Open clusters of stars are several dozen stars gravitationally bound.  Open cluster systems are important for the study of the dynamics of or Milky Way Galaxy.  They are the key objects to understanding the motion of the Galaxy’s spiral arms and distinguishing between star formation processes.  To investigate the motions of the spiral arms, it is critical to have accurate proper motion, radial velocities, distances and ages.

There are roughly 1,700 known open clusters, but only 1,000 have been studied to any degree.  The other 700 are known to exist, but nothing else is known about them. 

PARI has a collection of images of the clusters.  The images are on glass photographic objective prism plates which are archived in the National Plate Center at PARI.  The Open Cluster Project uses the images toclassify the stellar spectral types.  From that information the distances and ages of clusters are determined.  This study is long-term, and has many aspects.  Aspects include:  1) identification of the cluster stars;  2) determine the proper motions of the stars; 3) determine the spectral types of the cluster stars; and 4) measure the colors of the cluster stars.  We begain this project in 2006 by focus on just 4 clusters where listlte is known.  We are expanding this project in 2007 to 28 high galactic latitude clusters.

The first steps in this program have been the topics of two undergraduate summer interns.  Presented here are the abstracts by the two students as submitted to the American Astronomical Society for presentation at the January 2007 209th Meeting of the AAS and the January 2008 211th Meeting of the AAS

Roslaie McGurk, an undergraduate from the University of Washington in Seattle spent the summer at PARI studying open star clusters.  She presented her results at the 209th Meeting of the American Astronomical Society on January 9, 2007.  The abstract for her presentation is given in the next paragraph.

New information was obtained for the open star clusters Dolidze 27 (Dolidze, M.V. 1961, Astron. Cir., 223, 11) and Alessi 10 (Alessi B. S., Moitinho A., Dias W.S. 2003, A&A, 410, 565). Two 20-minute exposure objective prism plates were selected from the collection of Michigan Blue Sky Objective Prism Plates in the North American Astronomical Photographic Plate Center located at the Pisgah Astronomical Research Institute.  The plates were used to determine the spectral types for the stars in the two clusters.  Alessi 10 consists primarily of late B and early A type stars, whereas Dolidze 27 has primarily K-type stars.  Magnitudes of all stars in approximately 45 arcminute diameter areas centered on the clusters were measured from B and V images.  The images were taken in July 2006 using the 40 cm Panchromatic Robotic Optical Monitoring and Polarimetry Telescopes (PROMPT; Reichart et al. 2005, Il Nuovo Cimento C, vol. 28, Issue 4, p.767). Color magnitude diagrams (CMD) from the B and V magnitudes were used to analyze the membership of the clusters and to determine the reddening, distanced modulus, and distances to the probable members of the clusters.  E(B-V) = 0.55 +/- 0.02 mag, the distance modulus is 9.0 mag +/- 0.1 mag, and distance is 290 +/- 100 pc for Dolidze 27.  Alessi 10 is reddened by 0.22 +/- 0.01 mag, and has a distance modulus of 9.4 +/- 0.1 magnitudes, and a distance of 550 +/- 100 pc. Known proper motions from Hipparchos for the brighter stars in the regions were also used to determine cluster membership. The stars of Dolidze 27 had average proper motions of m_a = -7.50 mas and m_d = -13.94 mas, and the stars of Alessi 10 had m_a = 3.06 mas and m_d = -6.84 mas.

Michael Aubrey, an undergraduate from the University of North Carolina-Chapel Hill, spent the summer at PARI studying open star clusters.  He will be presenting his results at the 211th Meeting of the American Astronomical Society in January 2008.  The abstract for his presentation is given in the next paragraph.

Analysis of Open Star Clusters Collinder 394 and NGC 6645.  M. Aubrey (University of North Carolina at Chapel Hill and Pisgah Astronomical Research Institute Summer Intern) and M. Castelaz (Pisgah Astronomical Research Institute).  Data was collected for open star clusters Collinder 394 and NGC 6645 (Dias et al. 2003, in Galactic and Stellar Dynamics, C. Boily et al. (eds), EAS Publications Series, 10, 195).  Collinder 394 is well-studied, whereas only position and possible number of members are known for NGC 6645. Collinder 394 and NGC 6645 were imaged using the PROMPT Observatory (Reichart et al. 2005, Il Nuovo Cimento C, vol. 28, Issue 4, p.767) for BV photometry. The B and V magnitudes were measured for 28 cluster members in Collinder 394. For NGC 6645, B and V magnitudes were measured of all stars within 10 arcminutes of the cluster’s center. Cluster membership, distance modulus, interstellar reddening and distance to cluster were determined from the observed BV color magnitude diagrams (CMD).  The distance modulus of Collinder 394 was found to be 9.30mag ±0.2mag and a reddening of 0.30mag ±0.02mag, implying a distance of 630pc ±100pc. NGC 6645 was found to have a distance modulus of 9.8mag ±0.2mag and a reddening of 0.46mag ±0.02mag, implying a distance of 740 pc ±100pc. Of the 119 stars measured in the region of NGC 6645, 72 stars were found to be possible cluster members based on the CMD. The best match of a theoretical CMD (Bressan et al. 1996, AA Supp, 117, 113 to that observed for stars in NGC 6645 is the isochrone with temperature range of 6600 K and 8100 K and mass range from 0.86 to 1.04 solar masses. The age of NGC 6645 was found to be about 9.7 billion years from the turnoff on its color-magnitude diagram.  This make NGC 6645 one of the oldest open clusters.