194Lapj 94. . .55C the GALACTIC CLUSTERS MESSIER 46

.55C . 94. THE GALACTIC CLUSTERS MESSIER 46, MESSIER 50, AND NGC 2324 JAMES CUEFEY ABSTRACT 194lApJ The uncertainties introduced by interstellar absorption into determinations of the distances of galactic clusters emphasize the need for thorough investigation of the clusters in regions where obscuration is absent or is small. For this reason a number of the clusters in the rich and relatively uniform Milky Way field near Monoceros and Canis Major are being observed. The results for three of the clusters— Messier 46, Messier 50, and NGC 2324—confirm the smallness of the selective and probably also of the general absorption in this direction to a distance of approximately 3300 parsecs. The color excesses (XX 4300-6200) and distances, corrected for the small general absorption by means of the i/X law of scattering, are: Messier 46, 0.00 mag. and 1450 parsecs; Messier 50, 0.30 mag. and 1210 parsecs; and NGC 2324, 0.30 mag. and 3320 parsecs. Space densities, frequencies of color indices, and other data are given in the text. The color-magnitude relations show, in Messier 50, that the main sequence is well defined and normal in slope and that the giant branch is abrupt and extends from absolute magnitude +1 to — 2. In Messier 46 the slope of the main sequence appears to be significantly less steep than in other clusters. The presence of very blue Ao stars and also of the planetary nebula, NGC 2438, which is located within the apparent boundaries of the cluster, combine to make Messier 46 an unusual object. The distances obtained for the planetary nebula by various workers have, because of their wide dispersion, left the ques- tion of its physical membership in the cluster unsettled; the radial velocity of Messier 46 b4i-4 km/sec (obtained by Struve)—differs widely from that of the planetary (+77 km/sec) and shows definitely that the planetary is not a member of the cluster. NGC 2324, the distant cluster, is a remarkable object, for it contains negative color indices in spite of its low latitude and great distance; the colors show a normal main sequence and an indication of a giant branch. The absence of appreciable absorption to large distances, particularly in the case of NGC 2324, which is located only 35° from the anticenter direction, coupled with the absence of extragalactic nebulae from these fields, suggests that considerable absorption must occur at distances greater than 3300 parsecs. Either an increased density of absorbing material at the boundaries of the galaxy or a spiral arm con- taining obscuring clouds and extending to a distance much greater than the “average” radius of the system is suggested as a possible explanation. INTRODUCTORY REMARKS The irregular structure of the obscuration in the Milky Way has made increasingly evident the importance of the clear regions, for from the unobscured Milky Way fields must come the most definite and far-reaching of our knowledge concerning the distribu- tion of objects in our stellar system. Recent investigations by Shapley, Bok, and others have already shown the value of research in the unobscured fields. The uncertainties and limitations usually introduced by absorption are thereby largely avoided. The galactic clusters, also, are most profitably observed in the clear regions, and it seems likely that in some cases they may furnish powerful tools for investigating unobscured fields to distances as great as 4000 parsecs. The absence of appreciable absorption may be inferred from the low color excesses of some of the clusters; and, because of the finely divided nature of the interstellar material responsible for most of the obscuration, the color excesses are an indication at least, if not an actual measure, of the general absorption. The problem of determining the color excesses and distances of the galactic clusters from the observed relations between the color indices and the apparent magnitudes of cluster stars is, in the light of present knowledge of the nature of interstellar absorption, self-determinate, and the assumption of a uniform absorbing layer is no longer necessary. Formally the three essential unknowns may be obtained from the equations m0 + A = f(C + E), (1) (2) A =KE, (3) 55 © American Astronomical Society • Provided by the NASA Astrophysics Data System 56 JAMES CUFFEY where, of course, m0-M=s (log r - i) = f{C + E) - f(C) - A . The unknowns are r, the distance of the cluster; T, the absorption; and E, the color excess. Equation (i) represents the relation between color and apparent magnitude; m0 is the “unobscured” apparent magnitude of a given cluster star. Equation (2) is the relation between color and absolute magnitude for stars in the solar neighborhood or in some standard unobscured cluster of similar type whose distance is known from other data, spectral types, for example. Equation (3), which completes the problem, is fur- nished by the law of scattering; E is a constant, depending upon the wave lengths in which the photometric measurements were made. If the scattering varies as i/X and the colors are based upon magnitude differences between X 4300 and X 6200, then ^4 = 2E for red light (X 6200). Such a relation, although satisfactory when the absorption is small, may become unreliable if the absorption is large, for then deviations from the assumed i/X law and irregularities in the value of K from one obscuration to another might become important. That the form of the M-C relation is precisely the same for all clusters may, of course, be questioned. The color-magnitude diagrams do, indeed, show striking differences, especially for the brightest stars in the clusters. The main sequences in most of the clusters, however, appear to be very similar in form, and therefore it seems reasonable to conclude that the M-C relation may be assumed constant for main-sequence stars, but not for the giants. The spectrum-magnitude diagrams, which are essentially similar to the color-magnitude relations, have been shown by Trumpler1 to differ, although again mostly in the forms of the giant branches. The main-sequence stars in the spectral data seem to be characterized by a well-defined and, in most cases, constant relation between spectral type and absolute magnitude. Thus the conclusion that distances derived from the giant branches in the color-magnitude diagrams are entitled to low weight is confirmed by the spectral-type data. One additional conclusion to be drawn from the spectral material is that the brightest main-sequence stars in the majority of the galactic clusters are B and A stars. In only one cluster in the hundred classified by Trumpler are they of later type, and here they are F stars. The rarity of the f type of cluster is a fortunate circumstance, for, if spectral types were not available, such clusters might possibly give erroneously high color excesses on the basis of color-index data alone. B. Strömgren’s work2 shows that the form of the Russell-Hertzsprung diagram may, as suggested by Kuiper,3 depend upon the hydrogen content of the cluster as a whole. Trumpler’s and Rieke’s data, examined by Kuiper,3 indicated that for the main sequences observed in several clusters the form was closely the same and that the large differences occurred mainly in the giant branches. Our present empirical knowledge, therefore, appears to show reasonable constancy in the M-C relation for the main sequences in the galactic clusters. In the future, when abundant observational material becomes available, it may be possible to allow for the effects of variability in the M-C relations by adopting several, each one appropriate to clusters of one given type. DESCRIPTION OE REGION Messier 46 (NGC 2437), Messier 50 (NGC 2323), and NGC 2324 are situated in the rich and relatively uniform Monoceros-Canis Major region of the Milky Way. Close inspection of small-scale photographs4 reveals, however, that obscuration is not entirely 1 Lick Obs. Bull., 14, 154, 1930. 3 Ap. J., 86, 186, 1937. 2 Zs.f. Ap., 7, 222, 1933. 4 See Ross-Calvert Milky Way Atlas, Pis. 36 and 37. © American Astronomical Society • Provided by the NASA Astrophysics Data System PLATE III © American Astronomical Society • Provided by the NASA Astrophysics Data System PLATE IV % © American Astronomical Society • Provided by the NASA Astrophysics Data System .55C . 94. 194lApJ PLATE V # NGC 2324 © American Astronomical Society • Provided by the NASA Astrophysics Data System .55C . 94. GALACTIC CLUSTERS 57 absent from the field. Messier 50 appears to be at the edge of a small obscuring cloud, and Messier 46 is near to, but probably not obscured by, a very thin absorbing lane ex- 194lApJ tending from a small and poorly defined cloud to the southwest. NGC 2324, alone, seems to be in a field quite free from obscuration. The co-ordinates and other data previously published are shown in Table 1. The uncertainties in the distances tabulated, which were derived from the brightest stars and from angular diameters, are shown by the present survey of red color indices for 1119 stars in these clusters. TABLE 1 Distances 1900 Diam. Type5 Spectra Trumpler* Shapleyî Sp. Diam. h f7 37“2 —• i4°6\ Il2r 11B9-A1 1150 -1820 Messier 46. \2oo° +5-3/ 27 ra 710 610 fó^S11^ - 8?2\ 12m Messier 50. \ i89?5 + 0.1/ i6' ib-a 7B8—Aof 780 860 5oo:-8oo: fó^g^o + i°.2\ 12m NGC 2324. \ 181t + 4-8/ 8' 1720 4370 -6920 Lick Obs. Bull., 14, 154, IQSO- f HD stars. Î Star Clusters, Appen. B, 1930. PHOTOMETRIC DETAILS Magnitudes and red color indices were derived from polar comparison exposures made at the prime focus of the 36-inch, F/5 aluminized reflector of the Goethe Link Observatory.

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