Astron. Astrophys. 317, 953-961 (1997)

1. Introduction

For yet unknown reasons, some meteor streams soar into a spectacular display one year, while the next year rates are back to the normal low level of annual activity. The most intriguing examples are the Aurigids, Lyrids, and alpha-Monocerotids. These streams are associated with long period comets that were far from Earth at the time of the outbursts. It is still poorly understood what physical mechanism is responsible for these "far-comet type" meteor outbursts and the strong variation in rates from year to year, but the meteors are most likely from relatively fresh cometary ejecta of large cm-submm sized grains that have not yet diffused into the dust component that causes the annual meteor stream (e.g. Hughes 1982, Kresák 1993, Jenniskens 1994; 1995).

Although far-comet type outbursts do not occur in relation to the orbital period of the parent comet, outbursts do not manifest at random. The Lyrids, for example, were strong in 1803, possibly in 1863, in 1922, and in 1982, which forms a sequence of 60 year intervals or fractions thereof. By adding other accounts of possible outbursts of this stream, Guth (1947) inferred a period of 12 years and pointed out that in a majority of cases the planets Jupiter or Saturn were in conjunction with the stream at the time of the outburst. The alpha-Monocerotids were seen in 1925, 1935 and 1985, which suggests a ten year periodicity. Until now only two outbursts of Aurigids have been observed.

Guth suggested that the Lyrid maxima are some kind of resonance nodes on the comet's orbit, where occasional encounters with the planets cause periodic depletions of particle density. Another model was proposed by Poruban & Stohl (1992), who thought the outbursts were due to an isolated cloud of small particles from a secondary body of lesser orbital period that separated from the parent comet 1861 I (P/Thatcher). Both models are unlikely. The expected spread in orbital period of the meteoroids will quickly smear out any density variations and the presence of a meteoroid stream in a short period retrograde high inclination orbit is unlikely (Kresák 1958). Kresák proposed that a filament of dust may be brought accidentally in collision with the Earth as a result of planetary perturbations. Trails of dust have been discovered in the orbit of short period comets and have a width consistent with the duration of meteor outbursts (Sykes et al. 1986, Sykes & Walker 1992).

This paper presents observations of a third outburst of Aurigids that does not fit in a simple periodic pattern. The event is described in some detail in Sect. 2 and serves to illustrate far-comet type meteor outbursts. In Sect. 3, the result is added to outbursts of this and other such streams in order to examine the possible role of planetary perturbations. A correlation with the reflex motion of the Sun is found and discussed in the context of the trail hypothesis. The result has implications for interpreting historic accounts of meteor outbursts and allows a forecasting of future events, examples of which are given in Sect. 4.

© European Southern Observatory (ESO) 1997

Online publication: July 8, 1998
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