Arcastar is the Quenya rendition of the surname Tolkien (German tollkühn
"foolhardy, reckless, desperately brave, rushbold")
He signed as Arcastar Mondósaresse "Tolkien in Oxford" in a 1968 letter to the producer of the BBC documentary.
The first element seems to be related to the word arauca "swift, rushed" or from a related word of the same root (A-RAK).
The second element must be from stem STAR "stiff, hard"