The Radial Velocities of Globular Clusters

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Planet Hunting Techniques: Radial Velocity

Pilachowski , S. Barden , D.

Gclusters :: Bibliographic item "gc"

Willmarth , and C. The Astronomical Society of the Pacific. All rights reserved. Printed in U. Get permission to re-use this article. National Optical Astronomy Observatories,2 P. Received November 25 Accepted July Create citation alert. Buy this article in print. Journal RSS feed.

Sign up for new issue notifications. We report radial velocities for giant stars in the globular clusters M3, M13, M15, and M Spectra were obtained using the 4 m Mayall and 3. Observations spanned times from a few days to several months. Several stars show velocity variations which may result from binary companions or atmospheric motions. Radial velocity observations for the photometric variable V73 vZ in M3 confirm that the star varies with a period of 0.

Such binaries can absorb a substantial fraction of the binding energy within a star cluster.

Shaping Globular Clusters with Black Holes

A common illustration of this phenomenon is the effect of binaries on core collapse in globular clusters. Originally believed to have formed without any binary star systems, globular clusters are now known to contain a significant binary population as a result of both increased detection efficiency and more refined postcollapse cluster simulations Hut et al. Despite these recent observations, many questions remain in the search for an adequate understanding of cluster binaries: Does the frequency of binaries vary from cluster to cluster?

What parameters affect the proportion of binaries in clusters?

How are the binaries distributed radially in the clusters? How are binaries formed in clusters, and how do they evolve? Still other studies have concentrated on radial velocity variations among subgiant and giant populations in several globular clusters e. Generally, studies of giant stars, for which longer period binaries i. Further data are needed to confirm velocity variations, identify binary candidates, and eventually determine orbital parameters. Extant observations of dozens of giant stars in several globular clusters M3, M13, M15, and M92 obtained with the 4 m Mayall and 3.

The spectra were obtained for the purpose of studying abundance variations, mixing, and nucleosynthesis in cluster giants. Because most of the observations were obtained by the NOAO WIYN Queue Program, and not on dedicated observing runs, the observations span a range of time suitable for the detection of velocity variations from binary stars which may be present in the samples.

These data can also provide additional velocity measurements for stars observed in earlier surveys. We therefore undertook to determine radial velocities from these multifiber Hydra spectra to look for previously undetected binaries. The details of the observations and reductions of our Hydra multifiber spectra have been or will be discussed in papers related to the abundance analyses M Pilachowski et al. The observations obtained for each cluster and the selection of stars included by the Hydra configurations are described briefly below. The dates of all individual observations included in these programs can be found in Table 1.

Small dots represent cluster members, and large dots are the stars actually observed. Spectra of 87 M3 giants were obtained between April 18 and May 27, using the 3. Three separate Hydra configurations were used, with five to seven exposures per configuration, and many stars were included in more than one configuration. The stars included range from The resolution of the M13 spectra is lower than for other clusters because a different camera with less demagnification of the fiber diameter was used.

Many of the faintest stars were omitted by Pilachowski et al. Spectra of nearly three dozen giants in each of the globular clusters M15 and M92 were obtained through the 3. Numerous fibers were used to record the simultaneous sky spectrum to allow subtraction of the telluric Na D emission. The continuum of each spectrum was normalized. Examination of the spectra uncovered three nonmembers among the M15 sample. A more detailed discussion of the nonmembers in our sample of M15 giants will be included in a future paper on the abundances Pilachowski et al. No obvious nonmembers were identified among the M92 sample.

Density distribution

Since the observations were taken for a different purpose than velocity determinations, no velocity standards were observed to use as templates. The radial velocities of the individual stars are therefore determined relative to a cluster average velocity.

Globular Star Clusters

This approach produced velocities in agreement with those determined using the average giant template. Heliocentric corrections were applied, and the ensemble mean velocities are included in the observing log in Table 1. The uncertainties given for each ensemble mean velocity are the formal standard deviations calculated from the multiple observations on a single night and with a single grating setting. Standard deviations for some M13 and M15 configurations cannot be computed if only a single observation was obtained during the night.

A mean cluster velocity for M13 on HJD 2,, The number of stars included in each Hydra configuration is sufficient i. On a typical night several observations were obtained within a few hours for a single cluster and Hydra configuration, providing an opportunity to estimate the uncertainty of our individual stellar measurements. On some nights, as many as four or five observations were taken with a particular Hydra configuration.

In Figure 2 are plotted the calculated standard deviations for individual measurements as a function of V magnitude. The radial velocities of the individual stars in each cluster were converted to heliocentric velocities by adding them to the observed heliocentric velocity of the template. Tables 2A , 2B , 2C , and 2D list average velocity values for all stars in this study and the number of observations made from each star. In Tables 3A , 3B , 3C , and 3D we include sample data for the individual stars observed in each cluster that may be radial velocity variables and the velocities observed at each epoch.

The stars included in these sample tables are just those discussed in detail in the next section. In subsequent columns are the heliocentric Julian date of the observation, the velocity, and the average stellar velocity and standard deviation.

Parallax trends with magnitude and colour

For M3, M13, and M15, radial velocity measurements for individual stars are available from the literature. In Figure 3 we plot literature measurements versus our velocity determinations for detailed comparison. Note that the average velocity differences in Table 5 are a reflection of the level of systematic error which may affect our data. Symbols are defined in each panel. The combination of our observations and previous literature studies allows us to identify possible velocity variables in each of the clusters.

Variables can be identified either by their high standard deviation in the Hydra measurements compared with other stars of comparable magnitude or by changes over a longer baseline for stars observed previously. With the limited number of observations and time coverage available for globular cluster observations, distinguishing atmospheric motion from binary motion is difficult.

Hence we will omit from further consideration here stars within 1 mag of the red giant tip. Neglecting stars in the top magnitude of the giant branch, we identify three stars in our M3 data set with velocity standard deviations which suggest variable velocity: vZ , vZ , and vZ The measured velocities for these stars are given in Table 3A. Corwin , private communication has determined a period of 0. Two cycles are plotted for clarity. This is apparent in the larger standard deviations in velocities for fainter stars.

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