[vsnet-be 49] Re: Theta CrB Comparison Stars

[Thom Gandet <tgandet@mindspring.com>]

Erhan and Taichi,

   Thank you both for answering my question about Theta CrB.  You've
saved me much searching around, solved a minor mystery, and provided
a basis for using common comparison stars.   I'm also glad to know the
names that go with your initials.

   I used beta CrB and gamma Crb one night, when I made an observation
on the spur of the moment waiting for the CCD camera to cool down.  I
didn't know then that both are variable, ~0.05 mag. each; if one is at
maximum and one at minimum, there could be up to about a 0.1 mag error
in the estimate for Theta CrB, so at least one of those had to go.

    Looking Theta up in the VSNET archives I noticed an apparent zero-
point difference between your two sets of observations, of about 0.3
mag, which I, too, attributed to different comparison stars.  The
problem may be with Eps CrB, which is quite red.

   Be stars are so difficult to find good comparison stars for.

   Eps Her is good find - it's really the only star brighter than
Theta CrB that doesn't seem to be variable at some level.  It's some
distance from Theta CrB so differential extinction may contribute to
the zero-point difference, especially if the stars are observed at
larger zenith distances.  I can let you know something about what
that effect might be later today.  (Extinction effects may be
involved in Iot Ser also; it also seems an excellent find for the
faint end.)

   I tried using Chi Boo (=HR5676  V=5.26  B-V=+0.03) once.  It is
close and the right color, but it was too faint to make very
reliable estimates I thought.

   Beta CrB is, as Taichi pointed out, a good star to use to look
for major outbursts.  Many Be stars have small, rapid variations
that may be due to either intrinsic causes or to binarity; these
variations can be from 0.05 to 0.1 magnitude.  The bright ones are
too bright for effective CCD photometry and not many people seem
to do PEP on them.  I'm interested in developing methods of visual
magnitude estimation that would be of higher precision than current
methods.  The purpose there is to reduce the scatter and zero-point
differences to a bare minimum so that they may complement CCD and
PEP observations and fill in the gaps.

   May we all have good observing weather!


Cordially,
Thom Gandet
   

> Dear Thom
> My comparison stars are for Theta CrB:
> (Bright Star Catalog Mags)
> HR 5842 HD140159  21 Iot  Ser mV: 4.52 b-v: 0.04
> HR 5947 HD143107  13 Eps  CrB mV: 4.15 b-v: 1.23
> HR 6324 HD153808  58 Eps  Her mV: 3.92 b-v:-0.01
> 
> But there is a 0.3 or 0.4 mag difference between my and Mr. Kato's 
> observations.This may be because of comparison stars.
> 
> Thanks,
> 
> Erhan Eker


Taichi Kato wrote:
> 
> Re: Theta CrB Comparison Stars
> 
> > I see you both have been observing Theta CrB.  What comparison stars
> > have you been using and what magnitudes for the?  I want to add it to
> > my program of Be stars and want to use the same comparison stars that
> > have been used in the past.
> 
>    I have been using beta CrB with a magnitude of 3.6 (based on somewhat
> old reference, but the deviation from recent measures is about 0.1 mag).
> Beta CrB is a low-amplitude ACV-type variable, so it will not be an adequate
> comparison star for photoelectric photometry, but it looks suitable for
> visual monitoring for a possible major outburst of theta CrB.
> 
> Regards,
> Taichi Kato

-- 
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                      Lizard Hollow Observatory
                      Thomas L. Gandet, Director
                            PO Box 77021                   
                      Tucson, AZ 85703-7021  USA
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