On the V-type asteroids outside the Vesta family. I. Interplay of nonlinear secular resonances and the Yarkovsky effect: the cases of 956 Elisa and 809 Lundia
Among the largest objects in the main belt, asteroid 4 Vesta is unique in showing a basaltic crust. It is also the biggest member of the Vesta family, which is supposed to originate from a large cratering event about 1 Gyr ago (Marzari et al. 1996, A&A, 316, 248). Most of the members of the Vesta family for which a spectral classification is available show a V-type spectra. Due to their characteristic infrared spectrum, V-type asteroids are easily distinguished. Before the discovery of 1459 Magnya (Lazzaro et al. 2000, Science, 288, 2033) and of several V-type NEA (Xu et al. 1995, Icarus, 115, 1), all the known V-type asteroids were members of the Vesta family. Recently two V-type asteroids, 809 Lundia and 956 Elisa, (Florczak et al. 2002, Icarus, 159, 178) have been discovered well outside the limits of the family, near the Flora family. We currently know 22 V-type asteroids outside the family, in the inner asteroid belt (see Table 2). In this work we investigate the possibility that these objects are former family members that migrated to their current positions via the interplay of Yarkovsky effect and nonlinear secular resonances. The main dynamical feature of 956 Elisa and 809 Lundia is that they are currently inside the 2(g-g6)+s-s6 (z2 by Milani & Knežević 1994, Icarus, 107, 219) secular resonance. Our investigations show that members of the Vesta dynamical family may drift in three-body and weak secular resonances until they are captured in the strong z2 secular resonance. Only asteroids with diameters larger than 16 km can remain in one of the three-body or secular resonances long enough to reach the region of the z2 resonance. This two-step mechanism of capture into the z2 resonance could explain: i) the current resonant orbits of 956 Elisa and 809 Lundia; ii) why their size is significantly larger than that of the typical member of the Vesta family; and iii) provide a lower limit on the Vesta family age. We believe that other V-type asteroids could have followed the same path, and could currently be inside the z2 resonance. In an forthcoming article of this series we will investigate the role that other mechanisms of dynamical mobility, such as close encounters with massive asteroids, may have played in causing the current orbital distribution of the remaining 20 other V-type asteroids.