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Publications of the Astronomical Society of the Pacific 105: 588-594, 1993 June

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Publications of the Astronomical Society of the Pacific 105: 588-594, 1993 June Publications of the Astronomical Society of the Pacific 105: 588-594, 1993 June The Frequency of Binary Stars in the Young Cluster Trumpler 14 Laura R. Penny, Douglas R. Gies,1 William I. Hartkopf,1 Brian D. Mason, and Nils H. Turner1 Department of Physics and Astronomy, Center for High Angular Resolution Astronomy, Georgia State University, Atlanta, Georgia 30303-3083 Electronic mail: [email protected], [email protected], [email protected], [email protected], nils @ chara.gsu.edu Received 1992 December 1; accepted 1993 March 8 ABSTRACT. We present radial-velocity data for the six brightest members of the open cluster Trumpler 14 based on high-dispersion spectra obtained over a five-night interval. None of these O-type stars appear to be spectroscopic binaries with periods of the order of a week or less, and none are speckle binaries. This binary fraction is low for O-type stars, and we suggest that the lack of primordial hard binaries and their dynamical interactions may explain how the cluster has maintained a high spatial density even after several cluster crossing times. 1. INTRODUCTION and Johnson 1993). In this paper we report on a radial- velocity study of the brighter members of the cluster de- Massive O- and B-type stars are often bom in compact, signed to find the binary content and to determine whether dense clusters (for example, R 136: Elson et al. 1992, Wal- or not conditions favor dynamical ejection. At the outset of bom et al. 1992, Campbell et al. 1992; NGC 3603: Moffat the project, only the brightest star in the cluster, HD 93129 1983, Baier et al. 1985; the Orion Trapezium cluster: Her- A, had been the subject of a significant radial-velocity big and Temdrup 1986). In such environments, dynamical study by Conti et al. (1979) who found no evidence of interactions between the stars assume a new importance, orbital variability. Subsequently, a preliminary search for and Gies and Bolton (1986) argued that gravitational en- spectroscopic binaries was conducted by Levato et al. counters between binaries could produce high-speed escap- (1991), and these authors estimated the binary frequency ees, the so-called OB runaway stars. Leonard and Duncan to be 6/11 = 55% which is typical for O stars in clusters (1988, 1990) and Leonard (1991) studied this possibility and associations (26%-55%; Gies 1987). If the Levato et through numerical simulations of parsec-sized clusters that al. results are correct, they imply that Tr 14 does indeed contain a population of primordial massive binaries. They have favorable conditions for dynamical ejection (although found that these binaries tend to sink to the center of the no escapees were actually identified by Levato et al. ). Here cluster where they form hierarchical doubles. Gravita- we describe our spectroscopic (Sec. 2) and speckle (Sec. tional perturbations by other cluster members cause an 5) observations of the six brightest members of the cluster, increase in the eccentricity of such doubles, and eventually and measurements of their radial velocities (Sec. 3) and a binary-binary collision occurs at periastron. Leonard and projected rotational velocities (Sec. 4). Contrary to the Duncan demonstrate that this process can produce run- results of Levato et al. (1991), our measurements indicate away stars of sufficient number and speed to account for a low binary frequency, and we argue that this lack of the observed high-velocity population. Clarke and Pringle binaries may explain why Tr 14 has maintained a high (1992) have further explored the cluster ejection model, spatial density over a significant fraction of its dynamical and they find that the observed runaway star properties history (Sec. 6). can be explained if ( 1 ) massive stars form in small clusters of binaries with near unity mass ratios, and (2) the initial 2. SPECTROSCOPIC OBSERVATIONS mass function is under represented in low-mass stars. The dynamical influence of binaries on cluster evolution is ex- We obtained spectra with the CTIO 4-m telescope and cassegrain echelle spectrograph in the period 1987 Febru- plored in a more general way by Heggie and Aarseth -1 (1992). ary 14-18. We used the 31.6 grooves mm echelle grating -1 The young open cluster Trumpler 14 in the Eta Carinae and a 226 grooves mm cross disperser grating blazed at region is a potential site for binary-binary encounters since 8000 A (which we used in second order with a CUSO4 filter it contains many massive stars (including three of type to block competing orders). The spectra were made with 03) within a region 1 pc in diameter (Feinstein et al. 1973; blue train optics, blue collimator, and long camera. The Walbom 1973; Feinstein 1983; Morrell et al. 1988; Massey detector was the RCA4 CCD, a 320x512 array consisting of 30 μτη square pixels. We obtained spectra in two re- gions: on 1987 February 14, we made observations over the Visiting Astronomer, Cerro Tololo Inter-American Observatory, Na- tional Optical Astronomy Observatories, operated by the Association of range 4406-4688 A (seven echelle orders with some wave- Universities for Research in Astronomy, Inc., under contract with the length gaps between orders), and on the other four nights, National Science Foundation. we recorded spectra in the range 3915-4234 A (ten orders 588 © 1993. Astronomical Society of the Pacific © Astronomical Society of the Pacific · Provided by the NASA Astrophysics Data System FREQUENCY OF BINARIES IN TR 14 589 Table 1 interpolation. Each order was then rectified to a unit con- Target Stars tinuum (using "eccontinuum") by fitting a parabola (or a Spectral V sin i straight line in the case of the order recording Η<5 for the Star Classification B-V (km s"1) four cooler stars since this feature occupies most of the order) to line-free regions. Finally, the heliocentric Julian HD 93129 A 03 If* 7.3 0.22 130 HD 93129 Β 03 V((f)) 8.9 0.22 112 dates and solar corrections were calculated using "rvcor- HD 93128 03 V f 8.84 0.25 116 rect," and velocities were measured using rectified intensity FMM 20 06 V( f)) 9.61 0.28 56 FMM 8 06.5 V((f)) 9.40 0.17 46 versus heliocentric wavelength format spectra for each or- FMM 9 08 V 9.92 0.21 60 der. FMM 3 BO.5 IV-V 10.80 0.26 111 The final average spectra appear in Fig. 1. All the spec- tra of each target were transformed to a common wave- length grid and then summed to form a global average with no gaps). The spectra for the first (second) region spectrum. The global average was smoothed using a Gaus- were made with a 150 (225) μτη slit, equal to 1.0 (1.5) sian transfer function (truncated at 3σ) with a FWHM arcsec on the sky, and the spectra have a reciprocal dis- -1 =0.35 A. These merged spectra are plotted in Fig. 1 (sep- persion of 0.067 (0.060) A pixel and a resolution arated in intensity by 50% of the continuum for clarity) in (FWHM) of 0.15 (0.18) Â. a format similar to the illustrations in the spectral atlas of The primary targets of our survey are listed in Table 1 Walbom and Fitzpatrick (1990) [see their Fig. 4 and Fig. which identifies the stars by their designation in the HD 12(a) of Massey and Johnson 1993 for HD 93129 A]. The catalog or by the cluster member number assigned by Fein- redward portion of the diagram corresponds to data ob- stein et al. (1973; noted as FMM). The spectral types are tained on the first night (some wavelength gaps) while the from Walbom (1973, 1982), except for the final two en- blueward portion represents the sum of the final four tries (FMM 9, 3) which are types from Morrell et al. nights. Some discontinuités appear at the junction of indi- ( 1988). The photometric quantities are derived from Fein- vidual orders due to continuum placement errors at the stein et al. (1973), with the exception of the measures for edges. These spectra are available to other investigators the close pair HD 93129 A, Β which are taken from Wal- upon request. bom (1973). These measurements are in good agreement with those of Massey and Johnson (1993) for those stars which are free from image blending in their CCD photom- 3. RADIAL VELOCITY MEASUREMENTS etry. We also obtained spectra each night of the bright O stars HD 38666 (Mu Columbae) and HD 57682 which are We measured the radial velocities of the program stars considered constant radial-velocity objects by Garmany et by fitting parabolas to the lower (upper) half of the ab- al. ( 1980), and we used these targets to provide a check on sorption (emission) lines. Only features with a line depth the velocity stability of our observing scheme. We kept the significantly greater than the noise level were measured. exposure times short ( 10-20 min) to minimize the effect of All profiles and fits were inspected interactively to discard cosmic ray hits on the CCD, and we generally obtained problem lines or to adjust the portion of the line included three consecutive images that were combined subsequently in the fit in the case of partially blended lines. Our sample by median averaging. The S/N ratio (per pixel) of the of lines and adopted wavelengths are listed in Table 2; an resulting spectra falls in the range 40-90 depending on the "X" indicates which lines were measured for each star brightness of the target.
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