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195 9Apj. . .129. .2 8 7K ECLIPSING BINARIES in GALACTIC

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195 9Apj. . .129. .2 8 7K ECLIPSING BINARIES in GALACTIC 7K 8 .2 .129. ECLIPSING BINARIES IN GALACTIC CLUSTERS . AND 0-B ASSOCIATIONS 9ApJ. Robert P. Kraet 195 Yerkes Observatory, University of Chicago AND Arlo U. Landolt Goethe Link Observatory, Indiana University Received November 12, 1958 ABSTRACT Twenty-six eclipsing binaries are found to lie within the limits of galactic clusters; in addition, there aie 577 such stars in optical coincidence with known O-B associations. Of the latter, there are 105 stars brighter than Mv — +1, with log P ^ +1.4 lying in optical coincidence with the associations I Aur, I Car, I Cru, XI Cyg, I Per, and II Set. The period-frequency function of this group does not differ significantly from that found for the group of all eclipsing binaries with known spectral types brighter than Mv = +1 and with log P ^ +1.4. Though of fundamental importance in many stellar astronomy problems, both obser- vational and theoretical, the effective temperature scale remains far from definitely settled, particularly among the stars of early spectral type. The first generally accepted scale was one computed by Kuiper (1938), which, for the stars of early spectral type, was based on a model by Pannekoek (1936). Observational confirmation was afforded by the eclipsing binary ¡jl1 Sco, for which Kuiper computed an effective temperature of 15800° K from the then available luminosity, radius, and spectral type (B3). This value compared favorably with that computed from theory for a star of spectral type B3, viz., 18700° K. The result, however, was criticized by Kopal and Treuenfels (1951), who, from studies of mean parallaxes, concluded that the effective temperatures of early B-type stars should be lower than those computed by Kuiper. On the other hand, Keenan and Morgan (1951) adopted Kuiper’s scale with only minor modifications. Indeed, further studies of ¡j} Sco, the primary of which is now classified as B 1.5 V (Kukarkin, Parenago, Ephremov, and Kholopov 1958), lead to a considerably higher value of Te than the 21400° K given by Keenan and Morgan for that spectral type. If we adopt for this star Blaauw’s (1946) parallax, the absolute dimensions of the primary given by Stibbs (1948), and the bolo- metric correction computed by Miss Underhill (1957) for a star of type B1.5 V, we derive Te = 28300° K. This value is gratifyingly close to the Te — 28470° K given by Miss Underhill for her Model III, which is supposed to represent a star of spectral type B1.5 V. In view of the fact that our knowledge of the effective temperature scale among the B stars depends largely on the observations of a single eclipsing binary, we have made a search for eclipsing binaries in galactic clusters and O-B associations. Presumably, ac- curate luminosities for some of these stars could now be derived and, by a combination of photoelectric and spectrographic observations, absolute radii determined in a number of cases. The major uncertain factor is that of the bolometric corrections; these are best given by model atmosphere calculations, though information derived from rocket flights may also prove valuable. One such star, SZ Cam, which was mentioned by Kuiper (1938) as belonging to NGC 1502, has been studied by Wesselink (1941); however, no cluster photometry is available, and the spectral type is not known on the MK system. The positions of all eclipsing binaries listed by Kukarkin et al. (1958) were compared with the open cluster positions given by Trumpler (1930), Collinder (1931), and Haffner 287 © American Astronomical Society • Provided by the NASA Astrophysics Data System 7K 8 .2 288 ROBERT P. KRAFT AND ARLO U. LANDOLT .129. (1957). A total of 26 stars was found to lie within the limits of open clusters, and these 9ApJ. are listed in Table 1. The faintest star has = 14.0 at maximum light. In a few cases cluster photometry is already available, but, on the whole, the group has not been 195 well studied. The great majority of open clusters lies within a band of width 16° centered on the galactic equator, as do 23 of the stars listed in Table 1. Within this band, the open clusters cover 0.48 per cent of the total area. If we assume that the 1210 eclipsing vari- TABLE 1 Eclipsing Binaries in Galactic Clusters wpg Wpg Period Spec- Cluster Re- Cluster Star Type (Max.) (Min.) (days) trum Photometry marks* Pleiades AH Tau EW 11.8 12.5 0.333 Glp Johnson and Knuckles (1955) NGC 1502 SZ Cam EB 7.0 7.3 2.70 BOn NGC 2437 GIPup EA 13.6 15.2 NGC 2632 RY Cnc EA 12.5 15.3 1.09 Johnson (1952) TrlO BX Vel EA 13.2 15.0 1.34 C 213.... DX Vel EA 10.2 11.0 1.12 A5 IC 2602.. DU Car EA 13.5 14.6 4.97 NGC 3532 GV Car EA 8.9 9.4 4.29 A0 Koelbloed (un- published) Tr 18 EN Car EA 10; 4 10.7 1.53 B NGC 3766 BE Cen EA 8.5 9.4 3.69 B7 IX Cen EA 12.6 13.8 9.09 NGC 4103 AI Cm EA 9.2 9.95 1.42 OB Coma. ... RW Com EW 11.2 11.9 0.237 G2+G2 Johnson and Knuckles (1955) NGC 5316 V619 Cen E 12.5 14 16.45 Tr24.... V588 Sco EA 14.0 16.5? V589 Sco EA 12.8 13.8 2.61 NGC 6416 V496 Sco EA 11.1 12.3 2.19 F5 Tr 31.... V789 Sgr EA 11.9 13.0 5.10 IC 4756.. BU Ser EA 13.9 15.5 1.84 NGC 6871 V447 Cyg EA 13.1 14.5 2.21 V453 Cyg EA 8.3 8.6 3.89 B1 III NGC 7209 SS Lac EA 10.1 10.5 14.42 B7 CN Lac EB 12.4 12.9 0.637 G3 NGC 7243 TZ Lac EA 13.8 15.1 2.88 Becker and Stock (1954) NGC 7380 DH Cep E 8.9 9.0: 2.11 06+06 NGC 7790 QX Cas E 10.2 10.6 2.40 B2 Sandage (1958) * 1 Noted by Kuiper (1938); studied by Wesselink (1941); no cluster photometry. 2. Star is close to the edge of the cluster. 3. A coirected position for NGC 7790 has been given by Kraft (1958). ables brighter than 14.0 pg at maximum light which lie within this band are distributed at random, we would predict that 6 should lie accidentally within the clusters. Thus the fact that we find 23 in clusters indicates that the relationship is a physical one in most cases. We have found, as well, that 577 eclipsing binaries are in optical coincidence with the O-B associations, the dimensions and positions of which have been summarized by Schmidt (1958). We shall make no attempt to list even the brightest of these objects. So few spectral types are available that it is seldom possible to determine for a particular star whether its distance is compatible with that given by Schmidt for the association. In a few cases, two associations may overlap so that it is not possible to determine whether a given star belongs to one or the other, if to either. © American Astronomical Society • Provided by the NASA Astrophysics Data System 7K 8 .2 ECLIPSING BINARIES 289 .129. The distribution of eclipsing binaries as a function of galactic co-ordinates is exceed- 9ApJ. ingly spotty because surveys have been made to faint magnitudes only in certain limited regions (cf. Plaut 1953). Thus nothing can be said regarding the ratio of the number of 195 eclipsing binaries to all stars in association, as compared with non-association, fields. However, it is of interest to compare the period-frequency function in association fields with that of all eclipsing binaries having spectral types and which are brighter than some limiting magnitude. We make three assumptions: (1) the eclipsing binaries in optical coincidence with the associations are actually association members; (2) the group of eclipsing binaries having spectral types constitutes a random sample of all eclipsing binaries down to a certain apparent magnitude limit. We have chosen the latter group so as to include only stars more luminous than those corresponding to spectral type A2 V, and with wPg ^ +10. This eliminates virtually all stars of the W Ursae Majoris Fig. 1.—The period-frequency function of eclipsing binaries in association and non-association fields. The full line refers to the association fields. type; these objects are close to the plate limit in most of the association fields. From the 200 stars remaining, we eliminated those which had log P > 1.4 and had distances certainly greater than 1000 pc; of the remaining 158 stars, all of which contained at least one component of type O, B, or A, many did not have luminosity classes, but we as- sumed that those that did not were dwarfs. The majority of these 158 stars are almost certainly within 1000 pc of the sun, if the frequency function of the luminosity classes among the early-type stars (cf. Trumpler and Weaver 1953) is the same among binaries as among single stars. We shall therefore further assume that (3) this group of 158 stars constitutes a random sample of eclipsing binaries brighter than Mv = +1 with log P ^ + 1.4 and lying within 1000 pc of the sun, at least so far as the period-frequency function is concerned. One hundred and five eclipsing binaries down to this absolute magnitude limit were counted in the associations I Aur, I Car, XI Cyg, I Cru, I Per, and II Set, using the distances given by Schmidt (1958).
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