[vsnet-chat 2403] (fwd) GSC 3530.2757, a new RCB star?

[Taichi Kato <tkato>]

(fwd) GSC 3530.2757, a new RCB star?

From: bas@lowell.edu (Brian Skiff)
Date: Sun, 7 Nov 1999 16:42:20 -0700 (MST)

     I have followed with interest the work of the AAVSO gang on Lennart
Dahlmark's deep eclipser LD 282.  I will be sending to him the most recent
note to the discussion list from Dan Kaiser so that he can be brought up to
date on this object.  He is aware that Peter Guilbault had started work on
the star this time last year, and was keenly interested in the follow-up.
     In a letter from Dahlmark I received a few days ago, he poses another
possibly interesting challenge.  The star involved is GSC 3530-2757, located
at:  18 31 13.8  +46 58 35 (2000, GSC-ACT).  He reports that on most of a few
dozen exposures from 1967 to 1982, and on another 50 films from 1994 to 1999,
the star is sensibly constant at around mag. 11.5, similar to what's reported
in the GSC.  However, on three pairs of films (contemporaneous exposures on
103a-O (blue) and 103a-D + yellow filter) taken 14 July 1972, 28/29 July 1973,
and 21/22 June 1974, the star is completely absent ("no trace of it" he writes)
and must thus be fainter than about mag. 15.  This suggests an R CrB-like
behavior---but a two-year dimming?
     I examined the star on the POSS-I/II prints and films.  We happen to
have all three sets of POSS-II films on hand, and since the star falls in
the overlap region, there are three pairs of blue/red/far-red exposures all
taken on different dates.  The POSS-I prints and POSS-II red/blue pair show
that the star is "somewhat red", about what one would see in a K-giant, but
nothing unusual.  The IV-N "infrared" films however show the star significantly
brighter relative to other stars near it that are similar in brightness in the
red and blue.  On the Vehrenberg "Atlas Stellarum" plate (blue) taken in 1969,
the star also looks completely normal.
     At this point, someone who knows more than me about R CrB stars might be
able to say something sensible about the intervals of the dimmings and the
magnitude drops involved.  Some other mostly non-useful items:

---the star shows in USNO-A2.0 and A1.0 with only a red magnitude; the blue
   magnitude is given in both catalogues as "99.9" (in VizieR), indicating
   a problem with the blue scan.  The images look normal on the POSS-I prints
   in our library and on the DSS.
---the star is _not_ in AC2000, Tycho, TAC, or the BD; probably just a little
   too faint and/or red for those catalogues.
---lies just outside (of course) the Bergedorf spectral survey for SA 38;
   no other spectral surveys that I know of cover the region.
---nothing, not even an IRAS detection, in SIMBAD.

     Now you know all I know.  Dahlmark is in the midst of preparing another
list of new variables in a 15x20-degree region centered at 19h00m/+45, which
will include this star.  He says he will probably do this and one more northern
Milky Way field before stopping this work.  He is now 80 years old, and says
he's simply running out of energy.  We're discussing the disposition of his
film collection.  He has about 1000 4x5-inch astrograph films in two colors
covering much of the northern summer/autumn Milky Way over a 30-year interval,
plus a similar number of 20cm Schmidt camera films in 35mm format.  He also
has handwritten lists of magnitude estimates from these for nearly 400
variables.  The latter are not published for the most part (and no
lightcurves), and this is why most of the LD variables do not have GCVS
designations.  I think it would be really great to get his patrol films scanned
and photometrically calibrated so that data for the variables (new, known, and
ones he couldn't spot) could be redone more accurately.

\Brian

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