Spectral quantity in the object's rest frame?

[Markus Demleitner]

Dear Semantics folks,

I have recently had to UCD-annotate a column containing a spectral
quantity (actually, a wavelength, but that is beside the point here)
that was given in the object's rest frame.  Now, the pertinent UCDs
(em.wl, em.wavenumber, em.freq, em.energy) do not actually state the
frame, and undoubtedly for a scientifically complete characterisation
of a spectral column we need a full STC description.

On the other hand, telling "observed/barycentric/similarly
observer-related" from "rest frame/object-related" sounds like
something we can straightforwardly do and that might be useful in
discovery, where we won't have full STC for a long time (and perhaps
shouldn't even have it in the first place).

So... what about having modifiers "pos.obs" (say) and "pos.rest" that
would let people say in roughly which frame a spectral quantity (and
perhaps others; this *might* be relevant for periods, for instance)
is given?  Or can we already do that and I just fail to see how?


Incidentally, while grepping for "rest" in the UCD list, I
encountered this:

S | em.line.OIII                 	|  [OIII] line whose rest wl is 500.7 nm

I like the spectral characterisation here, although "rest wl" is
somewhat too terse for my taste.  So, I think I'd prefer "The [OIII]
line at 500.7 nm".  More to the point, though, I think it would be
nice if we annotated the remaining em.line UCDs with the wavelengths,
too, even though I give you that there is less ambiguity for most of
them.  So, for instance, I would like to have:

S | em.line.Lyalpha              	|  The Lyman alpha line at 121.5 nm

Should I prepare a UCD VEP that would do that?

Also, there is em.line.CO, which is different from all the others in
that it refers not to a specific line but a whole set of them.  The
current description is:

  CO radio line,  e.g 12CO(1-0) at 115GHz

I think we should stress that this is rather different from the
others and say:

  Any radio line of CO

or something like that.

I also think we should at least briefly think about whether this
is a pattern we want to avoid in the future (i.e., have em.line.X
refer to exactly one line, modulo fine structure and all that), or
whether "some line of element/molecule X" is actually something we
want to have UCD atoms for.  I see arguments in favour and against
both stances, so perhaps we should just leave it open.  Still, it
does not feel quite right to me that both kinds of things sit next to
each other.  I a way, em.line*s*.CO would feel less arbitrary.

Thanks,

            Markus