[vsnet-chat 4228] Re: Omega CMa comparison stars

[Thom Gandet <tgandet@mindspring.com>]

   As Fraser remarked, the many variables in southern CMA and nearby
Puppis that make finding bright comparison stars difficult.  I suggest
this comparison star close to Omega CMa be used in addition to 3 Pup:

   HR 2922 = HD60863    7h35m23s  -28º22'10" (2000)
                        V=4.639  B-V=-0.102  (from Tycho-1)  B8V

The other comparison stars that have been suggested are not reliable,
for reasons noted below.

   Delta Col is some distance from Omega CMa and is quite a bit
redder.  The same objection applies to 11 Puppis, which is nearly as
red as Delta Col.

   What is your reference for k Pup being a Be star, Sebastian?  It's
below our effective -20º Dec limit, so it's not being observed here.
I may know it by its HD number.  Neither "Kappa Pup" nor "k Pup"
is in any of the common catalogues; nor does SIMBAD recognize it.
However, "k Pup" is shown as a V=3.8 star in the AAVSO Atlas and it
appears on other atlases.  There are several stars in Puppis with
lower- and upper-case one character common names.  SIMBAD translates
them to upper-case, which accounts for it not recognizing "k Pup".
Could you provide alternate names or a position for k Pup?  Thanks.
 
   While an Ae/Be star may have an historical range of only 0.05
magnitude (or may be only "suspected" to be variable), they should not
be used as comparison stars.  Small "outbursts" are always possible
and may not be noticed; e.g., if the comparison brightens by 0.08
magnitude and not noticed as such, the variable will be reported as
having faded by 0.08 mag.  Ae/Be stars should not be used as
comparison stars especially as comparison stars for other Ae/Be
stars.  (Sort of like using one Mira variable as a comparison star
for another Mira.)

   A supergiant A-star (3 Pup) is likely to be a microvariable, 
especially if it shows emission lines in its spectrum.  It could be
used as long as stars for which no variability has ever been found,
such as HR 2922, are included as checks.  Unfortunately, 3 Pup is the
only potential comparison star that is brighter than Omega CMa and
is also similar in color.  One of those unfortunate, but necessary,
compromises when doing photometry of bright stars. 

Cordially,
Thom Gandet

Fraser Farrell wrote:
> 
> > I'm wondering which stars are being used as comparisons for
> > Omega CMa? Southern CMa seems to be littered with bright
> > low-amplitude variables, so I had considered including this
> > region in BVfSO. But I gave up, frustrated by the apparent
> > shortage of suitable comparison stars.
> >
> > So what comparisons are the various Omega CMa observers
> > using?
> >
> > I had a quick eyeball of Omega CMa last night and earlier
> > tonight, and both times it looked to be of equal magnitude
> > to Delta Col (3.85V).
> >
> >
> 

-- 
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                      Thomas L. Gandet, Director
                            PO Box 77021                   
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