New UCDs for VOEvent please

[Rob Seaman]

On Apr 26, 2005, at 3:28 AM, Rick Hessman wrote:

> I've added more VOEvent examples, several suggestions for IVOA/UCD to  
> the proposed
> object/event/process UCD list at
>
> 	http://monet.uni-sw.gwdg.de/twiki/bin/view/VOEvent/ 
> UnifiedContentDescriptors

Good start at a list.  A search facility would add immensely to its  
usefulness as the list grows.  This would also function as a validator  
for an astronomer's guess as to the proper syntax for a familiar class  
of objects.

> If something like this list has already been created, I'd appreciate  
> hearing about
> it before I put much more work into our own.

This would be a good opportunity for VO to reach out to the larger  
community.  It really isn't for us to mandate nomenclature.  An  
interactive dictionary/beastiary of astronomical objects and processes  
could be useful all by itself, and will be of obvious utility for  
various VO projects.  A common understanding of each "hypothesis" will  
indeed be key to VOEvent, for instance.

I'm not sure, however, that the ApJ keywords are the best model for  
constructing this content descriptor list.  Keywords are a way to  
partition the level of interest a member of the community may have in  
reading a paper.  A UCD is a way of partitioning the underlying  
physics.  For instance, given that the interest in working on this is  
arising out of VOEvent, it isn't surprising that there is a focus on  
processes more than objects, and that the selection of objects would be  
weighted toward time variability in various ways.  There are dozens of  
fine shadings between variable stars.  On the other hand, while one can  
distinguish between elliptical, irregular and spiral galaxies, there  
isn't any mechanism (yet) for specifying the precise type of each.

I'm leery of encouraging these descriptors to become longer than they  
are, but it seems inappropriate to specify generally descriptive  
classes, like:

	stars.variable.irregular
	stars.variable.long_period
	stars.variable.semi-regular

at the same level as members of prototypical classes, like:

	stars.variable.Cepheid
	stars.variable.RR_Lyr
	stars.variable.RS_CVn

(And where is stars.variable.W_UMa?  Just pointing out that the list  
will never be complete.)

Perhaps it should be stars.variable.class.Cepheid?  Or better yet,  
adjust in the other direction, maybe stars.variable.period.irregular?   
Or ideally, a general mechanism might be provided for parametrically  
classifying periodicity.

The challenge here is not only that the list will never be complete, it  
is that we should be encouraging researchers to actively augment and  
improve the list.  A workable classification scheme is often the first  
step in organizing a research program.  But the result of a research  
program is often to overturn the original classification scheme.  We  
don't want to provide a mechanism that is only useful for describing  
objects far removed from the cutting edge.

Rob Seaman
NOAO
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