Compositional Study of 51 Nemausa: A Possible Carbonaceous Chondrite-like Asteroid

Introduction: 51 Nemausa is a main-belt asteroid located at ~2.4 AU, with an IRAS diameter of ~150km, and a geometric albedo (pv) of 0.062 [1]. The asteroid has been classified taxonomically as a C-type asteroid with an unusual spectrum by [2] and a Ch-type by [3]. The C-type asteroids, which make up ~50% of the main belt asteroids, have low geometric albedos (pv < 0.065) [4] due to the possible presence of opaque phases on the surface. These asteroids are thought to be the parent bodies of carbonaceous chondrite meteorites. Carbonaceous chondrites, specifically CI1 and CM2 [5], contain hydrated minerals, such as Febearing phyllosilicates, which form at low temperatures [6]. The Ch-type asteroids have a ~0.70-μm absorption feature which suggests a hydrated mineral phase [3], likely by iron-bearing phyllosilicates [7], which have been found in carbonaceous chondrites. Several visible spectroscopic studies of 51 Nemausa have yielded the same ~0.70-μm absorption feature [8,9,10,11] which supports the idea of hydration of the asteroid’s surface. Though there have been several visible-wavelength spectroscopic studies done on 51 Nemausa, only a couple near-IR studies have been conducted to date. One study [12] shows a weak ~0.90-μm absorption feature which might be due to the presence of hydrated minerals [5]. Observations and Data Reduction: 51 Nemausa was remotely observed by V. Reddy on September 21, 2008 using the SpeX near-infrared spectrograph [13] at the NASA Infrared Telescope Facility on Mauna Kea, Hawai’i. The SpeX data was reduced using the Unixbased Image Reduction Analysis Facility and the Windows-based SpecPR spectral processing program [14]. SpecPR was also used to calculate the Band I center and area. Results: The near-IR spectrum of 51 Nemausa (Fig. 1) shows a very weak (~1-2%) Band I absorption feature with a band center of 0.94-μm ± 0.05-μm (Fig. 2). This correlates with the weak ~0.90-μm features found in the visible spectrum [3] (Fig. 3) and near-IR spectrum [12] of 51 Nemausa. Figure 4 shows the SMASS II visible spectrum normalized to the near-IR spectrum of 51 Nemausa. Both spectra (Figs. 1 and 2) show the ~0.9-μm absorption feature. There are also two additional, weak absorption features near ~1.9-μm and longward of ~2.1-μm which are possibly due to the presence of hydrated minerals. Figure 1. Near-infrared spectrum of 51 Nemausa: average of 14 spectra on September 21, 2008.