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

The Survival of Planetary Nebulae in the Intracluster Medium

The stellar population stripped from galaxies in clusters evolve under the extreme conditions imposed by the intracluster (IC) medium. Intracluster stars generally suffer very high systemic velocities, and evolve within a rarefied and extremely hot IC medium. We present numerical simulations which aim to explore the evolution and survival of IC Asymptotic Giant Branch (AGB) envelopes and Planetary Nebula (PN) shells. Our models reflect the evolution of a low-mass star under the observed conditions in the Virgo IC medium. We find that the integrated hydrogen-recombination line emission of a PN is dominated by the inner dense shell, whose evolution is unaffected by the environment. Ram pressure stripping affects mainly the outermost IC PN shell, which hardly influences the emission when the PN is observed as a point source. More importantly, we find that a PN with progenitor mass of 1 Msun fades to ~30% and 10% of its maximum emission, in 5,000 and 10,000 yr respectively, disclosing an actual PN lifetime t_PN several times shorter to what is usually adopted (25,000 yr). This result affects the theoretical calculation of the luminosity-specific density of IC PNe, which scales with t_PN. For t_PN=10,000 yr, our more conservative estimate, we obtain that the luminosity-specific density of PNe is in fair agreement with the value obtained from Red Giants. With our more realistic PN lifetime we infer a higher fraction (above 15%) of IC starlight in the Virgo core than current estimates.Comment: Accepted for publication in the Astrophysical Journal 14 pages, including 2 figure

The Circumstellar Extinction of Planetary Nebulae

We analyze the dependence of circumstellar extinction on core mass for the brightest planetary nebulae (PNe) in the Magellanic Clouds and M31. We show that in all three galaxies, a statistically significant correlation exists between the two quantities, such that high core mass objects have greater extinction. We model this behavior, and show that the relation is a simple consequence of the greater mass loss and faster evolution times of high mass stars. The relation is important because it provides a natural explanation for the invariance of the [O III] 5007 planetary nebula luminosity function (PNLF) with population age: bright Population I PNe are extinguished below the cutoff of the PNLF. It also explains the counter-intuitive observation that intrinsically luminous Population I PNe often appear fainter than PNe from older, low-mass progenitors.Comment: 12 pages, 2 figures, accepted for ApJ, April 10, 199

Dynamical properties of a dissipative discontinuous map: A scaling investigation

The effects of dissipation on the scaling properties of nonlinear discontinuous maps are investigated by analyzing the behavior of the average squared action \left as a function of the $n$-th iteration of the map as well as the parameters $K$ and $\gamma$, controlling nonlinearity and dissipation, respectively. We concentrate our efforts to study the case where the nonlinearity is large; i.e., $K\gg 1$. In this regime and for large initial action $I_0\gg K$, we prove that dissipation produces an exponential decay for the average action \left. Also, for $I_0\cong 0$, we describe the behavior of \left using a scaling function and analytically obtain critical exponents which are used to overlap different curves of \left onto an universal plot. We complete our study with the analysis of the scaling properties of the deviation around the average action $\omega$.Comment: 20 pages, 7 figure

Evaluation of performance impairment by spacecraft contaminants

The environmental contaminants (isolated as off-gases in Skylab and Apollo missions) were evaluated. Specifically, six contaminants were evaluated for their effects on the behavior of juvenile baboons. The concentrations of contaminants were determined through preliminary range-finding studies with laboratory rats. The contaminants evaluated were acetone, methyl ethyl ketone (MEK), methyl isobutyl ketone (MIBK), trichloroethylene (TCE), heptane and Freon 21. When the studies of the individual gases were completed, the baboons were also exposed to a mixture of MEK and TCE. The data obtained revealed alterations in the behavior of baboons exposed to relatively low levels of the contaminants. These findings were presented at the First International Symposium on Voluntary Inhalation of Industrial Solvents in Mexico City, June 21-24, 1976. A preprint of the proceedings is included

Planetary nebulae and stellar kinematics in the flattened elliptical galaxy NGC 1344

We present photometric and kinematic information obtained by measuring 197 planetary nebulae (PNs) discovered in the flattened Fornax elliptical galaxy NGC 1344 (also known as NGC 1340) with an on-band, off-band, grism + on-band filter technique. We build the PN luminosity function (PNLF) and use it to derive a distance modulus m-M=31.4, slightly smaller than, but in good agreement with, the surface brightness fluctuation distance. The PNLF also provides an estimate of the specific PN formation rate: 6x10^-12 PNs per year per solar luminosity. Combining the positional information from the on-band image with PN positions measured on the grism + on-band image, we can measure the radial velocities of 195 PNs, some of them distant more than 3 effective radii from the center of NGC 1344. We complement this data set with stellar kinematics derived from integrated spectra along the major and minor axes, and parallel to the major axis of NGC 1344. The line-of-sight velocity dispersion profile indicates the presence of a dark matter halo around this galaxy.Comment: 45 pages, 18 figures, accepted for publication in Ap