Astron. Astrophys. 319, 481-486 (1997)

3. Results for individual systems

3.1. NGC 2489

NGC 2489 is an intermediate-age cluster for which UBV photographic data have been published by Lindoff & Johansson (1968) and UBV CCD data by Ramsay & Pollaco (1992). Seven red giants in the cluster field have been observed. One (LJ 3) is non-member, and one (LJ 25) turned out to be a binary member. Twenty-three observations were obtained over an interval of 3968 days, which represents 2.5 cycles. While the phases between 0.1 and 0.95 are well covered (Fig. 1), the phases between 0.95 and 0.1 are represented by two points only. Both extrema have been observed, and the orbit appears well defined.

Fig. 1. Radial-velocity curve for LJ 25 in NGC 2489.

The minimum mass for the secondary from the mass function is 1.5 , assuming that the mass of the red giant primary is about 2.5 . However, since the secondary does not produce any visible effect on the combined colours it should also be at least 5 mag. fainter and hence have a maximum mass of about 1 . These conclusions are mutually inconsistent, but the actual luminosity of the secondary is quite uncertain because the Lindoff & Johansson (1968) UBV colours are based on photographic data, and the isochrone does in fact not fit the colour-magnitude diagram well, neither with CCD nor photographic data. New photometric data within a radius of 8 arcmin and a determination of metallicity would be useful.

3.2. NGC 2567

NGC 2567 is a young open cluster which has been studied by Lindoff (1968) in UBV photographic and by Ramsay & Pollaco (1992) in UBV CCD. Six red giants have been selected as candidates and one (L104) turned out to be a spectroscopic binary. The orbit is circular (Fig. 2), which is expected because the period is shorter than 100 days. Although the distribution in phase of the observations is not quite uniform, the orbit is well determined, thanks to the large number of measurements obtained (42). L104 is definitively a cluster member.

Fig. 2. Radial-velocity curve for L104 in NGC 2567.

With an age of log t 8.55, the mass of the red giant is 3.0 and the secondary has a mass of some 2.1 , as deduced from the photometric separation. Again, the data are photographic and the errors of the colours may be rather large. Therefore the real effect on the colour of the system is also uncertain. The minimum mass from the spectroscopic mass function is 1.85 . These two values would give a mass for the secondary around 2.0 .

The red giant binary L104 is located at the periphery of the cluster rather than close to the cluster centre. Due to the young age of this cluster, significant mass segregation is not expected according to the results of Raboud & Mermilliod (1994).

3.3. NGC 3033

NGC 3033 is a poorly studied open cluster. Vogt & Moffat (1972) have published UBV data for 19 stars. Two red giants have been observed with CORAVEL (VM 12 and 19), and both turned out to be binaries. Since the systemic velocities are different, however, only one of them at most can be a member of the cluster. Star VM 12 is too red to be a member, but VM 19 has about the right magnitude. Its colour is somewhat too blue, which could be due to the presence of its companion. Since the mean velocity of the cluster is not known, it is not possible to decide upon the membership of VM 19 with certainty, and it will be considered as a possible member only.

The radial-velocity curves for VM 12 and VM 19 are shown in Figs. 3 and   4. For VM 19, 1.9 cycles have been covered by the observations spanning an interval of 4104 days. Due to its sparseness, the global parameters of this cluster (distance, diameter, age, composition, etc.) are quite uncertain, and a more detailed discussion will await further observations.

Fig. 3. Radial-velocity curve for VM 12 in NGC 3033.

Fig. 4. Radial-velocity curve for VM 19 in NGC 3033.

3.4. NGC 5822

NGC 5822 was discussed in Paper III (Mermilliod & Mayor 1990). The radial-velocity observations of 28 red giants confirmed the membership of 21 stars, and eight spectroscopic binaries were discovered. Four orbits (for Boz 2, 31 80 and 151; Bozkurt 1974) were published in Paper II (Mermilliod et al. 1989). Orbits have now been determined for two more binaries: Boz 3 (Fig. 5) and Boz 312 (Fig. 6). In addition, the variability of stars Boz 4 and 276, suspected in Paper III from the available data, has now been confirmed; however, no orbit can be determined yet, because the periods are quite long and the first cycle has not yet been completed for any of them.

Fig. 5. Radial-velocity curve for Boz 3 in NGC 5822.

Fig. 6. Radial-velocity curve for Boz 312 in NGC 5822.

The minimum mass from the spectroscopic orbit for the secondary of Boz 3 is 0.5 , by assuming a primary mass of 2.2 . The upper mass limit derived from the photometric separation is 0.9 since Boz 3 shows no visible effect due to its companion, which we therefore assume is some 5 magnitudes fainter. With a magnitude difference of 5 magnitudes, the combined magnitude of the binary would be less than 0.01 mag. brighter than that of the primary alone.

The eccentricity of Boz 312 is the highest of this sample and we are indebted to H. Lindgren and Bo Reipurth for obtaining the critical observations close to the minimum which led to the revision of our preliminary orbit. The UBV colours of Boz 312 ( = 0.87, = 0.51) are appreciably bluer than the bulk of the red giants in NGC 5822 ( = 1.05, = 0.70). The photometric separation gave a primary mass of 2.2 and a secondary mass of 1.8 . In addition, the minimum mass from the spectroscopic mass function f(m) = 0.194 is 1.3 . Thus, the secondary mass should be between 1.3 ( = 1) and 1.8 ( = 0.807, corresponding to an inclination of ).

3.5. NGC 6134

A discussion of the 24 red giants in the intermediate-age open cluster NGC 6134, based on new photometric data in the UBV, DDO and Washington systems, and CORAVEL radial velocities, was published by Claria & Mermilliod (1992). Several spectroscopic binaries were discovered, but no orbital elements were published because only preliminary results were available at that time. The observations were pursued and reliable orbits eventually determined (Figs. 7 and   8). The systemic velocities of both stars L8 and L34 (star numbering from Lindoff 1972) are very close to the mean cluster velocity (-26.0 0.24 km s^-1), and both are cluster members.

Fig. 7. Radial-velocity curve for L8 in NGC 6134.

Fig. 8. Radial-velocity curve for L34 in NGC 6134.

The minimum secondary masses deduced from the spectroscopic mass function for L8 and L34 are 0.6 and 0.4 respectively, and neither star shows any clear effect due to the companion. If the secondaries are assumed to be main sequence stars at least 5 magnitudes fainter than the primary, the maximum masses are 0.9 and 1.0 respectively, again confining the the secondary mass within an interval of 0.5 .

3.6. NGC 6664

NGC 6664 contains the Cepheid EV Sct, and radial-velocity observations of 5 red giants and the Cepheid were discussed by Mermilliod et al. (1987). The membership of the Cepheid variable was not very well established due to the uncertain velocity of one of the giants, star #54 = Arp E (Arp 1958). Further observations have showed that star #52 = Arp C (Arp 1958) is also a binary and a probable member.

The mean velocity now determined for star #54 (18.1 0.1 km s^-1) is in good agreement with that of star #51 = Arp B (Arp 1958) (18.9 0.3 km s^-1) and the Cepheid EV Sct itself (17.8 0.2 km s^-1) as published by Mermilliod et al. (1987). Thus, the new results tend to support the membership of both the Cepheid EV Sct and the two red giants (Arp B and D) in the open cluster NGC 6664.

The radial-velocity curve of NGC 6664 #54 (Fig. 9) is rather well defined because the extrema have been well covered. Some phases could not be sampled because the period is close to two years. The mass function f(m) is quite large (0.673). With a primary mass of about 4.5 the minimum mass of the secondary is 3.5 . With such a massive companion, the combined colours of the binary system should place it in the middle of the Hertzsprung gap, and it is surprising that this is not observed: the UBV colours seem normal for a luminous red giant when reddening is taken into account. No MK type has been published for this star, and no double dip has been observed to indicate that the companion is also a red star. It is, however, true that the observed colour-magnitude diagram shows large scatter, probably due to differential reddening. A plausible explanation is that the secondary is itself a short period spectroscopic binary, formed by two early F-type stars, which would explain the important mass and the non-detection of the secondary, and the limited colour effect due to the large magitude difference. This star would therefore be a highly hierarchised triple system.

Fig. 9. Radial-velocity curve for star #54 in NGC 6664.

3.7. IC 2488

The first UBV study of IC 2488 was performed by Pedreros (1987). He derived a distance of 1450 pc and a colour excess E(B-V) = 0.26. The age is close to that of the Pleiades (log t = 8.0). No UBV data were known when the programme was initiated, so the red giant candidates were selected for observation on the basis of the classification given in the Henry Draper Extension catalogue. Thirteen red giants in the cluster field have been observed, but only three are true cluster members and one (P25) has been discovered to be a binary. Unfortunately, its systemic velocity differs by some 17 kms^-1 from the cluster mean velocity, so P25 is undoubtedly a non-member and no further analysis will be attempted. Fig. 10 shows its radial-velocity curve.

Fig. 10. Radial-velocity curve for star P25 in IC 2488.

© European Southern Observatory (ESO) 1997

Online publication: July 3, 1998
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