The Open Cluster NGC 457 Item Type text; Thesis-Reproduction (electronic) Authors Shuart, Ross Allen Publisher The University of Arizona. Rights Copyright © is held by the author. Digital access to this material is made possible by the University Libraries, University of Arizona. Further transmission, reproduction or presentation (such as public display or performance) of protected items is prohibited except with permission of the author. Download date 04/10/2021 00:46:31 Link to Item http://hdl.handle.net/10150/621903 THE OPEN CLUSTER NGC 457 by Ross Allen Shuart A Departmental Thesis Submitted to the Faculty of thu DEPARTMENT OF ASTRONOMY In Partial Fulfillment of the Requirements For the Degree of MASTER OF SCIENCE In the Graduate College THE UNIVERSITY OF ARIZONA 1 9 7 3 ACKNOWLEDGMENTS The author wishes to thank the following people who aided him in the preparation of this study: Drs. Jeffrey D. Rosendhal, Bart J. Bok, and Raymond E. White, who served as couuuittee members; Dr. Roberta M. Humphreys, who originally suggested the investigation and served as its first director; Drs. Helmut A. Abt, Arthur A. Hoag, and Peter Pesch, for informal discussions; and Mr. Ed Howell, for the photographic work. Special acknowledgment must go to Dr. Abt for supplying the author with unpublished radial velocity material and to Dr. Hoag for the loan and permission to reproduce a plate of NGC 457. TABLE OF CONTENTS Page LIST OF TABLES iv LIST OF ILLUSTRATIONS V ABSTRACT vi INTRODUCTION 1 PREVIOUS STUDIES OF NGC 457 4 Position 4 Catalogues 6 Proper Motions 7 Photometry 17 Polarization 36 Radial Velocities 37 PRESENT SPECTROSCOPIC WORK 42 Objectives 42 Observational Work 42 Discussion of Results 45 DISCUSSION 49 FUTURE WORK 52 REFERENCES 54 iii LIST OF TABLES Table Page 1. Lists of Cluster Stars 6 2. Mean Proper Motions for Different Magnitudes 9 3. Magnitude Dependences of Proper Motion Differences 12 4. Proper Motions of the Bright Supergiants 16 5. Photometric Studies of NGC 457 17 6. Differences in Photoelectric Photometry '19 7. Effect of Model Differences on the Scatter of Photometric Differences 21 8. Photometry of Selected Stars 23 9. Magnitude Dependence of Color Excess 28 10. Mean Color Excesses 30 11. Main Sequence Fitting of NGC 457 34 12. Polarization 36 13. Radial Velocity Observations 38 14. Adopted Radial Velocities 39 15. Observational Record 43 16. Present Spectral Types 44 17. MK Types and Spectroscopic Parallaxes 46 iv LIST OF ILLUSTRATIONS Figure Page 1. The Open Cluster NGC 457 5 2. Mean Proper Motions for Different Magnitudes 10 3. Vector Point Diagram of Proper Motions in Cluster Field . 15 4. Possible Systematic Effects in Differences in Photometry . 20 5. Color -Color Diagram of NGC 457 26 6. Distribution of Derived Color Excesses 29 7. Color -Magnitude Diagram of NGC 457 with Individual Color Excesses Removed 31 8. Color- Magnitude Diagram of NGC 457 with Average Color Excess Removed 32 9. Color- Magnitude Diagram of Stars with MK Spectral Classifications 47 v ABSTRACT Previous work on the open cluster NGC 457 is discussed. The presence of a magnitude equation creates difficulty in using the published proper motions to determine membership of the brighter stars. Analysis of the available photometry reveals no systematic differences larger than 0.03 magnitude. The photometric distance modulus of the cluster is (m -M)0 = 12.31 ± 0.25. The main sequence turnoff point, (B -V)T = -0.24, indicates an age of 1.3 x 107 years. MK classifications of previously unclassified cluster stars are used in a new evaluation of the spectroscopic distance, (m -M) = 11.98 0 ± 0.39, which is brought into agreement with the photometric distance. Membership of the supergiant star 0 Cas in the cluster seems likely, but cannot be proven. Some possible consequences of its member- ship are discussed. vi INTRODUCTION Open clusters provide a valuable link between the group and individual properties of stars. Most of the nearby open clusters are bright enough to permit moderately detailed study of the separate stars. Assuming cluster membership then allows the individual properties to be related to the common distance and hopefully common origin of the stars. Roughly 50 open clusters and a comparable number of associations include at least one supergiant. Some well known examples are NGC 2362 and GMa, NGC 4755 and w Cru, NGC 6231 and C Sco, and NGC 457 and 0 Cas. The relationship between supergiants and open clusters is important in several ways. Absolute magnitudes of supergiants are difficult to obtain by statistical parallax methods because of the small proper motions of these stars. Open clusters furnish an accurate means of calibrating the luminosities of supergiants, including supergiants which are Cepheids. Since the cluster provides a reference velocity, the atmospheric effects on observed radial velocities might also be studied, although this does not seem to have been done. Evolutionary models of supergiants can be checked against the age of the cluster, which can be determined from models of the main sequence stars. Aarseth (1968) has shown that, if present, very massive stars will dominate the dynamical evolution of a cluster. It is for these reasons that a cluster with an apparently asso- ciated supergiant deserves to be well studied. A comprehensive investi- gation of an open cluster must include astrometric, photometric, and 1 2 spectroscopic observations. All available data should be assembled and studied as a whole, paying careful attention to the possibility of various systematic effects. The cluster NGC 457 has three supergiants associated with it, but lias never been fully investigated in the manner described above. Although individual studies have supplied considerable amounts of data, the results have not been combined with proper consideration for possible systematic errors. Despite the substantial data available for NGC 457, problems remain. The spectroscopic and photometric distances previously derived have been discordant. The membership of 0 Cas in the cluster still remains an open question, but, in spite of this, several researchers in the field of supergiants have used this star as a calibration standard on the assumption that it is a cluster member. Work by de Vegt et al. (1968) has suggested the possiblity of a large motion for NGC 457 in a direction perpendicular to the galactic plane, which would imply that the cluster is not bound to the Galaxy. Because of these problems, NGC 457 is a candidate for a thorough review. Adequate photometry to about visual magnitude 14 exists for NGC 457, while good proper motions require a longer time base than is cur- rently available for the cluster. Therefore, no attempt was made to secure more data of these types. The requirements of slit spectroscopy, fairly large telescopes and long exposures, have created a lack of sub- stantial spectroscopic information about the stars in NGC 457. The need for such data, combined with the ready availability of the proper 3 equipment, indicated the type of new observational work which would be most useful in a thorough discussion of NGC 457. This work uses the standard techniques employed in the study of open clusters, relatively few modifications being necessary. The following sections discuss the data of each type (astrometric, photometric, and spectroscopic) in the order which seems most useful. The discussion in each section begins with a brief statement of what can be accomplished with the type of data under consideration and of what methods are used in that section. There follows a review of the previous work and a search for systematic errors. The data are then combined to produce the final results. At each stage, an attempt is made to eliminate stars that are not cluster members in order to reduce their effects on the conclusions reached in the subsequent parts of the discussion. PREVIOUS STUDIES OF NGC 457 Listed in the late eighteenth century catalogues of Sir William Herschel, the open cluster NGC 457 is today an object with large amounts Of data available. Proper motions have been determined for about 900 stars in the field, three -color photometry of varying quality has been done for some 600 stars, and spectral types have been determined for several dozen stars. However, radial velocities and MK luminosity classi- fications are known for only about a dozen stars. Figure 1 is a large -scale view of NGC 457. The caption identifies the three supergiants in its vicinity. Position The NGC equatorial coordinates of NGC 457, when precessed to 1950.0, become 1h 15m 55s, 1-58° 03.6, the rounded values of which are given in the catalogue of Alter et al. (1970) which designates the cluster as 0C1 -321. The equivalent LAU galactic coordinates are 1 = 12656, b = -4.35. With an approximate distance of three kiloparsecs, NGC 457 is located in the Perseus arm of the Galaxy. Alter (1944) previously gave equatorial coordinates of 1h 16.4, +58o Olt (1950.0). Inspection of the Palomar prints suggests that this position is correct. Unfortunately, most recent work quotes the position developed from the NGC coordinates. 4 Figure 1. The Open Cluster NGC 457 NGC 457 as photographed with the Kitt Peak 84 -inch reflector on 1968 January 7. This print is a 2.4x enlargement of a 30- minute exposure on 103a -D emulsion with a CG11 filter. The print scale is 5:'1 /mm. North is toward the binding and west is toward the top of the page. The supergiant 0 Cas is the brightest star in the field. HD 7902 (type B6) is located 55 mm up and 35 mm to the left of the lower right hand corner of the print. HD 236697 (type M2) is 53 mm up and 47 imu to the right of the lower left hand corner.