Astronomy Domine is one of Syds tunes from the early days.
Right.
I'm not sure what the story is with Ummagumma, could be a number of different scenarios, musically and otherwise. I don't believe Syd was credited with anything after the first 2 records.
I might be answering a question you might not be asking, if so, thanks for understanding:
The version of "Domine" on Ummagumma was recorded live in April '69, a period when Gilmour and Barrett were particularly close in that Gilmour lived almost next door to Syd and was helping him with his first solo project:
"The Jones-produced sessions started in April 1969 at EMI Studios. After the first of these sessions, Barrett brought in friends to help out: Humble Pie drummer, Jerry Shirley and Jokers Wild (Gilmour's old band) drummer, Willie Wilson. For the sessions, Gilmour played bass.". Also Syd had a history of simply wandering around stage only playing when he felt like it, as far back as late '67 early '68:
"Sometime around Christmas, David Gilmour (Barrett's old school friend) was asked to join the band as a second guitarist to cover for Barrett, whose erratic behaviour prevented him from performing, with the idea of retaining a five-member line-up of the band. For a handful of shows Gilmour played and sang while Barrett wandered around on stage, occasionally deciding to join in playing.". So even though he'd "officially" been out of the band for about a year I still kinda wonder if he "happened to be there" and was "allowed (or even invited) to sit in" on what was after all his own composition (he is credited on Ummagumma). At the time, crediting un-contracted artist performances on records was extremely problematic, it's why Clapton doesn't get a credit for "While My Guitar..." and George doesn't get a credit (in his own name) on "Badge". And that's assuming Syd's presence (if true) was even noted on the recording notes....
Didn't mean to imply that the band were "ripping him off", and didn't even realize Gilmour and Barrett were that close 'til you mentioned it and I did a bit of research. Seeing the timeline only further strengthened my 45-year old suspicions.
Quotes are from Wiki's Barrett page:
http://en.wikipedia.org/wiki/Syd_Barrett