Mathematics – Probability
Scientific paper
Sep 2008
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2008epsc.conf..731r&link_type=abstract
European Planetary Science Congress 2008, Proceedings of the conference held 21-25 September, 2008 in Münster, Germany. Online a
Mathematics
Probability
Scientific paper
Geminid Stream The Geminid meteor shower is one of the most intense annual showers. Its first reliable registrations date from 1862 [1]. An assumption exists that fireballs observed in 381-1163 AD originate from the Geminid stream [2]. The Geminid shower structure is known quite well. A qualitative model of the stream, which sums up a 20-years sequence of publications, explains most of the stream's features [3-4]. The activity profile for the Geminid shower has a special shape [5- 6], and the shape is ensured by a cometary model of the stream generation. The only essential discrepancy between the real and model streams is the stream's width. The real width of the Geminid stream is about 5° in solar longitude (for meteoroids mass about 10-4 g), and the model stream's width is about 2.5°. There were many attempts to calculate meteoroid bulk densities from meteor data, and the results lie in the rather broad range: from 0.1 to 8.0 g cm-3 (e.g. [7-8]). The Fe/Mg ratio in the Geminid meteoroids is similar to Halley type cometary material [9]. Na content in Geminids varies widely [9-10]. This fact was interpreted as an aging effect [9-10], so an instantaneous process of the Geminid formation was considered as unlikely. It may also reflect original inhomogeneity inside a parent body. Kasuga et al. [11] found that Na abundances of meteoroids do not depend on their perihelion distances at the interval [0.14 AU, 0.99 AU]. So the last alternative seems to be preferable. Asteroid (3200) Phaethon The identification of the asteroid (3200) Phaethon as the likely parent body for the Geminid meteoroid stream was first made by Whipple [12]. No cometary activity has ever been observed (e.g. [13-14]), so the link between Phaethon and the Geminids is based on the similarity of their orbits, but probability of a chance alignment is less than 0.001 [14]. Its Tisserand parameter TJ = 4.5 argues for an asteroidal origin, rather than a cometary one (most comets have TJ < 3). The colors of the asteroid are bluer than the Sun, and it was classified as an F-type asteroid or a B-type asteroid [15]. Licandro et al. [16] studied visible and near-infrared spectra of asteroid (3200) Phaethon, and found that the spectrum shows differences with the few comet nuclei properly observed at these wavelength. Surface of Phaethon probably contains hydrated minerals and is inhomogeneous in composition and/or physical properties. Licandro et al. [16] assume that Phaethon is an "activated" asteroid scattered from the main-belt to the NEO population. Asteroid (155140) 2005 UD Recently discovered Apollo asteroid 2005 UD is likely candidate for being a kin to Phaethon and/or the Geminid stream [17]. This is a km-order object, which colors are also blue (uncommon color among NEO), so the probability that two blue objects (Phaethon and 2005 UD) would be found by randomly is less than 0.002 [15]. The surface inhomogeneity of the asteroid may be associated with the fragmentation or collisional processes [18]. It was found that the Sextantid meteoroid stream seems to be associated more closely with 2005 UD than Phaethon [17]. The data provide no evidence for ongoing mass loss [15]. Asteroid 1999 YC Ohtsuka et al. [19] found recently another candidate for the Phaethon—Geminid complex, namely Apollo asteroid 1999 YC. Photometric observations [20] have shown that optical colors of the asteroid are consistent with those of taxonomic C-type asteroids and slightly redder than Phaethon and 2005 UD. No cometary activity has been found. Origin of the Complex Dynamical and spectral properties of Phaethon seem to support an asteroidal nature of the object. From the other side, modelling of the Geminid stream formation has shown that a model stream of collisional [21], or eruptive [22] origin does not agree with observations, whereas a cometary model [3] is in very good agreement with the observed structure features of the shower. How can a comet appear on the orbit, which even at aphelion is separated by more then 1 AU from the orbit of Jupiter? Lebedinets [23] suggested that the mechanism for the transformation of the initial comet's orbit is reactive drag. A single encounter with the Earth or Venus could also transform a cometary orbit into Geminid-type orbit [24]. According to Bottke et al. [25] the source for the asteroid Phaethon is mostly likely the main belt, but they cannot rule out possibility that Phaethon is an extinct comet gravitationally decoupled from Jupiter. Recently Hsieh and Jewitt [26] presented data showing that comets could originate from the main asteroid belt. Existence of the Geminid stream imposes a condition on the possible transfer mechanisms. A slow change EPSC Abstracts, Vol. 3, EPSC2008-A-00226, 2008 European Planetary Science Congress, Author(s) 2008 of the cometary orbit is hardly feasible, because a comet cannot live long in Geminid-type orbits. Transformation of the orbit of a parent body with its formed meteoroid stream as a whole is impossible. Discovery of two more members of the Phaethon— Geminid complex raises new questions. What was the mechanism for their decoupling? Could it be possible that their meteoroid streams have different ages? To answer these and many others questions we need more data.
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