"The work of known authors can be folk"
Only if it is taken over by the community, adapted and changed to the extent that it appears in distinct versions and is no longer recognised as the work of an individual author.
"The term does not cover composed popular music that has been taken over ready-made by a community and remains unchanged, for it is the re-fashioning and re-creation of the music by the community that gives it its folk character"
IFMC 1954.

More later,
Jim Carroll