answers to Martin's questions

[Tass Mailing List <tass@mail.alembic.net>]

Date: Wed, 13 Sep 2006 19:43:57 -0400
From: Michael Richmond <richmond@stupendous.cis.rit.edu>
To: tass@tass-survey.org
Cc: mwrsps@rit.edu
Subject: answers to Martin's questions




  Martin Nicholson asked about some specific stars.  I looked up
the information on each.

  First, he asked about known variable stars which didn't look
variable in the Mark IV database.

> V0699 Cyg - 304.2513  +39.1389  EA 1.55152 days
> 57 data points in TASS data
>
> Analysis in Peranso gave no indication of this period

   V0699 Cyg had an amplitude of 1.0 mag when discovered
   in 1952.  Measurements in 1990/91 showed that the amplitude
   had decreased to about 0.12 mag peak to peak, with a
   sinusoidal shape.  The TASS data show no evidence for this
   variation -- well, maybe a hint of it in the I-band measurements --
   but that's not surprising: it's too low an amplitude for our
   measurements.


> V0435 Cyg - 304.1125  +38.7611 EA/SD 6.771923 days
> 24 data points in TASS data
>
> Analysis in Peranso gave no indication of this period.

   This star has a nice eclipsing light curve with a
   period of 6.77 days.  The eclipse lasts for 0.12 of
   the period, or about 0.81 days.  The depth of the
   eclipse in V-band goes to V=14.5 from about V=12.8.

   The TASS data show 24 measurements.  None falls
   within the time of eclipse -- that is, when I computed
   the phase of each TASS measurement using the GCVS information,
   none were in the phases from start-eclipse to end-eclipse.
   This may be chance, or it may be due to the fact that
   during the eclipse, the star would be too faint to be
   detected in V-band.  For most of the Mark IV survey,
   we have required simultaneous detections in V and I
   for a measurement to be recorded.


> V1643 Cyg - 303.5850  +38.4619 M 297days
> 49 data points in TASS data
>
> Analysis in Peranso shows none of the characteristics of a Mira lightcurve

   The variable star happens to lie about 15 arcsec from a
   nearby star which is brighter in the optical, and is
   constant.  The light from the variable is being mixed with
   the light from the constant star.  Result: no real
   detection of the variable star.



  I found all this out by using SIMBAD to dig through the literature,
and using Aladin to make finding charts for each star.

  Now, on to the suspected variables Martin did find:

> I found 4 stars that looked variable to me around RS CYGNI.
> Have a look and see what you think.
>
>  1.  20 13 36.3 +38 43 02 (c0.7)
>  2.  20 12 27.4 +38 53 48 (c0.8)
>  3.  20 12 03.3 +39 07 50 (c0.8)
>  4.  20 14 51.5 +39 02 47 (c0.8)


   I examined the Mark IV data for each candidate.  In each case,
I looked at both the on-line engineering data, plus measurements
which had been subjected to ensemble analysis to remove some
systematic errors in the TASS photometry.

   In brief, candidate 1 shows variation in I-band because it is
close to the saturation point.  The other candidates appear to show
no significant variation, especially after one performs ensemble
analysis.  For example, the apparent variations in candidates
3 and 4 looked very much alike -- very probably due to systematic
errors in the measurements of _all_ stars in that region of
the sky during a series of night.  When I corrected for common
variations of _all_ stars in that region, almost all the
apparent variation disappeared.

   So, I'd conclude that none of these stars is actually
an intrinsic variable star at the level of more than
+/- 0.20 mag.

   Using the Mark IV data can be a tricky business,
unless one is seeing such hu-u-u-ge variations --
say, +/- 0.8 mag -- that the small errors are negligible.

                                Michael