1,425 research outputs found
Toward High-Precision Astrometry with WFPC2. I. Deriving an Accurate PSF
The first step toward doing high-precision astrometry is the measurement of
individual stars in individual images, a step that is fraught with dangers when
the images are undersampled. The key to avoiding systematic positional error in
undersampled images is to determine an extremely accurate point-spread function
(PSF). We apply the concept of the {\it effective} PSF, and show that in images
that consist of pixels it is the ePSF, rather than the often-used instrumental
PSF, that embodies the information from which accurate star positions and
magnitudes can be derived. We show how, in a rich star field, one can use the
information from dithered exposures to derive an extremely accurate effective
PSF by iterating between the PSF itself and the star positions that we measure
with it. We also give a simple but effective procedure for representing spatial
variations of the HST PSF. With such attention to the PSF, we find that we are
able to measure the position of a single reasonably bright star in a single
image with a precision of 0.02 pixel (2 mas in WF frames, 1 mas in PC), but
with a systematic accuracy better than 0.002 pixel (0.2 mas in WF, 0.1 mas in
PC), so that multiple observations can reliably be combined to improve the
accuracy by .Comment: 33 pp. text + 15 figs.; accepted by PAS
Accurate Internal Proper Motions of Globular Clusters
We have undertaken a long term program to measure high precision proper
motions of nearby Galactic globular cluster (GC) stars using multi-epoch
observations with the WFPC2 and the ACS cameras on-board the Hubble Space
Telescope. The proper motions are used to study the internal cluster
kinematics, and to obtain accurate cluster distances. In this paper, we also
show how the proper motions of the field stars projected in the direction of
the studied clusters can be used to set constraints on the Galaxy kinematics.Comment: 4 pages, 2 figures, A.S.P. Conf. Ser., in press in Vol. 296, 200
A preliminary survey of collapsed cores in globular clusters
This is a preliminary report on surface photometry of the major fraction of known globular clusters, to see which of them show the signs of a collapsed core. The authors find more than 20 examples, or one-fifth of the total. Core classifications are given for all clusters examined. The fraction of collapsed-core clusters may be an index of how long a cluster takes to reexpand after collapse
Deep HST/FOC Imaging of the Central Density Cusp of the Globular Cluster M15
Using the Faint Object Camera on the repaired Hubble Space Telescope, we have
observed two fields in the globular cluster M15: the central density cusp, and
a field at r = 20''. These are the highest-resolution images ever taken of this
cluster's dense core, and are the first to probe the distribution of stars well
below the main-sequence turnoff. After correction for incompleteness, we
measure a logarithmic cusp slope (d log \sigma / d log r) of -0.70 +- 0.05
(1-sigma) for turnoff (~ 0.8 \Msun) stars over the radial range from 0.3'' to
10''; this slope is consistent with previous measurements. We also set an
approximate upper limit of ~1.5'' (90% confidence limit) on the size of any
possible constant-surface-density core, but discuss uncertainties in this limit
that arise from crowding corrections. We find that fainter stars in the cusp
also have power-law density profiles: a mass group near 0.7 \Msun has a
logarithmic slope of -0.56 +- 0.05 (1-sigma) over the radial range from 2'' to
10''. Taken together, the two slopes are not well matched by the simplest
core-collapse or black-hole models. We also measure a mass function at r =
20'', outside of the central cusp. Both of the FOC fields show substantial mass
segregation, when compared with a mass function measured with the WFPC2 at r =
5'. In comparing the overall mass functions of the two FOC fields and the r =
5' field, we find that the radial variation of the mass function is somewhat
less than that predicted by a King-Michie model of the cluster, but greater
than that predicted by a Fokker-Planck model taken from the literature.Comment: 40 pages single-spaced, including 12 figures; to appear in AJ, April
199
HST Astrometry of M4 and the Galactic Constant V_0/R_0
From multi-epoch WFPC2/HST observations we present astrometric measurements
of stars in the Galactic globular cluster M4 (NGC 6121) and in the
foreground/background. The presence of an extragalactic point source allows us
to determine the absolute proper motion of the cluster, and, through use of the
field stars in this region only 18 degree from the Galactic center, to measure
the difference between the Oort constants, A-B. We find: (mu_alpha cos dec,
mu_dec)_J2000 = (-13.21 +/- 0.35, -19.28 +/- 0.35) mas/yr, and A-B = V_0/R_0 =
27.6 +/- 1.7 km / s / kpc.Comment: 20 pages, 6 figures, A.J.__ACCEPTED__, 1 April, 2003, (...!
Color and population gradients in the core of the postcollapse globular cluster M30
We have developed a new technique for seeking color gradients in multicolor CCD images of a globular cluster. Application of this technique to M30 confirms the radial color gradient reported by others. The B - V and V - R colors show linear gradients, when plotted against log r, that are many times their statistical errors. In the radial range ~ 3-100 arcsec, the B - V and V - R colors increase outwards by about (0.18 ± 0.015) mag/dex and (0.09 ± 0.01) mag/dex, respectively. Color-magnitude arrays of the central region show that the Balmer-line strength that accompanies the central blueness is due to an increased proportion of blue-horizontal-branch starlight in the center. This excess does not appear to be statistically significant in terms of star numbers, even though the color gradient is so highly significant.
The authors are divided among themselves as to whether the color and spectrum gradients observed here and in other clusters are due only to random statistical fluctuations, or whether they might have a
physical significance
A Re-examination of the "Planetary" Lensing Events in M22
We have carried out further analysis of the tentative, short-term
brightenings reported by Sahu et al. (2001), which were suggested to be
possible lensings of Galactic-bulge stars by free-floating planets in the
globular cluster M22. Closer examination shows that--unlikely as it may
seem--small, point-like cosmic rays had hit very close to the same star in both
of a pair of cosmic-ray-split images, which cause the apparent brightenings of
stars at the times and locations reported. We show that the observed number of
double hits is consistent with the frequency of cosmic rays in WFPC2 images,
given the number of stars and epochs observed. Finally, we point to ways in
which cosmic rays can be more directly distinguished.Comment: Accepted Dec 11, 2001 by ApJ Letter
Reaching the End of the White Dwarf Cooling Sequence in NGC 6791
We present new observations of the white dwarf sequence of the old open
cluster NGC 6791. The brighter peak previously observed in the white dwarf
luminosity function (WDLF) is now better delineated, and the second, fainter
peak that we suggested earlier is now confirmed. A careful study suggests that
we have reached the end of the white dwarf sequence. The WDs that create the
two peaks in the WDLF show a significant turn to the blue in the
color-magnitude diagram. The discrepancy between the age from the WDs and that
from the main sequence turnoff remains, and we have an additional puzzle in the
second peak in the WDLF. Canonical WD models seem to fail --at least at
~25%-level-- in reproducing the age of clusters of this metallicity. We discuss
briefly possible ways of arriving at a theoretical understanding of the WDLF.Comment: 29 pages, 10 figures (4 in low resolution), 1 table. Accepted (2007
December 19) on Ap
Stellar Kinematics of the Double Nucleus of M31
We report observations of the double nucleus of M31 with the f/48 long-slit
spectrograph of the HST Faint Object Camera. We obtain a total exposure of
19,000 sec. over 7 orbits, with the 0.063-arcsec-wide slit along the line
between the two brightness peaks (PA 42). A spectrum of Jupiter is used as a
spectral template. The rotation curve is resolved, and reaches a maximum
amplitude of ~250 km/s roughly 0.3 arcsec either side of a rotation center
lying between P1 and P2, 0.16 +/- 0.05 arcsec from the optically fainter P2. We
find the velocity dispersion to be < 250 km/s everywhere except for a narrow
``dispersion spike'', centered 0.06 +/- 0.03 arcsec on the anti-P1 side of P2,
in which sigma peaks at 440 +/- 70 km/s. At much lower confidence, we see local
disturbances to the rotation curve at P1 and P2, and an elevation in sigma at
P1. At very low significance we detect a weak asymmetry in the line-of-sight
velocity distribution opposite to the sense usually encountered. Convolving our
V and sigma profiles to CFHT resolution, we find good agreement with the
results of Kormendy & Bender (1998, preprint), though there is a 20%
discrepancy in the dispersion that cannot be attributed to the dispersion
spike. Our results are not consistent with the location of the maximum
dispersion as found by Bacon et al. We find that the sinking star cluster model
of Emsellem & Combes (1997) does not reproduce either the rotation curve or the
dispersion profile. The eccentric disk model of Tremaine (1995) fares better,
and can be improved somewhat by adjusting the original parameters. However,
detailed modeling will require dynamical models of significantly greater
realism.Comment: 29 pages, Latex, AASTeX v4.0, with 7 eps figures. To appear in The
Astronomical Journal, February 199
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