[vsnet-chat 2596] Re: errors in old spectral surveys

[Brian Skiff <bas@lowell.edu>]

     At the risk of possibly boring folks on the list---there's at least three
of us interested in it!---here are some more comments about the pre-War
spectral surveys following on John Greaves' and Mati Morel's responses.  I'll 
save some remarks about "fixing the databases" for another post.
     Although I based my comments about the Yale southern zone work on my own
comparison with modern surveys, one can also consult Nancy Houk's remarks on
the subject.  Houk is about halfway through reclassifying all the HD stars on
the MK system (the data for stars south of -12 Dec are published in four
volumes).  There's a presentation by her starting on page 70 of IAU Symposium
no. 50, "Spectral Classificiation and Multicolor Photometry", Fehrenbach and
Westerlund, eds., 1973.  The paper is a first discussion of progress on the
reclassification project.  In it Houk makes various comparisons with the
original HD data.  As part of that comparison, she also classified stars on
five of the plates Cannon used for the HD itself.  These plates would have had
Cannon's marks on them as to exactly the images examined.  Houk notes:  "It is
difficult to overstate the problems with focus on the HD plates.  Only the
central region of each plate is in good focus, and Miss Cannon classified many
stars which were quite out of focus.  On some no lines at all could be seen.
Consequently the HD type may be uncertain even for strongly exposed spectra."
Given that the non-HD Yale stars are roughly 1 or 2 magnitudes fainter than
the HD stars, underexposure would have been an additional problem.
     All spectral surveys before World War II and many afterwards were done
using astrographs with an objective-prism attached.  The fields of these lenses
are usually not flat, so only part of the field would be in good focus.  In
addition, because of the chromatic aberration of the lenses, only a narrow
range of wavelengths in the spectra would be in focus.  The usual workaround
would be to take a set of plates at the minimum focus, typically near H-gamma
(4340A), which would show most of the important lines for classification.
Another set would be taken with the focus adjusted so that the CaII H and K
lines toward the ultraviolet (between 3900 and 4000A) would be in focus.  This
set would allow the H&K strengths to be estimated (or measured), and allow
the classifier to examine the Balmer decrement (a luminosity criterion for hot
stars).  As a result of the color curve of the lens, this second set also
often had a region near H-beta in focus, including the TiO bandhead near 4700A,
which would be useful for red stars.
     There were other problems with the old astrographic lenses.  Most of them
had generally slow f/ratios.  The very largest ones, such as the excellent
astrographs at Crimea and Stockholm, were a mere 40cm aperture, huge beasts
even so!  And apart from the general chromatic aberration problems, in some
cases the combined thickness of the glass in the objective (triplet) and the
wedge of glass in the prism would mean most of the light blueward of 4000A was
soaked up, and only on bright stars were the important H&K lines recorded.
This led to B-type stars being misclassed as type F, since their hydrogen lines
are of similar strength (the early-B stars will show no H&K, except for
sometimes a weak interstellar K line, whereas in F stars H&K are quite strong),
and a moderately-reddened B star will have about the same overall spectral
curve as an unreddened F stars (and remember before 1930 there was no
convincing evidence of reddening at all!).  Plenty of B stars are given as F
(none of the reverse, luckily, that I've found) in the HD and HDE, in the
Bergedorf Durchmusterungen, in the Stockholm and Crimea surveys (which are
mostly reliable otherwise), and even in some of the surveys done at Case (which
are generally the best quality---taken with a fast Schmidt camera).
     I'm sure the folks who did the fainter non-HD stars appearing in the Yale
zone lists did the best they could in the circumstances.  Just as now, they
doubtless had a limited budget and time, and not all the plates they had to
work from were the best.  Probably also they considered that the types would
be used not on a per-star basis but only in a statistical ensemble, so a small
percentage of misclassified stars wouldn't ruin such analyses.
     My opinion is that the old spectral surveys retain their value, and have
even gained a bit of late.  Wherever these surveys cover, all the Tycho stars
and then some are included, often going two magnitudes fainter than Tycho,
nearly to the bottom of the GSC for some areas.  The main problem has been that
the stars are mostly identified only on charts, so their positions are not
readily recoverable without some software and some rather detailed work well
beyond the tedium-tolerance of most people (it isn't "science" after all, only
bookkeeping).  As some of you know, I have gone through a number of the
spectral surveys to obtain accurate positions and identifications (which are
indispensable for linkage to external lists like IRAS, x-ray/UV surveys, etc.).
If you're looking for long lists of spectral types that are not found at the
data centers, I've got around 25-30,000 of 'em stashed at the Lowell ftp area 
(http://ftp.lowell.edu/pub/bas/starcats).  Much of this has made its way into
SIMBAD thanks to the efforts of the folks in Strasbourg, particularly Fabienne
Woelfel.  Almost as much as adding reliable stuff to the database, the
identification work also reveals various errors in SIMBAD itself.  The main
value in going through these lists star-by-star may well be to turn up (and
fix!) such problems.
     This leads to Mati's comments about how to fix the databases, which I'll
discuss in another post.

\Brian

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