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

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 <sup>3

1</sup> 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

Contents

• 1. Introduction
• 2. Observations
  • 2.1. New Technology Telescope
  • 2.2. William Herschel Telescope
• 3. Data reduction
  • 3.1. William Herschel Telescope
  • 3.2. New Technology Telescope
• 4. Reflectance spectra of Hilda asteroids
  • 4.1. Absorption features
  • 4.2. The spectra
  • 4.3. Phase reddening
  • 4.4. Dependence of the taxonomy on asteroid size
• 5. Discussion
  • 5.1. Possible origin of the spectral slope - size relation
• 6. Conclusions
• Acknowledgements
• References

© European Southern Observatory (ESO) 1997

Online publication: June 5, 1998

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