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19 93MNRAS.264. . .93A Mon. Not. R. Astron. Soc. 264, 93 Mon. Not. R. Astron. Soc. 264, 93-105 (1993) .93A . Asteroids in the Taurid Complex 93MNRAS.264. 1 1 23 19 D. J. Asher, S. V. M. Chibe and D. I. Steel x Department of Physics, University of Oxford, Keble Road, Oxford 0X1 3RH 2Anglo-Australian Observatory, Private Bag, Coonabarabran, NSW2357, Australia 3 Department of Physics and Mathematical Physics, University of Adelaide, Adelaide, SA 5001, Australia Accepted 1993 February 18. Received 1993 February 16; in original form 1992 November 13 ABSTRACT We show that a statistically significant number of Earth-crossing asteroids are part of the Taurid Complex of interplanetary objects. We also identify another group which appears aligned with (2212) Hephaistos. In addition, we describe the kind of orbital evolution that such asteroids undergo and consider the implications for the history of these two complexes, which may have a common origin. Key words: comets: individual: P/Encke - comets: individual: Taurid Complex progenitor - meteoroids - minor planets. ary parent bodies which undergo further splitting or catastro- 1 INTRODUCTION phic disruption on colhsions in the asteroid belt. The The Taurid meteoroid complex contains objects at all size intention here is to extend this work by investigating Earth- ranges - submicron dust (e.g. Roosen, Berg & Farlow 1973; crossing asteroids, which are likely to have a role as second- Singer & Stanley 1980), and particles that produce radar and ary parent obj ects ( Stohl & Porubcan 1992). visual meteors in well-known broad showers (at night late in The association of meteoroids with some individual TC the calendar year and in the daytime in mid-year; see e.g. asteroids (as opposed to simply with the TC as a whole) has Stohl & Porubcan 1990). At higher masses there have been been demonstrated for radar meteors by Olsson-Steel detections very likely associated with the Taurids, for (1988), and for precise photographic meteors by Porubcan, example a swarm of meteoroids with individual masses up to Stohl & Yana (1992). Other associations of near-Earth ~ 105 g incident on the Moon in 1975 June (Dorman et al. asteroids with each other and with meteoroid streams, and 1978; Oberst & Nakamura 1987, 1991), many ordinary also with one or two Jupiter-family comets, further support fireballs ( < 109 g) and the Tunguska fireball ( ~ 1011 g) of the picture of the breakup of large parent bodies in the inner 1908 June 30 (Kresák 1978; Chyba, Thomas & Zahnle Solar system (Obrubov 1991). 1993). It is not surprising that, during its violent fragmenta- tion history, the putative original giant comet has also given 16 2 MEMBERSHIP OF THE TAURID rise to km-sized ( ~ 10 g, say) asteroids (Clube & Napier COMPLEX 1984). The largest known object in the Taurid Complex (TC) is Comet P/Encke ( ~ 1017 g), which has (or whose progenitor The linkage between four observed meteor showers and has) long been regarded as the parent body and the source of Comet P/Encke is well known, and it is now almost a decade most of the present-day zodiacal complex (Whipple 1967; since Clube & Napier (1984) proposed that some Apollo- Kresák 1980). The size of the TC is perhaps most convinc- type asteroids were likewise associated. Various new TC ingly demonstrated by the huge sporadic meteoroid stream associations have been suggested in the past few years surrounding the Taurid stream (Stohl 1986) and representing (Olsson-Steel 1987a; Steel 1992). However, the number of the intermediate evolutionary stage between the more Apollo asteroids discovered is increasing rapidly through structured Taurid stream and the zodiacal background. search programmes, giving us the chance to look for TC In a previous paper (Steel, Asher & Clube 1991) we asteroids in an ever-expanding data set, and the chance demonstrated the structure of the TC as delineated by repeatedly to improve the statistical significance of results meteor observations and used meteoroid orbital element demonstrating the existence of the km-sized asteroid com- distributions to model the evolution of the complex over the ponent of the TC. A preliminary version of tests of the form past ~ 104 yr, a time-scale conceivably supported by inde- discussed here has been given by Asher, Clube & Steel pendent (non-dynamical) evidence by means of the proposed (1993). association of the Farmington meteorite with the TC. Our Authors, following Southworth & Hawkins (1963), have preferred model was one where large fragments split from defined a ‘D-criterion’ of orbital similarity, with slight differ- the core object near perihelion, these then becoming second- ences between the definitions. Steel et al. (1991) used the © Royal Astronomical Society • Provided by the NASA Astrophysics Data System 94 D. /. Asher, S. K M C/wZ?e £). /. Steel .93A . following definition for Taurid meteoroids: Next, we consider whether these asteroids, selected only on the basis of similarity of orbital size, shape and inclination {a, e, i) to the TC, turn out to be aligned with the TC on the 2 2 + e 2 2 sin h~h D -(<h~ <h) (^i “ 2) + (1) basis of their longitude of perihehon, ar= Q+a>, the first of these terms being the longitude of the ascending node and 93MNRAS.264. with qx = 0.375 an, ex = 0.82 and i*! = 4°. The reason for this the second the argument of perihehon. We assume that or 19 formalism (q instead of 0) is that for meteoroids the peri- will be uniformly distributed, in the absence of a reason (i.e. helion distance q is generally more accurately determined one or more common progenitors) for groupings. Though than the semimajor axis a, the latter tending to reflect with this assumption may be invahdated by other factors (such as full weight the uncertainty in the meteoroid velocity; follow- discovery of selection effects), it appears reasonable in order ing this, Asher (1991) used a D-criterion involving q rather to establish whether any groupings exist, and this step is than a to demonstrate the existence of TC asteroids. On the more tractable than making assumptions about the a, e and / other hand, since a is known accurately for asteroids, in this distributions. Having written that, it is noteworthy that the study we use high TC value of the eccentricity (ej is more representative of cometary rather than typical Apollo-asteroidal orbits, and the perihehon distance {q^ is smaller than usual amongst the D2= (01 3 02) +(e,-e )2+|2 sin^Y^j , (2) 2 discovered Apollo orbits; that is, orbits selected using equation (2) are noteworthy in themselves as being atypical with a{ = 2.1 au and ex, ix as above. The D-criterion is of the bulk of Apollo asteroid orbits. essentially an empirical method of defining the difference A reasonable range of arto take as corresponding to the between two orbits, and the results are broadly the same TC, based on the core of the Taurid meteoroid stream/ either way. Indeed, it is useful to know that the result is meteor showers, is 140° ± 40° (see Steel et al. 1991). In Table independent of the precise method used (compare our 1 we see that two of the first five, five of the first 10, eight of results with those of Napier 1993). The important point here the first 15, 10 of the first 20 and 11 of the first 25 asteroids is that a longitude term should not be included because, are within this range. (Note that 1991 BA has ar= 189° and whilst appropriate for many (narrow) streams, the Taurids is thus outside this 100°-180° range; but see later for the have been widely dispersed in longitude, predominantly by reason for its inclusion in the first column of Table 1 ). These Jovian perturbations, and therefore a conventional longitude longitude alignments would happen by chance with prob- term in the D-criterion would have too large a contribution abilities 0.309, 0.050, 0.008, 0.006 and 0.013 respectively (see Steel et al. 1991). We shall consider longitude-selection (probabilities of occurring within that specific band inoT' if ot' after applying the D-criterion. were uniformly distributed from 0° to 360°). Applying a limit Noting that the inclination i varies by a factor of a few over of D = 0.15, as we did with our ^-containing D-criterion time-scales of order 103 yr, we use Brouwer’s (1947) secular (equation 1 here) in Steel et al. (1991), where we aimed to perturbation theory (computational details in Asher 1991) to restrict the study to the core of the meteoroid stream adjust i to be the minimum value that ever occurs, before complex, we would have five out of the first nine asteroids applying the D-criterion. This is not necessary in the case of and a probability of occurrence by chance of 0.030 (3 per TC meteoroids, because i is constrained to be low by the fact that the particle orbit must intersect the Earth’s orbit for a meteor to be produced (see Steel et al. 1991). There are Table 1. Asteroids with orbital elements {a, e, i) similar to the corresponding variations in eccentricity e, but these are Taurid Complex meteoroids; D is the D-criterion defined in much less significant with regard to the D-criterion than the equation (2). /-variations. In Asher et al. (1993) we neglected the effect due to varying e; here we include it, and find that the result- Aligned with Taurids Others 1991 AQ 2.16 0.50 0.77 3 0.05 222 ing change in D is less than 0.01 in nearly all cases. (2212) Hephaistos 2.16 0.36 0.84 12 0.06 236 We applied this D-criterion to a list of Earth-crossing 1984 KB 2.22 0.52 0.76 5 0.07 146 (2101) Adonis 1.87 0.44 0.76 1 0.10 32 asteroids complete to 1992 March.
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