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A New Program of
Multi-Year Continuous Phase
Coverage Photometry of
Alpha UMi
by
J. Donald Cline and Michael W. Castelaz
(Pisgah Astronomical Research Institute)
and
Mercedes Lopez-Morales (UNC-Chapel Hill)
202nd Meeting
of the American Astronomical Society
28 May 2003. Session 39.07
A Not-For-Profit Public Foundation
http://www.pari.edu
ABSTRACT
The well-known period-luminosity relationship of Cepheid variables
is used for extragalactic distance measurements. Understanding the
evolution of Cepheids and their migration off the instability strip is
therefore important. Several Cepheids are known to exhibit period
increase along with amplitude decrease. The most publicized
Cepheid showing these characteristics is  UMi. Alpha UMi is an F7
Ib-II to F8 Ib-II type, V = <2.0> magnitude Cepheid with a period of
~3.97 days and amplitude of V ~ 0.03 mag. Ferro (1983) describes
a rate of period increase and amplitude decrease beginning after
1945. Dinshaw et al. (1989) studied the radial velocity curve of 
UMi and concluded that it is evolving to the red edge of the
instability strip with pulsation stopping in 1995, and the possible
presence of starspots. Fernie, Kamper, & Seager (1993) also
suggested a cessation of Cepheid activity by Polaris before 1995.
However, in 1998, Kamper & Fernie (1998) note that Polaris was
still pulsating, and the decline in amplitude has stopped. They stated
that they had no explanation, and that continued monitoring of
Polaris is clearly important.
Polaris appears to be close to the red edge of the first-overtone
Cepheids and evolving into the area of fundamental pulsators. As the
evolution occurs, the amplitude of pulsation of  UMi may increase
again (Kamper & Fernie 1998). The behavior of the star in the
previous 50 years has been dramatic, and suggests the need for
continued monitoring. A systematic study over a period of years will
provide a fundamental contribution to our knowledge of stellar
structure and physical parameters of the Cepheid  UMi.
We present a new program to observe  UMi 24 hours a days, seven
days a week, using a 0.3 m telescope with initial results. The  UMi
observation program includes BVRI photometry of  UMi, a
comparison, and a check star.
I. INTRODUCTION
• The well-known period-luminosity
relationship of Cepheid variables is
used for extragalactic distance
measurements. Understanding the
evolution of Cepheids and their
migration off the instability strip is
therefore important.
• Several Cepheids are known to exhibit
period increase along with amplitude
decrease.
• The most publicized Cepheid showing
these characteristics is  UMi (hereafter
Polaris). Alpha UMi is an F7 Ib-II to
F8 Ib-II type, V = <2.0> magnitude
Cepheid with a period of ~3.97 days
and amplitude of V ~ 0.03 mag.
• Ferro (1983, ApJ, 274, 755)
describes a rate of period increase
and amplitude decrease beginning
after 1945.
• Dinshaw et al. (1989, AJ, 98, 2249)
studied the radial velocity curve of
Polaris and concluded that it is
evolving to the red edge of the
instability strip with pulsation
stopping in 1995, and the possible
presence of starspots.
• Fernie, Kamper, & Seager (1993, AJ,
416, 820) also suggested a cessation
of Cepheid activity by Polaris before
1995.
• However, in 1998, Kamper &
Fernie (1998, AJ, 116, 936) note
that Polaris was still pulsating,
and the decline in amplitude has
stopped. They stated that they had
no explanation, and that continued
monitoring of Polaris is clearly
important. Their Figure 3, shown
below, suggests the decline has
stopped.
• Cox (1998, ApJ, 496, 246) suggests
the period and amplitude changes are
due to
• rapid changes in the helium
composition gradient just below
the star’s hydrogen and helium
convection zones.
• The changes in helium gradients
“helium dredge-up episodes”
change surface layer structures
and periods.
• The episodes can occur in a few
days.
• Polaris appears to be close to the red
edge of the first-overtone Cepheids
and evolving into the area of
fundamental pulsators.
• As the evolution occurs, the
amplitude of pulsation of Polaris
may increase again (Kamper &
Fernie 1998).
• When the change in amplitude
and period behavior will occur is
unpredictable, and may in fact
take a long time.
• Nevertheless, the behavior of the star
in the previous 50 years has been
dramatic, and suggests that
continued monitoring is worth the
effort.
II. OBSERVATIONS
 In 2002 we began a project to build
and operate a 0.3-m optical telescope
and an SBIG STV CCD camera
dedicated to multi-year, continuous
phase coverage photometry of 
UMi. The project is funded by a
grant from the Small Research Grants
program of the American
Astronomical Society.
 A systematic study over a period of
years will provide a fundamental
contribution to our knowledge of
stellar structure and physical
parameters of the Cepheid  UMi.
 An extensive literature search shows
no other program dedicated to 24hour monitoring of Polaris.
II.A. The Polaris Observatory:
Telescope and Camera
 The Meade 0.30-m, f/10 telescope
selection based on
 Scale: 68.7 arcsec/mm
 Cost
 Aperture which is adequate for
night/day observations
 R232 computer control capability
 Ease of parts replacement because we
plan to use the telescope 24 hours a
days, seven days a week.
The V=8.7 mag lineof-sight star located
18 arcsec from
Polaris is separated
from Polaris with the
selected telescope fImage taken with PARI
ratio and camera pixel 0.3-m f/10 telescope and
size.
camera with 6.8 um pixels
 SBIG STV CCD camera selection based
on
 The STV 7.4 m pixel, which at the
telescope scale, is 0.60 arcsec.
 Typical seeing on the PARI Optical
Ridge is 1.5-2 arcsec (Cline,
Castelaz, & Powers 2000, BAAS,
196, 5201).
 Stellar seeing disks will cover
several pixels desirable for
photometry.
 Field of view (FOV) is 6.6
arcminutes.
The SBIG STV
is controlled by
RS 232
connection.
Polaris Obsevratory is located on
the PARI Optical Ridge
Optical Ridge located on PARI
Campus at an altitude of 910 m
0.25 m
0.30 m
(1.8 m)
Polaris
OVIEW
Observatory
(1.1 m) 0.25 m
0.30 m
0.20 m
Webcams, All Sky, Weather Station
o
o
o
o
o
o
o
POLARIS Observatory: 30 cm f/10 telescope
and SBIG STV
One 30 cm Telescope and CCD (SBIG T10XME)
Two 25 cm Telescopes and CCDs (SBIG STV
and SBIG ST-7)
Two 20 cm Telescopes and CCDs (Apogee AP4)
Two 12.7 cm Telescopes and SBIG STVs
All Sky camera (SBIG AllSky)
Webcams and Weather Station
The Polaris Telescope is temporarily polar mounted
in one PARI’s observatory buildings, waiting for
completion of the Polaris Observatory. The
telescope was tested and used to begin the Polaris
Monitoring Program from this site.
II.B. The Polaris Observatory:
Observing Strategy
 We are using the classical variablecomparison-check photometry method
because the CCD FOV is 6.6 arcminutes,
and there are no stars of comparable
brightness to Polaris within that FOV.
 Because the amplitude of variation of
Polaris is several hundredths of a
magnitude, our goal is to consistently
achieve <0.005 mag accuracy.
 One observation consists of a series of
four images of each star.
 Each series of images are taken at 10
minute intervals.
 Observations began in May 2003
III. Test of Observing Strategy
 We tested whether we need to follow
the steps outlined for the the Observing
Strategy, or if we can follow a simpler
method of simply taking a continuous
series of images of one FOV centered
on Polaris.
 For this test, we took a series of images
centered on Polaris using a 0.2-m
telescope with a 2048x2048 CCD
camera with a scale of 2.2 arcsec/pixel,
and I filter 13 May 2003 UT.
 Differential photometry between the
Polaris and a nearby 8th mag star in the
I filter.
 A single 1.5 second exposure was taken
every 10 minutes.
 Table 1 shows 2 hours of test
measurements of the continuous series
of images. Since the exposure time was
so short, the signal-to-noise of the 8th
magnitude star is small compared to that
of Polaris.
 Mean differential magnitude is 4.990
and the Std.Dev. is 0.058 mag
TABLE 1. Test Measurements
JD 2452772+
0.55253
0.56139
0.56807
0.57685
0.58346
0.59007
0.59669
0.60330
0.60992
0.61653
Differential Magnitude
4.944
4.910
4.894
5.043
5.043
5.060
4.926
5.034
5.058
4.994
III. Discussion
 The Standard Deviation is much larger
than what we want to achieve. The large
StdDev argues for
 Measuring Polaris, comparison, and
check stars, each in their own fields,
so that similar magnitude stars can be
used in the differential photometry.
 Series of images to increase S/N.
 Completing one set of image series
every 10 minutes, adequately
sampling the 3.97 day light curve of
Polaris.
 The observations will ulitmately result
in the photometric measurements of
the lightcurve of Polaris. We plan to
continue observations for the lifetime
of the instruments.