ISOPHOT polarization measurements of the asteroids (6) Hebe and (9) Metis at 25 mu m

Citation
Jsv. Lagerros et al., ISOPHOT polarization measurements of the asteroids (6) Hebe and (9) Metis at 25 mu m, ICARUS, 142(2), 1999, pp. 454-463
Citations number
49
Language
INGLESE
art.tipo
Article
Categorie Soggetti
Space Sciences
Journal title
ICARUS
ISSN journal
0019-1035 → ACNP
Volume
142
Issue
2
Year of publication
1999
Pages
454 - 463
Database
ISI
SICI code
0019-1035(199912)142:2<454:IPMOTA>2.0.ZU;2-U
Abstract
The subsurface thermal IR emission of asteroids becomes linearly polarized when scattered across the surface, The disk integrated mid-IR Stokes parame ters are computed by extending the new thermophysical model by Lagerros (19 96, Astron. Astrophys. 315, 625-632; 1998, Astron. Astrophys. 332, 1123-113 2). The small scale surface roughness is approximated by hemispherical segm ent craters covering a smooth surface, Analytical solutions are used far th e multiple scattered solar and thermally emitted radiation inside the crate rs. The surface roughness enhances the emission toward the solar direction, but randomizes the scattering planes which lowers the polarization, In gen eral the predicted degree of linear polarization increases with higher refr active index, higher absorption coefficient, and a more elongated shape of the asteroid. The asteroids (6) Hebe and (9) Metis were observed at lambda = 25 mu m with ISOPHOT onboard the Infrared Space Observatory (ISO), The model absolute f luxes are in good agreement with the photometric results. Although no linea r polarization was detected, the upper limits together with the extended mo del enable us to put useful constraints on the regolith properties of the t arget asteroids. The derived detection limits are compared to model polariz ation, by spanning a range in surface roughness, refractive index, and ther mal inertia parameter space. The Metis observations favor a low refractive index and high surface roughness, but the Hebe observations are inconclusiv e since they coincided with a minimum in the polarization curve. (C) 1999 A cademic Press.