[vsnet-chat 2348] Re: Standard clusters

[bas@mailhost.lowell.edu (Brian Skiff)]

     As you've probably alreaedy noticed, there really isn't a suitable open
cluster in the region you describe unless you get right into the thick of
the Milky Way, where backgrounds will inevitably be quite crowded.
     I'd like to suggest instead either of two globular clusters, especially
the very outer fringes of them.  Two that more-or-less fit the bill are
M68 = NGC 4590 and NGC 5897.  They are centered at:  12 39 28  -26 44.6 and
15 17 25  -21 00.6 (2000), respectively.  Both are nearly unreddened.  M68
of course lies at moderately-high galactic latitude, so the field won't have
that many stars apart from the globular.  Alistair Walker has already pretty
well clobbered this object and created a photometric sequence of secondary-
standard quality (see AJ 108, 555), although I think in just BVI (no R).
ALcaino et al.'s sequence might be examined to find what range of colors are
available among the field stars.  M68 has a total mangitude of V=7.3, and the
tip of giant branch (i.e. the brightest stars) are near V=12.6, only slightly
fainter than M5, for instance.
     NGC 5897 is somewhat fainter at total V=8.4, and tip of the giant branch
at V=13.3.  It lies superposed on the outer parts of the galactic bulge
north of the plane.  My thought here is that the giant-branch tip stars would
provide all the red stars you could want---assuming there's some constant
ones among them!---and that having some moderately-low latitude field stars
makes it likely that some fairly blue A/F stars exist in the same field in 
the desired magnitude range.  Again, Sandage gives UBV for a good sample of
fieldd stars, and from those a range of colors might be available.
     Another possibility might be to center the sequence on a bright, well-
known active galaxy, such as AP Librae.  Here the range of colors might be
limited to the usual F/G dwarfs, but you might luck out with a couple of
blue halo stars.

\Brian

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