[vsnet-chat 4627] Re: Photometry of variables involved with other stars

[aah@nofs.navy.mil]

Gianluca asked about measuring variable stars
in the presence of close companions.
Nogami-san asked me during the last campaign to measure
the magnitudes of the V446 Her grouping; I reported those
to vsnet-chat.  There are actually 4 stars involved.
I intend to write a short IBVS on these stars, but just
haven't finished it yet.
  The best method is to psf-fit the group, but I've
found that psf-fitting crowded images works best when
you have decent signal/noise, not likely on faint stars
like the companions to this DN.
  The next best method is to use very small apertures
such that the companions are excluded.  This only works
if you have decent pixel scale and good seeing.
  A third method is the image subtraction techniques
pioneered by Alard and his colleagues.  This is a complex
technique but has resulted in some pretty fantastic results.
  The final method is to measure the combined grouping
with a bigger aperture and then subtract out the contributions
of the 3 companions.  This has to be done in intensity space
and not magnitude space, but is possible.  A good example
is Smak J. 1966, Acta Astronomica 16,11, where he deconvolved
the close cepheid binary CEab Cas.  I've observed the V446 Her
grouping often enough to say that the other stars are constant.
The difficulty will arise in trying this with unfiltered measures
since you don't know the contribution of each group member.  If
you do it in one of the standard BVRI filters, you can use my
determinations of the group members.
Arne

VSNET Home Page

Return to Daisaku Nogami

---

vsnet-adm@kusastro.kyoto-u.ac.jp