New UCDs for VOEvent please

[Patricio F. Ortiz]

Hi,

I didn't want to jump into this discussion, but what the heck :-)

OK, I'm concerned about the usage of this new proposal. UCDs were born to
identify quantities listed in tables (essentially a way to let automated
software recognize the nature of the columns in such table). We left out
any attempt to include object types, after all, a magnitude is something
which applies to a planet, star, galaxy, QSO, whatever, and so with many
things like spectral features.

It is obvious that in the context of the VO, we would like to have some
way to attach a label to the kind of object we are talking about. And the
proposed schemes sound doable, not easy, not 100% reliable, but workable.

My questions to anyone interested are:

"how do you envision using these new UODs (Unified Object-type
Descriptor)?"  By that, I mean, in which part of the metadata would you
like to insert theses descriptors?

Catalogues like USNO, 2mass, etc, contain a mixture of objects, and even
homogeneous catalogues (like galaxies catalogues) will contain several
flavours of galaxies, hence my concern.

An object type may not be unique... How do we handle those cases? (after
all, these are flags/labels to make our life easier. If the core of a
galaxy is an AGN, what does the descriptor look like?
UCDs of physical quantities are much easier to handle in that sense! (as
noted below regarding the unicity of a UCD).

How deep do we want to describe an object? Say that the same AGN has
exhibited variations in HBeta flux (ok, kick for the incorrectness :-) :-)
Do we want to describe such variability? For some people/applications it
will be important, for others not.

Although Frederic says that a SN will always be a SN, sure, but what
about before? If Eta Car went SN next month, how would we label it?

Furthermore, objects may not change in human-lifetime scales, but
our perception can... I recall in the mid 80's that the core of the
Tarantula Nebula (R136??) was thought to be a super-massive star, only to
be discovered later, using instrument with higher angular resolution
that it was a compact star cluster... I'm nearly sure there are other
examples floating around.

For the historians, as Frederic says, should we think of attaching a
time stamp to any UOD/object pair? After all, if tomorrow we have to
change it, should we erase the previous knowledge?

Just my 2 pences :-)

Patricio


On Wed, 27 Apr 2005, Frederic V. "Rick" Hessman wrote:
> > I'm becoming more skeptical that IVOA standard UCDs are appropriate  
> > for representing the often slippery nature of astronomical "processes"  
> > and "objects".  The question is whether the characterization of  
> > astronomical objects/processes represents "metadata".  The whole point  
> > of the precision of a UCD specification (e.g.,  
> > phot.flux;em.optical;meas.error;stat.max) is to provide a solid  
> > foundation for building a sound scientific argument - but such an  
> > argument typically results in drawing new insights and building new  
> > logical connections - often using new vocabulary.  On the other hand,  
> > even the most well established nomenclature regarding astronomical  
> > objects and processes is subject to revision and extension as better  
> > data and more profound theory collide.
> 
> Ah, but the point of UCDs is not to let computers write papers but to  
> be able to let computers organize and re-organize information using  
> terms which we can easily formulate/manipulate.  At a pretty trivial  
> level, it can be something as simple as saying what the point of an  
> observation is/was (the reason why a target classification is needed in  
> RTML) or to describe what is roughly happening so that a computer can  
> respond to it (the reason it's needed in VOEvent), but a broader use  
> within the VO is obvious. While we're all spoiled by the services  
> provided by Simbad, resolving astronomical names and querying  
> catalogues won't permit us to combine information to the fullest unless  
> we can express what it is we have and what we want in a fashion which  
> goes far beyond the present IVOA/UCD (crafted really only for VOTable).
> 
> Thus, RTML and VOEvent would certainly get by using their own little  
> classification lists, but the adoption of a more generally used UCD for  
> astronomical objects and processes is bound to become necessary.   The  
> earlier we start discussing it, the faster we will be able to profit  
> from it.
> 
> Fortunately, a supernova will always remain a supernova and a spiral  
> galaxy won't loose it's spirals within our lifetime, so the evolution  
> of UCD's will be in the direction of the addition of new terms - as  
> they become needed - and the passive abandonment of old UCD's which no  
> longer serve any but a purely historical purpose -  well, we should let  
> historians of science keep their access to old terms (they may actually  
> want to know what a W UMa star was/is ;-)
> 
> > 	3. Only a single UCD should be appropriate for a given concept.
> > 	4. UCDs should be complete, describing all concepts of interest.
> 
> Rots of ruck!  A UCD which attempts (!) to describe what it is we do  
> will never be complete and unambiguous.  The point is not that it can  
> be perfect but that it exists at all.
> 
> > However much effort is expended to constrain and mandate standard  
> > usage, the users will seek ways to subvert the dominant paradigm.   
> > This isn't just the nature of users, it is the nature of science as  
> > poetry.  Careful support for alternate usage is the key to controlling  
> > anarchy.
> 
> Frankly, I imagine that most users won't even see it.   We're doing  
> this to help stupid computers become more useful and not to constrain  
> the science of our colleagues.
> 
> Rick

---
Patricio F. Ortiz			pfo at star.le.ac.uk
Department of Physics & Astronomy	Phone: +44 (0)116 252 2015
University of Leicester			
Leicester, LE1 7RH, UK