Observing the Mira Variable Star S Leo Using CCD Photometry Lindsey A. Whitesides¹, Florin D. Ciocanu¹, Dr. Richard Dietz² ¹Frontiers of Science Institute ²Department of Physics, University of Northern Colorado Abstract Many amateur astronomers observe variable stars and submit the information obtained to databases in order to expand the amount of data available to learn more about star mechanics. In this study, the Mira variable star S Leo was observed using CCD telescopes operated by Global Rent a Scope. By analyzing the results in VPhot, accurate data was obtained and a partial light curve was created. The results were submitted to AAVSO for further use by other astronomers. Abstract Many amateur astronomers observe variable stars and submit the information obtained to databases in order to expand the amount of data available to learn more about star mechanics. In this study, the Mira variable star S Leo was observed using CCD telescopes operated by Global Rent a Scope. By analyzing the results in VPhot, accurate data was obtained and a partial light curve was created. The results were submitted to AAVSO for further use by other astronomers. Introduction In this study, the Mira variable star S Leo was observed using CCD telescopes in Nerpio, Spain and Mayhill, New Mexico, USA. A partial light curve was constructed to discover the day at which S Leo was at its faintest. The results were compared with the trend of the observations taken from the American Association of Variable Star Observers (AAVSO). The goal was to achieve data accurate to within 1/100 th of a magnitude and submit the data collected to AAVSO. This project taught us how to remotely control CCD telescopes, use the program VPhot to analyze the data gathered, make accurate measurements of the physical characteristics of variable stars and to contribute the knowledge gained to the scientific community. A variable star changes in magnitude (brightness) over time. S Leo is a Mira variable. Mira variables are red giants that pulsate over long periods between 150 days and several years. They vary in amplitudes from 2.5 to 10 magnitudes. As magnitude is a logarithmic scale, this accounts for a star varying in luminosity by 10 to 10,000 fold. These stars are relatively cool in comparison to other stars at 3500 K. Charge-coupled device (CCD) telescopes were used in this study. CCDs are silicon chips that operate via the photoelectric effect. A photon of light hits the silicon atom and excites the electrons. The pattern of excited electrons is transferred to the computer and stored as an image. CCDs transformed the way astronomers conduct photometry, the technique of measuring ­the intensity of an astronomical object's electromagnetic radiation. From a series of images taken by CCDs, a light curve can be generated, revealing a range of magnitude and a period for the star. Astronomy is a unique science because the amateurs are as important as the astrophysicists. Using the CCD telescopes, backyard astronomers produce highly accurate observations which are submitted to databases such as AAVSO and combined with results from hundreds of other astronomers. Observing variable stars is important because it helps professional astronomers to determine the long term behavior of a star and to correlate and create variable star models for better understanding of the universe. Introduction In this study, the Mira variable star S Leo was observed using CCD telescopes in Nerpio, Spain and Mayhill, New Mexico, USA. A partial light curve was constructed to discover the day at which S Leo was at its faintest. The results were compared with the trend of the observations taken from the American Association of Variable Star Observers (AAVSO). The goal was to achieve data accurate to within 1/100 th of a magnitude and submit the data collected to AAVSO. This project taught us how to remotely control CCD telescopes, use the program VPhot to analyze the data gathered, make accurate measurements of the physical characteristics of variable stars and to contribute the knowledge gained to the scientific community. A variable star changes in magnitude (brightness) over time. S Leo is a Mira variable. Mira variables are red giants that pulsate over long periods between 150 days and several years. They vary in amplitudes from 2.5 to 10 magnitudes. As magnitude is a logarithmic scale, this accounts for a star varying in luminosity by 10 to 10,000 fold. These stars are relatively cool in comparison to other stars at 3500 K. Charge-coupled device (CCD) telescopes were used in this study. CCDs are silicon chips that operate via the photoelectric effect. A photon of light hits the silicon atom and excites the electrons. The pattern of excited electrons is transferred to the computer and stored as an image. CCDs transformed the way astronomers conduct photometry, the technique of measuring ­the intensity of an astronomical object's electromagnetic radiation. From a series of images taken by CCDs, a light curve can be generated, revealing a range of magnitude and a period for the star. Astronomy is a unique science because the amateurs are as important as the astrophysicists. Using the CCD telescopes, backyard astronomers produce highly accurate observations which are submitted to databases such as AAVSO and combined with results from hundreds of other astronomers. Observing variable stars is important because it helps professional astronomers to determine the long term behavior of a star and to correlate and create variable star models for better understanding of the universe. Methods Global Rent A Scope (GRAS) is an organization that operates 14 CCD telescopes in 3 locations: 10 in Mayhill, New Mexico; 4 in Victoria, Australia; and 3 in Nerpio, Spain. In this project, telescopes G1, located in New Mexico and G7, located in Spain, were used. In this study, the images used were taken by UNC students under the supervision of Dr. Dietz but the process was replicated by imaging another star. After selecting either G1 or G7, a plan was generated. In the generating of the plan, the coordinates of S Leo (expressed in right ascension and dseclination), the picture count, type of filter, and exposure time were chosen. S Leo has a R.A. of 11h 10m 50s and a Dec. of +05° 27’ 34”. The picture count was set to 1, a Visual or V-Filter was used, and the exposure time was set to 120 or 180 seconds. The options to plate-solve the image and send it directly to VPhot were selected. Methods Global Rent A Scope (GRAS) is an organization that operates 14 CCD telescopes in 3 locations: 10 in Mayhill, New Mexico; 4 in Victoria, Australia; and 3 in Nerpio, Spain. In this project, telescopes G1, located in New Mexico and G7, located in Spain, were used. In this study, the images used were taken by UNC students under the supervision of Dr. Dietz but the process was replicated by imaging another star. After selecting either G1 or G7, a plan was generated. In the generating of the plan, the coordinates of S Leo (expressed in right ascension and dseclination), the picture count, type of filter, and exposure time were chosen. S Leo has a R.A. of 11h 10m 50s and a Dec. of +05° 27’ 34”. The picture count was set to 1, a Visual or V-Filter was used, and the exposure time was set to 120 or 180 seconds. The options to plate-solve the image and send it directly to VPhot were selected. Results The table displays the information gathered about S Leo from February to April. The magnitude increased and then decreased, which is to say, S Leo became fainter and then brighter again. According to the partial light curve constructed from the data obtained, the minimum magnitude occurred around the first day of April. For every picture, the magnitude of S Leo was found. The approximate error and standard deviation of the given magnitude was also provided. On average throughout the eight images, the error was about This means that the magnitude given could be incorrect by plus or minus the error. The data gathered was plotted onto the light curve of S Leo from AAVSO during the same time frame. From AAVSO, the data was gathered that the period is 190 days and the range of apparent magnitude is When the inverse-square law calculation was computed for the distance of Earth to S Leo, in the correct amount of significant figures, the result was 12,000 light years. Results The table displays the information gathered about S Leo from February to April. The magnitude increased and then decreased, which is to say, S Leo became fainter and then brighter again. According to the partial light curve constructed from the data obtained, the minimum magnitude occurred around the first day of April. For every picture, the magnitude of S Leo was found. The approximate error and standard deviation of the given magnitude was also provided. On average throughout the eight images, the error was about This means that the magnitude given could be incorrect by plus or minus the error. The data gathered was plotted onto the light curve of S Leo from AAVSO during the same time frame. From AAVSO, the data was gathered that the period is 190 days and the range of apparent magnitude is When the inverse-square law calculation was computed for the distance of Earth to S Leo, in the correct amount of significant figures, the result was 12,000 light years. Discussion The results of this study were successful. The average error (0.0074) of the data produced was below the desired 1/100 th of a magnitude. This low of an error makes it accurate enough to be submitted to AAVSO for future reference. We were also successful in generating a partial light curve from the data collected. Our partial light curve was compared with prevalidated data provided by the contributions of other astronomers, creating a precise correlation. Constructing a regression through the observed values enabled us to find a minimum brightness of S Leo and attribute a date to it (around April 1). Although the final data turned out well there were a couple issues with the images. Out of the 14 pictures, only eight were usable. Two images were of poor quality, creating too large of an error in order to be useful. The other four unusable pictures were not plate-solved, meaning comp star data could not be retrieved. If this project were to be improved, more images should be taken. The two months and 23 days time period is not enough to construct a complete light curve. Despite the limitations of our data, the results were significant because the error was low enough to contribute to the scientific community for further research. Discussion The results of this study were successful. The average error (0.0074) of the data produced was below the desired 1/100 th of a magnitude. This low of an error makes it accurate enough to be submitted to AAVSO for future reference. We were also successful in generating a partial light curve from the data collected. Our partial light curve was compared with prevalidated data provided by the contributions of other astronomers, creating a precise correlation. Constructing a regression through the observed values enabled us to find a minimum brightness of S Leo and attribute a date to it (around April 1). Although the final data turned out well there were a couple issues with the images. Out of the 14 pictures, only eight were usable. Two images were of poor quality, creating too large of an error in order to be useful. The other four unusable pictures were not plate-solved, meaning comp star data could not be retrieved. If this project were to be improved, more images should be taken. The two months and 23 days time period is not enough to construct a complete light curve. Despite the limitations of our data, the results were significant because the error was low enough to contribute to the scientific community for further research. References Bucheim, R.K. (2007). The sky is your laboratory: Advanced astronomy projects for amateurs. Chichester, UK: Springer-Praxis Clayton, M.L. & Feast M.W. (August 11, 1969). Absolute magnitudes of Mira variables from statistical parallaxes. Monthly Notices of the Royal Astronomical Society, vol. 146, p doi: 1969MNRAS C O’Connell, Robert (September 8, 2003). Magnitude and color system. Retrieved from Richmond, M. Introduction to CCDs. Retrieved from Robertson, B.S.C. & Feast M.W. (July 16, 1980). The bolometric, infrared and visual absolute magnitudes of Mira variables. Monthly Notices of the Royal Astronomical Society, Monthly Notices, vol. 196, July 1981, p doi: 1981MNRAS R Roger A. Freedman, William J. Kaufmann III (2008). Universe: Stars and Galaxies (3 rd edition). New York, NY: W.H. Freeman and Company Templeton, M. (April 14, 2011). Variable stars and the stories they tell. Retrieved from Templeton, M. (September 13, 2010). Stellar evolution. Retrieved from References Bucheim, R.K. (2007). The sky is your laboratory: Advanced astronomy projects for amateurs. Chichester, UK: Springer-Praxis Clayton, M.L. & Feast M.W. (August 11, 1969). Absolute magnitudes of Mira variables from statistical parallaxes. Monthly Notices of the Royal Astronomical Society, vol. 146, p doi: 1969MNRAS C O’Connell, Robert (September 8, 2003). Magnitude and color system. Retrieved from Richmond, M. Introduction to CCDs. Retrieved from Robertson, B.S.C. & Feast M.W. (July 16, 1980). The bolometric, infrared and visual absolute magnitudes of Mira variables. Monthly Notices of the Royal Astronomical Society, Monthly Notices, vol. 196, July 1981, p doi: 1981MNRAS R Roger A. Freedman, William J. Kaufmann III (2008). Universe: Stars and Galaxies (3 rd edition). New York, NY: W.H. Freeman and Company Templeton, M. (April 14, 2011). Variable stars and the stories they tell. Retrieved from Templeton, M. (September 13, 2010). Stellar evolution. Retrieved from Acknowledgements This project would not have been possible without the generous support of many people. First and foremost, we would like to extend our deepest gratitude to our research sponsor, the Bacon Family Foundation, for funding this project, and our scholar sponsors, Ball Aerospace Technologies & Corporation and Xcel Energy Inc., for funding our participation in the FSI program. Just as important, we would like to thank our research mentor Dr. Dietz for all the time that he spent helping and teaching us, as well as his willingness to try new things. A huge thank you goes to Lori Ball, the program administrator; our advisor, Nick True; our teachers Abby Davidson, Nick True, Nathan Kirkley, and Zabedah Saad; and our resident advisors Karen Allnutt and Klaus Broeker for all the ways that they helped us. Lastly, we would like to thank our families for being supportive and allowing us to have this experience. Acknowledgements This project would not have been possible without the generous support of many people. First and foremost, we would like to extend our deepest gratitude to our research sponsor, the Bacon Family Foundation, for funding this project, and our scholar sponsors, Ball Aerospace Technologies & Corporation and Xcel Energy Inc., for funding our participation in the FSI program. Just as important, we would like to thank our research mentor Dr. Dietz for all the time that he spent helping and teaching us, as well as his willingness to try new things. A huge thank you goes to Lori Ball, the program administrator; our advisor, Nick True; our teachers Abby Davidson, Nick True, Nathan Kirkley, and Zabedah Saad; and our resident advisors Karen Allnutt and Klaus Broeker for all the ways that they helped us. Lastly, we would like to thank our families for being supportive and allowing us to have this experience. DateMagnitudeError Standard Dev. Exposure Time s s s s s s s s Next, a reservation was made on the GRAS website. The reservation was made based on the local time of the telescope. Other factors such as weather forecast and distance from moon (minimum 65-70° away) were considered when making the reservation. After the pictures were taken, they were either manually or automatically transferred to VPhot, a browser-based software that enabled us to accurately measure S Leo’s brightness. Once the picture was ready, the General Catalogue of Variable Stars (GCVS) was used to identify S Leo and then comparison stars were downloaded from AAVSO. Aperture radius and sky annulus must be changed so that the entire radius of the star is enclosed in the aperture radius and the sky annulus includes nothing but the background sky. After a series of operations, the most stable 3 stars were selected. The most “well behaved” comp star is designated to be the check star which acts like a target star with known magnitude for differentiation. Finally, the Photometry Report was viewed and the results were collected. Next, a reservation was made on the GRAS website. The reservation was made based on the local time of the telescope. Other factors such as weather forecast and distance from moon (minimum 65-70° away) were considered when making the reservation. After the pictures were taken, they were either manually or automatically transferred to VPhot, a browser-based software that enabled us to accurately measure S Leo’s brightness. Once the picture was ready, the General Catalogue of Variable Stars (GCVS) was used to identify S Leo and then comparison stars were downloaded from AAVSO. Aperture radius and sky annulus must be changed so that the entire radius of the star is enclosed in the aperture radius and the sky annulus includes nothing but the background sky. After a series of operations, the most stable 3 stars were selected. The most “well behaved” comp star is designated to be the check star which acts like a target star with known magnitude for differentiation. Finally, the Photometry Report was viewed and the results were collected.