Rotation state of 495 Eulalia and its implication

¹ Institute of Astronomy, Charles University, V Holešovičkách 2, 180 00 Prague 8, Czech Republic
e-mail: vokrouhl@cesnet.cz
² Astronomical Institute, Academy of Sciences of the Czech Republic, Fričova 1, 251 65 Ondřejov, Czech Republic
³ Blue Mountains Observatory, 94 Rawson Pde, Leura, NSW 2780, Australia
⁴ Palmer Divide Observatory/Space Science Institute, Colorado Springs, CO 80908, USA
⁵ Southwest Research Institute, 1050 Walnut St, Suite 300, Boulder, CO 80302, USA

Received: 13 July 2015
Accepted: 14 October 2015

Abstract

Context. The low-albedo part of the Nysa-Polana-Hertha asteroid complex has recently been found to consist of at least two families. The larger of them has been associated with asteroid 495 Eulalia, hereafter named the Eulalia family. The unstable location of this body very close to Jupiter’s 3:1 mean motion resonance (J3/1 resonance) at the periphery of the associated family in the space of proper orbital elements makes this case peculiar.

Aims. We consider the possibility that 495 Eulalia was originally positioned farther from the J3/1 resonance when the family formed via a catastrophic impact than it is today. It was then transported to its current orbit by the Yarkovsky thermal forces over hundreds of millions of years. This requires that 495 Eulalia had a prograde rotation state.

Methods. We use photometric observations and lightcurve inversion methods to determine the rotation pole of 495 Eulalia. Numerical simulation accounting for perturbations from the Yarkovsky effect then reveals the possible pathways of Eulalia orbital evolution.

Results. We find that both of the possible pole solutions are prograde, in accordance with our initial hypothesis. In studying the long-term evolution of Eulalia’s spin state, we show that the obliquity can oscillate over a large interval of values yet always remain <90°. We estimate that Eulalia could have migrated by as much as ~0.007 au toward the J3/1 resonance within the past 1 Gyr. Our numerical runs show that it could have originated in the orbital zone well aligned with other family members in proper eccentricity, whichafter it gained its current orbit by chaotic evolution along the J3/1 resonance.