Defining the Flora Family: Reflectance Properties and Age

The Flora family resides in the densely populated inner main belt, bounded in semimajor axis by the ν₆ secular resonance and the Jupiter 3:1 mean motion resonance. The presence of several large families that overlap dynamically with the Floras (e.g. the Vesta, Baptistina, and Nysa-Polana families), and the removal of a significant fraction of Floras via the nearby ν₆ resonance have historically complicated the Flora family's distinction in both proper orbital elements and reflectance properties. Here we use orbital information from AstDyS, color information from SDSS, and albedo information from WISE, to obtain the characteristic orbital and reflectance properties of the Floras, by sampling the core of the family in multidimensional phase space. We find the characteristic Flora SDSS colors to be a* = 0.127 ± 0.012 and i-z = -0.038 ± 0.008; the characteristic Flora albedo is p_V = 0.295 ± 0.006. These properties allow us to select a high-purity sample of Floras with similar orbital and reflectance properties as required for a detailed dynamical study. We then use the young Karin family, for which we have an age determined via direct backward integration of members' orbits, to calibrate the Yarkovsky drift rates for the Flora family without having to estimate the Floras' material properties. The size-dependent dispersion of the Flora members in semimajor axis (the "V" plot) then yields an age for the family of 940⁺¹⁶⁰_-120 My. We discuss the effects on our age estimate of two independent processes that both introduce obliquity variations among the family members on short (My) timescales: 1) the capture of Flora members in spin-orbit resonance, and 2) YORP-driven obliquity variation through YORP cycles. Accounting for these effects does not significantly change the age determination.