William F. van Altena

Space Velocities of Globular Clusters. III. Cluster Orbits and Halo Substructure

We have compiled a catalog of absolute proper motions of globular clusters from various sources. The sample consists of 38 clusters, from which most of the southern ones (15 clusters) were measured in our previous papers in this series. We have integrated orbits assuming two different Galactic potential models adopted from the literature and have calculated orbital parameters. The uncertainties associated with the orbital parameters were derived in a Monte Carlo approach, and we conclude that, overall, at the present level of measurement errors, orbital differences due to Galactic potential models are not significant. Three metal-poor clusters are found to have orbits similar to prototypical metal-rich disk clusters. These clusters are NGC 6254 (M10), NGC 6626 (M28), and NGC 6752. We interpret this as a potentially significant constraint on the formation of the disk. It is thus possible that part of the inner metal-poor halo is the low-metallicity tail of the thick disk. In this case, the ages of these clusters indicate that the formation of the disk partially overlapped with that of the halo. The clusters classified as young halo or red horizontal-branch by Zinn show a radially anisotropic velocity distribution, their orbits are of high total energy, with apocentric radii larger than 10 kpc and highly eccentric. In this sense they may represent an accreted component of our Galaxy. We also discuss ω Cens orbit characteristics in the view of an accreted origin. We investigate the effect of the orbital motion on the internal dynamics of clusters. Adopting the formalism from Gnedin & Ostriker and their destruction rates due to two-body relaxation, we find that, in most cases, this internal process is more important than the destruction processes due to disk and bulge shocking. Hubble Space Telescope (HST) observations argue that NGC 6397s luminosity function is depleted at the faint end, and this is blamed on its high total destruction rate. We propose a list of clusters with similar destruction rates that may also have depleted luminosity functions. We also note the bias toward deriving higher destruction rates in studies that statistically assign tangential velocities based on a kinematic model of the globular cluster system, in contrast to the rates derived from the measured tangential velocities. Clusters prone to such biases are those that have circular orbits (kinematically thick-disk clusters) and some of those with orbits of high total energy.

Space Velocities of Southern Globular Clusters. II. New Results for 10 Clusters

Absolute proper motions have been measured for 10 clusters, in addition to the sample of five clusters presented in an earlier paper. These newly measured clusters are NGC 1904 (M79), NGC 2298, NGC 4590 (M68), NGC 5139 (ω Cen), NGC 5897, NGC 6093 (M80), NGC 6121 (M4), NGC 6144, NGC 6809 (M55), and NGC 7099 (M30). The correction to absolute proper motion is determined based on galaxies, except for one field with high extinction, for which we used the Hipparcos system. This latter correction proves to be very useful for fields at low Galactic latitude, provided magnitude-dependent systematics are well controlled. The errors in absolute proper motion per cluster range between 0.4 and 1.0 mas yr-1. Space velocities are also determined, and the kinematics are briefly discussed; a companion paper will present a detailed analysis of orbits in conjunction with other physical parameters of clusters.

The Southern Proper Motion Program. II. A Catalog at the South Galactic Pole

We describe the Yale/San Juan Southern Proper Motion (SPM) Catalog 1.0, which provides positions, absolute proper motions, and photographic BV photometry for 58,880 objects at the South Galactic pole. The sky coverage of the SPM Catalog 1.0 is about 720 deg2 in the magnitude range 5 < V < 18.5. About 55% of all catalog stars are randomly chosen, whereas the remainder contains numerous astrophysically interesting objects drawn from various lists and databases. The accuracy of positions and absolute proper motions is 40–150 mas and 3–8 mas yr-1, respectively, depending on the stars magnitude. The high-end astrometric accuracy applies to the stars brighter than V = 15. A great deal of effort was put into correcting positions and proper motions for the magnitude-dependent systematic errors. The catalog can be used for Galactic structure studies and deep CCD-frame astrometric calibration and as a database for stars, galaxies, and other objects at the south Galactic pole. The catalog is supplemented by calibrated CCD BV photometry in selected 13 × 13 areas.

Speckle observations of binary stars with the WIYN Telescope. II. Relative astrometry measures during 1998-2000

Five hundred twelve relative astrometry measures are presented for 253 double stars, including 53 double stars discovered by Hipparcos. In 15 cases, relative astrometry is reported for the first time for newly confirmed pairs. In addition, 20 high-quality nondetections of companions are reported for stars suspected of being nonsingle by Hipparcos. Observations were taken using a fast-readout CCD camera system at the WIYN 3.5 m telescope at Kitt Peak, Arizona. In comparing these measures with ephemeris predictions for binary stars with very well known orbits, we find that the measurement precision is better than 3 mas in separation and 1° in position angle per individual observation. Measurement precision and detection capabilities are fully discussed, and confirmed orbital motion is reported in four cases of the Hipparcos double star discoveries.

Speckle Observations of Binary Stars with the WIYN Telescope. I. Measures During 1997

Two hundred seventy-seven position angle and separation measures of 154 double stars are presented. Three of the systems were previously unknown to be double, and 16 other systems were discovered earlier this decade by the Hipparcos satellite. Measures are derived from speckle observations taken with the Wisconsin-Indiana-Yale-NOAO (WIYN) 3.5 m telescope located at Kitt Peak, Arizona. Speckle images were obtained using two different imaging detectors, namely, a multianode microchannel array (MAMA) detector and a fast-readout CCD. A measurement precision study was performed on a sample of binaries with extremely well known orbits by comparing the measures obtained here to the ephemeris predictions. For the CCD, the root mean square (rms) deviation of residuals was found to be 3.5 milliarcseconds (mas) in separation and 12 in position angle, while the residuals of the MAMA data varied depending on the magnification used and seeing conditions but can be comparable or superior to the CCD values. In addition, the two cameras were compared in terms of the detection limit in total magnitude and magnitude difference of the systems under study. The MAMA system has the ability to detect some systems with magnitude differences larger than 3.5, although reliable astrometry could not be obtained on these objects. Reliable astrometry was obtained on a system of magnitude difference of 5.3 with the CCD system.

The Southern Proper Motion Program. I. Magnitude Equation Correction

As with virtually all photographic plate material, the astrograph plates from which the Southern Proper Motion (SPM) catalog is being compiled suffer from magnitude-dependent systematic shifts in the image positions, i.e., a magnitude equation. If left uncorrected, these would adversely affect both the primary goals of the SPM catalog, the absolute proper motions, and the faint secondary reference frame. Using the diffraction-grating images present on the plates, a procedure is adopted that allows a spatially variant magnitude equation correction to be derived individually for each SPM plate. Application of the method to ~120 SPM fields, primarily around the south Galactic pole and the -30° and -35° declination zones, demonstrates its effectiveness. Indeed, the method proves superior, in the case of the SPM, to a direct calibration of the magnitude equation using the newly available Tycho Catalogue. The result of the diffraction-grating technique is final SPM positions and proper motions that are largely magnitude equation–free, as verified by numerous internal and external checks.

A Large Local Rotational Speed for the Galaxy Found from Proper Motions: Implications for the Mass of the Milky Way

Predictions from a Galactic structure and kinematic model are compared with the absolute proper motions of about 30,000 randomly selected stars with 9 < BJ ≤ 19 derived from the Southern Proper-Motion (SPM) program toward the south Galactic pole. The absolute nature of the SPM proper motions allows us to measure not only the relative motion of the Sun with respect to the local disk but also, and most importantly, the overall state of rotation of the local disk with respect to galaxies. The SPM data are best fitted by models having a solar peculiar motion of 5 km s-1 in the V-component (pointing in the direction of Galactic rotation), a large LSR speed of 270 km s-1, and a disk velocity ellipsoid that points toward the Galactic center. We stress, however, that these results rest crucially on the assumptions of both axisymmetry and equilibrium dynamics. The absolute proper motions in the U-component indicate a solar peculiar motion of 11.0 ± 1.5 km s-1, with no need for a local expansion or contraction term. The implications of the large LSR speed are discussed in terms of the gravitational mass of the Galaxy inferred from the most recent and accurate determination for the proper motion of the LMC. We find that our derived value for the LSR is consistent both with the mass of the Galaxy inferred from the motion of the Clouds [(3-4) × 1012 M☉ to ~50 kpc] and with the timing argument, based on the binary motion of M31 and the Milky Way and on that of Leo I and the Milky Way (≥1.2 × 1012 M☉ to ~200 kpc).

WIYN Open Cluster Study. VII. NGC 2451A and the Hipparcos Distance Scale

We provide new evidence that NGC 2451A is undoubtedly a young open cluster, although sparsely populated. New cluster membership has been derived from relative proper motions of 5868 stars. In total, 136 stars down to V ~ 15 have membership probability Pμ ≥ 2%. New CCD BV photometry indicates that about 70 stars are indeed main-sequence stars of NGC 2451A. This is also supported by our measurements of radial velocities. A total of 34 very likely cluster members yield a mean heliocentric radial velocity for NGC 2451A equal to +22.9 km s-1. The high quality of our BV photometry, a confirmation of cluster membership from proper motions and radial velocities, and a recently obtained metallicity estimate for several cluster stars allow us to perform a precise isochrone fit. The Yale isochrones, updated by the latest available input physics, have been fitted to the clusters color-magnitude diagram, yielding a distance modulus V0 - MV in the range 6.35 to 6.38, which is in excellent agreement with the distance modulus (m - M)0 = 6.38 derived from the Hipparcos data recently by van Leeuwen and Robichon et al. For NGC 2451A the isochrone fit yields an age of 60 ± 20 Myr; hence, the cluster appears to be somewhat younger than the Pleiades. We also present alternative evidence suggesting that the cluster could be slightly older than the Pleiades. The most important result of this study is an excellent match between the main-sequence fitting and Hipparcos distances to NGC 2451A. If we consider noticeable similarities between NGC 2451A and Pleiades, then the Hipparcos distance anomaly for Pleiades may not require an explanation of astrophysical nature.

The first definitive binary orbit determined with the Hubble Space Telescope fine guidance sensors: Wolf 1062 (Gliese 748)

The M dwarf binary, Wolf 1062 (Gliese 748), has been observed with the Hubble Space Telescope (HST) Fine Guidance Sensor 3 in the transfer function scan mode to determine the apparent orbit. This is the first orbit defined fully and exclusively with HST, and is the most accurate definitive orbit for any resolved, noneclipsing system. The orbital period is 2.4490 ± 0.0119 yr and the semimajor axis is 01470 ± 00007—both quantities are now known to better than 1%. Using the weighted mean of seven parallax measurements and these HST data, we find the system mass to be 0.543 ± 0.031 M⊙, where the error of 6% is due almost entirely to the parallax error. An estimated fractional mass from the infrared brightness ratio and infrared mass-luminosity relation yields a mass for the primary of 0.37 M⊙, and the secondary falls in the regime of very low mass stars, with a mass of only 0.17 M⊙.

A Deep Proper-Motion Survey in Kapteyn Selected Areas. I. Survey Description and First Results for Stars in the Tidal Tail of Sagittarius and in the Monoceros Ring

We describe a high-precision, deep (to V ~ 19-21), absolute proper-motion survey that samples ~50 lines of sight in the Kapteyn selected areas along declination zones -15°, 0°, and 15°. In many fields the astrometric baseline reaches nearly a century. We demonstrate that these data provide typical per star precisions between ~1 and 3 mas yr^(-1) to the above magnitude limits, with the absolute reference frame established by numerous extragalactic sources in each survey field. Combined with existing and ongoing photometric and radial velocity surveys in these fields, these astrometric data will enable, among other things, accurate detailed dynamical modeling of satellite interactions with our Galaxy. In this contribution we describe the astrometric part of our survey and show preliminary results along the trailing tail of the Sagittarius dwarf galaxy, and in the Monoceros Ring region.