[vsnet-chat 2787] Re: V915 Sgr

[Brian Skiff <bas@lowell.edu>]

Taichi (and anyone else interested),
     I'm not quite sure what's going on with V915 Sgr.  Luyten's source
position precesses to:  18 43 14  -29 00.9 (2000), and he lists a photo-blue
range of 12.3 to 13.5.  Modern sources including the GCVS 4.1 seem to agree
that V915 is the fairly bright star of ordinary color you mentioned previously.
These include Herbig (PASP 70,605), Morel (IBVS 3096), and the Pena et al.
paper already mentioned.  The Pena paper is kind of odd, since it is describing
two objects that have no relation whatsoever to each other.  As best I can tell
from looking at the short-V and POSS-II scans, the nebula they mention simply
does not exist.  It is merely the early-A star HD 172355 (A0/1V according to
Houk); there is no other mention of such an object in SIMBAD, and it almost
certainly would have been noticed if it were as bright as is indicated.
Further the colors shown at the two epochs for what they say is V915 appear
to be just the opposite of what one would expect:  very red when bright, but
far bluer when faint.
     Despite their claim that there could be no error in identification, I'd
be willing to bet Pena et al. measured two different stars, that _neither_ of
them is V915, and that one of them is the bright red star CD-29 15226. 
Although Luyten's positions are none too good, I note that the CD star is
only 1'.3 due south of his position.
     The implication is of course that:  Luyten's object = the original V915,
and the star showing the RCB-like behavior is a different variable.  It could
be that the 282d period given by the GCVS (derives from Astron. Tsirk. 171) is
for the red CD star.  This is probably a case for the Nikolai Samus and the
Sternberg group to look into.
     I notice also that the RCB-like star lies just outside the IRAS position
error-ellipse, but the large IRAS 12mu flux (and consistent 12/25mu flux ratio)
suggests the ID is certain (i.e. IRAS 18383-2906 = GSC 6870-0090).  The image
shown in the Herbig paper essentially proves that this star gets quite faint.

\Brian



V915 Sgr candidates:
Luyten        18 43 14     -29 00.9
GCVS 4.1      18 41 31.4   -29 04 12
Tycho-2       18 41 32.59  -29 04 03.6  V=11.49+/-0.13, B-V=0.65+/-0.23
IRAS          18 41 30.7   -29 04 06 (+/- 22"x7")

brighter red star:
CD-29 15226   18 41 15.94  -29 03 09.9  V=10.74+/-0.07, B-V=1.6+/-0.3

Pena et al. "nebula":  HD 172355, 18 41 01.69  -29 18 56.7 (Tycho-2)

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