Proposal for a research project to be carried. out in Physics 400 (Senior Research) and. IDIS 493 (Honors Thesis).

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1 Proposal for a research project to be carried out in Physics 400 (Senior Research) and IDIS 493 (Honors Thesis). Variable Star CCD Photometry and Analysis Amber L. Stuver Submitted in fulfillment of requirements for Physics 491. December 16, 1998

2 Abstract My research will focus initially on the techniques required to properly observe and analyze a variable star. The technique as planned includes selecting a variable star to observe and selecting a constant magnitude star in the same star field as the variable star, and observing the frequency of the variable star by taking CCD (charge-coupled device) images though the CCD camera. The software that accompanies the camera I plan to use has utilities that will assign the variable star a comparive magnitude with respect to the constant magnitude star I select as my reference. First, a short period star (preferably a member of the Cepheid populations) will be observed and its light curve will be plotted in order to refine my photometry techniques. A longer period variable star (perhaps of the Beta Canis Majois group) and/or an eclipscing binary system will be observed, weather permitting. Once the observations are complete, a Fourier analysis of the light curve will be performed in order to reveal any periodicities that are not obvious from the light curve and a hypothesis formation will be attempted in respect to what is causing the varying luminosity of the variable star.

3 Stuver 1 Introduction Variable stars are stars that vary in luminosity over a period of time which can range from several hours to many days and each star has its own characteristic period. 1 The purpose for the research being proposed here is to observe a variable star and analyze the luminosity variation over the star s period. CCD photometry will be implemented due to its sensitivity and convenient digital format. The data collected will be processed first by the CCD camera s software in order to plot the star s light curve and then this light curve will undergo a Fourier analysis to analyze the periodicities of the light curve. From this Fourier analysis, a hypothesis will be attempted in order to account for the luminosity variations. Background A CCD camera detects photons and these photons induce a charge which is then recorded and then interpreted by the camera to produce a digital image. The quantum efficiency (the photoelectric effects that induce a charge on the CCD) of CCD cameras is extremely high and thus makes the camera extraordinarily sensitive to the few photons that have traversed the light years and penetrated the earth s atmosphere. Along with the camera s sensitivity, CCD cameras are very linear and, by this, can measure light intensity accurately. 2 These two features make CCD astrophotography preferable over traditional silver astrophotography. Figure 1 illustrates the power of the CCD camera by

4 Stuver 2 showing an image of the horse-head nebula which is very faint but this 900 second exposure can resolve the nebula and stars around it well. Procedure Since a CCD camera is an area detector, it can cover a decent fraction of the telescope s field of view and can therefore observe more than one star. Therefore, a choice field of view would be one that contains a variable star and a constant magnitude star so that both are visible in the image. Using this comparative method allows for atmospheric variations from one night to another to be all but eliminated. Since the CCD has an intrinsic noise involved with it, a dark frame (a frame that is the same temperature and exposure time as the image being processed but not exposed to any photons) must be digitally subtracted from the actual raw image in order to eliminate the noise in the picture. This is done using the software that accompanied the CCD camera through a process called dark subtraction. It has also been determined that the most efficient temperature to take exposures at is approximately 25 o centigrade (since the intrinsic noise is temperature dependent). This was determined by plotting the average pixel value of ten second dark frames against the temperature, ranging from 40 o centigrade to room temperature, of the respective dark frames as illustrated in Figure 2. As shown there, reducing the temperature to below 25 o centigrade reduced the efficiency of the camera since it takes a great deal more energy to only decrease the noise marginally after that point.

5 Stuver 3 The software mentioned previously interprets the input data from the camera and has a utility that allows a star to be assigned a unit magnitude and every other star in that same image can be compared to that magnitude. 3 The numerical values from these comparative magnitudes are displayed through this utility and a light curve will be developed from these magnitudes. The observing techniques will first be tested by plotting a light curve for a short period variable star, possibly a member of the Dwarf Cepheids or the Beta Canis Majoris stars. Members of these groups have characteristic periods of between two to five hours, respectively, on average. Once the technique has been refined, a longer period variable star, perhaps a population I or population II Cepheid which have characteristic periods of between 5-10 days and days, respectively, will be observed. 4 As of this point in time, I plan to begin my prototype curve with β Canis Majoris which has a characteristic period of 5 approximately hours. Once the data has been gathered, a Fourier analysis will be carried out in order to detect any periodicities that are not obvious from the star s light curve. Further analysis of the Fourier analysis will lead to a formulation of a hypothesis as to what is causing the variation in the luminosity of the variable star observed. Resources The primary piece of apparatus will be a CCD (charged-coupled device) camera (specifically a model ST-6 CCD imaging camera manufactured by the Santa Barbara Instrument Group). Along with the CCD camera, a telescope around 6 inches in diameter

6 Stuver 4 and between 50 to 100 inches in focal length will be needed, a laptop computer to receive the data input from the CCD camera, and Fourier analysis software. The budget is therefore minimal since all of the equipment is already possessed by Frostburg State University and can be provided.

7 Figure 1 Stuver 5

8 Stuver 6 Figure 2 Temperature vs. Average Pixel Value for a 10 Second Dark Exposure Average pixel value Temperature (degrees C)

9 Stuver 7 Figure Captions: Figure 1: This is a CCD image of the horse-head nebula which is very difficult to resolve with moderate sized telescopes. This image was taken by the manufactures of the camera and is a 900 second exposure. Figure 2: This average pixel value for a 10 second dark frame vs. temperature plot shows how lowering the temperature below 25 o centigrade reduces the efficiency of the camera since it takes a great deal more energy to only to decrease the noise marginally. Works Cited 1. Pasachoff, Jay M., Donald H. Menzel, Stars and Planets, (New York: Houghton Mifflin, 1992), pp Zelik, Michael, Elske v. P. Smith. Introductory Astronomy and Astrophysics, (Philadelphia: Saunders, 1987), p Model ST-6 Professional CCD Imaging Camera, (California: Santa Barbara Instrument Group, n.d). p Reference 2, pp