ISOPHOT Polarization Measurements of the Asteroids (6) Hebe and (9) Metis at 25 μm
✍ Scribed by J.S.V. Lagerros; T.G. Müller; U. Klaas; A. Erikson
- Publisher
- Elsevier Science
- Year
- 1999
- Tongue
- English
- Weight
- 144 KB
- Volume
- 142
- Category
- Article
- ISSN
- 0019-1035
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✦ Synopsis
The subsurface thermal IR emission of asteroids becomes linearly polarized when scattered across the surface. The disk integrated mid-IR Stokes parameters are computed by extending the new thermophysical model by Lagerros (1996, Astron. Astrophys. 315, 625-632; 1998, Astron. Astrophys. 332, 1123-1132). The small scale surface roughness is approximated by hemispherical segment craters covering a smooth surface. Analytical solutions are used for the multiple scattered solar and thermally emitted radiation inside the craters. The surface roughness enhances the emission toward the solar direction, but randomizes the scattering planes which lowers the polarization. In general the predicted degree of linear polarization increases with higher refractive index, higher absorption coefficient, and a more elongated shape of the asteroid. The asteroids (6) Hebe and (9) Metis were observed at λ = 25 µm with ISOPHOT onboard the Infrared Space Observatory (ISO). The model absolute fluxes are in good agreement with the photometric results. Although no linear polarization was detected, the upper limits together with the extended model enable us to put useful constraints on the regolith properties of the target asteroids. The derived detection limits are compared to model polarization, by 1 The ISOPHOT results are 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) and with the participation of ISAS and NASA.