Introduction

According to generally accepted opinion, meteoroid streams are formed as a result of the disintegration of cometary nuclei. The presence of meteor showers associated with some near-Earth asteroids (NEAs) give evidence that such asteroids have a cometary origin, i.e. they are extinct cometary nuclei [1,2,3,4,5]. The existence of asteroids identifiable with extinct or dormant comets (2060 Chiron, 4015 Willson-Harrington, 1986 TF Parker-Hartley) confirm the cometary origin for some NEAs.

Investigation of NEA-meteor showers associations is important not only for confirmation or denial of NEA cometary origins, but also for the receipt of important information about NEA sources - comets from outer regions of the Solar System, and real asteroids from the main belt.

The calculation of theoretical meteor radiants is the first step in revealing the generic relationship between a given near-Earth object (comet or asteroid) and its possible meteor showers. However, methods for the determination of the theoretical radiants of comets and asteroids approaching the Earth's orbit close than 0.1-0.3 AU, which were used by different authors, e.g., [6,7,8] until recently, did not take into account the meteoroid stream evolution and could only roughly predict one or two radiants of the given comet or asteroid.

As follows from basic principles of meteoroid stream formation and evolution [9,10,3,4,11] related meteor showers can be possessed also by those comets whose orbits are located presently at distances more than 0.3 AU from the Earth's orbit (Table 1) but which crossed it in the past. The orbit of the parent body for the moments of its crossings of the Earth's orbit can be determined by studying its evolution under the gravitational perturbing action of the major planets.

2. FORMATION OF METEOROID STREAMS

Ejection velocities of meteoroids from their parent bodies and radiation pressure (for small particles) cause an initial dispersion in orbital elements of ejected meteoroids. Because of differences in the semi-major axes (and orbital periods) between the meteoroids and their parent body, some meteoroids lag behind the parent body, while others, overtaking it, spread along the entire orbit and form a complete loop in a comparatively short time [12,13].

After the meteoroids are distributed along the orbit of the parent body, due to differences in the planetary perturbing action on stream meteoroids of different semi-major axes and eccentricities, the rates and cycles of variations in the angular orbital elements (a), Q, i) will be different for different meteoroids. As a result, the orbits of different meteoroids will be at different evolutionary stages as distinguished by their arguments of perihelia, that is the stream meteoroids occupy all evolutionary tracks of their parent body. This process increases considerably the size of the meteoroid stream, first of all, its thickness (the breadth of a stream is determined by the value of the meteoroids' orbital semi-major axes).

If the Earth's orbit is assumed to be circular, then it may be intersected by those stream meteoroids which have the orbital node at r=l AU, i.e. satisfying the expression:

As shown earlier [10,3,4] the number of meteor showers produced by a meteoroid stream is determined by the Earth-crossing class of the parent-body orbit. For example, if it is a quadruple crosser of the Earth's orbit (i.e. during one cycle of variations of the perihelion argument of its orbit under the perturbing action of the major planets a parent body crosses the Earth's orbit four times) the meteoroids of the stream that separated from parent can produce four meteor showers: two at the pre-perihelion intersections and two at the post-perihelion intersections with the Earth. Crossing before perihelion gives rise to two nighttime showers, and after the perihelion to two daytime showers. These two pair of showers are formed by the same meteoroid stream, each pair consisting of a northern and a southern branch.

3. METEOR SHOWERS ASSOCIATED WITH TAURID COMPLEX ASTEROIDS

The object of the present paper is to reveal the meteor showers associated with the Taurid Complex asteroids, which has Encke's comet as a member and according to Clube & Napier [14] and Asher et al. [15] have a common cometary origin. Possible association of daytime fireballs and some Taurid complex asteroids (4486 Mithra, 1990 SM and 1991 BA) was suggested by Hasegawa [16]. But the existence of observable associated meteor showers is the only substantial index that a given NEA is a candidate for comet origin.

Asher et al. [15] assumed that those near-Earth objects (NEOs) belong to the Taurid complex asteroids if their longitudes of perihelion 7i=£2-t-co lie within the limits of 100° < n < 190°, and if their orbital parameters (a, e, i) satisfy the criterion £><0.2, where

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