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1958Apj. . .128 . .150S CEPHEIDS in GALACTIC CLUSTERS. I. CF CASS

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1958Apj. . .128 . .150S CEPHEIDS in GALACTIC CLUSTERS. I. CF CASS .150S . CEPHEIDS IN GALACTIC CLUSTERS. I. CF CASS IN NGC 7790 .128 . Allan Sandage Mount Wilson and Palomar Observatories Carnegie Institution of Washington, California Institute of Technology 1958ApJ. Received May 5, 1958 ABSTRACT Light-curves in three colors have been measured photoelectrically for the classical cepheid CF Cass. This star is very probably a member of the galactic cluster NGC 7790. The double cepheid system CE Cass a and CE Cass b is also probably a cluster member, but no photometric data were obtained for the components A new eclipsing star has been found which may also be a member Both components of the binary are probably Bl V stars with Mv ~ —36 Fragmentary data are given for the star, but no period was found Photometric data for 133 stars in NGC 7790 are given The apparent modulus of the cluster ism—M = 14.35 ± 015, the reddening is E(B — V) = 0 52 ± 0 04, E{U—B) = 0 38 ± 0 03 The true modulus is m—M = 128 ± 015 The distance is 3630 ± 250 parsecs in the direction / = 83 ?5, b = — 0?8 The cepheid CF Cass has a period of 4 87513 days, an amplitude of A F = 0 55 mag , normal mean color indices of (B —F)o = 0 72 ± 0 04, (U—B)0 = 0 49 ± 0 03, and mean absolute magnitudes oí MB = -2.50 ± 0.15, and My = -3 22 ± 0 15 I. INTRODUCTION This is the first of a series of papers on cepheid variable stars in galactic clusters. The papers will be written by Arp, Irwin, Kraft, and Sandage. The aim of the series is to give the photometric and spectroscopic data for each of at least eight clusters which contain cepheids so that normal colors and absolute magnitudes of the cepheids can be found. A final discussion of the colors and of the period-luminosity relation will be made when all the cluster data are available. This should occur after the 1958-1959 observing season for the right ascension region of 0h to 6h. The individual papers will appear in the order in which they have been completed. Photometric material is now complete on CF Cass in NGC 7790, EV Set in NGC 6664, S Nor in NGC 6087, and U Sgr in M25. The results for NGC 7790 appear in this paper. The spectroscopic data for NGC 129, NGC 6664, and NGC 7790 are reported by Kraft in Paper II. The data for NGC 6664 are given by Arp in Paper III. Two subsequent papers by Irwin will give data on NGC 6087 and M25. In 1954, Olin Eggen called my attention to the galactic cluster NGC 7790, which contains the three cepheids CE Cass, CE Cass a, and CE Cass b. The double star CE Cass a and b is unique. Each component of the pair is a cepheid with periods of 4.4462 and 5.1275 days (G. A. Starikova, quoted in Russian Variable Star Catalogue). The separation of the stars is 2" at a position angle of 266°. The third cepheid in the cluster is CF Cass, which is a normal variable of period 4.87522 days (Parenago and Kukarkin 194°)- The recent search for cepheids in galactic clusters by Kraft (1957) and by van den Bergh (1957) failed to list CF Cass or CE Cass as probable members of NGC 7790. This is because the right ascension for NGC 7790 is, as quoted by Dreyer, Shapley, and Trumpler, incorrect by about lm30s (Kraft 1958). The three variables are actually located within the cluster area as outlined by the bright member stars, and there is little question of membership. Although radial velocities are not available to check membership, the general agreement of the positions of CF Cass in the color-magnitude diagram of NGC 7790, compared with M25 and NGC 6087 (Irwin 1958), with NGC 6664 (Arp 1958), and with NGC 129 (Arp, Sandage, and Stephens 1958), make it almost certain that CEa, CEb, and CF Cass belong to NGC 7790. 150 © American Astronomical Society • Provided by the NASA Astrophysics Data System 1958ApJ. .128 . .150S © American Astronomical Society • Provided by the NASA Astrophysics Data System Fig. 1.—The identification chart for stars in and near NGC 7790. The lettered stars are the photoelectric standards of Table 1. The numbered stars are the photographic stars of Table 2. The plate is a 1-minute exposure on Eastman 103a-D behind a Schott GG11 filter taken with the .150S . CEPHEIDS IN GALACTIC CLUSTERS 151 .128 . Olin Eggen knew of the probable connection of these three variables with NGC 7790 in 1952, and he generously made finding charts and other information available to me in 1954. 1958ApJ. II. PHOTOMETRIC DATA NGC 7790 forms a double cluster with NGC 7788. The positions for these clusters are R.A. = 23h54m42s, Dec. = +61°10' (1960) for NGC 7788 and R.A. = 23h56m26s, Dec. = +61o00' (1960) for NGC 7790. The data for 7790 are given in this paper. The data for 7788 are given in a following paper. The usual combination of photoelectric and photographic techniques was used to obtain the color-magnitude diagram, the reddening, and the true and apparent distance modulus for NGC 7790. 1. The Photoelectric Observations Three-color photoelectric observations were made in NGC 7790 of thirty-three stars on ten nights between July 13, 1956, and October 7, 1956, with the 60-inch telescope on Mount Wilson. The photometer used has been described by Walker (1954). The photo- electric filters were Corning 9863 in the ultraviolet, 1 mm of of Schott BG12 plus 2 mm of Schott GG13 in the blue and 2 mm of Schott GG11 in the yellow. An end-on EMI was used for the photomultiplier. Deflections were read from a Brown strip-chart re- corder and were reduced in the usual way to outside the atmosphere, with nightly extinction coefficients determined from two or more extinction stars of different color index. Iteration of the extinction values was performed by the method of conditioned coefficients. All data were transformed to the Johnson U, B, V system with coefficients found from observations of thirty-four Johnson standards taken from Tables 3 and 5 of Johnson and Morgan’s paper (1953). A total of one hundred and twenty-six separate observa- tions of these Johnson standard stars was made from September, 1954, to October, 1956. The transformation from the instrumental magnitudes to the U, B, V system was very good, with probable errors in V, B— F, and U — B all less than +0.01 mag. in the mean. Table 1 gives the F, B—V, and U—B values for the thirty-three photoelectric stars in NGC 7790. The stars are identified in Figure 1. 2 The Photographic Observations Photographic plates in ultraviolet, blue, and visual wave lengths were taken of NGC 7790 with the 60-inch and 200-inch telescopes. The plate and filter combinations were Eastman 103a-O behind 2 mm of Schott UG2 for the ultraviolet, Eastman 103a-O behind 2 mm of Schott GG13 for the blue, and Eastman 103a-D behind 2 mm of Schott GG11 for the visual. The ultraviolet plates were taken with the 60-inch diaphragmed to 32 inches. The blue and visual plates were taken with the 200-inch with the/3.67 Ross corrector lens. Two plates in the ultraviolet and three plates each in the blue and visual were meas- ured by Miss Cynthia Stephens with the iris-diaphragm Eichner photometer at the California Institute of Technology. Two plates in each of the ultraviolet, blue, and visual wave lengths were measured by Sandage. The dispersion of the residuals of Stephens minus Sandage for measurements on the same plate was a = 0.039 mag , with no significant systematic difference. If each operator has the same personal error, then <7 (Stephens) = a (Sandage) = 0.028 mag. The probable error of a single measurement due to the personal equation is therefore ±0.019 mag. This error of measurement is about 1.5 times higher than we have found in other similar investigations. But it is still small enough that the final photographic values are accurate to ±0.02 mag. as computed below. The magnitudes and colors for the thirty-three photoelectric standards were read back through the photographic calibration-curves. The means are given in the right- © American Astronomical Society • Provided by the NASA Astrophysics Data System 152 ALLAN SANDAGE hand section of Table 1. The residuals of these values from the photoelectric values show no dependence on either V or B—V. Consequently, there is no magnitude or color equa- tion between the photoelectric and the photographic results. Therefore, all data are on Johnson’s Z7, B, V system to within the accuracy of the measurements. Colors and magnitudes for one hundred additional stars were obtained by photo- graphic interpolation from the two plates in the U and the three plates in the B and V. These stars are identified in Figure 1. The area around the cluster has been divided into three regions. Region 1 is a circle of 111" centered on star E. Region 2 is the annulus between circles of radii 111" and 161" centered on star E. Region 3 is the area outside regions 1 and 2. The adopted F, B—V, and V—B values for the hundred additional stars are given in Table 2. The distribution of the residuals of each measurement from TABLE 1 Photometric Standards in NGC 7790 Photoelectric Data From Photographic Plates Star Region* B-V U-B B-V U-B A 11 08 +0 24 -0 59 38 11 08 +0 24 -0 63 B 12 16 +0 38 - 11 3 12 13 +0 41 -F 02 C 12 47 +0 55 + 21 7 12 50 +0 56 -F 18 L>t 12 59 +0 44 + 26 2 12 68 +0 31 E 12 79 +0 37 - 08 1 12 78 +0 36 - 07 F 12 82 + 1 98 1 12 82 +2 26 G 13 13 +0 55 + 26 1 13 02 +0 62 + 31 H 13 15 +0 49 + 29 2 13 13 +0 47 + 42 I 13 18 + 1 20 + 85 1 13 09 + 1 32 + 67 J 13 20 +0 53 + 31 3 13 30 +0 46 + 36 K 13 21 + 1 52 3 12 98 + 1 72 L.
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