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