Redetermination of the space weathering rate using spectra of Iannini asteroid family members

We have obtained moderate S/N (∼85) spectra at a realized resolution of R∼100R∼100 for 11 members of the Iannini family, until recently the youngest known family at under 5 million years of age [Nesvorný, D., Bottke, W.F., Levison, H.F., Dones, L., 2003. Astrophys. J. 591, 486–497, 720–771]. The spectra were acquired using the Echellette Spectrograph and Imager in its low-resolution prism mode on the Keck II telescope. The family members belong to the S-complex of asteroids with perhaps some K class members. The Iannini family members' average spectral slope, defined as the slope of the best-fit line constrained to pivot about 1 at 550 nm, is (0.30±0.04)/μm(0.30±0.04)/μm, matching the (0.26±0.03)/μm(0.26±0.03)/μm reported by Jedicke et al. [Jedicke, R., Nesvorný, D., Whiteley, R.J., Ivezić, Ž., Jurić, M., 2004. Nature 429, 275–277] using SDSS [Ivezić, Ž., Jurić, M., Lupton, R.H., Tabachnik, S., Quinn, T., 2002. In: Tyson, J.A., Wolff, S. (Eds.), Survey and Other Telescope Technologies and Discoveries. In: Proc. SPIE, vol. 4836. SPIE, Bellingham, pp. 98–103] color photometry. Using our spectra for this family as well as new observations of Karin family members [Vernazza, P., Birlan, M., Rossi, A., Dotto, E., Nesvorný, D., Brunetto, R., Fornasier, S., Fulchignoni, M., Renner, S., 2006. Astron. Astrophys. 460, 945–951] and new classifications of some older families we have revised the space weathering rate of S-complex asteroids originally determined by Jedicke et al. [Jedicke, R., Nesvorný, D., Whiteley, R.J., Ivezić, Ž., Jurić, M., 2004. Nature 429, 275–277]. Following Jedicke et al. [Jedicke, R., Nesvorný, D., Whiteley, R.J., Ivezić, Ž., Jurić, M., 2004. Nature 429, 275–277] we parameterize the space weathering rate of the principal component color of the spectrum (PC1PC1), which is correlated with the spectral slope, as PC1(t)=PC1(0)+ΔPC1[1−exp−α(t/τ)]PC1(t)=PC1(0)+ΔPC1[1−exp−(t/τ)α]. Our revised rate suggests that the characteristic time scale for space weathering is τ=570±220 Myrτ=570±220 Myr and that new S-complex clusters will have an initial color of PC1(0)=0.31±0.04PC1(0)=0.31±0.04. The revised time scale is in better agreement with lab measurements and our measurements support the use of space weathering as a dating method. Under the assumption that all the spectra should be identical, since the members all derive from the same parent body and are presumably covered with similar regolith, we combined them to obtain a high-S/N composite spectrum for the family. The combined spectrum is within the S-complex.