[vsnet-chat 2789] V915 Sgr & Pena paper (Greaves)

[Taichi Kato <tkato>]

V915 Sgr & Pena paper (Greaves)

Date: Sat, 08 Apr 2000 15:04:04 +0000
From: JG <jgts@jgws.totalserve.co.uk>
Subject: V915 Sgr & Pena paper

Very close [eye hurting] examination of the two images given in the Pena
paper reveals subtle differences.  Granted that I am looking at a
digitially scanned version [via adsabs.harvard.edu] of the original
paper, which itself is diplaying a _copies_ of the original plates, but
it is immediately obvious that the right hand one appears under exposed
compared to the left hand one.

HOWEVER, a more important point is that the two images appear to be at
different colours!

This is evidenced by checking the _alignment_ of certain stars in both
images.

The left hand image does not show GSC 6870 0090 [=V915 Sgr] but does
show GSC 6870 0260 [Brian Skiff's red star], whilst the right hand image
shows GSC 6870 0090 but not [or barely] GSC 6870 0260.

A check is given by GSC 6870 0376 which doesn't appear in the left hand
image, although a redder similar brightness USNO star just North and a
bit West does, whilst it does appear in the right hand image, with the
aforementioned USNO star no longer present.

In fact, using colours as [gu]estimated via USNO A2 blue and red
magnitudes I'd suggest that rather than the right hand image being under
exposed compared to the left, we have a situation where the left hand
image could be more red sensitive than the right.

Notwithstanding Pena et al appear to have identified V915 Sgr's position
correctly on the plates and colour doesn't seem to be enough reason for
this variable to appear on one and not the other [if anything it should
be on the redder one according to Tycho1 BT-VT).

As an aside, I note that although quite a few adjacent fainter and
redder stars appear in the AC2000 at mean epoch around 1913, V915 Sgr
does not.  This is only suggestive, however.

A red star only 3' or so to the WNW that can be absent on blue sensitive
plates is not useful in situations where a star's position is not well
known, especially when fields are being checked prior to observating
runs.  Whether the star GSC 6870 0260 is sufficiently red to account for
this discrepancy between plates or whether it is actually variable I do
not know, though the difference between the images for nearby GSC 6870
0336 suggests colour difference may be sufficient.

Finally, re Pena et al's 'reflection nebula' around HD 172355: use of
the Aladin server provided by the CDS reveals only a red plate survey
for this region, and you'd not particular expect a reflection nebula
around an A type star to show up on these.

However, using Aladin's 'zoom' facility two distinct stars very adjacent
to HD 172355 are revealed, so adjacent they do not even appear in USNO
A2 though more than bright enough.  Their brightness and position [along
with that of one or two much fainter ones] more than adequately
reproduce the shape shown by the magnified image of HD 172355 in Pena et
al if one takes into account that the individual stars have not been
resolved in that particular case.

Cheers

John

John Greaves
UK

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