10,009 research outputs found
Faint blue objects on the Hubble Deep Field North & South as possible nearby old halo white dwarfs
Using data derived from the deepest and finest angular resolution images of
the universe yet acquired by astronomers at optical wavelengths using the
Hubble Space Telescope (HST) in two postage-stamp sections of the sky (Williams
et al. 1996a,b), plus simple geometrical and scaling arguments, we demonstrate
that the faint blue population of point-source objects detected on those two
fields (M\'endez et al. 1996) could actually be ancient halo white dwarfs at
distances closer than about 2 kpc from the Sun. This finding has profound
implications, as the mass density of the detected objects would account for
about half of the missing dark matter in the Milky-Way (Bahcall and Soneira
1980), thus solving one of the most controversial issues of modern astrophysics
(Trimble 1987, Ashman 1992). The existence of these faint blue objects points
to a very large mass locked into ancient halo white dwarfs. Our estimate
indicates that they could account for as much as half of the dark matter in our
Galaxy, confirming the suggestions of the MACHO microlensing experiment (Alcock
et al. 1997). Because of the importance of this discovery, deep follow-up
observations with HST within the next two years would be needed to determine
more accurately the kinematics (tangential motions) for these faint blue old
white dwarfs.Comment: Accepted for publication on The Astrophysical Journal, Part 1. 8
pages (AAS Latex macros V4.0), 1 B&W postscript figure, 2 color postscript
figure
Galactic structure towards the Open Clusters NGC 188 and NGC 3680
We present the first comparisons of a newly developed Galactic Structure and
Kinematic Model to magnitude and color counts, as well as relative proper
motions, in the fields of the open clusters NGC 188 [(l, b)= (122.8 deg, +22.4
deg)] and NGC 3680 [(l,b)= (286.8 deg, +16.9 deg)]. In addition to determining
the reddening toward these two clusters, it is shown that starcounts at
intermediate Galactic latitudes in the range 11< V< 17 allow us to constrain
the model scale-height for disk subgiants. We obtain a mean value of 250 +/- 32
pc, in agreement with previous determinations of the scale-height for
red-giants. We are also able to constrain the scale-height of main-sequence
stars, and the distance of the sun from the Galactic plane, ruling out the
possibility of a value of +40 pc, in favor of a smaller value. Comparisons with
the observed proper-motion histograms indicate that the velocity dispersion of
disk main-sequence stars must increase with distance from the Galactic plane in
order to match the observed proper-motion dispersion. The required increase is
consistent with the values predicted by dynamical models, and provides a clear
observational evidence in favor of such gradients. The shape of the observed
proper-motion distribution is well fitted within the Poisson uncertainties.
This implies that corrections to absolute proper motion (and, therefore, space
velocities) for open clusters may be obtained using our model when no inertial
reference frame is available. Using this approach, the derived tangential
motions for NGC 188 and NGC 3680 are presented.Comment: Tex type, 29 pages, 9 postscript figures. Accepted for publication in
The Astronomical Journa
A catalog of planetary nebulae in the elliptical galaxy NGC 4697
We present a catalog of 535 planetary nebulae discovered in the flattened
elliptical galaxy NGC 4697, using the FORS1 Cassegrain spectrograph of the Very
Large Telescope of the European Southern Observatory at Cerro Paranal, Chile.
The catalog provides positions (x, y coordinates relative to the center of
light of NGC 4697, as well as RA, Dec.), and, for almost all PNs, the magnitude
m(5007) and the heliocentric radial velocity in km/s.Comment: 16 pages, 5 figures, ApJS in pres
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