Active asteroid (6478) Gault: a blue Q-type surface below the dust?

Abstract

We present near-infrared spectroscopy of the sporadically active asteroid (6478) Gault collected on the 3 m NASA/Infrared Telescope Facility observatory in late 2019 March/early April. Long-exposure imaging with the 0.5 m NEEMO T05 telescope and previously published data simultaneously monitored the asteroid activity, providing context for our measurements. We confirm Gault is a silicate-rich (Q- or S-type) object likely linked to the (25) Phocaea collisional family. The asteroid exhibits substantial spectral variability over the 0.75-2.45 $\mu$m wavelength range, from unusual blue (s'=-13.5+/-1.1% $\mu$m-1 to typical red (s'=+9.1+/-1.2% $\mu$m-1) spectral slope, that does not seem to correlate with activity. Spectral comparisons with samples of ordinary chondrite meteorites suggest that the blue color relates to the partial loss of the asteroid dust regolith, exposing a fresh, dust-free material at its surface. The existence of asteroids rotating close to rotational break-up limit and having similar spectral properties as Gault further supports this interpretation. Future spectroscopic observations of Gault, when the tails dissipate, will help further testing of our proposed hypothesis.

8 pages, 5 figures, 2 tables, accepted for publication in ApJL

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• The Lingering Death of Periodic Near-Sun Comet 323P/SOHO
• Spinning and color properties of the active asteroid (6478) Gault
• Broadband photometry of asteroid 6478 Gault: activity and morphology
• A Fireball and Potentially Hazardous Binary Near-Earth Asteroid (164121) 2003 YT${}_1$
• The appearance of a 'fresh' surface on 596 Scheila as a consequence of the 2010 impact event