798 research outputs found
Evaluating irreversible social harms
In this paper we investigate how irreversible social harms should be evaluated from an ethical perspective. First, we define a general notion of irreversibility, drawing on discussions in ecology and economics. This notion is relational in the sense that 'irreversibility' is always 'irreversibility for a certain party'. We also note that a change may be more or less difficult to reverse, with full reversibility and irreversibility as two extremes. Second, we examine what can make an irreversible change a harm, and why these kinds of harms have particular ethical significance. Here we draw on discussions from ethics, particularly regarding the Capability Approach. We also show how our notion of irreversibility connects to, and can add to, discussions in the fields of development studies and disaster management, particularly on the concept of resilience. Third, we suggest how potentially irreversible harms can be recognised and dealt with in policy-making. Finally, we show how our framework can be applied by evaluating the land acquisition process of two biofuel producers in Tanzania
The late stages of evolution of helium star-neutron star binaries and the formation of double neutron star systems
With a view to understanding the formation of double neutron-stars (DNS), we
investigate the late stages of evolution of helium stars with masses of 2.8 -
6.4 Msun in binary systems with a 1.4 Msun neutron-star companion. We found
that mass transfer from 2.8 - 3.3 Msun helium stars and from 3.3 - 3.8 Msun in
very close orbits (P_orb > 0.25d) will end up in a common-envelope (CE) and
spiral-in phase due to the development of a convective helium envelope. If the
neutron star has sufficient time to complete the spiraling-in process before
the core collapses, the system will produce very tight DNSs (P_orb ~ 0.01d)
with a merger timescale of the order of 1 Myr or less. These systems would have
important consequences for the detection rate of GWR and for the understanding
of GRB progenitors. On the other hand, if the time left until the explosion is
shorter than the orbital-decay timescale, the system will undergo a SN
explosion during the CE phase. Helium stars with masses 3.3 - 3.8 Msun in wider
orbits (P_orb > 0.25d) and those more massive than 3.8 Msun do not go through
CE evolution. The remnants of these massive helium stars are DNSs with periods
in the range of 0.1 - 1 d. This suggests that this range of mass includes the
progenitors of the galactic DNSs with close orbits (B1913+16 and B1534+12). A
minimum kick velocity of 70 km/s and 0 km/s (for B1913+16 and B1534+12,
respectively) must have been imparted at the birth of the pulsar's companion.
The DNSs with wider orbits (J1518+4904 and probably J1811-1736) are produced
from helium star-neutron star binaries which avoid RLOF, with the helium star
more massive than 2.5 Msun. For these systems the minimum kick velocities are
50 km/s and 10 km/s (for J1518+4904 and J1811-1736, respectively).Comment: 16 pages, latex, 12 figures, accepted for publication in MNRA
Theoretical Examination of the Lithium Depletion Boundary
We explore the sensitivity in open cluster ages obtained by the lithium
depletion boundary (LDB) technique to the stellar model input physics. The LDB
age technique is limited to open clusters with ages ranging from 20 to 200 Myr.
Effective 1-sig errors in the LDB technique due to uncertain input physics are
roughly 3% at the oldest age increasing to 8% at the youngest age. Bolometric
correction uncertainties add an additional 10 to 6% error to the LDB age
technique for old and young clusters, respectively. Rotation rates matching the
observed fastest rotators in the Pleiades affect LDB ages by less than 2%. The
range of rotation rates in an open cluster are expected to ``smear'' the LDB
location by only 0.02 mag for a Pleiades age cluster increasing to 0.06 mag for
a 20 Myr cluster. Thus, the observational error of locating the LDB (~7-10%)
and the bolometric correction uncertainty currently dominate the error in LDB
ages. For our base case, we formally derive a LDB age of 148 +- 19 Myr for the
Pleiades, where the error includes 8, 3, and 9% contributions from
observational, theoretical, and bolometric correction sources, respectively. A
maximally plausible 0.3 magnitude shift in the I-band bolometric correction to
reconcile main sequence isochrone fits with the observed (V-I) color for the
low mass Pleiades members results in an age of 126 +- 11 Myr, where the error
includes observational and theoretical errors only. Upper main-sequence-fitting
ages that do not include convective core overshoot for the Pleiades (~75 Myr)
are ruled out by the LDB age technique.Comment: 35 pages, 9 figures, accepted Ap
The Evolution of Relativistic Binary Progenitor Systems
Relativistic binary pulsars, such as B1534+12 and B1913+16 are characterized
by having close orbits with a binary separation of ~ 3 R_\sun. The progenitor
of such a system is a neutron star, helium star binary. The helium star, with a
strong stellar wind, is able to spin up its compact companion via accretion.
The neutron star's magnetic field is then lowered to observed values of about
10^{10} Gauss. As the pulsar lifetime is inversely proportional to its magnetic
field, the possibility of observing such a system is, thus, enhanced by this
type of evolution. We will show that a nascent (Crab-like) pulsar in such a
system can, through accretion-braking torques (i.e. the "propeller effect") and
wind-induced spin-up rates, reach equilibrium periods that are close to
observed values. Such processes occur within the relatively short helium star
lifetimes. Additionally, we find that the final outcome of such evolutionary
scenarios depends strongly on initial parameters, particularly the initial
binary separation and helium star mass. It is, indeed, determined that the
majority of such systems end up in the pulsar "graveyard", and only a small
fraction are strongly recycled. This fact might help to reconcile theoretically
expected birth rates with limited observations of relativistic binary pulsars.Comment: 24 pages, 10 Postscript figures, Submitted to The Astrophysical
Journa
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