30,458 research outputs found
Simulating a White Dwarf-dominated Galactic Halo
Observational evidence has suggested the possibility of a Galactic halo which
is dominated by white dwarfs (WDs). While debate continues concerning the
interpretation of this evidence, it is clear that an initial mass function
(IMF) biased heavily toward WD precursors (1 < m/Msol < 8), at least in the
early Universe, would be necessary in generating such a halo. Within the
framework of homogeneous, closed-box models of Galaxy formation, such biased
IMFs lead to an unavoidable overproduction of carbon and nitrogen relative to
oxygen (as measured against the abundance patterns in the oldest stars of the
Milky Way). Using a three-dimensional Tree N-body smoothed particle
hydrodynamics code, we study the dynamics and chemical evolution of a galaxy
with different IMFs. Both invariant and metallicity-dependent IMFs are
considered. Our variable IMF model invokes a WD-precursor-dominated IMF for
metallicities less than 5% solar (primarily the Galactic halo), and the
canonical Salpeter IMF otherwise (primarily the disk). Halo WD density
distributions and C,N/O abundance patterns are presented. While Galactic haloes
comprised of ~5% (by mass) of WDs are not supported by our simulations, mass
fractions of ~1-2% cannot be ruled out. This conclusion is consistent with the
present-day observational constraints.Comment: accepted for publication in MNRA
Galactic Cannibalism: the Origin of the Magellanic Stream
We are in a privileged location in the Universe which allows us to observe
galactic interactions from close range -- the merger of our two nearest dwarf
satellite galaxies, the LMC and SMC. It is important to understand the local
merger process before we can have confidence in understanding mergers at high
redshift. We present high resolution Nbody+SPH simulations of the disruption of
the LMC and SMC and the formation of the Magellanic Stream, and discuss the
implications for galaxy formation and evolution.Comment: 2 pages, 1 figure, to appear in "The Evolution of Galaxies II: Basic
Building Blocks", (2002) ed. M. Sauvage et al. (Kluwer
The Complex Links Between Governance and Biodiversity
We argue that two problems weaken the claims of those who link corruption and the exploitation of natural resources. The first is conceptual. Studies that use national level indicators of corruption fail to note that corruption comes in many forms, at multiple levels, and may or may not affect resource use. Without a clear causal model of the mechanism by which corruption affects resources, one should treat with caution any estimated relationship between corruption and the state of natural resources. The second problem is methodological: Simple models linking corruption measures and natural resource use typically do not account for other important causes and control variables pivotal to the relationship between humans and natural resources. By way of illustration of these two general concerns, we demonstrate that the findings of a well known recent study that posits a link between corruption and decreases in forests, elephants, and rhinoceros are fragile to simple conceptual and methodological refinements
High-resolution N-body Simulations of Galactic Cannibalism: The Magellanic Stream
Hierarchical clustering represents the favoured paradigm for galaxy formation
throughout the Universe; due to its proximity, the Magellanic system offers one
of the few opportunities for astrophysicists to decompose the full
six-dimensional phase-space history of a satellite in the midst of being
cannibalised by its host galaxy. The availability of improved observational
data for the Magellanic Stream and parallel advances in computational power has
led us to revisit the canonical tidal model describing the disruption of the
Small Magellanic Cloud and the consequent formation of the Stream. We suggest
improvements to the tidal model in light of these recent advances.Comment: 6 pages, 4 figures, LaTeX (gcdv.sty). Refereed contribution to the
5th Galactic Chemodynamics conference held in Swinburne, July 2003. Accepted
for publication in PASA. Version with high resolution figures available at
http://astronomy.swin.edu.au/staff/tconnors/publications.htm
Importance of Baryon-Baryon Coupling in Hypernuclei
The coupling in --hypernuclei and coupling in --hypernuclei produce novel
physics not observed in the conventional, nonstrange sector. Effects of
conversion in H are reviewed.
The role of coupling suppression in the
--hypernuclei due to Pauli blocking is highlighted, and the
implications for the structure of B are explored.
Suppression of conversion in He is hypothesized as the reason that the
matrix element is small. Measurement of H is
proposed to investigate the full interaction. The
implication for analog states is discussed.Comment: 17 pages LATEX, 1 figure uuencoded postscrip
The complex links between governance and biodiversity
We argue that two problems weaken the claims of those who link corruption and the exploitation of natural resources. The first is conceptual. Studies that use national level indicators of corruption fail to note that corruption comes in many forms, at multiple levels, and may or may not affect resource use. Without a clear causal model of the mechanism by which corruption affects resources, one should treat with caution any estimated relationship between corruption and the state of natural resources. The second problem is methodological: Simple models linking corruption measures and natural resource use typically do not account for other important causes and control variables pivotal to the relationship between humans and natural resources. By way of illustration of these two general concerns, we demonstrate that the findings of a well known recent study that posits a link between corruption and decreases in forests, elephants, and rhinoceros are fragile to simple conceptual and methodological refinements.Environmental Economics and Policy,
On Dwarf Galaxies as the Source of Intracluster Gas
Recent observational evidence for steep dwarf galaxy luminosity functions in
several rich clusters has led to speculation that their precursors may be the
source of the majority of gas and metals inferred from intracluster medium
(ICM) x-ray observations. Their deposition into the ICM is presumed to occur
through early supernovae-driven winds, the resultant systems reflecting the
photometric and chemical properties of the low luminosity dwarf spheroidals and
ellipticals we observe locally. We consider this scenario, utilising a
self-consistent model for spheroidal photo-chemical evolution and gas ejection
via galactic superwinds. Insisting that post-wind dwarfs obey the observed
colour-luminosity-metallicity relations, we conclude that the bulk of the ICM
gas and metals does not originate within their precursors.Comment: 43 pages, 8 figures, LaTeX, also available at
http://msowww.anu.edu.au/~gibson/publications.html, to appear in ApJ, Vol
473, 1997, in pres
Broad Absorption Line Variability in Radio-Loud Quasars
We investigate C IV broad absorption line (BAL) variability within a sample
of 46 radio-loud quasars (RLQs), selected from SDSS/FIRST data to include both
core-dominated (39) and lobe-dominated (7) objects. The sample consists
primarily of high-ionization BAL quasars, and a substantial fraction have large
BAL velocities or equivalent widths; their radio luminosities and
radio-loudness values span ~2.5 orders of magnitude. We have obtained 34 new
Hobby-Eberly Telescope (HET) spectra of 28 BAL RLQs to compare to earlier SDSS
data, and we also incorporate archival coverage (primarily dual-epoch SDSS) for
a total set of 78 pairs of equivalent width measurements for 46 BAL RLQs,
probing rest-frame timescales of ~80-6000 d (median 500 d). In general, only
modest changes in the depths of segments of absorption troughs are observed,
akin to those seen in prior studies of BAL RQQs. Also similar to previous
findings for RQQs, the RLQs studied here are more likely to display BAL
variability on longer rest-frame timescales. However, typical values of
|Delta_EW| and |Delta_EW|/ are about 40+/-20% lower for BAL RLQs when
compared with those of a timescale-matched sample of BAL RQQs. Optical
continuum variability is of similar amplitude in BAL RLQs and BAL RQQs; for
both RLQs and RQQs, continuum variability tends to be stronger on longer
timescales. BAL variability in RLQs does not obviously depend upon their radio
luminosities or radio-loudness values, but we do find tentative evidence for
greater fractional BAL variability within lobe-dominated RLQs. Enhanced BAL
variability within more edge-on (lobe-dominated) RLQs supports some geometrical
dependence to the outflow structure.Comment: 27 pages, 16 figures, 6 tables, accepted to MNRAS, full Appendix A at
http://www.macalester.edu/~bmille13/balrlqs.htm
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