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Astron. Astrophys. 361, 369-378 (2000)

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2. Cometary nuclei in the protosolar cloud

In this paper, we deal with the problem of how the comets present in the parent cloud of the solar system could remain at large heliocentric distances after the cloud collapse. However, we need to discuss the creation of cometary nuclei in the interstellar clouds as this is the rationale for our study.

The similarity of material in dense interstellar molecular clouds and cometary material, mentioned in Sect. 1, as well as the necessity of a relatively high abundance of dust in the star-forming regions (Lattanzio 1984; Delsemme 1991; Mouschovias 1996; Nakano et al. 1996) lead us to assume that molecular clouds are an appropriate birth-place of comets. The idea of interstellar cloud comet origin has already been proposed by several authors (omitting the theories which have been rejected, e.g. that by Lyttleton (1948), we can mention for example: Hasegawa (1976), Khanna & Sharma (1983), Yabushita (1985) or Greenberg (1998)). A particular argument in favour of this concept was given by Clube & Napier (1985). They noticed the results of previous studies (Cardelli & Bohm-Vitense 1982; Phillips et al. 1982; Mentese 1982; Tarafdar et al. 1983) viz. that the diffuse interstellar nebulae are deficient in almost all elements including most refractories (C, Mg, Fe, Ti) and volatiles (N, S, O, Ar) relative to solar abundances. It seems reasonable to suppose that the large gas phase depletions, which are difficult to reconcile simply with loss to grains (Greenberg 1974; Tinsley & Cameron 1974), are due to comet formation.

In spite of the fact that a number of interstellar theories have been published, we cannot disregard a certain weakness of this concept of comet origin: no specific physical mechanism of creation of cometary nuclei in an interstellar-cloud environment, even if relatively dense, has been suggested and worked out in detail. Nevertheless, new observational facts have recently revealed a variety of unexpected phenomena and structures in the interstellar clouds. As an example, we can mention the evaporating gaseous globules discovered few years ago in the Hubble Space Telescope WFCP2 images (Hestler et al. 1996). The discovery was made in the M16 nebula, which is the site of very active recent star formation. Such globules are overdense regions appropriate for a condensation of macroscopic bodies. The globules in M16 are very probably only a sample of these objects. They became visible due to the radiation of nearby O stars, which had disrupted the star formation. In the molecular clouds, we can expect a number of dark sites with relatively high density, which are similar to those in M16.

Another possibility of comet creation (already mentioned in Sect. 1) is provided by the mechanism suggested by Hills (1982). Not only the pressure due to radiation from the protosun and neighbouring protostars, but from any luminous star may have forced the coagulation into comets of the dust grains, if the star crosses a dense interstellar region. Let us assume a region of radius [FORMULA]AU. García-Sánchez et al. (1999) found that the rate of close approaches by star systems (single or multiple stars) within a distance D (in parsecs) from the Sun is given by [FORMULA]Myr-1. Applying this result to the assumed region, we can estimate a rate of passages through the region of about 750 star systems per Gyr. Some stars are luminous enough to form cometary nuclei along cylindric surfaces around their paths.

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© European Southern Observatory (ESO) 2000

Online publication: September 5, 2000