[vsnet-chat 5550] Re: The curious case of BD+59 224

[Brian Skiff <Brian.Skiff@lowell.edu>]

     Thanks to Doug West for his efforts with regard to the peculiar star
BD+59 224.  Since both B-V and V-R give essentially the same information
about stars (specifically:  temperature), having both of them does not really
provide much new information compared to just one or the other color-index. 
The same would apply to any of the three usual BVRI color indices.  This is
the main reason why Arne and I (probably others) have suggested to amateur
CCD photometrists that for generic variable-star monitoring, getting V and
just one of the BVRI colors (B-V, V-R, or V-I --- doesn't matter which one)
is entirely sufficient to characterize the variations.  There's no need for
all BVRI colors because any pair will tell you just about everything that all
four will, and get you that information in a lot less time and more
accurately, given that most folks are at relatively poor photometric sites.
Note that even if you extended this out to JHK there's still no more
information in most cases, since J-K (or I-K etc) still tell you mainly about
star temperature.  Yes, special cases, but for nightly long-term stuff, just
V and any one other filter is sufficient.
     As I mentioned in my original note, the U-B color would be much more
diagnostic in the case of BD+59 224 since it is sampling the spectrum below
the Balmer jump, and would show up a hot component if there is one in this
system.
     The available information, including Doug's BVR colors, the other
published B-V color, the IRAS and MSX detections (strong 12mu-only source),
and the Dearborn red-light spectral classification, all point to there being
a cool luminous star involved.  That's the easy part.  What's puzzling are the
reasonably reliable reports of earlier-type spectral features and the emission
lines.  These suggest there must be another hotter component---a hot star
perhaps with a disc in a close binary---so I've been somewhat idly wondering
what sort of pathology is involved.  If there is simply an M giant (say) with
transient emission lines, then U-B will be somewhat larger in value than B-V,
say something close to U-B = 2.0.  If there's a hot star involved, then U-B is
likely to be much less red, say around 0.5, and to me that makes it likely
that this is a VV Cep-type system that's simply been overlooked heretofore.
There's a smaller possibility that there's something more pathological going
on, and it might deserve more thorough study.
     Since Aaron Price asked:  a red star like this is likely to be variable
at small amplitude, but I'm pretty sure just finding the variability will not
tell us much, since most of the variability in VV Cep systems (assuming that's
what this is) comes from the ordinary Mira/semiregular pulsation of the cool
component.  If there's an eclipse (possible but unlikely) or something
disruptive goes on with the stream or whatever connecting the stars, then the
variations might be more interesting.  If you are interested in just
discovering the variability, go ahead and do so!  You might anticipate a range
of a few tenths of a magnitude on a timescale of a few months if things are
typical.
     Some other comments on Doug's report:

>>  Astrometry of field:
>>  GSC 4030:591 [= BD+59 225]    01 17 26.82 +60 22 11.6
>>  GSC 4030:149 [= BD+59 224]    01 17 19.08 +60 23 04.1
>>  Positions based on 8 GSC 1.1 stars.  Estimated error 0.5".

     Since the stars are both in Tycho-2 with positions to a few tens of
milliarcsec, new astrometry is not really all that useful.  Your offsets from
Tycho-2 are 0".43 for the first star (though it is a close pair with large
delta-m), and 0".70 for the second peculiar star, so the estimated uncertainty
of 0".5 is about right.
     It would probably be good to switch to some later catalogue than GSC v1.1,
since it is not on the ICRS and has sometimes-large systematic errors (1"-4")
is some regions.  GSC-ACT is probably the easiest thing to obtain:  this will
cut your astrometric errors in half and avoid systematic problems.


>>  Differential Photometry (without color transformation):
>>  GSC 4030:591  V=10.28 +/-0.06, B-V= -0.06+/-0.15, V-Rc=0.23+/-0.07
>>  BD+59 224     V= 9.38 +/-0.06, B-V=  1.31+/-0.15, V-Rc=0.97+/-0.07

     The blue star (BD+59 225 = GSC 4030-0591) has photoelectric UBV photometry
from two sources:

1964ArA.....3..339S:   V=10.12  B-V=0.26
1965JO.....48..171B:   V=10.10  B-V=0.31  U-B=0.21  (V-R ~0.20)

...so it looks as though your data for that star show too faint in V,
and too blue in B-V.
     The red star is possibly variable, but I wouldn't expect colors to
change much:

V=9.38  B-V=1.31  V-R=0.97    DWest
V=9.85  B-V=1.62 (V-R ~0.9)   1964ArA.....3..339S


     In re your comparison stars, Tycho-2 data is going to be much better
for such comparisons because the large systematic errors of Tycho-1 are
largely absent, and the internal errors are better than Tycho-1, but still
underestimated by _half_ for stars with V > 9.5:

>>  Comparison stars for photometry:
>>  GSC 4030:427  [BD+59 227]  B=11.57, V=10.48, Rc=9.88  [B-V=1.09, V-R=0.60]
>>  B&V from Tycho 1, Rc from conversion formula below.
>>  SAO 11690 [HD 7796]  B=9.59, V=9.47, Rc=9.39   [B-V=0.12, V-R=0.08]


Tycho-2 data (GSC 4030-0427):  V=10.45 +/- 0.10   B-V=1.09 +/- 0.11
                               inferred V-R=0.60 is about right

...there is published photoelectric UBV data for the other bluer star HD 7796:
Tycho-2 data:         V=9.45 +/- 0.03   B-V=0.14 +/- 0.05
1965JO.....48..171B:  V=9.45            B-V=0.18
                      V-R=0.08 is okay, something like 0.13 might be better



All for now.

\Brian

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