[vsnet-chat 2591] Re: "Hot" cool carbon stars

[Brian Skiff <bas@lowell.edu>]

     Before getting too excited about the F-type spectra reported for the
two southern carbon stars, and drawing from them unwarranted astrophysical
interpretations, be sure to look for the most likely cause of the anomaly:
human error.  In the case of FK Pup, the reason it has an 'HD' type without
an HD number is that it was classified by either Cannon & Mayall at Harvard
or by folks at McCormick Observatory (Vyssotsky et al.) as part of the Yale
zone catalogue astrometric work many years after the original HD catalogue and
its extensions.  This stuff was not published except in the Yale zone volumes
themselves.  Having gone over many regions containing these stars comparing
them with more recent objective-prism surveys, I can assure you the types are
none too reliable.  The stars they examined were often underexposed and/or
out-of-focus on the plates, and thus the classifier would frequently give a
type based on little more than the general shape of the spectrum as a function
of wavelength (rather than working from line ratios as would be the correct
method).  Thus gross errors are common in the set of classifications done for
the Yale zones.
     Another significant factor relating to the carbon stars is that since the
classifications were done on blue-sensitive plates, most likely the carbon
stars were too faint and simply not present at all on the plates used.  Yet the
classifiers knew there was a zone-catalogue star close by, so they tried to
find something to fill the bill.  I'd be willing to bet in both cases under
discussion that within a few arcminutes there is a decently bright star of
roughly F type---in other words, the classificiation is correct but it is for
some other star....
     I just looked in SIMBAD around FK Pup, and sure enough not too far away is
GSC 7133-3997, which is reported in the Tycho catalogue a full magnitude
brighter (in blue!) than the carbon star and with B-V color 0.4 --- hey, that's
just the color of an F star!  Now look at the field of CGCS 2792:  just 20"
away is a blue star (GSC 8960-1845 = PPM 358078) that's 0.7 mag. brighter (in
blue) than the carbon star.  Indeed SIMBAD duly assigns the 'F0' type to this
star, not the carbon star, so somebody's noticed this one before.  QED.
(I don't insist on collecting on my bet.)

     In re the IRAS LRS classifications:  as Mr. Greaves notes, the
astrophysical zoo is more complicated than what was possible to discern
reliably from the IRAS data; nearly all the LRS spectra are quite low S/N.
Thus the machine-derived 'region' types are little more than a guide, and
frankly are best ignored.  All the objects he mentions have substantial
bibliographies shown in SIMBAD, including post-IRAS follow-up spectroscopy,
and there is no ambiguity about what they are.

     In re visible-light identifications in star catalogues:  the GSC can be
useful for such searches, but more often you'll want to use the USNO-A series
catalogues, since this contains half a billion stars, goes something like
seven magnitudes fainter, and includes some star-color information.  USNO-A2.0
can be searched using the Strasbourg VizieR utility (catalogue i/252 in their
system).  There are plenty of stars of extreme color not in A2.0, so be careful
even here.

\Brian

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