The mid-infrared spectrum of the zodiacal and exozodiacal light

Abstract

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 midinfrared 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 (> 10µm radius), low-albedo (< 0.08), rapidly-rotating, grey 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.-2 -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.

EXPLORATIONS BEYOND THE SNOW LINE:SPITZER/IRS SPECTRA OF DEBRIS DISKS AROUND SOLAR-TYPE STARS

“…Similarly, most of the other stars with excesses in other surveys with the IRS show no evidence for small grains, suggesting that the grains in these systems are larger than ∼10 μm (Beichman et al 2006a;Chen et al 2006). These grains may be similar to those in our own zodiacal cloud which are predominantly larger than 10-100 μm with some smaller silicate grains, yielding only a weak 10 μm emission feature (e.g., Reach et al 2003).…”

Kuiper belts around nearby stars

“…According to the results from Draine & Lee (1984) the power-law relation holds for λ 0 < ∼ 3 μm, and we choose a = 1.0 μm in our calculation. The albedos of micron-sized dust grains are generally below 0.1, as shown by modelling (Shen et al 2009) and observations of cometary and zodiacal dust (Lasue & Levasseur-Regourd 2006;Reach et al 2003), thus we use A = 0, and note that an albedo of almost 0.20 would be required to reduce R dust with 10%. The results are presented in the last column of Table 2.…”

0.8–13 Micron Spectroscopy of V838 Monocerotis and a Model for Its Emission

“…What sets V838 Mon's feature apart from the others is the fact that the central absorption occurs at 10.3 m rather than at 9.7-9.8 m, where most interstellar medium (ISM) and circumstellar absorption reach their maximum. A few sources are known to show a 10.3 m feature, most being in emission: some interplanetary dust particles (Sandford & Walker 1985), portions of the Carinae homunculus (Polomski et al 1999), exozodiacal emission (Reach et al 2003), and the broad silicate emission in supergiants and AGBs (Speck et al 2000). Recently, Lynch, Russell, & Sitko (2002a) reported a weak 10.3 m feature in the spectrum of C/1999 T1 (McNaught-Hartley), which they argued may have been due to hydrated silicates.…”