Dana I. Casetti-Dinescu

THE SOUTHERN PROPER MOTION PROGRAM. IV. THE SPM4 CATALOG

We present the fourth installment of the Yale/San Juan Southern Proper Motion Catalog, SPM4. The SPM4 contains absolute proper motions, celestial coordinates, and B, V photometry for over 103 million stars and galaxies between the south celestial pole and -20{sup 0} declination. The catalog is roughly complete to V = 17.5 and is based on photographic and CCD observations taken with the Yale Southern Observatorys double astrograph at Cesco Observatory in El Leoncito, Argentina. The proper-motion precision, for well-measured stars, is estimated to be 2-3 mas yr{sup -1}, depending on the type of second-epoch material. At the bright end, proper motions are on the International Celestial Reference System by way of Hipparcos Catalog stars, while the faint end is anchored to the inertial system using external galaxies. Systematic uncertainties in the absolute proper motions are on the order of 1 mas yr{sup -1}.

KINEMATIC DECOUPLING OF GLOBULAR CLUSTERS WITH THE EXTENDED HORIZONTAL BRANCH

About 25% of the Milky Way globular clusters (GCs) exhibit unusually extended color distribution of stars in the core helium-burning horizontal-branch (HB) phase. This phenomenon is now best understood as due to the presence of helium-enhanced second-generation subpopulations, which has raised the possibility that these peculiar GCs might have a unique origin. Here we show that these GCs with extended HB are clearly distinct from other normal GCs in kinematics and mass. The GCs with extended HB are more massive than normal GCs and are dominated by random motion with no correlation between kinematics and metallicity. Surprisingly, however, when they are excluded, most normal GCs in the inner halo show clear signs of dissipational collapse that apparently led to the formation of the disk. Normal GCs in the outer halo share their kinematic properties with the extended HB GCs, which is consistent with the accretion origin. Our result further suggests heterogeneous origins of GCs, and we anticipate this to be a starting point for more detailed investigations of Milky Way formation, including early mergers, collapse, and later accretion. Subject headings: Galaxy: formation — globular clusters: general — stars: horizontal-branch

SPACE VELOCITIES OF SOUTHERN GLOBULAR CLUSTERS. V. A LOW GALACTIC LATITUDE SAMPLE

We have measured the absolute proper motions of globular clusters NGC 2808, 3201, 4372, 4833, 5927, and 5986. The proper motions are on the Hipparcos system, and they are the first determinations ever made for these low Galactic latitude clusters. The proper-motion uncertainties range from 0.3 to 0.5 mas yr-1. The inferred orbits indicate that (1) the single metal-rich cluster in our sample, NGC 5927, dynamically belongs to the thick disk; (2) the remaining metal-poor clusters have rather low-energy orbits of high eccentricity, and among these there appear to be two pairs of dynamically associated clusters; (3) the most energetic cluster in our sample, NGC 3201, is on a highly retrograde orbit?which had already been surmised from radial velocity alone?with an apocentric distance of 22 kpc; and (4) none of the metal-poor clusters appear to be associated with the recently detected SDSS streams or with the Monoceros structure. These are the first results of the Southern Proper Motion program where the second-epoch observations are taken with the recent CCD camera system installed on the double astrograph at El Leoncito, Argentina.

THE THREE-DIMENSIONAL VELOCITY STRUCTURE OF THE THICK DISK FROM SPM4 AND RAVE DR2

We analyze the three-dimensional kinematics of a sample of ~4400 red clump stars ranging between 5 and 10?kpc from the Galactic center and up to 3 kpc from the Galactic plane. This sample is representative for the metal-rich ([Fe/H] = ?0.6 ? +0.5) thick disk. Absolute proper motions are from the fourth release of the Southern Proper Motion Program and radial velocities from the second release of the Radial Velocity Experiment. The derived kinematical properties of the thick disk include the rotational velocity gradient ?V ?/?z = ?25.2 ? 2.1?km?s?1?kpc?1, velocity dispersions ?km?s?1, and velocity-ellipsoid tilt angle ? Rz = 86 ? 18. Our dynamical estimate of the thin-disk scale length is R thin = 2.0 ? 0.4 kpc and of the thick-disk scale height is z thick = 0.7 ? 0.1 kpc. The observed orbital eccentricity distribution compared with those from four different models of the formation of the thick disk from Sales et?al. favors the gas-rich merger model and the minor merger heating model. Interestingly, when referred to the currently accepted value of the LSR, stars more distant than 0.7 kpc from the Sun show a net average radial velocity of 13 ? 3?km?s?1. This result is seen in previous kinematical studies using other tracers at distances larger than ~1 kpc. We suggest this motion reflects an inward perturbation of the locally defined LSR induced by the spiral density wave.

PROPER-MOTION STUDY OF THE MAGELLANIC CLOUDS USING SPM MATERIAL

Resumen en: Absolute proper motions are determined for stars and galaxies to V = 17.5 over a 450 square degree area that encloses both Magellanic Clouds. The proper ...

Velocity Shear of the Thick Disk from SPM3 Proper Motions at the South Galactic Pole

The kinematical properties of the Galactic thick disk are studied using absolute proper motions from the Third Yale/San Juan Southern Proper Motion Catalog and Two Micron All Sky Survey near-infrared photometry for a sample of ~1200 red giants in the direction of the south Galactic pole. The photometrically selected sample is dominated by thick-disk stars, as indicated by the number-density distribution that varies with distance from the Galactic plane as a single-valued exponential over the range 1 kpc < z < 4 kpc. The inferred scale height of the thick disk is 0.783 ± 0.048 kpc. The kinematics of the sample are also consistent with disklike motion. The U-velocity component is roughly constant, reflecting the Suns peculiar motion, while a considerable shear is seen in the mean rotational velocity, V. The V-velocity profiles dependence on z is linear, with a gradient of dV/dz = -30 ± 3 km s-1 kpc-1. The velocity dispersions in both U and V show a lesser gradient of about 9 ± 3 km s-1 kpc-1. We demonstrate that the derived velocity and velocity-dispersion profiles are consistent with the assumptions of dynamical equilibrium and reasonable models of the overall Galactic potential.

SPACE VELOCITIES OF SOUTHERN GLOBULAR CLUSTERS. VII. NGC 6397, NGC 6626 (M28), AND NGC 6656 (M22)

We have measured the absolute proper motions of globular clusters NGC 6397, NGC 6626 (M22), and NGC 6656 (M28) as part of our ongoing Southern Proper-Motion Program. The reference system is the ICRS via Hipparcos stars for these three low-Galactic-latitude clusters. Formal errors range between {approx}0.3 and 0.7 mas yr{sup -1}. Notable is the result for NGC 6397, which differs by 2.5 mas yr{sup -1} from two Hubble Space Telescope determinations while agreeing with previous ground-based ones. We determine orbits for all three clusters in an axisymmetric and barred model of the Galaxy and discuss these in the context of globular-cluster formation. M22 is a well-known cluster with an iron abundance spread; such clusters are now believed to have formed in massive parent systems that can retain ejecta of core-collapsed supernovae. We find that the five currently accepted globular clusters with iron/calcium abundance spread show orbits unrelated to each other, thus suggesting at least five independent, massive progenitors that have contributed to the build-up of the Milky-Way halo.

SPACE VELOCITIES OF SOUTHERN GLOBULAR CLUSTERS. VI. NINE CLUSTERS IN THE INNER MILKY WAY

(abridged) We have measured the absolute proper motions of nine low-latitude, inner Galaxy globular clusters, namely NGC 6273 (M 19), NGC 6284, NGC 6287, NGC 6293, NGC 6333 (M 9), NGC 6342, NGC 6356, NGC 6388 and NGC 6441. These are the first determinations ever made for these clusters. The proper motions are on the ICRS via Hipparcos. The proper-motion errors range between 0.4 and 0.9 mas/yr, and are dominated by the number of measurable cluster members in these regions which are very crowded by the bulge/bar and the thick disk. This samle contains five metal poor ([Fe/H < -1.0) and four metal rich clusters; seven clusters are located within 4 kpc from the Galactic center, while the remaining two, namely NGC 6356 and NGC 6284 are in the background of the bulge at 7.5 kpc from the Galactic center. By combining proper motions with radial velocities and distances from the literature we derive 3D velocities. In a number of cases, distance uncertainties make the kinematical classification ambiguous. The two metal rich clusters NGC 6388 and NGC 6441 have velocities incompatibile with membership in the thick disk or the bar of the Milky Way. They can be though of as members of a kinematically hot system in the inner Galaxy. Curiously, both clusters have similar velocity components. Together with their similar Galactic location and peculiar but similar stellar-population characteristics, these two clusters may share a common origin. Their velocities are also very low indicating that the two clusters are now at/near apocenter, and they will not leave the inner ~4 kpc of the Galaxy.

PROPER MOTIONS IN KAPTEYN SELECTED AREA 103: A PRELIMINARY ORBIT FOR THE VIRGO STELLAR STREAM

We present absolute proper motions in Kapteyn Selected Area (SA) 103. This field is located 7° west of the center of the Virgo Stellar Stream (VSS), and has a well-defined main sequence representing the stream. In SA 103, we identify one RR Lyrae star as a member of the VSS, according to its metallicity, radial velocity, and distance. VSS candidate turnoff and subgiant stars have proper motions consistent with that of the RR Lyrae star. The three-dimensional velocity data imply an orbit with a pericenter of ~11 kpc and an apocenter of ~90 kpc. Thus, the VSS comprises tidal debris found near the pericenter of a highly destructive orbit. Examining the six globular clusters at distances larger than 50 kpc from the Galactic center, and the proposed orbit of the VSS, we find one tentative association, NGC 2419. We speculate that NGC 2419 is possibly the nucleus of a disrupted system of which the VSS is a part.

CONSTRAINTS ON THE MAGELLANIC CLOUDS' INTERACTION FROM THE DISTRIBUTION OF OB STARS AND THE KINEMATICS OF GIANTS

Young, OB-type candidates are identified in a ~7900?deg2 region encompassing the Large and Small Magellanic Clouds (LMC/SMC) periphery, the Bridge, part of the Magellanic Stream (MS), and Leading Arm (LA). Selection is based on UV, optical, and IR photometry from existing large-area surveys and proper motions from the Southern Proper Motion 4 (SPM4) catalog. The spatial distribution of these young star candidates shows (1) a well-populated SMC wing which continues westward with two branches partially surrounding the SMC, (2) a rather narrow path from the SMC wing eastward toward the LMC which is offset by 1?-2? from the high-density H I ridge in the Bridge, (3) a well-populated periphery of the LMC dominated by clumps of stars at the ends of the LMC bar, and (4) a few scattered candidates in the MS and two overdensities in the LA regions above and below the Galactic plane. Additionally, a proper-motion analysis is made of a radial-velocity-selected sample of red giants and supergiants in the LMC, previously shown to be a kinematically and chemically distinct subgroup, most likely captured from the SMC. SPM4 proper motions of these stars also indicate they are distinct from the LMC population. The observational results presented here, combined with the known orbits of the Clouds and other aspects of the LMC morphology, suggest an off-center, moderate to highly inclined collision between the SMC and the LMCs disk that took place between 100 and 200?Myr ago.