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Astron. Astrophys. 321, 652-659 (1997)

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1. Introduction

The origin of ordinary chondrites (hereinafter OCs) is an important problem of modern planetary science. The two main hypotheses, largely debated in recent years, are that OC s either come from (at least some subset of) normal S-type asteroids in the main belt, or from a hitherto unobserved primordial population of small main-belt objects (Q-types). The supporters of the first hypothesis, led by C.R. Chapman (Chapman 1996, Gaffey et al. 1989, Gaffey et al. 1993) believe that the inconsistencies between the spectral properties of OC s and S-type asteroids can be explained by a process of space weathering experienced by S-type asteroids and strongly suggested by the results of GALILEO observations of asteroids Ida and Gaspra. The alternative hypothesis is mainly supported by J.F. Bell (Bell 1986, Bell et al. 1989), and is based on the known fact that S-type spectra do not match those of OCs.

In recent years Gaffey et al. (1993) have pointed out that a subsample of the S-type class, the S(IV) subclass according to their classification, exhibits spectra that do not exclude OC assemblages. They are considered to be composed of an olivine-orthopiroxene (Ca-poor) surface assemblage. Gaffey et al. (1993) analysed a set of 39 S-type asteroids out of a sample of 144, classifying 11 of them as belonging to the subset S(IV). It is notable that they are concentrated near the 3:1 Kirkwood gap. Among the most important objects belonging to the S(IV) subclass there are the relatively large asteroids (6) Hebe and (7) Iris. Farinella et al. (1993) estimated the relatively efficiency of a large number of asteroids located close to either the 3:1 mean-motion resonance with Jupiter or the [FORMULA] secular resonance with Saturn, as plausible meteorite sources. Their analysis took into account both the closeness to the resonances and the sizes of the objects, since these influence strongly the kind of collisional evolution and the produced fluxes of collisional fragments. They concluded that (6) Hebe should be a very efficient source of meteorites, and due to its S(IV) classification it should be considered as a possible major source of OC s. Also (7) Iris was found to be among the objects characterised by a relatively high delivery efficiency.

Following these arguments we have observed (7) Iris through rotationally-resolved spectroscopy and have also integrated its present orbit backward and forward, in order to estimate the typical ejection velocities needed for collisional fragments from this object to be injected into the 3/1 resonance (located at the heliocentric distance of 2.501 AU), taking into account also the dynamical evolution of the parent body.

The paper is organised as follows: in Sect. 2 we review the main properties of (7) Iris, while in Sect. 3 we address the problem of the delivery of fragments from this asteroid to the 3/1 resonance. The results of our spectroscopic campaign are shown in Sect. 4, and the main conclusions of this paper are discussed in Sect. 5.

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© European Southern Observatory (ESO) 1997

Online publication: June 30, 1998