The Blue Stragglers of M67 and Other Open Clusters

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Abstract

Results are presented of a study aimed at establishing the frequency of spectroscopic binaries in a sample of blue stragglers, belonging to five old- and intermediate-age open clusters: NGC 752, NGC 2360, NGC 2420, NGC 2682 or M67, and NGC 7789. The observational material, which for the M67 subsample spans 10 years, consists of high-resolution, low signal-to-noise echelle spectra obtained with equivalent systmes at the Multiple Mirror Telescope and at the 1.5-m Tillinghast Reflector, both atop Mt. Hopkins (Arizona). In order to obtain reliable radial velocities for these early-type and/or rapidly rotating objects with the cross-correlation method, synthetic spectra matching the observed spectra were employed as templates. The goodness of the grid of computed spectra wsa estimated to be excellent judging from the degree of agreement in high-resolution, high signal-to-noise observed spectra of the Sun, Procyon, and Sirius with their theoretical counterparts. Using this technique it was possible to determine for the first time the spectroscopic orbit for the M67 blue straggler Fagerholm 190 (P 4.2 days). It is proposed that this is a case where mass transfer between the components has been involved, that the system is still in the final stages of mass transfer, and that the original primary--now the secondary--is an abnormal subgiant evolving towards a white dwarf. The rather large orbital eccentricity (e 0.21) remains an open question, since according to current theories a system with such a period should have been circularized by now. If the mass-transfer process that made F190 into a blue straggler started recently, by using the additional information provided by the turnoff mass we can put very strict limits on the original and current masses of both components. Very long-period variations (several thousand days) were detected in three other blue stragglers of M67; since only one of these has a preliminary orbit available, it is not clear whether these systems could also be explained by the mass-transfer mechanism, or a binary-binary interaction scenario should be invoked. Three large-amplitude spectroscopic binaries were discovered among the blue stragglers of the remaining clusters. Some blue stragglers do not show any signifianct variation at all. It is found that the contamination of the blue-straggler sample by field stars is significant in the case of NGC 7789, and to a lesser extent also for NGC 2420. The frequency of spectroscopic binaries among the blue stragglers studied exceeds 40%, approximately twice the current value accepted for the field or normal cluster members. Blue stragglers tend to be more concentrated towards the center of the clusters than single stars, where the frequency of binaries is expected to increase due to mass segregation; in that case, the derived value would be within the expected range. Nevertheless, more than one mechanism seems to be required to account for all the blue stragglers.

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