The Luminosity Function of Omega Centauri

Deep HST-WFPC2 observations of the stellar population in the globular cluster Omega Cen (NGC 5139) have been used to measure the luminosity function of main sequence stars at the low-mass end. Two fields have been investigated, located respectively ~4.6' and ~7' away from the cluster center, or near the half-light radius of this cluster (~4.8'). The color-magnitude diagrams derived from these data show the cluster main sequence extending to the detection limit at I ~ 24. Information on both color and magnitude is used to build the luminosity functions of main sequence stars in these fields and the two independent determinations are found to agree very well with each other within the observational uncertainty. Both functions show a peak in the stellar distribution around M_I ~ 9 followed by a drop at fainter magnitudes well before photometric incompleteness becomes significant, as is typical of other globular clusters observed with the HST. This result is at variance with previous claims that the luminosity function of Omega Cen stays flat at low masses, but is in excellent agreement with recent near-IR NICMOS observations of the same cluster.Comment: To appear in The Astronomical Journal. Contains 7 pages, 4 figures, prepared with the AAS LaTeX macr

The Space Motion of the Globular Cluster NGC 6397

As a by-product of high-precision, ultra-deep stellar photometry in the Galactic globular cluster NGC 6397 with the Hubble Space Telescope, we are able to measure a large population of background galaxies whose images are nearly point-like. These provide an extragalactic reference frame of unprecedented accuracy, relative to which we measure the most accurate absolute proper motion ever determined for a globular cluster. We find mu_alpha = 3.56 +/- 0.04 mas/yr and mu_delta = -17.34 +/- 0.04 mas/yr. We note that the formal statistical errors quoted for the proper motion of NGC 6397 do not include possible unavoidable sources of systematic errors, such as cluster rotation. These are very unlikely to exceed a few percent. We use this new proper motion to calculate NGC 6397's UVW space velocity and its orbit around the Milky Way, and find that the cluster has made frequent passages through the Galactic disk.Comment: 5 pages including 3 figures, accepted for publication in the Astrophysical Journal Letters. Very minor changes in V2. typos fixe

Spectroscopy of New High Proper Motion Stars in the Northern Sky. I. New Nearby Stars, New High Velocity Stars, and an Enhanced Classification Scheme for M Dwarfs

We define an enhanced spectral classification scheme for M dwarf stars, and use it to derive spectral classification of 104 northern stars with proper motions larger than 0.5"/yr which we discovered in a survey of high proper motion stars at low galactic latitudes. The final tally is as follows: 54 M dwarfs, 25 sdK and sdM subdwarfs, 14 esdK and esdM extreme subdwarfs, and 11 DA and DC white dwarfs. Among the most interesting cases, we find one star to be the coolest subdwarf ever reported (LSR2036+5059, with spectral type sdM7.5), a new M9.0 dwarf only about 6pc distant (LSR1835+3259), and a new M6.5 dwarf only 7pc from the Sun (LSR2124+4003). Spectroscopic distances suggests that 27 of the M dwarfs, 3 of the white dwarfs, and one of the subdwarfs (LSR2036+5059) are within 25pc of the Sun, making them excellent candidates for inclusion in the solar neighborhood census. Estimated sky-projected velocities suggest that most of our subdwarfs and extreme subdwarfs have halo kinematics. We find that several white dwarfs and non metal-poor M dwarfs also have kinematics consistent with the halo, and we briefly discuss their possible origin.Comment: 53 pages, 18 figures, 5 tables, accepted by The Astronomical Journal; this updated version includes radial velocities for the stars and an expanded discussio

Very Large Telescope Observations of the peculiar globular cluster NGC6712. Discovery of a UV, H-alpha excess star in the core

We present results from multi-band observations in the central region of the cluster NGC6712 with the ESO-Very Large Telescope. Using high resolution images we have identified three UV-excess stars. In particular two of them are within the cluster core, a few arcsec apart: the first object is star "S" which previous studies identified as the best candidate to the optical counterpart to the luminous X-ray source detected in this cluster. The other UV object shows clearcut H-alpha emission and, for this reason, is an additional promising interacting binary candidate (a quiescent LMXB or a CV). The presence of two unrelated interacting binary systems a few arcsec apart in the core of this low-density cluster is somewhat surprising and supports the hypothesis that the (internal) dynamical history of the cluster and/or the (external) interaction with the Galaxy might play a fundamental role in the formation of these peculiar objects.Comment: 15 pages, 3 figures. ApJL in pres

The Proper Motion of NGC 6522 in Baade's Window

We have detected seven stars with a common proper motion which are located within 2.5 arcminutes of the globular cluster NGC 6522 in the Baade's Window field of the Galactic bulge. We argue that these stars are members of the cluster, and derive a weighted mean proper motion and heliocentric radial velocity of mu_l = 1.4 +/- 0.2 mas/yr, mu_b = -6.2 +/- 0.2 mas/yr, v = -28.5 +/- 6.5 km/s. We rederive the distance to NGC 6522 (0.91 +/- 0.04 R_0, where R_0 is the Galactocentric distance) and metallicity ([Fe/H] = -1.28 +/- 0.12), making use of recent revisions in the foreground extinction toward the cluster (A_V = 1.42 +/- 0.05). We find the spatial velocity of the cluster and conclude that the cluster stays close to the Galactic center, and may have experienced significant bulge/disk shocking during its lifetime.Comment: submitted to AJ, 21 pages, 5 figure

Parallax and Luminosity Measurements of an L Subdwarf

We present the first parallax and luminosity measurements for an L subdwarf, the sdL7 2MASS J05325346+8246465. Observations conducted over three years by the USNO infrared astrometry program yield an astrometric distance of 26.7+/-1.2 pc and a proper motion of 2.6241+/-0.0018"/yr. Combined with broadband spectral and photometric measurements, we determine a luminosity of log(Lbol/Lsun) = -4.24+/-0.06 and Teff = 1730+/-90 K (the latter assuming an age of 5-10 Gyr), comparable to mid-type L field dwarfs. Comparison of the luminosity of 2MASS J05325346+8246465 to theoretical evolutionary models indicates that its mass is just below the sustained hydrogen burning limit, and is therefore a brown dwarf. Its kinematics indicate a ~110 Myr, retrograde Galactic orbit which is both eccentric (3 <~ R <~ 8.5 kpc) and extends well away from the plane (Delta_Z = +/-2 kpc), consistent with membership in the inner halo population. The relatively bright J-band magnitude of 2MASS J05325346+8246465 implies significantly reduced opacity in the 1.2 micron region, consistent with inhibited condensate formation as previously proposed. Its as yet unknown subsolar metallicity remains the primary limitation in constraining its mass; determination of both parameters would provide a powerful test of interior and evolutionary models for low-mass stars and brown dwarfs.Comment: Accepted to ApJ 10 September 2007; 13 pages, 5 figures, 3 tables, formatted in emulateapj styl

The Anisotropic Distribution of M 31 Satellite Galaxies: A Polar Great Plane of Early-Type Companions

The highly anisotropic distribution and apparent alignment of the Galactic satellites in polar great planes begs the question how common such distributions are. The satellite system of M31 is the only nearby system for which we currently have sufficiently accurate distances to study the three-dimensional satellite distribution. We present the spatial distribution of the 15 presently known M31 companions in a coordinate system centered on M31 and aligned with its disk. Through a detailed statistical analysis we show that the full satellite sample describes a plane that is inclined by -56 deg with respect to the poles of M31 and that has an r.m.s. height of 100 kpc. With 88% the statistical significance of this plane is low and it is unlikely to have a physical meaning. The great stellar stream found near Andromeda is inclined to this plane by 7 deg. There is little evidence for a Holmberg effect. If we confine our analysis to early-type dwarfs, we find a best-fit polar plane within 5 deg to 7 deg from the pole of M31. This polar great plane has a statistical significance of 99.3% and includes all dSphs (except for And II), M32, NGC 147, and PegDIG. The r.m.s. distance of these galaxies from the polar plane is 16 kpc. The nearby spiral M33 has a distance of only about 3 kpc from this plane, which points toward the M81 group. We discuss the anisotropic distribution of M31's early-type companions in the framework of three scenarios, namely as remnants of the break-up of a larger progenitor, as tracer of a prolate dark matter halo, and as tracer of collapse along large-scale filaments. (Abridged)Comment: 14 pages, 5 figures, accepted for publication in the Astronomical Journa

Discovery of an M8.5 Dwarf With Proper Motion mu=2.38 arcsec/yr

We report the discovery of LSR1826+3014, a very faint (V=19.36) star with a very large proper motion (mu=2.38 arcsec/yr). A low resolution red spectrum reveals that LSR1826+3014 is an ultra-cool red dwarf with spectral type M8.5 V and with a radial velocity v_rad=+77+/-10 km/s. LSR1826+3014 is thus the faintest red dwarf ever discovered with a proper motion larger than 2 arcsec/yr. Optical and infrared photometry suggest that the star is at a distance d=13.9+/-3.5 pc from the Sun, which implies it is moving relative to the local standard of rest with a total velocity of 175+/-25 km/s. Numerical integration of its orbit suggests that LSR1826+3014 is on a halo-like galactic orbit.Comment: 12 pages, including 1 table and 3 figures, accepted for publication in The Astrophysical Journal Letter

Mass Segregation in the Globular Cluster Palomar 5 and its Tidal Tails

We present the stellar main sequence luminosity function (LF) of the disrupted, low-mass, low-concentration globular cluster Palomar 5 and its well-defined tidal tails, which emanate from the cluster as a result of its tidal interaction with the Milky Way. The results of our deep (B ~ 24.5) wide-field photometry unequivocally indicate that preferentially fainter stars were removed from the cluster so that the LF of the cluster's main body exhibits a significant degree of flattening compared to other globular clusters. There is clear evidence of mass segregation, which is reflected in a radial variation of the LFs. The LF of the tidal tails is distinctly enhanced with faint, low-mass stars. Pal 5 exhibits a binary main sequence, and we estimate a photometric binary frequency of roughly 10%. Also the binaries show evidence of mass segregation with more massive binary systems being more strongly concentrated toward the cluster center.Comment: 14 pages, 12 figures, accepted for publication in the Astronomical Journa

Deep ACS Imaging in the Globular Cluster NGC6397: Dynamical Models

We present N-body models to complement deep imaging of the metal-poor core-collapsed cluster NGC6397 obtained with the Hubble Space Telescope. All simulations include stellar and binary evolution in-step with the stellar dynamics and account for the tidal field of the Galaxy. We focus on the results of a simulation that began with 100000 objects (stars and binaries), 5% primordial binaries and Population II metallicity. After 16 Gyr of evolution the model cluster has about 20% of the stars remaining and has reached core-collapse. We compare the color-magnitude diagrams of the model at this age for the central region and an outer region corresponding to the observed field of NGC6397 (about 2-3 half-light radii from the cluster centre). This demonstrates that the white dwarf population in the outer region has suffered little modification from dynamical processes - contamination of the luminosity function by binaries and white dwarfs with non-standard evolution histories is minimal and should not significantly affect measurement of the cluster age. We also show that the binary fraction of main-sequence stars observed in the NGC6397 field can be taken as representative of the primordial binary fraction of the cluster. For the mass function of the main-sequence stars we find that although this has been altered significantly by dynamics over the cluster lifetime, especially in the central and outer regions, that the position of the observed field is close to optimal for recovering the initial mass function of the cluster stars (below the current turn-off mass). More generally we look at how the mass function changes with radius in a dynamically evolved stellar cluster and suggest where the best radial position to observe the initial mass function is for clusters of any age.Comment: 34 pages, 11 figures, submitted to AJ, companion paper to 0708.403