486 research outputs found
Europa: Prospects for an ocean and exobiological implications
As far as we know, Earth is the only planet in our solar system that supports life. It is natural, therefore, that our understanding of life as a planetary phenomenon is based upon Earth-like planets. There are environments in the solar system where liquid water, commonly believed to be a prerequisite for biological activity, may exist in a distinctly non-Earth-like environment. One such location is Europa, one of the Galilean satellites of Jupiter. The possibility that liquid water exists on Europa presents us with some interesting exobiological implications concerning the potential of the satellite to support life. Topics include the following: an ocean on Europa; thermal evolution of Europa; Europa's three models; exobiological implications; early conditions of Europa; low-temperature abiotic chemistry; possibility of the emergence of life on Europa; prerequisites for the habitability of Europa; energy sources for biosynthesis and metabolic activity; habitability of Europa by anaerobic life; and habitability by aerobic life
Accretional heating of the satellites of Saturn and Uranus
Voyager images of the satellites of Saturn and Uranus have shown that these bodies are characterized by remarkable diversity and surprisingly complex geologic histories. Despite their small sizes, a number of the satellites show unambiguous evidence for resurfacing. The goal was to develop a detailed model for heating of these small satellites, and then to explore the consequences of variations in the free parameters in the model. Specifically an attempt was made to determine for what range of conditions melting will occur in these satellites. Along with varying a number of model parameters, the important effects of inclusion of small amounts of ammonia and methane in the system were considered
Five New Millisecond Pulsars From a Radio Survey of 14 Unidentified Fermi-LAT Gamma-ray Sources
We have discovered five millisecond pulsars (MSPs) in a survey of 14
unidentified Fermi-LAT sources in the southern sky using the Parkes radio
telescope. PSRs J0101-6422, J1514-4946, and J1902-5105 reside in binaries,
while PSRs J1658-5324 and J1747-4036 are isolated. Using an ephemeris derived
from timing observations of PSR J0101-6422 (P =2.57 ms, DM=12 pc cm-3), we have
detected {\gamma}-ray pulsations and measured its proper motion. Its
{\gamma}-ray spectrum (a power law of {\Gamma} = 0.9 with a cutoff at 1.6 GeV)
and efficiency are typical of other MSPs, but its radio and {\gamma}-ray light
curves challenge simple geometric models of emission. The high success rate of
this survey-enabled by selecting {\gamma}-ray sources based on their detailed
spectral characteristics-and other similarly successful searches indicate that
a substantial fraction of the local population of MSPs may soon be known.Comment: 6 pages, 3 figures, 2 tables, accepted by ApJ
Radio Observations of the Hubble Deep Field South Region III: The 2.5, 5.2 and 8.7 GHz Catalogues and Radio Source Properties
Deep radio observations of a wide region centred on the Hubble Deep Field
South have been performed, providing one of the most sensitive set of radio
observations acquired on the Australia Telescope Compact Array to date. A
central rms of ~10 microJy is reached at four frequencies (1.4, 2.5, 5.2 and
8.7 GHz). In this paper the full source catalogues from the 2.5, 5.2 and 8.7
GHz observations are presented to complement Paper II, along with a detailed
analysis of image quality and noise. We produce a consolidated catalogue by
matching sources across all four frequencies of our survey. Radio spectral
indices are used to investigate the nature of the radio sources and identify a
number of sources with flat or inverted radio spectra, which indicates AGN
activity. We also find several other interesting sources, including a broadline
emitting radio galaxy, a giant radio galaxy and three Gigahertz Peaked Spectrum
sources.Comment: Accepted by AJ. 13 figures and 13 table
A fast Monte Carlo algorithm for site or bond percolation
We describe in detail a new and highly efficient algorithm for studying site
or bond percolation on any lattice. The algorithm can measure an observable
quantity in a percolation system for all values of the site or bond occupation
probability from zero to one in an amount of time which scales linearly with
the size of the system. We demonstrate our algorithm by using it to investigate
a number of issues in percolation theory, including the position of the
percolation transition for site percolation on the square lattice, the
stretched exponential behavior of spanning probabilities away from the critical
point, and the size of the giant component for site percolation on random
graphs.Comment: 17 pages, 13 figures. Corrections and some additional material in
this version. Accompanying material can be found on the web at
http://www.santafe.edu/~mark/percolation
Further X-ray observations of EXO 0748-676 in quiescence: evidence for a cooling neutron star crust
In late 2008, the quasi-persistent neutron star X-ray transient and eclipsing
binary EXO 0748-676 started a transition from outburst to quiescence, after it
had been actively accreting for more than 24 years. In a previous work, we
discussed Chandra and Swift observations obtained during the first five months
after this transition. Here, we report on further X-ray observations of EXO
0748-676, extending the quiescent monitoring to 1.6 years. Chandra and
XMM-Newton data reveal quiescent X-ray spectra composed of a soft, thermal
component that is well-fitted by a neutron star atmosphere model. An additional
hard powerlaw tail is detected that changes non-monotonically over time,
contributing between 4 and 20 percent to the total unabsorbed 0.5-10 keV flux.
The combined set of Chandra, XMM-Newton and Swift data reveals that the thermal
bolometric luminosity fades from ~1E34 to 6E33 (D/7.4 kpc)^2 erg/s, whereas the
inferred neutron star effective temperature decreases from ~124 to 109 eV. We
interpret the observed decay as cooling of the neutron star crust and show that
the fractional quiescent temperature change of EXO 0748-676 is markedly smaller
than observed for three other neutron star X-ray binaries that underwent
prolonged accretion outbursts.Comment: Moderate textual revisions according to referee report, accepted for
publication in MNRA
Massive stars as thermonuclear reactors and their explosions following core collapse
Nuclear reactions transform atomic nuclei inside stars. This is the process
of stellar nucleosynthesis. The basic concepts of determining nuclear reaction
rates inside stars are reviewed. How stars manage to burn their fuel so slowly
most of the time are also considered. Stellar thermonuclear reactions involving
protons in hydrostatic burning are discussed first. Then I discuss triple alpha
reactions in the helium burning stage. Carbon and oxygen survive in red giant
stars because of the nuclear structure of oxygen and neon. Further nuclear
burning of carbon, neon, oxygen and silicon in quiescent conditions are
discussed next. In the subsequent core-collapse phase, neutronization due to
electron capture from the top of the Fermi sea in a degenerate core takes
place. The expected signal of neutrinos from a nearby supernova is calculated.
The supernova often explodes inside a dense circumstellar medium, which is
established due to the progenitor star losing its outermost envelope in a
stellar wind or mass transfer in a binary system. The nature of the
circumstellar medium and the ejecta of the supernova and their dynamics are
revealed by observations in the optical, IR, radio, and X-ray bands, and I
discuss some of these observations and their interpretations.Comment: To be published in " Principles and Perspectives in Cosmochemistry"
Lecture Notes on Kodai School on Synthesis of Elements in Stars; ed. by Aruna
Goswami & Eswar Reddy, Springer Verlag, 2009. Contains 21 figure
Statistical Properties of Turbulence: An Overview
We present an introductory overview of several challenging problems in the
statistical characterisation of turbulence. We provide examples from fluid
turbulence in three and two dimensions, from the turbulent advection of passive
scalars, turbulence in the one-dimensional Burgers equation, and fluid
turbulence in the presence of polymer additives.Comment: 34 pages, 31 figure
Eco-evolutionary dynamics on deformable fitness landscapes
Conventional approaches to modelling ecological dynamics often do not include evolutionary changes in the genetic makeup of component species and, conversely, conventional approaches to modelling evolutionary changes in the genetic makeup of a population often do not include ecological dynamics. But recently there has been considerable interest in understanding the interaction of evolutionary and ecological dynamics as coupled processes. However, in the context of complex multi-species ecosytems, especially where ecological and evolutionary timescales are similar, it is difficult to identify general organising principles that help us understand the structure and behaviour of complex ecosystems. Here we introduce a simple abstraction of coevolutionary interactions in a multi-species ecosystem. We model non-trophic ecological interactions based on a continuous but low-dimensional trait/niche space, where the location of each species in trait space affects the overlap of its resource utilisation with that of other species. The local depletion of available resources creates, in effect, a deformable fitness landscape that governs how the evolution of one species affects the selective pressures on other species. This enables us to study the coevolution of ecological interactions in an intuitive and easily visualisable manner. We observe that this model can exhibit either of the two behavioural modes discussed in the literature; namely, evolutionary stasis or Red Queen dynamics, i.e., continued evolutionary change. We find that which of these modes is observed depends on the lag or latency between the movement of a species in trait space and its effect on available resources. Specifically, if ecological change is nearly instantaneous compared to evolutionary change, stasis results; but conversely, if evolutionary timescales are closer to ecological timescales, such that resource depletion is not instantaneous on evolutionary timescales, then Red Queen dynamics result. We also observe that in the stasis mode, the overall utilisation of resources by the ecosystem is relatively efficient, with diverse species utilising different niches, whereas in the Red Queen mode the organisation of the ecosystem is such that species tend to clump together competing for overlapping resources. These models thereby suggest some basic conditions that influence the organisation of inter-species interactions and the balance of individual and collective adaptation in ecosystems, and likewise they also suggest factors that might be useful in engineering artificial coevolution
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