Stellar Radial Velocities in the Old Open Cluster M67 (Ngc 2682). I. Memberships, Binaries, and Kinematics

Abstract

We present results from 13776 radial-velocity (RV) measurements of 1278 candidate members of the old (4 Gyr) open cluster M67 (NGC 2682). The measurements are the results of a long-term survey that includes data from seven telescopes with observations for some stars spanning over 40 years. For narrow-lined stars, RVs are measured with precisions ranging from about 0.1 to 0.8 km s −1 . The combined stellar sample reaches from the brightest giants in the cluster down to about 4 mag below the main-sequence turnoff (V 16.5 = ), covering a mass range of about 1.34 M  to 0.76 M  . Spatially, the sample extends to a radus of 30 arcmin (7.4 pc in projection at a distance of 850 pc or 6-7 core radii). We find M67 to have a mean RV of 33.64 + km s −1 (with an internal precision of ±0.03 km s −1 ) well separated from the mean velocity of the field. For stars with 3  measurements, we derive RV membership probabilities and identify RV variables, finding 562 cluster members, 142 of which show significant RV variability. We use these cluster members to construct a color-magnitude diagram and identify a rich sample of stars that lie far from the standard single star isochrone, including the well-known blue stragglers, sub-subgiants and yellow giants. These exotic stars have a binary frequency of (at least) 80%, more than three times that detected for stars in the remainder of the sample. We confirm that the cluster is mass segregated, finding the binaries to be more centrally concentrated than the single stars in our sample at the 99.8% confidence level (and at the 98.7% confidence level when only considering main-sequence stars). The blue stragglers are centrally concentrated as compared to the solar-type main-sequence single stars in the cluster at the 99.7% confidence level. Accounting for measurement precision, we derive an RV dispersion in M67 of 0.80 ± 0.04 km s −1 for our sample of single main-sequence stars, subgiants and giants with V 15.5  . When corrected for undetected binaries, this sample yields a true RV dispersion of 0.59 0.06 0.07 -+ km s −1 . The radial distribution of the velocity dispersion is consistent with an isothermal distribution within our stellar sample. Using the cluster RV dispersion, we estimate a virial mass for the cluster of 2100 550 610 -+ M  .

On the mass of the Galactic star cluster NGC 4337

“…Then we estimate the dynamical mass using the stars velocity dispersion both with he assumption of virial equilibrium and taking into account possible non-stationarity of the cluster and Galactic gravitational field. Note that virial equilibrium is a commonly accepted condition for star clusters (Davies et al 2011, Tofflemire et al 2014, Geller et al 2015. In anticipation of the results, we found that the dynamical (virial) mass is a factor of 5 larger than the luminous mass, and we discuss different possible explanations.…”

A Chandra X-Ray Census of the Interacting Binaries in Old Open Clusters—Collinder 261

“…The multiplicative factor 2 is included to scale the mass estimate to the half-mass radius. The value for M 67 has been updated with respect to the van den Berg et al (2013) value, to account for an updated mass estimate (Geller et al 2015). For NGC 6819, new membership information from Platais et al (2011) has been included.…”

“…For NGC 6819, new membership information from Platais et al (2011) has been included. References -1 van den Berg et al (2004), Geller et al (2015), 2 Gosnell et al (2012), Platais et al (2013) 3 van den Berg et al (2013), Platais et al (2011) …”

Isochrones of M67 with an Expanded Set of Parameters

“…The best fit age, distance modulus, and reddening value for each set of parameters was then determined using 2-dimensional χ 2 fitting between the points of the M67 CMD (Geller et al, 2015) and the isochrone. Only M67 stars with radial velocity membership probability of 90% or greater were used (from Geller et al, 2015). Allowing the distance modulus, reddening, and age to be free parameters, a χ 2 value for both the magnitude, V, and the color, B − V, was determined for each star in M67.…”

“…The cluster has a metallicity around solar, with [Fe/H] = 0.00 ± 0.06 (Heiter et al, 2014) and a reddening of E(B − V) = 0.041 ± 0.004 (Taylor 2007). A color magnitude diagram (CMD) of M67 can be seen in Figure 1 (data from Geller et al, 2015). It is of importance to note that the M67 CMD has a distinctive "hook" feature below the main sequence turn-off.…”