A New Program of Multi-Year Continuous Phase

Coverage Photometry of Alpha UMi

J. Donald Cline and Michael W. Castelaz

(Pisgah Astronomical Research Institute)

202nd Meeting of the American Astronomical

Society

28 May 2003. Session 39.07

by

andMercedes Lopez-Morales (UNC-Chapel Hill)

A Not-For-Profit Public Foundationhttp://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.

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 24-hour monitoring of Polaris.

II. OBSERVATIONS

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 line-of-sight star located 18 arcsec from Polaris is separated from Polaris with the selected telescope f-ratio and camera pixel size.

Image taken with PARI 0.3-m f/10 telescope and 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

o POLARIS Observatory: 30 cm f/10 telescope and SBIG STV

o One 30 cm Telescope and CCD (SBIG T10XME)o Two 25 cm Telescopes and CCDs (SBIG STV

and SBIG ST-7)o Two 20 cm Telescopes and CCDs (Apogee AP4)o Two 12.7 cm Telescopes and SBIG STVso All Sky camera (SBIG AllSky)o Webcams and Weather Station

0.20 m

OVIEW

0.30 m 0.25 m

0.30 m (1.1 m)

(1.8 m)

Webcams, All Sky, Weather Station

0.25 m

Polaris Observatory

Optical Ridge located on PARI Campus at an altitude of 910 m

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 variable-comparison-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

JD 2452772+ Differential Magnitude0.55253 4.9440.56139 4.9100.56807 4.8940.57685 5.0430.58346 5.0430.59007 5.0600.59669 4.9260.60330 5.0340.60992 5.0580.61653 4.994

TABLE 1. Test Measurements

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.