--200 C atoms (responsible for the 7.7\mum C--C stretching band) in the immediate vicinity of the nucleus have been preferentially destroyed. (abstract continues) Comment: To appear in The Astrophysical Journal; 37 pages, 2 tables, and 11 figures
    04/2010;
  • Source
    Article: Complex Organic Materials in the HR 4796A Disk?
    M. Koehler, I. Mann, Aigen Li
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    ABSTRACT: The red spectral shape of the visible to near infrared reflectance spectrum of the sharply-edged ring-like disk around the young main sequence star HR 4796A was recently interpreted as the presence of tholin-like complex organic materials which are seen in the atmosphere and surface of Titan and the surfaces of icy bodies in the solar system. However, we show in this Letter that porous grains comprised of common cosmic dust species (amorphous silicate, amorphous carbon, and water ice) also closely reproduce the observed reflectance spectrum, suggesting that the presence of complex organic materials in the HR 4796 disk is still not definitive. Comment: 6 pages, 3 figures; To be published in The Astrophysical Journal Letters
    08/2008;
  • Article: CCE measurements and annealing studies on proton-irradiated p-type MCz silicon diodes
    H. Hoedlmoser, M. Moll, M. Koehler, H. Nordlund
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    ABSTRACT: Magnetic Czochralski (MCz) silicon has recently been investigated for the development of radiation tolerant detectors for future high-luminosity HEP experiments. A study of p-type MCz Silicon diodes irradiated with protons up to a fluence of has been performed by means of Charge Collection Efficiency (CCE) measurements as well as standard CV/IV characterizations. The changes of CCE, full depletion voltage and leakage current as a function of fluence are reported. A subsequent annealing study of the irradiated detectors shows an increase in effective doping concentration and a decrease in the leakage current, whereas the CCE remains basically unchanged. Two different series of detectors have been compared differing in the implantation dose of p-spray isolation as well as effective doping concentration (Neff) of the p-type bulk presumably due to a difference in thermal donor (TD) activation during processing. The series with the higher concentration of TDs shows a delayed reverse annealing of Neff after irradiation.
    Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment.
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