52 research outputs found
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
The gravitational force and potential of the finite Mestel disk
Mestel determined the surface mass distribution of the finite disk for which
the circular velocity is constant in the disk and found the gravitational field
for points in the plane. Here we find the exact closed form solutions for
the potential and the gravitational field of this disk in cylindrical
coordinates over all the space. The Finite Mestel Disk (FMD) is characterized
by a cuspy mass distribution in the inner disk region and by an exponential
distribution in the outer region of the disk. The FMD is quite different from
the better known exponential disk or the untruncated Mestel disk which, being
infinite in extent, are not realistic models of real spiral galaxies. In
particular, the FMD requires significantly less mass to explain a measured
velocity curve
Calibration of radii and masses of open clusters with a simulation
Context: A recent new approach to apply a simple dynamical mass estimate of
tidally limited star clusters is based on the identification of the tidal
radius in a King profile with the dynamical Jacobi radius. The application to
an unbiased open cluster catalogue yields significantly higher cluster masses
compared to the classical methods. Aims: We quantify the bias in the mass
determination as function of projection direction and cluster age by analysing
a simulated star cluster. Methods: We use direct -body simulations of a star
cluster including stellar evolution in an analytic Milky Way potential and
apply a best fit to the projected number density of cluster stars. Results: We
obtain significantly overestimated star cluster masses which depend strongly on
the viewing direction. The overestimation is typically in the range of 10-50
percent and reaches a factor of 3.5 for young clusters. Mass segregation
reduces the derived limiting radii systematically.Comment: 9 pages, 10+1 figures, accepted by Astronomy and Astrophysic
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
Faraday Rotation in the Tail of the Planetary Nebula DeHt 5
We present 1420 MHz polarization images of a 5x5 degree region around the
planetary nebula (PN) DeHt 5. The images reveal narrow Faraday-rotation
structures on the visible disk of DeHt 5, as well as two wider, tail-like,
structures "behind" DeHt 5. Though DeHt 5 is an old PN known to be interacting
with the interstellar medium (ISM), a tail has not previously been identified
for this object. The innermost tail is approximately 3 pc long and runs away
from the north-east edge of DeHt 5 in a direction roughly opposite that of the
sky-projected space velocity of the white dwarf central star, WD 2218+706. We
believe this tail to be the signature of ionized material ram-pressure stripped
and deposited downstream during a >74,000 yr interaction between DeHt 5 and the
ISM. We estimate the rotation measure (RM) through the inner tail to be -15 +/-
5 rad/m^2, and, using a realistic estimate for the line-of-sight component of
the ISM magnetic field around DeHt 5, derive an electron density in the inner
tail of n_e = 3.6 +/- 1.8 cm^-3. Assuming the material is fully ionized, we
estimate a total mass in the inner tail of 0.68 +/- 0.33 solar masses, and
predict that 0.49 +/- 0.33 solar masses was added during the PN-ISM
interaction. The outermost tail consists of a series of three roughly circular
components, which have a collective length of approximately 11.0 pc. This tail
is less conspicuous than the inner tail, and may be the signature of the
earlier interaction between the WD 2218+706 asymptotic giant branch (AGB)
progenitor and the ISM. The results for the inner and outer tails are
consistent with hydrodynamic simulations, and may have implications for the PN
missing-mass problem as well as for models which describe the impact of the
deaths of intermediate-mass stars on the ISM.Comment: 30 pages (single-column preprint format), 5 figures. Accepted for
publication in the Astrophysical Journa
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
The global mass function of M15
Data obtained with the NICMOS instrument on board the Hubble Space Telescope
(HST) have been used to determine the H-band luminosity function (LF) and mass
function (MF) of three stellar fields in the globular cluster M15, located ~7'
from the cluster centre. The data confirm that the cluster MF has a
characteristic mass of ~0.3 Msolar, as obtained by Paresce & De Marchi (2000)
for a stellar field at 4.6' from the centre. By combining the present data with
those published by other authors for various radial distances (near the centre,
at 20" and at 4.6'), we have studied the radial variation of the LF due to the
effects of mass segregation and derived the global mass function (GMF) using
the Michie-King approach. The model that simultaneously best fits the LF at
various locations, the surface brightness profile and the velocity dispersion
profile suggests that the GMF should resemble a segmented power-law with the
following indices: x ~ 0.8 for stars more massive than 0.8 Msolar, x ~ 0.9 for
0.3 - 0.8 Msolar and x ~ -2.2 at smaller masses (Salpeter's IMF would have
x=1.35). The best fitting model also suggests that the cluster mass is ~5.4
10^5 Msolar and that the mass-to-light ratio is on average M/L_V ~ 2.1, with
M/L_V ~ 3.7 in the core. A large amount of mass (~ 44 %) is found in the
cluster core in the form of stellar heavy remnants, which may be sufficient to
explain the mass segregation in M15 without invoking the presence of an
intermediate-mass black hole.Comment: 12 pages, 10 figures, accepted for publication in A&
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
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