(208) Lacrimosa: A case that missed the Slivan state?

Abstract

Context.The largest asteroids in the Koronis family (sizes ≥25 km) have very peculiar rotation state properties, with the retrograde- and prograde-rotating objects being distinctly different. A recent re-analysis of observations suggests that one of the asteroids formerly thought to be retrograde-rotating, 208 Lacrimosa, in reality exhibits prograde rotation, yet other properties of this object are discrepant with other members this group.Aims.We seek to understand whether the new spin solution of Lacrimosa invalidates the previously proposed model of the Koronis large members or simply reveals more possibilities for the long-term evolutionary paths, including some that have not yet been explored.Methods.We obtained additional photometric observations of Lacrimosa, and included thermal and occultation data to verify its new spin solution. We also conducted a more detailed theoretical analysis of the long-term spin evolution to understand the discrepancy with respect to the other prograde-rotating large Koronis members.Results.We confirm and substantiate the previously suggested prograde rotation of Lacrimosa. Its spin vector has an ecliptic longitude and latitude of (λ,β) = (15° ± 2°, 67° ± 2°) and a sidereal rotation periodP= 14.085734 ± 0.000007 h. The thermal and occultation data allow us to calibrate a volume equivalent size ofD= 44 ± 2 km of Lacrimosa. The observations also constrain the shape model relatively well. Assuming uniform density, the dynamical ellipticity is Δ = 0.35 ± 0.05. Unlike other large prograde-rotating Koronis members, Lacrimosa spin is not captured in the Slivan state. We propose that Lacrimosa differed from this group in that it had initially slightly larger obliquity and longer rotation period. With those parameters, it jumped over the Slivan state instead of being captured and slowly evolved into the present spin configuration. In the future, it is likely to be captured in the Slivan state corresponding to the proper (instead of forced) mode of the orbital plane precession in the inertial space.