173 research outputs found
Stellar Forensics II: Millisecond Pulsar Binaries
We use the grid of models described in paper~I to analyse those millisecond
pulsar binaries whose secondaries have been studied optically. In particular,
we find cooling ages for these binary systems that range from to
. Comparison of cooling ages and characteristic spin down ages
allows us to constrain the initial spin periods and spin-up histories for
individual systems, showing that at least some millisecond pulsars had
sub-Eddington accretion rates and long magnetic field decay times.Comment: Latex, 14 pages, and 15 postscript figures. Accepted by Monthly
Notice
The Pulsar Kick Velocity Distribution
We analyse the sample of pulsar proper motions, taking detailed account of
the selection effects of the original surveys. We treat censored data using
survival statistics. From a comparison of our results with Monte Carlo
simulations, we find that the mean birth speed of a pulsar is 250-300 km/s,
rather than the 450 km/s foundby Lyne & Lorimer (1994). The resultant
distribution is consistent with a maxwellian with dispersion . Despite the large birth velocities, we find that the pulsars with long
characteristic ages show the asymmetric drift, indicating that they are
dynamically old. These pulsars may result from the low velocity tail of the
younger population, although modified by their origin in binaries and by
evolution in the galactic potential.Comment: Latex, 10 pages, and 11 postscript figures. Accepted by Monthly
Notice
Stellar Forensics I: Cooling Curves
The presence of low mass, degenerate secondaries in millisecond pulsar
binaries offers the opportunity to determine an age for the binary system
independent of the rotational properties of the pulsar. To this end, we present
here a detailed calculation of the evolution of a grid of low mass () helium core white dwarfs. We investigate the effects of different
Hydrogen layer masses and provide results for well-known optical band-passes.
We supplement the OPAL opacity calculations with our own calculations for low
effective temperatures () and also provide fitting formulae
for the gravity as a function of mass and effective temperature. In paper II we
shall apply these results to individual cases.Comment: Latex, 10 pages, and 16 postscript figures. Accepted by Monthly
Notice
Response of the Atlantic overturning circulation to South Atlantic sources of buoyancy
The heat and salt input from the Indian to Atlantic Oceans by Agulhas Leakage
is found to influence the Atlantic overturning circulation in a low-resolution
Ocean General Circulation Model. The model used is the Hamburg Large-Scale
Geostrophic (LSG) model, which is forced by mixed boundary conditions. Agulhas
Leakage is parameterized by sources of heat and salt in the upper South
Atlantic Ocean, that extend well into the intermediate layers.
It is shown that the models overturning circulation is sensitive to the applied
sources of heat and salt. The response of the overturning strength to changes in
the source amplitudes is mainly linear, interrupted once by a stepwise change.
The South Atlantic buoyancy sources influence the Atlantic overturning strength
by modifying the basin-scale meridional density and pressure gradients. The nonlinear,
stepwise response is caused by abrupt changes in the convective activity
in the northern North Atlantic.
Two additional experiments illustrate the adjustment of the overturning circulation
upon sudden introduction of heat and salt sources in the South Atlantic.
The North Atlantic overturning circulation responds within a few years after the
sources are switched on. This is the time it takes for barotropic and baroclinic
Kelvin waves to reach the northern North Atlantic. The advection of the anomalies
takes 3 decades to reach the northern North Atlantic.
The model results give support to the hypothesis that the re-opening of the
Agulhas Gap at the end of the last ice-age, as indicated by palaeoclimatological
data, may have stimulated the coincident strengthening of the Atlantic overturning
circulation
Maximum black-hole spin from quasi-circular binary mergers
Black holes of mass M must have a spin angular momentum S below the Kerr
limit chi = S/M^2 < 1, but whether astrophysical black holes can attain this
limiting spin depends on their accretion history. Gas accretion from a thin
disk limits the black-hole spin to chi_gas < 0.9980 +- 0.0002, as
electromagnetic radiation from this disk with retrograde angular momentum is
preferentially absorbed by the black hole. Extrapolation of
numerical-relativity simulations of equal-mass binary black-hole mergers to
maximum initial spins suggests these mergers yield a maximum spin chi_eq <
0.95. Here we show that for smaller mass ratios q = m/M << 1, the superradiant
extraction of angular momentum from the larger black hole imposes a fundamental
limit chi_lim < 0.9979 +- 0.0001 on the final black-hole spin even in the
test-particle limit q -> 0 of binary black-hole mergers. The nearly equal
values of chi_gas and chi_lim imply that measurement of supermassive black-hole
spins cannot distinguish a black hole built by gas accretion from one assembled
by the gravitational inspiral of a disk of compact stellar remnants. We also
show how superradiant scattering alters the mass and spin predicted by models
derived from extrapolating test-particle mergers to finite mass ratios.Comment: final version accepted in PRD, new Fig.4 and discussio
Peculiar Multimodality on the Horizontal Branch of the Globular Cluster NGC 2808
We present distributions of colors of stars along the horizontal branch of
the globular cluster NGC 2808, from Hubble Space Telescope WFPC2 imaging in B,
V, and an ultraviolet filter (F218W). This cluster's HB is already known to be
strongly bimodal, with approximately equal-sized HB populations widely
separated in the color-magnitude diagram. Our images reveal a long blue tail
with two gaps, for a total of four nearly distinct HB groups. These gaps are
very narrow, corresponding to envelope-mass differences of only \sim 0.01 Msun.
This remarkable multimodality may be a signature of mass-loss processes, subtle
composition variations, or dynamical effects; we briefly summarize the
possibilities. The existence of narrow gaps between distinct clumps on the HB
presents a challenge for models that attempt to explain HB bimodality or other
peculiar HB structures.Comment: LaTeX, including compressed figures. To appear in ApJL. Larger (851k)
PostScript version, including high-quality figures, available from
http://astro.berkeley.edu/~csosin/pub
Hydropower dependency and climate change in sub-Saharan Africa: A nexus framework and evidence-based review
In sub-Saharan Africa, 160 million grid-connected electricity consumers live in countries where hydropower accounts for over 50% of total power supply. A warmer climate with more frequent and intense extremes could result in supply reliability issues. Here, (i) a robust framework to highlight the interdependencies between hydropower, water availability, and climate change is proposed, (ii) the state-of-the art literature on the projected impacts of climate change on hydropower in sub-Saharan Africa is reviewed, and (iii) supporting evidence on past trends and current pathways of power mix diversification, drought incidence, and climate change projections is provided. We find that only few countries have pursued a diversification strategy away from hydropower over the last three decades, while others' expansion plans will reinforce the dependency. This will occur irrespective of the fact that some of the largest river basins have experienced a significant drying during the last century. Agreement is found on likely positive impacts of climate change on East Africa's hydropower potential, negative impacts in West and Southern Africa, and substantial uncertainty in Central Africa. Irrespective of the absolute change in gross technical potential, more frequent and intense extremes are projected. One possible paradigm to increase resilience and fulfil the pledges of the Paris Agreement is a synergetic planning and management of hydropower and variable renewables
Discovery of Extended Blue Horizontal Branches in Two Metal-Rich Globular Clusters
We have used WFPC2 to construct B, V color-magnitude diagrams of four
metal-rich globular clusters, NGC 104 (47 Tuc), NGC 5927, NGC 6388, and NGC
6441. All four clusters have well populated red horizontal branches (RHB), as
expected for their metallicity. However, NGC 6388 and 6441 also exhibit a
prominent blue HB (BHB) extension, including stars reaching as faint in V as
the turnoff luminosity. This discovery demonstrates directly for the first time
that a major population of hot HB stars can exist in old, metal-rich systems.
This may have important implications for the interpretation of the integrated
spectra of elliptical galaxies.
The cause of the phenomenon remains uncertain. We examine the possibility
that NGC 6388 and 6441 are older than the other clusters, but a simple
difference in age may not be sufficient to produce the observed distributions
along the HB. The high central densities in NGC 6388 and 6441 suggest that the
existence of the blue HB (BHB) tails might be caused by stellar interactions in
the dense cores of these clusters, which we calculate to have two of the
highest collision rates among globular clusters in the Galaxy. Tidal collisions
might act in various ways to enhance loss of envelope mass, and therefore
populate the blue side of the HB. However, the relative frequency of tidal
collisions does not seem large enough (compared to that of the clusters with
pure RHBs) to account for such a drastic difference in HB morphology. While a
combination of an age difference and dynamical interactions may help, prima
facie the lack of a radial gradient in the BHB/RHB star ratio seems to argue
against dynamical effects playing a role.Comment: LaTeX, includes one Postscript figure. To appear in ApJ
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