An Examination of the Age of and Distance to NGC 1496 Andrew Wade and Curtis Roberts Sullivan South High School, Kingsport, TN Teacher: Thomas Rutherford, RBSE 2005 ABSTRACT Using the 0.9-meter SARA telescope (equipped with a CCD camera) located at the Kitt Peak National Observatory; images were obtained of the open cluster NGC 1496 in B and V photometric filters. From this data the age of and distance to the cluster were determined. NGC 1496 was found to lie at a distance of 1.4 x 10 3 parsecs (4600 light years) from Earth. In addition, the cluster, which contains no evident spectral class O or B stars, is approximately 400 million years old. INTRODUCTION An open cluster is a group of gravitationally-bound stars that formed in a single gas cloud. Members of an open cluster are approximately the same age, lie at approximately the same distance from the earth, and are composed of similar chemicals (Frommert 2007). The study of open clusters is crucial to astronomy because it provides insight into the nature of the formation of stars. All data used in this study were collected remotely on the night of February 25, 2010 using a Charged-Coupled Device (CCD) mounted on the SARA 0.9-meter telescope at Kitt Peak National Observatory (Tucson, AZ). Previous research of NGC 1496 includes photoelectric UBV and photographic RGU photometry by del Rio and Huestamendía in 1987. OBSERVATIONS AND DATA REDUCTION Six 60-second exposures of the cluster were taken in two filters (B and V). These images were then calibrated and stacked using AutoStar Suite in order to increase the signal to noise ratio of the data (LeFevre 2003). The two resulting images (one in each filter) were then analyzed photometrically using the software MaximDL. The RBSE Journal 1 2010 V4
Image 1: An image of the stacked data of the cluster in the B filter is shown above. The image s color has been inverted in order to facilitate the nature of the photometric analysis involved with MaximDL. Image 2: This study used the numbering system created by del Rio (1987). Some numbered stars were not included because of limitations involving photometry. The RBSE Journal 2 2010 V4
To determine the magnitudes of the other cluster members, NGC1496-22 was used as a reference in both filters. Star 22 was chosen because its magnitude was known from the WEBDA site; however, the software should calculate the same magnitudes for the entire data set if any other reference star had been used. After determining the other members magnitudes using MaximDL, the data was entered into a spreadsheet (Microsoft Excel) in order to efficiently subtract the V magnitudes from the B magnitudes. This gave the color index of each star. However, corrections to the data had to be made in order to account for interstellar extinction and reddening, which make the cluster members seem fainter and farther away than they actually are. In order to accomplish this, 0.45 was subtracted from each color index value (WEBDA), and 1.4 was subtracted from each V magnitude value (since the total absorption in the V band is approximately 3.1 times greater than the reddening) (Garmany 2007). ANALYSIS AND RESULTS After accounting for reddening in the (B-V) calculations, the stars were classified by spectral type. Table 1 presents the criteria used to classify the cluster members. Table 1: The color index of a star may be used to estimate its spectral type. It may also be used to estimate the star s lifespan (Washington 2005, Clemens 2007). Spectral Type Color Index (B-V) Lifespan (years) O -0.4 <1 x 10 6 B -0.2 3 x 10 7 A 0.2 4 x 10 8 F 0.5 4 x 10 9 G 0.7 1 x 10 10 K 1.0 6 x 10 10 M 1.6 >1 x 10 11 The cluster members were next plotted according to the Hertzsprung-Russell (H-R) model (Comins 2003). Then the Schmidt-Kaler ZAMS (Zero-Age-Main-Sequence) graph (1982) was juxtaposed onto the graph. By examining the H-R diagram in relation to the ZAMS line, the turnoff point of NGC 1496 became apparent. The star that sits on the tip of the main sequence is approximately the same age as the cluster itself (Rieke 2002). Using this method, the age of NGC 1496 is no greater than 400 million years. The RBSE Journal 3 2010 V4
Figure 1: (X=color index (b-v), Y=Brightness (v)) According to the graph, the tip of the main sequence appears to lie around NGC 1496-1. This star s color index is approximately 0.062, classifying it as a spectral type A star. Based on this observation, the age of the cluster is estimated to be no greater than 400 million years. Stars 21 and 46 appear to be outliers in the data set. This could imply that they are not cluster members, but are in fact closer to the earth than is the actual cluster. Since all stars in a cluster lie at approximately the same distance from Earth (Frommert 2007), we can determine the distance of the entire cluster from Earth based on a few stars. However, the absolute magnitude of these stars must be known. Thus 3 standard stars, Vega (α Lyrae), Procyon (α Canis Minoris), and Capella (α Aurigae) ( Stellar Brightness, The Colour of Stars ), with known absolute magnitudes and color indices equal to that of three of the cluster members-- stars 2, 7, and 15, respectively, were chosen and the distance modulus equation applied to them (Seeds 2005): d=10 (m-m+5)/5 Equation 1 where m=apparent V magnitude (11.484, 14.061, 9.786), M=absolute V magnitude (0.6, 2.6, 0.4), and d=distance from Earth (in parsecs). The resulting distances from each calculation were then averaged in order to determine the cluster s distance from Earth, which is 1400 parsecs, or 4600 light years. The RBSE Journal 4 2010 V4
DISCUSSION There are some differences with the data observed in this study and del Rio s determination of the age of the cluster. After converting (G-B) data to (B-V) data, del Rio determined the age of the cluster to be 630 million years (1987) while the current study arrived at an age of 400 million years. This discrepancy may be due to differences in photometric analysis as well as in the method of image capture (CCD vs. photoelectric photometer). The distance to the cluster observed in del Rio s paper also varies from the findings in this study (1230 pc versus 1400 pc) (1987), although not by a great deal. SUMMARY Using modern methods of photometry to analyze images taken of NGC 1496 using the 0.9-meter SARA telescope, the age of the cluster (400 million years) and its distance from Earth (1400 pc) were determined. These results do vary from those of previous studies (del Rio 1987); however, this may be due to differences in the methods used to collect data. ACKNOWLEDGEMENTS The researchers would like to thank the following people for their contributions to the project: Dr. Gary Henson (ETSU), for collecting the images using the SARA telescope, the Southeastern Association for Research in Astronomy (SARA); Curtis Roberts, for assisting with various aspects of the project; Mr. Thomas Rutherford, for advising and guiding this endeavor; and Ms. Melanie Calhoun, for her cooperation and support of the project. REFERENCES Clemens, Christina and Rachel Reece. The Age and Distance of the Open Cluster NGC 2345. RBSE Journal. 2007. 23 March 2010. The Colour of Stars. [Online] http://outreach.atnf.csiro.au/education/senior/astrophysics/photometry_colour.html December 24, 2004. Comins, Neil and William Kaufmann III. Discovering the Universe. 65 th ed. New York: W.H. Freeman and Company, 2003. 277. Del Rio, G. and G. Huestamendía. 1988. Photoelectric UBV and Photographic RGU Photometry of the Open Clusters NGC 1496 and NGC 1513. Astronomy and Astrophysics Supplement Series. Vol 73, 425. Distances to Open Clusters. [Online] http://www.math.lsa.umich.edu/mmss/courses ONLINE/Astro/Ex1.3/ 2002. Frommert, Hartmut and Christine Kronberg. Open Star Clusters. [Online] http://messier.obspm.fr/open.html August 27, 2007. Garmany, Katy. 2007 March 22. A Photometry Question [email.] Accessed 2010 March 29. Kaisler, Denise. Cosmic Intrigue. Astronomy (2000): 42-46. The RBSE Journal 5 2010 V4
Kitt Peak National Observatory. [Online] http://www.noao.edu/kpno/ 2010. LeFevre, Paul.. Aligning and Stacking Images. [Online] http://www.lefevre.darkhorizons.org/articles/proctutorialchap3.htm 2003. Rieke, Marcia. Star Clusters, Introduction to the Milky Way. [Online] http://ircamera.as.arizona.edu/astr_250/lectures/lecture_18.htm March 18, 2002. Schmidt-Kaler, Th. 1982, Landolt-Börnstein, Numerical Data and Functional Relationships in Science and Technology, New Series, Group VI, vol. 2(b), ed. K. Schaifers, & H.H. Voigt (Berlin: Springler Verlag), 14 Stellar Brightness. [Online] http://www.astro.wisc.edu/~dolan/constellations/extra/brightest.html 2010. University of Washington. Cluster Color-Magnitude Diagrams and the Age of Stars. 16 May 2005. 29 March 2010. The RBSE Journal 6 2010 V4
APPENDIX 1. The following table presents each star used in this study. Some cluster members were omitted. Star Name X Position Y Position B Magnitude V Magnitude Color Index (B V) B V (red) V (red) Spectral Class NGC 1496 1 0 0 12.535 12.023 0.512 0.062 10.623 A 2 6.7 6.5 13.452 12.884 0.568 0.118 11.484 A 3 13.6 6.9 13.913 13.365 0.548 0.098 11.965 A 4 17.3 11.7 13.5 12.887 0.613 0.163 11.487 A 5 12.1 20 15.141 14.347 0.794 0.344 12.947 A 6 1.1 27.4 14.241 13.641 0.6 0.15 12.241 A 7 10.4 32.6 16.397 15.461 0.936 0.486 14.061 F 8 1.1 43.7 14.291 13.621 0.67 0.22 12.221 A 9 4.8 41.6 13.562 12.303 1.259 0.809 10.903 G 10 41.5 34.1 16.086 15.315 0.771 0.321 13.915 A 11 36.4 47.2 15.366 14.607 0.759 0.309 13.207 A 12 14.4 60.7 16.398 15.547 0.851 0.401 14.147 F 15 58.4 120.9 12.31 11.186 1.124 0.674 9.786 G 16 92.7 89.6 16.185 15.304 0.881 0.431 13.904 F 17 63.5 75.7 16.601 15.651 0.95 0.5 14.251 F 20 68.5 103.1 15.797 14.905 0.892 0.442 13.505 F 21 114.1 106.6 14.486 13.721 0.765 0.315 12.321 G 22 68.1 63.6 15.79 14.89 0.9 0.45 13.49 F 23 82.6 65.8 13.719 13.161 0.558 0.108 11.761 A 24 96.1 57.2 13.957 13.005 0.952 0.502 11.605 F 25 103.6 35.9 16.83 14.776 2.054 1.604 13.376 M 26 111 49.6 15.668 14.659 1.009 0.559 13.259 F 27 118.9 55.3 15.017 14.377 0.64 0.19 12.977 A 28 121.4 71 14.616 13.252 1.364 0.914 11.852 M 29 141.3 47.9 15.33 14.68 0.65 0.2 13.28 A 30 52.8 27.7 14.903 14.111 0.792 0.342 12.711 A 31 44.3 24.5 14.154 13.557 0.597 0.147 12.157 A 32 41.9 15.3 16.132 15.277 0.855 0.405 13.877 F 33 34 20.5 15.239 14.577 0.662 0.212 13.177 A 34 36.7 0.5 12.346 11.436 0.91 0.46 10.036 F 35 44.2 23.4 14.652 14.032 0.62 0.17 12.632 A 36 22.3 47.6 13.196 12.645 0.551 0.101 11.245 A 37 84.3 41.3 15.617 14.893 0.724 0.274 13.493 A 38 96.3 59.1 15.239 14.393 0.846 0.396 12.993 F 39 92.5 63.1 16.218 15.423 0.795 0.345 14.023 A 40 100 74.8 15.873 14.981 0.892 0.442 13.581 F 42 65 105.6 14.337 13.73 0.607 0.157 12.33 A 43 33.2 96.5 14.423 13.795 0.628 0.178 12.395 A The RBSE Journal 7 2010 V4
44 20 113.9 14.926 14.073 0.853 0.403 12.673 F 45 31.3 153.3 14.939 14.025 0.914 0.464 12.625 F 46 12 154.6 15.473 14.69 0.783 0.333 13.29 A 47 6.5 79.1 15.009 14.085 0.924 0.474 12.685 F 48 34.4 41.5 14.356 13.766 0.59 0.14 12.366 A 49 57.1 43.5 15.616 14.883 0.733 0.283 13.483 A 50 68.7 57.2 16.372 15.495 0.877 0.427 14.095 F 51 78.4 34.3 16.583 14.768 1.815 1.365 13.368 M 52 135.8 86.8 13.712 13.123 0.589 0.139 11.723 A 53 143.3 48.2 14.905 14.192 0.713 0.263 12.792 A 54 143.2 39.8 14.874 14.232 0.642 0.192 12.832 A 55 91.1 10.7 15.355 14.674 0.681 0.231 13.274 A The RBSE Journal 8 2010 V4