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Date: Tue, 14 Dec 93 21:06:22 MST
From: bas@lowell.edu (Brian Skiff account lowell)
Subject: Comparison sequence for KU Cas
Submitted to the Information Bulletin on Variable Stars.
\Brian Skiff
*****************************************************************
Photometry of Stars in the Field of the Dwarf Nova KU Cassiopeiae
KU Cassiopeiae is a relatively faint dwarf nova that reaches mag. 13
during outbursts, which occur about every two months. A precise position and
large-scale identification chart are provided by Bruch et al. (1987). The
position for KU Cas is: 1 31 02.6 +57 54 14 (J2000), which differs
significantly from the (precessed) position in the GCVS4.
At the request of Charles Scovil of the American Association of Variable
Star Observers, I made photoelectric measurements of several stars in the
field to improve the magnitudes for an existing sequence of comparison stars
on an AAVSO chart. Since the quiescent brightness is quite faint (about mag.
18), the sequence stars concentrate on those likely to be useful during
eruptions; the six stars span the range 11.6 < V < 14.6.
The observations were made on 4 and 7 September 1992 UT using the Lowell
Observatory 53cm photometric telescope. The stars were observed through a
19-arcsec diaphragm with Str\"omgren \it{y} and \it{b} filters. Each
observation consisted of a minimum of four 10s integrations of `star' and two
10s integrations on `sky', with greater numbers of integrations for the
fainter stars. All the standard stars are from the lists of Landolt (1983a,
1983b, 1992). The V magnitudes were adopted directly, sometimes supplemented
with values from Menzies et al. (1991). The $b-y$ colors were determined
using the same telescope against primary Str\"omgren standards. The redder
Landolt stars were necessary in order to properly calibrate the red and
reddened stars commonly found in low-latitude fields such as this one.
The data for each night was reduced separately using linear transformations.
Atmospheric extinction was estimated on these nights from measurements taken
on other nights near this time. Average per star residuals (standard
deviation) were $\pm$0.003 in V and $\pm$0.003 in $b-y$ on 4 September, and
$\pm$0.006 and $\pm$0.005 on the 7th.
The standards observed are listed in Table 1 along with the adopted and
mean observed V and $b-y$ colors, and the numbers of observations `n'. The
mean deviations of the observed averages from the assumed values listed in
the table are: V = 0.001 $\pm$ 0.004; $b-y$ = 0.000 $\pm$ 0.004.
Table 1. Standard Star Observations
Name V $b-y$ V $b-y$ n
(std) (obs)
HD 315 6.440 -0.078 6.439 -0.080 1
HD 5319 8.046 0.601 8.046 0.596 1
HD 7615 6.693 0.023 6.688 0.027 2
BD-00 0288 8.831 0.711 8.831 0.710 3
HD 11983 8.192 0.959 8.191 0.963 1
HD 16581 8.195 -0.033 8.203 -0.038 1
HD218155 6.783 -0.005 6.784 -0.002 1
HD222732 8.857 0.735 8.860 0.737 1
The results for the KU Cas field are given in Table 2, listed in order
of decreasing brightness. Identifications and J2000 positions are provided
for all except the very faintest star from the Guide Star Catalog. Each star,
again except the faintest, was observed on two nights. The uncertainties
listed in the second line of each entry are the standard deviations of the
means of the two observations; for the faint star it is the standard deviation
on the batch of integrations taken. The two measures of GSC 3678-0321
differed by 0.1 magnitudes, indicating a possible variable. The $b-y$ color
suggests the star could be a $\delta$ Scuti-type variable.
Table 2. Photometry of Stars Near KU Cas
Star RA (2000) Dec V $b-y$ n Remarks
GSC 3678-0321 1 30 57.0 +57 56 18 11.648 0.236 2 suspect var
.073 .001
GSC 3678-0036 1 31 19.0 +57 56 16 12.830 0.914 2
.013 .000
GSC 3678-0187 1 31 14.1 +57 57 16 12.931 0.381 2
.011 .016
GSC 3678-1147 1 31 11.5 +57 50 50 13.150 0.539 2
.006 .001
GSC 3678-0002 1 31 08.8 +57 57 28 13.867 0.608 2
.021 .028
anon 1 31 01 +57 54.3 14.645 0.964 1 not in GSC: lies
.042 .078 ~15$''$ NW of KU Cas
For the convenience of observers, a chart based on the GSC is shown in
Figure 1, centered on the Bruch et al. position of the variable. The
comparison stars are indicated by their V magnitudes rounded to the nearest
tenth of a magnitude (deicmal point omitted) in the style of visual
variable-star charts.
Brian A. Skiff
Lowell Observatory
1400 West Mars Hill Road
Flagstaff AZ 86001-4499
USA
e-mail (Internet): bas@lowell.edu
References:
Bruch, A., Fischer, F. J., and Wilmsen, U. 1987, Astr. & Ap. Suppl. 70, 481.
Lnadolt, A. U. 1983a, Astron. J., 88, 439.
Landolt, A. U. 1983b, Astron. J., 88, 853.
Landolt, A. U. 1992, Astron. J., 104, 340.
Menzies, J. W., Marang, F., Laing, J. D., Coulson, I. M., and Englebrecht,
C. A. 1991, Mon. Not. Roy. Astr. Soc., 248, 652.
Figure caption:
Figure 1. The field of KU Cas showing stars from the GSC. V magnitudes are
indicated to the nearest tenth of a magnitude with the decimal point omitted.
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