Cometary activity of the objects of the Taurid Complex

Introduction
One of the most important meteor shower is that of the Taurids, which is associated with comet 2P/Encke. Although meteors are usually produced by very small particles, some recent Taurid fireballs have been associated to much larger meteoroids or even to asteroid-sized objects. It has been suggested, in fact, that, in addition to comet 2P/Encke, other kilometer-sized objects, asteroids and comets, have orbits very similar to those of meteoroids which produce the Taurid meteor shower (Napier, 1980; Seargent, 2017). All these large and small bodies are collectively called Taurid Complex (TC). According to Clube and Napier (1984) this complex could be the result of the fragmentation of a large comet (with a size of about 100 kilometer), occurred 10,000-20,000 years ago.

Procedure and results
In our work, as a first step, the belonging to the TC of a large number of candidates, proposed in the literature by various authors, has been checked, using the often adopted modified D criterion (Asher, Clube and Steel, 1993). This criterion uses the D parameter which measures the deviations of the orbital parameters a (semi-major axis), e (eccentricity) and i (inclination) with respect to the reference ones, representing the average of the orbital parameters of the various meteor streams belonging to the complex (Steel, Asher and Clube, 1991). Note that the longitude of perihelion does not enter in the definition of D for the reasons reported by Asher, Clube and Steel (1993).
Adopting a threshold value D_t = 0.25 for the membership to the TC (Ferrin and Orofino, 2021), we have found that 88, out of the proposed 141 objects, satisfy the modified D criterion.

It is important to note that the average values of a, e and i of the 88 selected objects show not entirely negligible differences compared to the average values of the meteor streams belonging to the TC. This is not surprising, however, since the TC is composed by two components (see the Introduction): a first one, made up by the meteor showers particles, and a “background” of larger objects (asteroids or comets). The former are groups of very small bodies (meteroids and micrometeoroids) whose motion, precisely due to their small sizes, is affected not only by gravitational forces but also by non-gravitational ones (due i.e. to Pointing-Robertson effect or to radiation pressure) that heavily perturb the Keplerian motion of these particles so much that they are often very quickly removed from their original orbits. The objects of the second component, instead, have sizes large enough that their orbits are unaffected by non-gravitational forces and are only affected by the gravitational perturbations of Jupiter. As a consequence, the dynamic behaviors of the two components are not exactly the same.

Among the objects belonging to the TC there are four recognized comets (2P/Encke, 169P/NEAT, P/2003 T12 and D/1766 G1 Helfenzrieder) along with a number of asteroids with sizes more than a kilometer (such as Hephaistos, Oljiato, Cuno, Mithra). In particular, it has been found that asteroid Hephaistos and comet 169P/NEAT have their own sub-group inside the TC (see Figure 1).
According to our results, another important member of the complex was very probably also the small asteroid that exploded in the terrestrial atmosphere over Tunguska in 1908.

Figure 1.  Polar diagram showing the distribution of the longitude of perihelion of the orbit of the TC members. The objects are placed at slightly different distances to the center, in order to see the different groups distinctly. This diagram shows the close relationship between 2P/Encke and asteroid Hephaistos.

 

As a second step the cometary nature of the objects listed as asteroids has been analyzed, searching for photometric traces of activity, especially if close to perihelion. This has been accomplished using the Secular Light Curve (SLC) formalism (see Ferrin and Orofino, 2021, and references therein): out of the 88 members of TC, 51 have useful SLCs and 34 of these (67%) exhibit cometary activity and thus are activated asteroids.

Our results give a photometric support to the hypothesis that the objects of the TC are related to comet 2P/Encke and that the disintegration event postulated by Clube and Napier (1984) really took place.

The hypothesized scenario starts with a large comet (the parent body) of minimum size of about 30 km. This lower limit has been evaluated starting from the present total volume of the TC objects and extrapolating backward this value, by means of a simple model of mass loss due to the cometary activity. Because of fragmentation, the real original diameter of the parent body was greater than 30 km, reaching 120 km (see Ferrin and Orofino, 2021, for details).

A heterogeneous structure of the original body, similar to a rubble pile (that is made by elementary, rocky or carbonaceous blocks, held together inside an icy matrix and surrounded by a crust of silicatic or carbonaceous/organic grains) can explain why objects members of the TC, like Oljato, show a cometary activity (and so an important ice content) coupled with a mostly silicatic superficial composition, suggested by the spectral analysis. 

Conclusions
Our work highlights the importance of the study of the objects of the TC, due to the risk of collision with the Earth. Understanding their nature and their characteristics is, in fact, necessary for the choice of the strategies of planetary defense against these dangerous objects.

References
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Clube, S.V.M., Napier, W. M. (1984). MNRAS 211, 953-968.
Ferrin, I., Orofino, V. (2021). PSS, submitted.
Napier, W.M. (2010). MNRAS 405, 1901-1906.
Seargent, D.A.J. (2017).  Weird comets and asteroids. Springer. Cham, Switzerland.
Steel, D. I.,  Asher, D. J., Clube, S. V. M., (1991). MNRAS 251, 632-648.