Low dispersion spectra of lunar impact flashes in 2018 Geminids

Lunar impact flashes have been observed at collisions of meteoroids against the non-sunlit lunar surface at speeds exceeding 10 km s^-1. We detected 13 flash candidates between 6.2 and 9.9 in R-magnitude on December 15, 2018 during the Geminids meteor activity. Two or three observatories confirmed eleven of them. We obtained their spectra in the wavelength range between 400 and 870 nm. They are continuous and red, with best-fitted single blackbody spectra indicating the temperatures of about 2000-4000 K. The temperatures for a few successive movie frames at 16 ms or 25 ms intervals decrease with time. Incandescent ejecta, consisting of melt droplets or dust, and the radiant floor of an impact crater could be the source of these flashes, except for the initial stages. At the beginning of some flashes, we found an excess of fluxes at short wavelengths of less than about 600 nm. The composites of two blackbody spectra may fit the spectra better where their temperatures are about 2000 K and 6000 K. The contribution of a high-temperature vapor plume, generated at the very beginnings of the impact phenomena, could be important.