165 research outputs found
One- and two- dimensional models are equally effective inmonitoring the crustâs thermal response to advection bylarge-scale thrusting during orogenesis
Depto. de MineralogĂa y PetrologĂaFac. de Ciencias GeolĂłgicasTRUEpu
Probing Dark Matter
Recent novel observations have probed the baryonic fraction of the galactic
dark matter that has eluded astronomers for decades. Late in 1993, the MACHO
and EROS collaborations announced in this journal the detection of transient
and achromatic brightenings of a handful of stars in the Large Magellanic Cloud
that are best interpreted as gravitational microlensing by low-mass foreground
objects (MACHOS). This tantalized astronomers, for it implied that the
population of cool, compact objects these lenses represent could be the elusive
dark matter of our galactic halo. A year later in 1994, Sackett et al. reported
the discovery of a red halo in the galaxy NGC 5907 that seems to follow the
inferred radial distribution of its dark matter. This suggested that dwarf
stars could constitute its missing component. Since NGC 5907 is similar to the
Milky Way in type and radius, some surmised that the solution of the galactic
dark matter problem was an abundance of ordinary low-mass stars. Now Bahcall et
al., using the Wide-Field Camera of the recently repaired Hubble Space
Telescope, have dashed this hope.Comment: 3 pages, Plain TeX, no figures, published as a News and Views in
Nature 373, 191 (1995
The contribution of Oxygen-Neon white dwarfs to the MACHO content of the Galactic Halo
The interpretation of microlensing results towards the Large
Magellanic Cloud (LMC) still remains controversial. White dwarfs have been
proposed to explain these results and, hence, to contribute significantly to
the mass budget of our Galaxy. However, several constraints on the role played
by regular carbon-oxygen white dwarfs exist. Massivewhite dwarfs are thought to
be made of a mixture of oxygen and neon. Correspondingly, their cooling rate is
larger than those of typical carbon-oxygen white dwarfs and they fade to
invisibility in short timescales. Consequently, they constitute a good
candidate for explaining the microlensing results. Here, we examine in detail
this hypothesis by using the most recent and up-to-date cooling tracks for
massive white dwarfs and a Monte Carlo simulator which takes into account the
most relevant Galactic inputs. We find that oxygen-neon white dwarfs cannot
account for a substantial fraction of the microlensing depth towards the LMC,
independently of the adopted initial mass function, although some microlensing
events could be due to oxygen--neon white dwarfs. The white dwarf population
contributes at most a 5% to the mass of the Galactic halo.Comment: 10 pages, 4 figures. Accepted for publication in Astronomy &
Astrophysic
Use of thermal modeling to assess the tectono-metamorphic history of the Lugo and Sanabria gneiss domes, Northwest Iberia
The Lugo and Sanabria domes in Northwest Iberia have well constrained metamorphic and structural histories.
Both occur in the Iberian autochthon and resulted from late-Variscan extensional collapse following crustal thickening
related to the Variscan collision. The two domes developed beneath large thrust sheets, are cored by
sillimanite-orthoclase anatectic gneiss, preserve evidence of a steep thermal gradient (1 oC MPa-1), and exhibit a distinct
decrease in metamorphic grade to the east in the direction of nappe movement. Geochronological evidence indicates
that the lower crust melted within 30 Ma of initial crustal thickening and that dome formation occurred within
50 Ma.
The histories of the two domes are considered as the basis for one-dimensional finite-difference models of thermal
response to changes in crustal thickness. Results from thermal models suggest that thickening was limited to the
crust, provide a numeric explanation for timing and nature of granite magmatism, and indicate that high-temperature
metamorphism and crustal anatexis may result directly from thermal relaxation, eliminating the need for significant
mantle thermal contribution. Also, the models show that small differences in thickness of large, wedge-shaped thrust
sheets can explain distinct P-T paths experienced by different limbs of the domes
Can Galactic Observations Be Explained by a Relativistic Gravity Theory?
We consider the possibility of an alternative gravity theory explaining the
dynamics of galactic systems without dark matter. From very general assumptions
about the structure of a relativistic gravity theory we derive a general
expression for the metric to order . This allows us to compare the
predictions of the theory with various experimental data: the Newtonian limit,
light deflection and retardation, rotation of galaxies and gravitational
lensing. Our general conclusion is that the possibility for any gravity theory
to explain the behaviour of galaxies without dark matter is rather improbable.Comment: 12p, REVTeX 3.
White dwarfs with hydrogen-deficient atmospheres and the dark matter content of the Galaxy
The nature of the several microlensing events observed by the MACHO team
towards the Large Magellanic Cloud (LMC) is still a subject of debate. Low-mass
substellar objects and stars with masses larger than ~M_{sun} have been ruled
out as major components of a Massive Astrophysical Compact Halo Object (MACHO)
Galactic halo, while stars of half a solar mass seem to be viable candidates.
Main sequence stars have been already discarded, and there are tight
restrictions on the role played by white dwarfs with hydrogen-dominated
atmospheres. In this paper we evaluate the contribution to the dark matter
content of the Galaxy of white dwarfs with hydrogen-deficient atmospheres. For
this purpose we use a Monte Carlo simulator which incorporates up-to-date
evolutionary sequences of white dwarfs with hydrogen-rich and
hydrogen-deficient atmospheres. We also take into account detailed descriptions
of the thick disk and the halo of our Galaxy as well as of a reliable model of
the LMC. We find that the contribution of white dwarfs with hydrogen-deficient
atmospheres moderately increases the theoretical estimate of the optical depth
with respect to the value obtained when only hydrogen-rich white dwarfs are
considered. We also find that the contribuiton of the thick disk population of
white dwarfs is comparable to the halo contribution. However, the contributions
of both the halo and the thick disk white-dwarf populations are still
insufficient to explain the number of events observed by the MACHO team.
Finally, we find that the contribution to the halo dark matter of the entire
population under study is less than 10% at the 95% conficence level.Comment: 12 pages, 5 figures, accepted for publication in A&
Unlocking the secrets of stellar haloes using combined star counts and surface photometry
The stellar haloes of galaxies can currently be studied either through
observations of resolved halo stars or through surface photometry. Curiously,
the two methods appear to give conflicting results, as a number of surface
photometry measurements have revealed integrated colours that are too red to be
reconciled with the halo properties inferred from the study of resolved stars.
Several explanations for this anomaly have been proposed - including dust
photoluminescence, extinction of extragalactic background light and a
bottom-heavy stellar initial mass function. A decisive test is, however, still
lacking. Here, we explain how observations of the halo of a nearby galaxy,
involving a combination of both surface photometry and bright star counts, can
be used to distinguish between the proposed explanations. We derive the
observational requirements for this endeavour and find that star counts in
filters VI and surface photometry in filters VIJ appears to be the optimal
strategy. Since the required halo star counts are already available for many
nearby galaxies, the most challenging part of this test is likely to be the
optical surface photometry, which requires several nights of exposure time on a
4-8 m telescope, and the near-IR surface photometry, which is most readily
carried out using the upcoming James Webb Space Telescope.Comment: 14 pages, 4 figures; v.2 matches published version (minor changes
only
Study of Constrained Minimal Supersymmetry
Taking seriously phenomenological indications for supersymmetry, we have made
a detailed study of unified minimal SUSY, including effects at the few percent
level in a consistent fashion. We report here a general analysis without
choosing a particular unification gauge group. We find that the encouraging
SUSY unification results of recent years do survive the challenge of a more
complete and accurate analysis. Taking into account effects at the 5-10% level
leads to several improvements of previous results, and allows us to sharpen our
predictions for SUSY in the light of unification. We perform a thorough study
of the parameter space. The results form a well-defined basis for comparing the
physics potential of different facilities. Very little of the acceptable
parameter space has been excluded by LEP or FNAL so far, but a significant
fraction can be covered when these accelerators are upgraded. A number of
initial applications to the understanding of the SUSY spectrum, detectability
of SUSY at LEP II or FNAL, BR(), Width(), dark
matter, etc, are included in a separate section. We formulate an approach to
extracting SUSY parameters from data when superpartners are detected. For small
tan(beta) or large both and are entirely bounded from
above at O(1 tev) without having to use a fine-tuning constraint.Comment: Michigan preprint UM-TH-93-24, LaTeX, 60 pages without figures.
Complete paper with inline figures available by anonymous ftp to
williams.physics.lsa.umich.edu in /pub/preprints/UM-TH-93-24.ps.Z
(uncompresses to 10MB / 77 pages), or by e-mailing reques
- âŠ