[vsnet-chat 434] Hipparcos and the amateur observer

[bas@lowell.Lowell.Edu (Brian Skiff)]

     Questions have naturally arisen about the effect the Hipparcos/Tycho
database will have on the astronomical community.  Here are some personal
reactions from having used the data for the past few weeks for several
purposes.

Mr. Mattiazzo asked:
Q:  With the recent release of the Hipparchos catalogue, how will the
worldwide variable-star community cope with the data?

A:  You can't really cope with all the data at once:  probably most astronomers
will cope by simply ignoring it!  They will wait for the results to be merged
into familiar catalogues, such as the "Bright Star Catalogue" or the next
edition of the GCVS, and deal with it from there.

Q:  Will the data replace existing variable star charts?

A:  This will be a complicated decision.  For the sake of continuity, the
AAVSO (at least) is loath to change the magnitudes of comparison sequences on
long-established charts, at least as long the sequence is reasonably
consistent.  The Tycho photometry becomes quite noisy below about V mag. 10.0,
so any chart changes will pertain only to the brightest variables---which are
the ones with those long observational histories using "old" charts.
     There are at least two reasons why I would argue that such charts be
fixed anyway.  One is simply that the photometry is now well enough in hand
that the sequences won't ever have to be changed again.  The second is that
with the increased use of CCDs, amateur observers will require much higher
precision in the magnitudes of the comparison stars, i.e. 0.01 mag. rather
than the 0.1 mag. precision that is customary on charts for visual use.
In addition, CCD observers will need star colors to correct for instrumental
effects and put their measurements on a standard system.
     From about mag. 10.0 on down, there is still the need for magnitudes and
colors for comparison stars for the use of visual and CCD observers.

Q:  How will the new variable star discoveries be incorporated into existing
catalogues?

A:  Bascially, it will happen the same way any other list of new variables is
dealt with.  The variable-star group in Moscow will assign formal designations
to each star, issued on name-lists, and then they will be incorportated into
the major secondary catalogues and on-line databases from there.  Later this
year names-lists will be issued for both the Hipparcos/Tycho stars (about
8000 new variables) as well as the OGLE microlensing survey (several thousand
more stars, mostly eclipsing binaries and hot pulsators such as RR Lyraes).

     With so many new variables, and with so many observations of the known
ones, variable-star observers might well ask whether there's anything left
to do!  The asnwer is simply:  plenty!  Even among bright stars (say brighter
than mag. 9), Hipparcos did not catch all the variables, particularly among
Algol-type eclipsing binaries.  Fainter than mag. 9, the per-observation
errors in the photometry are quite large, reaching about 0.4 mag. by V mag.
10.0.  This means only the large-amplitude variables could be picked out from
the noise, and even their lightcurves are very ill-defined, since the scatter
in the data is much worse than even visual observations.  How many dwarf
novae did they observe?  One?  I read somewhere that even their observations
of SS Cygni might be for another star in the field!  So particularly in the
realm between about mag. 10 and 15, where the new deep surveys take over,
there's still almost no one but amateurs working.
     Perhaps the most important thing to remember about Hipparcos vis-a-vis
variable-star observing is that Hipparcos is not observing _now_.  As the
lightcurves of Miras evolve, as novae appear, as eclipsing binaries have
sudden period shifts, as dwarf novae outburst, there's still no one (and no
instrument) out there making amateur observers obsolete.

\Brian Skiff  (bas@lowell.edu)

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