VI.A Zodiacal IR Emission

The most relevant feature of the exozodiacal measurement problem in the IR is the strong 10 µm foreground. The preferred wavelength band for these measurements is around 10 µm because of: a) favorable contrast ratio between the exozodiacal signal and the star, b) presence of a terrestrial atmospheric window, and c) existence of spectral features of substances such as O₃ and CH₄ that might indicate a non-equilibrium atmosphere and the presence of life (ExNPS 1996). However, the 10 µm exozodiacal signal will be deeply embedded in foreground terrestrial and local zodiacal emission.

Figure 8: Thermal model of a 1-zodi cloud around a solar-type star viewed from 10 pc based on the COBE zodiacal model. a) Disk vertical optical depth (t^) and temperature versus distance from the central star, compared with a possible profile of the b Pic disk. b) Thermal emission surface brightness versus distance from the star for several wavelengths around 10 µm.

Figure 8a shows the face-on optical depth of the smoothly varying portion of the solar system zodiacal dust as a function of radius from near the Sun to 100 AU (Traub et al. 1996) for the COBE model parameters (Kelsall et al. 1998). The COBE model spatial distribution ~(0.71 x 10^-7)(r_AU^-0.39) was assumed valid from near the Sun to 100 AU. That figure also shows typical temperatures in the cloud assuming emissivity = 1 (cf. Table 1). The inset scale gives angular radii for an observer at a distance of 10 pc; thus the Earth lies at a radius of 100 mas and the solar radius is 0.44 mas. Figure 8b displays the corresponding face-on surface brightness (also called specific intensity or spectral radiance) in frequency units at wavelengths of 8, 10, and 12 µm. Note the steep fall-off with radius caused by weak blackbody flux at 10 µm for temperatures below about 300 K.

The IRAS zodiacal model has a steeper radial gradient, ~(1.12 x 10^-7)(r_AU^-0.80), than the COBE model. The COBE and IRAS models are both constrained by observations relevant to material between about 0.9 and 2 AU and are similar in that region. Extrapolations of these models down to a few solar radii is evidently dangerous: they differ there by almost a factor of 10 in surface density. Also, to the extent that much of the dust originates in the asteroid belt or in solar-activated comets, there is probably a substantial break in density beyond 3-5 AU.

Figure 9 shows the integral power density from the Sun's surface outward for the IRAS model. Note that 50% of the power comes from the region inside 0.1-0.2 AU, depending on the wavelength, and 90% of the total power is included inside a radius of 0.5-1.5 AU. For the COBE model (not shown) the corresponding radii are 0.2-0.5 AU for the 50% points and 1.0-2.0 AU for the 90% points. An exozodiacal dust-detecting interferometer needs to be sensitive in this range of radii.

Figure 9: Integral power versus distance from the star for a 1-zodi cloud around a solar-type primary viewed from 10 pc, based on the IRAS zodiacal model.