list of valid characters in object names

Your question is a little ambiguous.  All characters are allowed in object
names, but the parser will only recognize some of them if they are quoted in
backticks.

The ones it recognizes without the backticks are ones that the C isalnum()
(or iswalnum()) function declares to be alphanumeric, plus . and _.  Your
list above are all allowed, but locales are allowed to declare other
characters to be alpha.  I wouldn't recommend using anything else, because
your code won't be recognized in other locales.

The definitive reference for this is the source code, specifically
src/main/gram.y.  There's some discussion in the Intro to R manual, section
1.8.