The composite radiation of galactic star clusters Item Type text; Thesis-Reproduction (electronic) Authors Vanek, Polly Elizabeth Hanson, 1927- 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 25/09/2021 13:11:27 Link to Item http://hdl.handle.net/10150/551646 THE COMPOSITE RADIATION OF GALACTIC STAR CLUSTERS bgr Polly H* Vanek A Thesis Submitted to the Faculty of the DEPARTMENT CF ASTRONOMY: In Partial Fulfillment of the Requirements For the Degree of MASTER CF SCIENCE In the Graduate College THE UNIVERSITY CF ARIZONA 1963 v STATEMENT BY AUTHOR This thesis has been submitted in partial fulfillment of re­ quirements for an advanced degree at The University of Arizona and is deposited in The University Library to be made available to bor­ rowers under rules of the Library* Brief quotations from this thesis are allowable without special permission, provided that accurate acknowledgment of source is made* Requests for permission for extended quotation from or reproduction of this manuscript in whole or in part may be granted by the head of the major department or the Dean of the Graduate College when in their judgment the proposed use of the material is in the interests of scholarship* In all other instances, however, permission must be obtained from the author* SIGNED: APPROVAL BY THESIS DIRECTOR This thesis has been approved on the date shown below: o /V / y CjJ ARTHUR A. HOAG Date Director, Flagstaff Station U.S. Naval Observatory ABSTKACT The composite, reddening-corrected, B-V color-index, (B-V)0, is computed for each of ten galactic clusters. NGG 129 is further investi­ gated in order to determine the maximum possible range in (B-V)o. This is less than about 0^15 • It is shown that only those stars within 4 mag. of the brightest assumed cluster member are needed to obtain (B-V)q to within about 0?06. The color-age relation for these clusters is shown to be linear, with the equation (&-V)T = -0.07 + 0.67 (B-V)o. ±.02 ±.02 (m.e.) The slope of this equation is the same as that of the color-age relation obtained from the observed integrated colors of Kron and Mayall (i960), with the observed curve shifted 0?13 to the red of the computed curve. It is suggested that the color-age relation might be used to predict, in most cases to within about 0?1, the (B-V)t of galactic clusters which are so remote that their ages cannot be determined relia­ bly by present methods. iii CONTENTS Section 1* The Observed Color-Age Relation ••••••»••! Section 2, The Computed Color-Age Relation Section 3. The (5?) of NGC 1 2 9 ................................. 6 o a) The effect on (B-V)o of including only those stars within 4 nag. of the brightest assumed cluster member b) The effect on (B-V) of different determinations < * * * * • • • • ...............................16 c) The effect on (B-V)o of the Cepheid DL Gas. 16 d) The effect on (B-V)o of assumed members for which there are no UBV observations ...... 17 e) The effect on (B-¥)q of observational selection • .17 f) The effect on (B-V)q of various assumptions concerning cluster membership ........IB Section 4. Conclusion ................ 19 Section 5. Acknowledgments • • • .........................21 iv LIST OF TABIES I. Clusters used to determine the computed color-age relation • • 5 II# Stars within 4 mag. of DL Gas in the field of NGC 129# • • • 8 III# The effect on (B-V) of including only those stars within o 4 mag# of the brightest assumed cluster member . .... 16 IV# Effect on (B-V)o of the Cepheid DL Gas# ........17 V. The effect of observational selection on (B-V)o, * .... 18 VI. The effect on (B-V)q of various assumptions concerning cluster membership. *.....*.......19 V LIST OF FIGURES 1, The observed colon-age relation for galactic clusters * , . * 2 2# The computed color-age relation for galactic clusters • # • • 4 3. Observations of stars in the field of NGC 129 •••••••15 4. The range in (B-V)o for NGC .............. .. ................ 20 vi 1. THE OBSERVED COLOR-ACE RELATION Kron and Mayall (I960) give photoelectric observations, on their PVI system, of the integrated colors of 137 star clusters. Of these, 159 are globular clusters, in external galaxies as well as in our own. The remaining 28 are galactic clusters that are members of our own system. Kron and Mayall (I960) present a three-color (two-color-index) diagram of the observations, uncorrected for reddening, and suggest that such three-color photometry of integrated light may provide a classifica­ tion, perhaps according to age, of both galactic and globular clusters. They state that the corrected integrated magnitudes are probably reliable to 0?5 for galactic clusters as well as for sparse globular clusters. These data are used in obtaining the observed color-age relation for galactic clusters, plotted in Figure 1. The abscissa, (B-V)0, is the reddening-corrected composite B-V color-index, obtained by the use of their transformation equation relating B-V to P-V, along with the color excess, Eg_y, determined for each cluster by Johnson, Hoag, Iriarte, Mitchell, and Ballam (1961). The ordinate, (B-V)^, is the color-index of the point on the zero-age main sequence at which the cluster sequence turns up, and is therefore an indicator of the cluster age. These values are also from Johnson et al. (1961). The equation of the linear regression line is (B-V)t = -0.16 + 0.67 (B-V^. -•07 —.22 (sue.) 1 P!g. 1. TW for galaetie cl\»Ur«# 3 2. THE COMPUTED COLOR-AGE RELATION Another approach to the problem is suggested by Johnson (1959), who computes the integrated colors for the Eyades and for M6? from observations of individual cluster members. He indicates that UBV photometry, alone, could not distinguish between types of globular clusters; but he makes no statement with regard to galactic clusters. An approach similar to his may be used to obtain the color-age relation of Figure 2. Computations of the composite B-V color-indices were made on the IBM Type-650 computer of the Numerical Analysis Labora­ tory of the University of Arizona. The computer program first, for each star, corrects the observed V magnitude for the visual absorption, Ay, and corrects the B-V color-index for Eg_y> using the method of Hiltner and Johnson (1956), next converts these values into V and B intensity ratios, then sums these ratios for all assumed cluster members, and finally reconverts these sums into a composite color-index, (B-V)q. The computer went through this procedure about a quarter of a million times. Approximately 3000 individual computer runs were made. The computer program operates in IBM 407 print-time, at the rate of 150 lines per minute. Table I presents the data and sources for the ten clusters studied. Data for NGC 129 are discussed more thoroughly in the next section. (B-V)q is the computed composite B-V color-index for stars within 4 mag. of the brightest assumed cluster member. A(b-V)q is the difference between this value and the (B-V)q for all stars in the com­ puter run, including the faintest ones. A(B-V)0 is ^ 0 % 5 in all cases. (A-S) +0.2 +0.6 +0.7 Tir. 2, The iiMimf >1 colw •§» relation for gilaetlo elwtere. Berlsental lines represent the oomepondlng yalues of A(B-V) . The polygem repr*sente the eartrmee reace la seine# for NOC 129, ead is ahoro to larger eeele In Figure 4, Table I* Clusters used to determine the computed color-age relation Number of V, B-V e b -v Cluster stars reference reference Remarks (B-V)t e b -v A(B-V)0 Pleiades 224 14— Tables la,lb 0.04 10 (1) —#12 +.05 -.12 Hyades 185 13— Tables 2a,2b,2c 0.00 20 (2) +.37 +.02 +.10 Coma 41 13— Table 5 0.00 20 +.18 +.01 +.05 Praesepe 133 8— Table 3a 0.00 20 (3) +.37 +.01 +.15 M36 39 15— Table 6 0.24 10 (4) -.20 .00 -.24 M39 14 9— Table 1 0.00 9 (5) +.01 -.06 M67 59 16— Table 2 0.06 20 +.70 -.01 +.40 NGC 2362 26 15— Table 7 0.12 10 (4) -.26 -.01 -.28 NGC 6633 73 4— Table 1 0.17 4 (6) +.18 .00 +.15 138 7— Tables 1,2 0.58 7 +.13 -.04 —.16 NGC 129 192 2— Tables 1,1a,2 0.53 2 (7) +.18 -.02 -.13 222 3— Table 5 0.61 3 (7) +.26 —.03 -.07 Remarks: (1) Included only nCluster Members* or KProbable Members.* (2) Excluded as Non-members Stars 136, 150, I64, (3) Included only "Cluster Members.* (4) Included only stars not designated as “Non-members.” (5) Included only those stars for which "membership is of high probability on the basis either of proper-motion criteria or of radial velocity criteria and for which membership is not of low probability on the basis of the other criteria•* (6) Included only stars designated as "Definite Members (7) (B-V),j, estimated from the authors * color-magnitude diagrams. 6 (B-V)g, is the color-index of the 'turn-up point estimated by Johnson et al.