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The mid-infrared spectrum of the zodiacal and exozodiacal light

Reach, William T. and Morris, Patrick and Boulanger, François and Okumura, Koryo (2003) The mid-infrared spectrum of the zodiacal and exozodiacal light. Icarus, 164 (2). pp. 384-403. ISSN 0019-1035.

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The zodiacal light is the dominant source of the mid-infrared sky brightness seen from Earth, and exozodiacal light is the dominant emission from planetary and debris systems around other stars. We observed the zodiacal light spectrum with the mid-infrared camera ISOCAM over the wavelength range 5–16 μm and a wide range of orientations relative to the Sun (solar elongations 68°–113°) and the ecliptic (plane to pole). The temperature in the ecliptic ranged from 269 K at solar elongation 68° to 244 K at 113°, and the polar temperature, characteristic of dust 1 AU from the Sun, is 274 K. The observed temperature is exactly as expected for large (>10 μm radius), low-albedo (<0.08), rapidly-rotating, gray particles 1 AU from the Sun. Smaller particles (<10 μm radius) radiate inefficiently in the infrared and are warmer than observed. We present theoretical models for a wide range of particle size distributions and compositions; it is evident that the zodiacal light is produced by particles in the 10–100 μm radius range. In addition to the continuum, we detect a weak excess in the 9–11 μm range, with an amplitude of 6% of the continuum. The shape of the feature can be matched by a mixture of silicates: amorphous forsterite/olivine provides most of the continuum and some of the 9–11 μm silicate feature, dirty crystalline olivine provides the red wing of the silicate feature (and a bump at 11.35 μm), and a hydrous silicate (montmorillonite) provides the blue wing of the silicate feature. The presence of hydrous silicate suggests the parent bodies of those particles were formed in the inner solar nebula. Large particles dominate the size distribution, but at least some small particles (radii ∼1 μm) are required to produce the silicate emission feature. The strength of the feature may vary spatially, with the strongest features being at the lowest solar elongations as well as at high ecliptic latitudes; if confirmed, this would imply that the dust properties change such that dust further from the Sun has a weaker silicate feature. To compare the properties of zodiacal dust to dust around other main sequence stars, we reanalyzed the exozodiacal light spectrum for β Pic to derive the shape of its silicate feature. The zodiacal and exozodiacal spectra are very different. The exozodiacal spectra are dominated by cold dust, with emission peaking in the far-infrared, while the zodiacal spectrum peaks around 20 μm. We removed the debris disk continuum from the spectra by fitting a blackbody with a different temperature for each aperture (ranging from 3.7″ to 27″); the resulting silicate spectra for β Pic are identical for all apertures, indicating that the silicate feature arises close to the star. The shape of the silicate feature from β Pic is nearly identical to that derived from the ISO spectrum of 51 Oph; both exozodiacal features are very different from that of the zodiacal light. The exozodiacal features are roughly triangular, peaking at 10.3 μm, while the zodiacal feature is more boxy, indicating a different mineralogy.

Item Type:Article
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URLURL TypeDescription Paper
Reach, William T.0000-0001-8362-4094
Additional Information:© 2003 Elsevier. Received 3 July 2002, Revised 15 January 2003, Available online 14 June 2003. This work is based on observations with ISO, an ESA project with instruments funded by ESA Member States (especially the PI countries: France, Germany, the Netherlands and the United Kingdom) with the participation of ISAS and NASA. We gratefully acknowledge help from Joris Blommaert with ISOCAM calibration issues. The search for variations in the zodiacal spectrum was inspired by Alain Léger. The research described in this paper was carried out at the California Institute of Technology under a contract with the National Aeronautics and Space Administration.
Group:Infrared Processing and Analysis Center (IPAC)
Funding AgencyGrant Number
Subject Keywords:Zodiacal light; Infrared observations; Interplanetary dust
Issue or Number:2
Record Number:CaltechAUTHORS:20170408-172640095
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Official Citation:William T Reach, Patrick Morris, François Boulanger, Koryo Okumura, The mid-infrared spectrum of the zodiacal and exozodiacal light, Icarus, Volume 164, Issue 2, 2003, Pages 384-403, ISSN 0019-1035, (
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:76346
Deposited By: 1Science Import
Deposited On:29 Mar 2018 22:23
Last Modified:03 Oct 2019 17:00

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