4,528 research outputs found
Zinc-oxygen primary cell yields high energy density
Zinc-oxygen primary cell yields high energy density for battery used as an auxiliary power source in space vehicle systems. Maximum reliability and minimum battery weight is achieved by using a stacking configuration of 23 series-connected modules with 6 parallel-connected cells per module
The Missing Link: Bayesian Detection and Measurement of Intermediate-Mass Black-Hole Binaries
We perform Bayesian analysis of gravitational-wave signals from non-spinning,
intermediate-mass black-hole binaries (IMBHBs) with observed total mass,
, from to and
mass ratio 1\mbox{--}4 using advanced LIGO and Virgo detectors. We employ
inspiral-merger-ringdown waveform models based on the effective-one-body
formalism and include subleading modes of radiation beyond the leading
mode. The presence of subleading modes increases signal power for inclined
binaries and allows for improved accuracy and precision in measurements of the
masses as well as breaking of extrinsic parameter degeneracies. For low total
masses, , the observed chirp
mass ( being the
symmetric mass ratio) is better measured. In contrast, as increasing power
comes from merger and ringdown, we find that the total mass
has better relative precision than . Indeed, at high
(), the signal resembles a
burst and the measurement thus extracts the dominant frequency of the signal
that depends on . Depending on the binary's inclination, at
signal-to-noise ratio (SNR) of , uncertainties in can be
as large as \sim 20 \mbox{--}25\% while uncertainties in are \sim 50 \mbox{--}60\% in binaries with unequal masses (those
numbers become versus in more symmetric binaries).
Although large, those uncertainties will establish the existence of IMBHs. Our
results show that gravitational-wave observations can offer a unique tool to
observe and understand the formation, evolution and demographics of IMBHs,
which are difficult to observe in the electromagnetic window. (abridged)Comment: 17 pages, 9 figures, 2 tables; updated to reflect published versio
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
High Energy Variability Of Synchrotron-Self Compton Emitting Sources: Why One Zone Models Do Not Work And How We Can Fix It
With the anticipated launch of GLAST, the existing X-ray telescopes, and the
enhanced capabilities of the new generation of TeV telescopes, developing tools
for modeling the variability of high energy sources such as blazars is becoming
a high priority. We point out the serious, innate problems one zone
synchrotron-self Compton models have in simulating high energy variability. We
then present the first steps toward a multi zone model where non-local, time
delayed Synchrotron-self Compton electron energy losses are taken into account.
By introducing only one additional parameter, the length of the system, our
code can simulate variability properly at Compton dominated stages, a situation
typical of flaring systems. As a first application, we were able to reproduce
variability similar to that observed in the case of the puzzling `orphan' TeV
flares that are not accompanied by a corresponding X-ray flare.Comment: to appear in the 1st GLAST symposium proceeding
No nearby counterparts to the moving objects in the Hubble Deep Field
Ibata et al (1999) have recently discovered faint, moving objects in the
Hubble Deep Field. The quantity, magnitudes and proper motions of these objects
are consistent with old white dwarfs making up the Galactic dark halo. We
review a number of ground-based proper motion surveys in which nearby dark halo
white dwarfs might be present, if they have the colours and absolute magnitudes
proposed. No such objects have been found, whereas we argue here that several
times more would be expected than in the Hubble Deep Field. We conclude it is
unlikely that hydrogen atmosphere white dwarfs make up a significant fraction
of the halo dark matter. No limits can be placed yet on helium atmosphere
dwarfs from optical searches.Comment: 7 pages, 4 figures, MNRAS LaTeX forma
MACHOs, White Dwarfs, and the Age of the Universe
(Abridged Abstract) A favored interpretation of recent microlensing
measurements towards the Large Magellanic Cloud implies that a large fraction
(i.e. 10--50%) of the mass of the galactic halo is composed of white dwarfs. We
compare model white dwarf luminosity functions to the data from the
observational surveys in order to determine a lower bound on the age of any
substantial white dwarf halo population (and hence possibly on the age of the
Universe). We compare various theoretical white dwarf luminosity functions, in
which we vary hese three parameters, with the abovementioned survey results.
From this comparison, we conclude that if white dwarfs do indeed constitute
more than 10% of the local halo mass density, then the Universe must be at
least 10 Gyr old for our most extreme allowed values of the parameters. When we
use cooling curves that account for chemical fractionation and more likely
values of the IMF and the bolometric correction, we find tighter limits: a
white dwarf MACHO fraction of 10% (30%) requires a minimum age of 14 Gyr (15.5
Gyr). Our analysis also indicates that the halo white dwarfs almost certainly
have helium-dominated atmospheres.Comment: Final version accepted for publication, straight TeX formate, 6 figs,
22 page
The representation of female journalists and the female voice in the South African newspapers: a case study of the Cape Times
Over the past 15 years, there has been limited research regarding gender and journalism in a South African context. Existing research implies that there is almost complete gender balance across all media. Yet, despite the gender parity in the workforce, women are underrepresented in senior and key decision-making positions, indicating that a glass ceiling still exists for female journalists in South Africa. Furthermore, newsrooms are still dominated by patriarchal practices, norms and values. This study investigated the representation of female journalists in South African daily newspapers via one case study: the Cape Times. It shed light on what type of stories female journalists report on in comparison to their male colleagues and identified patterns in female reporting. The study also investigated how transformative policies may have affected newsrooms. Further, attention was paid to the female voice in the print media in terms of the use of female sources and women as the subject of news stories. The data for this study was generated through a content analysis and interview research. The content analysis was conducted on articles of the Cape Times newspaper and was based on three research questions regarding representation, female voice in the media and topic assignment/ contribution. As part of the interview research, in-depth informant interviews were conducted with female journalists and editors; the aim was to collect industry insight and opinions regarding the representation of female journalists and female voices in South African newspapers. The study revealed that the assignment of stories has been characterised by constant fluctuations over the past 20 years. It became apparent that effort is put into gender balance not only when it comes to the representation of journalists, but also the assignment of beats. However, the study indicated a significant difference between the representation of female journalists and the female voice in the print media. It also highlighted issues of gender inequality in reporting. The study revealed that while South Africa's newsrooms might be progressing towards gender balance, patriarchal structures and views are still embedded in the print media. These findings are congruent with the existing academic literature. This research further revealed that structures and issues of the newsroom or the media reflect issues and structures of South African society. Past research in a South African context has been very limited, making this study one of a few of its kind and its findings are indicative for other print media, filling a gap in the literature
Chemical Abundance Constraints on White Dwarfs as Halo Dark Matter
We examine the chemical abundance constraints on a population of white dwarfs
in the Halo of our Galaxy. We are motivated by microlensing evidence for
massive compact halo objects (Machos) in the Galactic Halo, but our work
constrains white dwarfs in the Halo regardless of what the Machos are. We focus
on the composition of the material that would be ejected as the white dwarfs
are formed; abundance patterns in the ejecta strongly constrain white dwarf
production scenarios. Using both analytical and numerical chemical evolution
models, we confirm that very strong constraints come from Galactic Pop II and
extragalactic carbon abundances. We also point out that depending on the
stellar model, significant nitrogen is produced rather than carbon. The
combined constraints from C and N give from
comparison with the low C and N abundances in the Ly forest. We note,
however, that these results are subject to uncertainties regarding the
nucleosynthesis of low-metallicity stars. We thus investigate additional
constraints from D and He, finding that these light elements can be kept
within observational limits only for \Omega_{WD} \la 0.003 and for a white
dwarf progenitor initial mass function sharply peaked at low mass (2).
Finally, we consider a Galactic wind, which is required to remove the ejecta
accompanying white dwarf production from the galaxy. We show that such a wind
can be driven by Type Ia supernovae arising from the white dwarfs themselves,
but these supernovae also lead to unacceptably large abundances of iron. We
conclude that abundance constraints exclude white dwarfs as Machos. (abridged)Comment: Written in AASTeX, 26 pages plus 4 ps figure
Direct Detection of Giant Close-In Planets Around the Source Stars of Caustic-Crossing Microlensing Events
We propose a direct method to detect close-in giant planets orbiting stars in
the Galactic bulge. This method uses caustic-crossing binary microlensing
events discovered by survey teams monitoring the bulge to measure light from a
planet orbiting the source star. When the planet crosses the caustic, it is
more magnified than the source star; its light is magnified by two orders of
magnitude for Jupiter size planets. If the planet is a giant close to the star,
it may be bright enough to make a significant deviation in the light curve of
the star. Detection of this deviation requires intensive monitoring of the
microlensing light curve using a 10-meter class telescope for a few hours after
the caustic. This is the only method yet proposed to directly detect close-in
planets around stars outside the solar neighborhood.Comment: 4 pages, 2 figures. Submitted to ApJ Letter
- …