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Astron. Astrophys. 323, 606-619 (1997)


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A study of Hilda asteroids

II. Compositional implications from optical spectroscopy *

M. Dahlgren 1, C.-I. Lagerkvist 1, 4, A. Fitzsimmons 2, I.P. Williams 3 and M. Gordon 3

1 Astronomical observatory, Box 515, S-75120 Uppsala, Sweden (Mats.Dahlgren@astro.uu.se, Claes-Ingvar.Lagerkvist@astro.uu.se)
2 APS Division, Department of Pure and Applied Physics, The Queen's University of Belfast, Belfast BT7 1NN, Northern Ireland (A.Fitzsimmons@Queens-Belfast.ac.uk)
3 Astronomy Unit, School of Mathematical Sciences, Queen Mary and Westfield College, Mile End Road, London E1 4NS, UK (I.P.Williams@qmw.ac.uk, M.K.Gordon@qmw.ac.uk)
4 DLR, German Aerospace Research Establishment, Rudower Chausse 5, D-12489 Berlin, Germany

Received 9 April 1996 / Accepted 16 January 1997

Abstract

We obtained 49 reflectance spectra of 23 Hilda asteroids in the wavelength ranges 3850-7650 and 3850-10150 Å during two observing runs in 1992. The primitive D-types dominate and comprise 34 % of the numbered Hilda asteroids, while 28 % and 2 % are P and C-types, respectively. This is in contrast to the results of earlier investigations by Tedesco & Gradie (1982) and Gradie et al. (1989) who found fewer D-type asteroids in the Hilda group. We confirm the result by Dahlgren & Lagerkvist (1995) that there is a spectral slope - asteroid size relation among the Hilda asteroids, implying a size dependent surface composition. No evidence of absorption features are seen in the spectra of the P and D-types, in particular no features due to phyllosilicates are found, implying that no aqueous alteration of the surfaces has occurred. The absence of C-types within the Hilda group suggests that the C population is confined to smaller heliocentric distances than thought before, with very few C asteroids further out than the Cybele group. The phase reddening effect is absent for the P and D-types observed. Though it is not a Hilda member one spectrum of 279 Thule was obtained and is also presented.

Key words: asteroids

* Partly based on observations collected at the European Southern Observatory, La Silla (Chile).

Send offprint requests to: M. Dahlgren

© European Southern Observatory (ESO) 1997

Online publication: June 5, 1998

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