Astrophysical Properties of Red Giants in Three Open Clusters Older Than the Hyades*

J. Astrophys. Astr. (1983) 4, 117-134 Astrophysical Properties of Red Giants in Three Open Clusters Older than the Hyades* J. J. Clariá**and Ε. Lapasset** Observatorio Astronómico, Universidad Nacional de Córdoba, Laprida 854, 5000 Córdoba, Argentina Received 1983 March 31; accepted 1983 May 20 Abstract. Results from CMT1T2 1T2 broad-band and DDO intermediate-band photometry are presented for G and Κ giants in the old open clusters NGC. 2482, NGC 3680, and IC 4651. Two independent photometric criteria have been used to separate red field stars from the physical members of the clusters. Recent calibrations of the DDO and CMT1T2 systems have been used to derive reddening, distance moduli, metallicities, effective tempe- ratures, and surface gravities. Rough estimates of masses have also been made. The giants of NGC 2482 and IC 4651 have CN strengths nearly identical to the Hyades giants, while those of NGC 3680 are slightly richer in CN than the nearby Κ giants. CMT1T2 abundance analysis in NGC 2482 and NGC 3680 yield [Fe/H]MT = – 0.1 ± 0.1 as derived from the iron lines, while abundances derived from the CNO –. contaminated (C – Μ) index are 0.4 dex higher. Both CMT1T2 and DDO data support the conclusion that 1C 4651, with [Fe/H] = + 0.2 ± 0.1, is on the metal- rich side of the distribution of intermediate and old open clusters. Finally, the mass results suggest that the clump stars in NGC 3680 and. IC 4651 could have undergone mass loss before reaching their helium core burning phase of evolution. Key words: clusters, open – photometry, CMT1T2/DDO – stars, abundances – stars, late-type I. Introduction The role of G and Κ giants for photometric abundance studies in stellar clusters is well known. In particular, globular clusters and open clusters older than the Hyades are im- portant as indicators of the evolution of the chemical composition of the galactic disk (Sandage & Eggen 1969) and of variations in metal content with position in the galaxy. In a recent paper, Janes (1979a) has used UBV and DDO photometry of distant field Κ giants and 41 open clusters to estimate the variation of metallicity across the galactic * Supported in part by the Consejo Nacional de Investigaciones Cientificas y Tecnicas (CONICET) of Ar- gentina. ** Visiting astronomer of Cerro Tololo Inter-American Observatory supported by the National Science Foundation under contract No. AST 74-04128. 118 J. J. Clariá and Ε. Lapasset disk. The metallicity gradient of d[Fe/H]/dR = – 0.05 found by Janes in the solar vi- cinity is somewhat lower than that of ~ – 0.1 found by Clariá (1980) from a preliminary UBV-DDO study of G and Κ giants in several open clusters. There is, however, some contradictory information concerning the real nature of this abundance gradient. In fact, while some authors have found that the heavy-element abundance in the galactic disk decreases with increasing distance from the galactic centre (D’Odorico, Peimbert & Sabbadin 1976; Mayer 1976 Janes 1979a; Clariá 1980) others have not obtained the same results (Clegg & Bell 1973; Heifer & Jennens 1975). There are rea- sons for suspecting that the apparently contradictory results might perhaps be reflect- ing the existence of a more pronounced gradient in the CNO elements than in the heavier ones. Since the DDO abundance parameters for G and Κ stars are affected by the presence of CN and CH features, it is clear that such parameters depend on the ra- tio [Fe/CNO], which in turn may be a function of position and/or age in the galaxy. On the observational side, therefore, it should, be interesting to use any observational technique capable of separating the effects of blanketing by the molecular CN and CH bands from those of the metallic lines in the ultraviolet. Such a separation can be carried out by using the CMT1T2 system, usually known as the CMT1T2 Washing- ton system. This relatively new broad-band system was designed by G. Wallerstein and developed by Canterna (1976). The system employs a RCA C31034 Ga-As tube whose high quantum efficiency in the near-infrared enables observations of stars ap- proximately 1.5 mag fainter than those for which (R – I) measurements can be taken with an S-l phototube on the same telescope. Although the Washington system was primarily designed for G and Κ gaints, its applicability to other stars and/or stellar groups has been described by Canterna (1976) and Canterna & Harris (1979). We present in this paper CMT1T2 observations of G and Κ giants in three open clusters older than the Hyades: NGC 2482 (C 0752-241), NGC 3680 (C1123-429), and 1C 4651 (C1720-499). The broadband observations in the first two clusters were supplemented by observations in the DDO intermediate-band photometric system. Existing DDO data of 1C 4651 (Smith 1982) have been used. The primary purpose of this paper is to use both CMT1T2 and DDO data as well as published UBV values in or- der to determine some of the main astrophysical properties of these three clusters. Some preliminary results have been previously published by Clariá & Lapasset (1982). In Section 2 of this paper the photoelectric observations are presented. Two photometric criteria for evaluating cluster membership as well as the procedures and calibrations used to derive the astrophysical properties are described in Section 3; in Sec- tion 4 the results on individual clusters and interpretation of them are presented. Fi- nally, the main conclusions are summarized in Section 5. 2. Photoelectric observations of the cluster stars Photoelectric photometry in the CMT1T2 system was obtained on three nights in April 1980 with the 41-cm reflector of the Cerro Tololo Inter-American Observatory (CTIO). Measurements were done for three giants in NGC 2482, nine giants in NGC 3680, and thirteen giants in 1C 4651. The CMT1T2 filters recommended by Canterna were used together with a Ga-As RCA C31034 photomultiplier cooled by dry ice. Pulse counting electronics was used for all measurements. Mean extinction coefficients at CTIO were employed. Standard stars (Canterna 1976) were observed each night for conversion to the original Washington system. The colour transformation Red giants in open clusters 119 120 J. J. Clariá and E. Lapasset slopes agree well with those found by Canterna for CTIO, and the internal mean errors of a single observation are ± 0.009, ± 0.008, ± 0.012 for the colours (C – M), (M – T1), (T1 – T2), respectively, and ± 0.015 for the T1 magnitude. These errors are practically independent of the Τ1 magnitude. The data for the programme stars are given in Table 1. Columns (l) – (5) give the star names and UBV data. The observed CMT1T2 colours and T1 magnitudes together with their mean errors are listed in columns (7) – (10). For single measurements, the standard error is estimated for the appropriate T1 level. Columns (6) and (11) give the numbers n1 and n2 of UBV and CMTlT2 observations, respectively. The sources for the star identifications as well as for the UBV data are given in the footnotes. DDO photometry of giants in NGC 2482 and NGC 3680 was also obtained with the aim of determining additional information about their abundance properties. The DDO observations were made with the CTIO 91-cm telescope and an S-20 photomultiplier. Only the four primary DDO filters were used. Transformation to the DDO standard system was done by nightly observing equatorial standards from the list of McClure (1976). Mean DDO extinction coefficients were used. The uncorrected DDO colour indices of the observed stars in NGC 2482 and NG C 3680 are listed in columns (3) – (5) of Table 2 together with their mean errors. Column (6) lists the number of nights on which each star was observed. The average values of the mean errors obtained from the internal consistency of the multiple observations of each star are ± 0.008, ± 0.010 and ± 0.012, respectively, for the three colour indices in Table 2. A comparison of the present DDO photometry for NGC 3680 with that of McClure (1972) indicates them to be nearly identical. For the nine stars observed in common with McClure, the mean residuals (McClure minus present study) are ΔC(45 – 48) = 0.002 ± 0.008 (s.d.), ΔC(42 – 45) = 0.003 ± 0.009 (s.d.), and ΔC(41 – 42) = 0.001 ± 0.010 (s.d.). 3. Analysis of the data 3.1 Membership Criteria Although the positions of the programme stars in the colour-magnitude (c-m) and [(U – B), (B – V)] diagrams are consistent with cluster membership, it is well known that the UBV criteria alone are insufficient evidence of their physical connection with the cluster. The best method for separating red field stars from the physical members Table 2. DDO data for NGC 2482 and NGC 3680. Red giants in open clusters 121 of the clusters is probably on the basis of proper motions and/or radial velocities, but these data are not available for the clusters of the present study. We have found that cluster membership of red evolved stars can be assigned with fair certainty using only BV and DDO photometry. Firstly, the probability of membership can be estimated from how closely the computed reddening of a given star agrees with that obtained from the main sequence stars. Assuming Whitford’s (1958) reddening curve to be valid, McClure & Racine (1969) have derived the relation (1) where A1 = – 2.380, A2 = – 1.420, A3 = 2.175, A4 = 1.841 and all the photometric indices involved are the observed (reddened) ones.

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