Portrait of the Polana-Eulalia family complex: Surface homogeneity revealed from near-infrared spectroscopy

The inner asteroid belt is an important source of near-Earth asteroids (NEAs). Dynamical studies of the inner asteroid belt have identified several families overlapping in proper orbital elements, including the Polana and Eulalia families that contain a large fraction of the low-albedo asteroids in this region.

We present results from two coordinated observational campaigns to characterize this region through near-infrared (NIR) spectroscopy. These campaigns ran from August 2012 to May 2014 and used the NASA Infrared Telescope Facility and the Telescopio Nazionale Galileo. The observations focused on objects within these families or in the background, with low albedo (p_v ≤ 0.1) and low inclination (i_P ≤ 7°). We observed 63 asteroids (57 never before observed in the NIR): 61 low-albedo objects and two interlopers, both compatible with S- or E- taxonomical types.

We found our sample to be spectrally homogeneous in the NIR. The sample shows a continuum of neutral to moderately-red concave-up spectra, very similar within the uncertainties. Only one object in the sample, asteroid (3429) Chuvaev, has a blue spectrum, with a slope (S^‧ = - 1.33 ± 0.21%/1000 Å) significantly different from the average spectrum (S^‧ = 0.68 ± 0.68%/1000 Å). This spectral homogeneity is independent of membership in families or the background population. Furthermore, we show that the Eulalia and Polana families cannot be distinguished using NIR data. We also searched for rotational variability on the surface of (495) Eulalia which we do not detect. (495) Eulalia shows a red concave-up spectrum with an average slope S^‧ = 0.91 ± 0.60%/1000 Å, very similar to the average slope of our sample.

The spectra of two targets of sample-return missions, (101955) Bennu, target of NASA's OSIRIS-Rex and (162173) 1999 JU₃ target of the Japanese Space Agency's Hayabusa-2, are very similar to our average spectrum, which would be compatible with an origin in this region of the inner belt.