.655K THE STAR CLUSTER NGC 6791* .142. T. D. Kinman Lick Observatory, University of California 65ApJ. Received January 11, 1965 19 ABSTRACT ABF color-magnitude diagram of NGC 6791 for 13 < F < 20 is similar to that of the well-evolved cluster NGC 188. Intrinsic colors are derived from the classification of sixteen spectra of cluster stars and gave Eb~v = 0 22 ± 0.02 mag. Implicit in this value is a correction which assumes equal line blanketing in the cluster stars and the MK standards used. No line weakening was observed in the cluster spectra, but the smaller color range in the quasi-horizontal part of the subgiant sequence at +3 < Mv < +4 may indicate a slight metal/hydrogen deficiency in NGC 6791 with respect to NGC 188. The true modu- lus of NGC 6791 is w — iif = 13.55 corresponding to a height of 1 kpc above the galactic plane at a distance of 5.1 kpc. The estimated mass is 3700 Mo and the integrated Mv =—54. The relative frequencies of stars in the cluster sequences are given after correction for field stars. A sequence of blue subgiants is identified in the cluster and membership is verified from radial velocities; no satisfactory explanation of the evolutionary state of these stars was found A concentration of stars in the red-giant branch at Mv = +0.5 with an incipient blueward extension probably indicates the pres- ence of cluster stars in a post-red-giant stage of evolution. The calculated integrated spectral type of this cluster is about G4; its radial velocity is —68 km/sec. Classification by low-dispersion spectra alone may therefore lead to some confusion between this type of cluster and disk-globular clusters of the latest spectral type. The estimated departure of the radial velocity component of NGC 6791 from that ex- pected from galactic rotation is —95 km/sec compared with —26 km/sec for NGC 188 This, taken with its greater height above the galactic plane, suggests that NGC 6791 has a galactic orbit that is inter- mediate between NGC 188 and the globular clusters. I. INTRODUCTION NGC 6791 was described by Baade (1931) as a rich, open star cluster of diameter 13' whose brightest stars were of 16-17 mag. Settiducati and Weaver (1960) included it as No. 46 in their first catalogue of newly found star clusters, but no other observations appear to have been published. The faintness of the cluster stars, the large cluster diameter, and the galactic coordinates together suggest that NGC 6791 might be a well- evolved star cluster like M67 and NGC 188 unless the interstellar extinction in its direc- tion is large. Preliminary direct blue and visual photographs of the cluster taken with the 120-inch reflector were calibrated by a photographic transfer and measured with an iris astro- photometer. The resulting rough color-magnitude diagram showed (a) a strong concen- tration of stars of similar color at F ^ 17 such as might be expected near the main sequence turn-off of a well-evolved cluster; and (b) an absence of many very faint cluster stars, suggesting that the object is not a globular cluster. An appreciable number of galaxies were visible on the plates, and this suggested that the interstellar extinction was unlikely to be large. It therefore seemed desirable to make more precise photometric observations of this cluster since it is significantly richer than either M67 or NGC 188 and an improvement in the delineation of the poorly populated sequences of the evolved cluster stars seemed possible. This paper describes two-color (BV) photometry and low- dispersion spectroscopy (200 Â/mm) of stars in this cluster (19h17“3, +37°39' [1900]) (/ii = 70?0, 611 = +10?9). II. PHOTOMETRIC OBSERVATIONS AND REDUCTIONS The magnitudes and colors of the cluster stars were obtained by photographic inter- polation from a photoelectric sequence. The photoelectric sequence (13 < V < 20) was measured with the prime-focus photometer of the 120-inch reflector and was referred to * Contributions from the Lick Observatory, No. 182. 655 © American Astronomical Society • Provided by the NASA Astrophysics Data System .655K 656 T. D. KINMAN .142. eight local standards that were kindly measured by Dr. J. Smak with the Crossley re- flector; in addition, transfers were also made from BV sequences in NGC 188 (Sandage 65ApJ. 1962a), M13 (Arp and Johnson 1955), and NGC 6882/5 (Hoag, Johnson, Iriarte, Mitch- 19 ell, Hallam, and Sharpless 1961). The sequence stars were chosen for their isolation in the field and were mostly situated in an annular zone T'-IO' from the cluster center in order to avoid crowding errors. De- tails of these observations are given in Appendix I. Since NGC 6791 is in a moderately rich star field, it was necessary to consider which region in the cluster would give the maximum contrast between the cluster sequences and the field stars in a color-magnitude diagram. A discussion of this point is given in Ap- pendix II and leads to the photographic measures being made within the following radial distances from the cluster center: r < 4' for F < 15 , r < 2' for F < 17 , f < 1' for F < 20 . Transfer errors may therefore be expected since the photographic and photoelectric measures were made in different regions. Such errors will depend partly upon image structure and therefore upon the seeing, star magnitude, exposure time, and the telescope focal ratio. They may also be produced by non-uniformity of the emulsion and its processing. The plates were therefore taken with a wide range of exposure times (Ap- pendix I), both with the Ross corrector at full aperture//5.1 and with no corrector at //8.33 (i.e., with the telescope diaphragmed to 72-inch). Transfer errors, if present, might then be expected to show up as systematic differences between the magnitudes derived from different plates. It was particularly important in the case of this cluster to be able to make a quantita- tive estimate of the contribution by field stars to the measured color-magnitude diagram. A series of plates centered approximately 9' north, south, east, and west of the cluster center were therefore obtained so that the field-star density could be estimated as a func- tion of F and B — V. The photographic plates were measured with a modified Sartorius iris diaphragm astrophotometer (Oliver and Kinman 1964). A few plates were remeasured after changing the wedge setting of the astrophotometer, so that different parts of the star image would be effective in the two measures. Differences in image structure between the standards and the unknown stars might then be expected to cause systematic differences between the two sets of measures; such differences as were found were no larger than could be expected from the random errors of measurement. First, 119 secondary standards were measured on 13 blue and 6 visual plates. These stars are marked in Figure 1, and their magnitudes and colors are given in Table 1. These stars were chosen for their isolation in the field so that plates taken in poor seeing could be included. It was hoped that any asymmetrical field errors produced by the asym- metrical diffraction pattern of the prime-focus cage would tend to be smoothed out with this number of plates. The distribution of photoelectric and secondary standards was roughly symmetrical about the cluster center. The identification of the stars near the cluster center is given in Figure 2 and their magnitudes and colors are in Table 2. Only plates taken with relatively good seeing were suitable for the measurement of these stars. An attempt was made to obtain the magni- tudes of all stars within 1' of the cluster center. If an astrophotometer measurement was not possible because of crowding, a visual estimate was made. Qualities “a” (best), “b,” “c,” and “d” (worst) were assigned to the magnitudes obtained for each star on the basis of the freedom from crowding by neighboring stars. The numbers which precede the quality estimates in Table 2 are the number of blue and visual measures, respectively. © American Astronomical Society • Provided by the NASA Astrophysics Data System .655K .142. 1965ApJ. ,. * * . « Fig. 1.—NGC 6791 photographed with the Lick 120-inch ’ *'* . ‘ '• • reflector (25 min with Kodak 103a-D emulsion and Schott GG11 filter). The circles are of radius 2'and 4'. The 119 num- * ; .' * ’ bered stars outside the 4' circle are the secondary standards. Photoelectric standards: J-Z are shown; a, f, k, g, m, n, b, c, * *M d, h are identical with 4, 9, 17, 24, 31, 58, of the secondary standards and 12, 16, 19, 24 of the 2,-4'zone; e, i, j are shown v- in Fig. 4. © American Astronomical Society, • Provided by the N ASA Astrophysics Data System Fig. 2.—NGC 6791: an enlargement of the central portion of Fig. 1 giving the identification of measured stars near the cluster center. The circles are of radius 1' and 2'. © American Astronomical Society • Provided by the NASA Astrophysics Data System .655K TABLE 1 .142. SECONDARY STANDARDS IN NGC 6791 65ApJ. PHOTOGRAPHIC MEASURES 19 STAR B-V STAR V B-V 1 13.35 1.33 4,9 51 17.93 .88 6,5 2 13.54 1.03 4,9 52 17. 43 .77 6,7 3 14.47 .90 4,9 53 18.08 .98 6,5 4 13.19 .55 4,9 54 18.44 .76 6.5 5 14.24 1.11 4,9 55 17.83 .66 6,7 6 14.93 1.42 4,9 56 17.66 .84 6,7 7 13.92 .92 4,9 57 17.39 .92 6.7 8 14.22 .75 4.9 58 16.91 .08 6.8 9 15.02 .58 4.10 59 17.36 1.05 6.6 10 13.73 1.20 4.9 60 17.56 .79 6,6 11 14.