[vsnet-chat 3086] (fwd) eg & GC vars... (Greaves)

[Taichi Kato <tkato>]

(fwd) eg & GC vars... (Greaves)

Date: Wed, 14 Jun 2000 13:49:15 +0000
From: JG <jgts@jgws.totalserve.co.uk>
Subject: eg & GC vars... [chat]

Brian Skiff wrote:

>      In generating charts for LMC fields, beware that both the GSC and
>
> USNO-Ax.x have lots of errors of both in omitting brighter stars,
> showing
> stars where there is none (spurious detections), and in Ax.x having
> magnitudes that are completely erroneous.  Probably neither catalogue
> will
> be suitable for chart purposes.  The current best bet would be the
> recently-issued UCAC.

Useful info from Brian [yet agin!].  I _believe_ Bill Gray is _hoping_
to make UCAC1 data usable from Guide 8, when the latter appears.
_However_, copyright, technical and/or time problems may preclude this.

Suffice to say that people in the southern hemisphere are finally
getting their "revenge", and enjoying more up to date and rigorous data
than us northerners of late ;-)

Taichi Kato wrote :

>    Until very recently, we usually had no way to access to Sawyer Hogg
>
> catalog.  We had to consult many literatures before reaching
> particular
> globular cluster variables.  The present electronic availability of
> Clement's catalog is already a real progress in this field!

whilst Brian Skiff commented :

>     The general lack of accurate coordinates for the variables in
> globulars
> is (in my opinion) a serious lacuna in the literature.  The system of
> ad hoc
> x,y offsets is an anachronism inadequate for modern observing.  I'd
> like
> to suggest therefore that since John has started the work that he
> publish it.  The x,y coord systems do not always have their origin at
> the
> center of the cluster, but instead are sometimes for a specific star
> in the
> field or for a specific RA/Dec (usually to only 1' accuracy!).  This
> varies
> cluster-to-cluster, so has to be determined from Clement's files for
> each
> case.  The lack of coordinates has resulted in papers claiming large
> numbers
> of supposedly new variables in at least one globular found by Space
> Telescope
> folks which in fact were mostly known.  In completing such work, it
> will
> be necessary both to make a large number of cross-IDs and also to
> generate
> new positions for objects with poor or incorrect x,y coordinates---if
> you
> want to do it right.

and both are quite right!

However, I should note that all I actually did was convert arcsec
offsets into decimal degrees and do some simple arithmetic against a
list of Globular Cluster J2000 "central" positions, also converted into
decimal degrees.

More was intended, but I've hit nothing but a plethora of brick walls
since, and the project has gone nowhere rapid.

The Clement's files are just about all I found, and the fact that some
of the systems are centred on a star instead of the globular's centre is
new to me, but certainly explains a thing or two!

[An example of weirdity : M3 V141is the nonmmber RV CVn.  Positions
according to CGVS & GCVS suggests that the Clement/Sawyer-Hogg data may
have nonlinear plate distortion effects as you move from plate centre.
Then again, when I check the field it seems that GCVS position for RV
CVn is within 8 arcsecs of GSC 2004 1159.  These may be different
objects, but we all know that the GCVS positions can be a bit out. Then
I find that ROTSE1 J134018.16+281822.1 is virtually coincident with GSC
2004 1159.  All the magnitudes [14-16] are roughly consistent, given the
differing passbands.  Indeed, the ROTSE1 star is listed as an EW star of
a period that is the same as listed for the EW star RV CVn.  I could
then go on and check Brian's ROTSE1 IDs on VSNET survey, look at survey
plates, etc, and probably finally resolve this one.  And that's just for
a field star away from the gobular centre, and a nonmember to boot!  Oh,
and I didn't mention that it is also listed as galaxy MIG 1804623, of BT
mag 16.6 (which is _total integrated_ blue mag, _not_ _Tycho_ blue
mag)...]

Apparently, even if someone had complete and total access to all the
literature, everywhere, the case would often still be uncertain.  It
seems that authors were not always clear on star identification, and the
written text doesn't always tally with the plates when the latter are
taken from the archives and examined!

Quite a lot of the variables in the CGVS do not even have listed
magnitudes of any kind, just a position.

Accordingly, some professionals are observationally re-assessing some of
these objects with moderately "simple" equipment and facilities, of the
sort some amateurs have access to nowadays, hence a separate email on
this to follow...

...even the listed epochs and periods for some of the cepheids are not
necessarily rigorous, as they are based on scatterings of irregularly
timed plates.


A simple task I could, ought, and probably will, do, is to cross ID ASAS
variables with the GCVS Vol V.  I intend to use the assumption, mostly
well founded, that the SAI GCVS team rigorously checked the positions in
GCVS vol V, and also the assumption that the ASAS astrometry is quite
good, to create a working list good to the 0.001 degree level.

The number of objects preclude anything more systematic on my part, with
regards survey plate checks etc, due to the amount of online time that
that would need.  Anyway, after awhile churlishness sets in on my part,
cos really this should have been the ASAS teams' job in the first
place!!!!!

Cheers

John

John Greaves
UK

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