Determination of the near-UV absorption in the spectra of primitive asteroids

C-complex asteroids are dark, low-albedo objects, mainly containing carbon components. Their spectra are usually featureless, especially in the visible-near-infrared (VNIR) wavelengths. However, for some taxonomic types, absorption has been found in the near-UV (NUV) range. Laboratory studies have shown similarities to spectra of chondrite meteorites, demonstrating that this absorption is a result of charge transfer involving iron ions (Fe2+ and Fe3+)[9,10,14], particularly in hydrated silicates and clay minerals [6]. These minerals exhibit strong NUV absorption, even in low-albedo asteroids. This option can be used to detect the presence of hydrated minerals (and hence water) and organic minerals on C-complex asteroids. Detecting hydrated minerals on asteroids is crucial for understanding their history and the processes that have shaped them. Moreover, spectroscopy of the UV spectrum can show a link to space weathering [7] - the process by which the surfaces of asteroids are altered by exposure to solar radiation and micrometeoroid impacts. Observations can help characterize the effects of space weathering on asteroid surfaces, providing insights into their age and history.

Our spectroscopic studies of C-complex asteroids are performed with the 10-m Southern African Large Telescope (SALT), located in Sutherland, South Africa. It is equipped, among all, with the RSS spectrograph, which we used to get asteroid spectra with the resolution of R=800. One of the unique characteristics of SALT is its high throughput in the NUV region, caused by the NaCl correction lens. In November 2023 we started observations of primitive asteroids with SALT, and during the conference, we will present spectra of several of them, obtained in the 0.32-0.9 μm range. Additionally, we will discuss the observational strategy, including searching for solar analogue stars in the NUV range in the southern hemisphere.

Figure 1. Reflectance spectrum of asteroid (1101) Clematis obtained with SALT on 20th Dec 2023.  The original asteroid spectrum was divided by the spectrum of a solar analogue star BD+002717 (confirmed by [11] to be almost identical to the Sun in NUV range).  The result has been normalized to 1 at the 0.55 μm wavelength.

Acknowledgments: All observations reported in this abstract were obtained with the Southern African Large Telescope (SALT). This work has been done under the SALT programs 2023-2-SCI-025 and 2024-1-SCI-012 (PI: T.  Kwiatkowski). Polish participation in SALT is funded by grant No. MEiN nr 2021/WK/01

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