ULTRAVIOLET SPECTRAL REFLECTANCE OF 16 ASTEROIDS BY SWIFT

Remote spectroscopy plays an important role in studying the surface physics of asteroids. Compared with longer wavelengths, the ultraviolet (UV) waveband has been demonstrated to be able to be more sensitive to mineral properties, grain sizes, and space weathering effects by laboratory measurements, space missions, and remote observations [Butterworth et al. 1985, Cloutis et al. 2008, Becker et al. 2020]. However, although it has been shown that UV spectra can be extremely useful in the compositional analysis of planetary surfaces [Cloutis et al., 2008], only the IUE dataset from more than a decade ago and a few scattered UV observations exist for asteroids’ UV dataset. Instrumental noises of IUE render the spectra only reliable between 240nm and 320nm, which leaves many observational gaps in near-UV waveband.

Besides HST, the Neil Gehrels-Swift observatory is currently the only in-orbit facility that has the UV capability to measure asteroids’ UV spectra. Swift’s UV grism has an effective wavelength range from 170nm to 500nm for the first order, which is optimized around 260nm. From 2010 to 2012, we observed 16 asteroids with Swift’s UV grism. To more widely compare with general observations such as optical and infrared spectra, the 16 asteroids were selected to variously cover the broadly used taxonomy (cf. Bus 2002). To study rotation effects, every asteroid was exposed for five or six times with an interval from about 20min to 100min.

We have developed a program with the UVOTPY software [Kuin et al. 2015] to analyze the data. The extracted UVOT UV spectra reliably covered the waveband between 200nm and 500nm, which well fill the observational gaps, fitly link near UV observations to visible observations, and therefore will benefit comparative study between different wavebands. In the range of 300-400nm, there are significantly more laboratory studies of meteorites and planetary analog materials at wavelengths >300 nm in the RELAB spectral database that can be used to better constrain asteroids’ surface composition. With the observed flux spectra, we derived lightcurves to study asteroids’ rotation effects, and reflectance spectra to characterize the surface conditions.