GSC 0279 0321 is probably not an eclipser (also note on colour index use)

[jg <jg@jgws.freeisp.co.uk>]

Hi Michael K and all

Did you know that TASS churns out two colour photometry?  So do I. 
You'd think I'd make use of it then!

Anyway, Michael K asked for thoughts on the variability of GSC 0279
0321.  I'm going to use the TASS photometry of it to suggest some points
that may help for future analyses / observerations etc that Michael and
others may do.

Type EA eclipsers can have periods ranging from 0.2 days to, well
decades I suppose, and a wide range of amplitudes.

The TASS V-I (properly VJ - Ic here) photometry can be used to show that
the above star isn't one of these.

The plot of V-I for the two minima in the TASS photometry is quite flat,
albeit a bit scattered.

Michael, EA stars are predominantly different in colour for the two
eclipsing stars.  Thus they undergo a drift in colour index (B-V is a
colour index, V-I is a colour index, etc) during eclipse.  The highest
WS statistic star from collected.big is RW Mon, a known EA type star. 
So, if you plot the TASS V, I and V-I for that you'll note the evident
curve parallelling (<-?spelling?) the V and I minima. Do note that it is
shallower.

Pulsators change in colour too, of course, but far less evidently.  For
your example here, the second highest WS star is BE Mon, a delta
Cepheid, to illustrate all this to yourself also plot V-I of this object
from the TASS dataset for it and notice how very flat that is, and
pretty constant in value twixt nights despite V and I varying.

GSC 0279 0321 shows a flat, if scattered, V-I during the times of minima
and is therefore not an EA star.


Type EB eclipsers have minimum periods of around a day, which'd fit
here.  Unfortunately in this instance these eclipsers consist of two
similar stars, so a flat V-I is not diagnostic.  The (VJ-Ic)TASS colour
index is around 0.7.  Even if you can't look that up directly, note that
by convention the vast majority of "visual" spectrum colour indices are
defined such that they equal 0.0, or very nearly so, for A0 stars
(specifically Vega = alpha Lyrae).  EB eclipsers are predominantly B or
A type stars (maybe some early F????).  This star is simply too "red"
(too "late" in spectral class) to be an EB eclipsing binary.  [Note that
redness, blueness, earlyness and lateness in photometry and spectroscopy
are astronomy-speak conventions that have no physical or evolutionary
meaning, they are just short hand terms for linear ranges with two
extremes/ends.  So you can end up reading of incongruous things like
"are bluer at infrared wavelengths"].


Type EW eclipsers can have periods of up to a day long.  Both stars are,
again, usually quite similar.  In this instance spectral type ranges
from F-G and into K, maybe even a few type M lurking around.  This GSC
0279 0321 colour index from the TASS data fits within this range.

Rather than a one day period here though, I'd think it to be more likely
near to the periodicity you've found. The diagnostic re
eclipser/pulsator here would have to be a full light curve.  You will
have seen in IBVSs that despite EW stars appearing near sinusoidal they
are quite different from the sine waves a symmetric pulsator displays.
And if the light curve turns out to be an asymmetric sine wave, you're
very likely to have a pulsator.  (The techy terms are "extrinsic" and
"intrinsic" variables denoting whether the variability is caused by
mechanisms external or internal to the star(s).  Eclipsers are therefore
extrinsic variables, pulsators intrinsic.  Possibly I should be saying
extrinsic instead of eclipser, intrinsic instead of pulsator, which are
not proper terms).


Hope that helps.  Don't take any of it as "gospel", but it should be
generally accurate.



NOTE : I don't know whether selectun and gnuplot are amenable to on the
fly creation of V-I values for plotting or whether this would have to be
done separately, as the data output does not include V-I, but the colour
index would make an immediate diagnostic tool for EA stars where patchy
lightcurves have hints of minima and maxima in the TASS CD datasets but
the light curve is not well sampled.  There are quite a few of these
amongst the variable star candidates.  Maybe it should be included in
the data, this would however make the .cal and .big files bigger, due to
including an extra column of information.  Not certain as to whether it
would also be increasing processing time as I am not even remotely
familiar with perl, whether it's a compiler or an interpreter, nor how
fast it works, so I'm not sure whether it is worth the effort.  It is
certainly not primary but derived data, but then again where does
primary data end and derived data start?

Ironically, this would not so much be to choose EA stars as to discard
them.  Some people simply find them less interesting, but on a more
objective point, it looks like there's a one day selection effect  in
the periods (or two minima 2 d or 1/2 d that can appear 1 to be one day
in gapped data).  EA stars with these periods will need a great deal of
observational follow up to sort them out, whereas pulsating stars will
probably soon reveal their nature, even if their period turn out to be a
large fraction submultiple of a day.  If you don't have a full
_continuous_ lightcurve for an eclipser it is hardy to be certain that
you haven't missed a secondary eclipse in some cases, whereas gapped
data for pulsators can be folded and solved quite nicely in similar
circumstances.

Just thoughts.


Cheers

John

John Greaves
UK


PS The HAT paper contains about the 8th different way I've ever seen the
GSC number denoted (re graphs in it), though this is the first time I've
ever seen the two sets of numbers running together without a delimiter
of any kind.  This is _non-preferred_, as the GSC numbers are not one
number but two.