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Evolutionarily stable sexual allocation by both stressed and unstressed potentially simultaneous hermaphrodites within the same population.
Factors influencing allocation of resources to male and female offspring continue to be of great interest to evolutionary biologists. A simultaneous hermaphrodite is capable of functioning in both male and female mode at the same time, and such a life-history strategy is adopted by most flowering plants and by many sessile aquatic animals. In this paper, we focus on hermaphrodites that nourish post-zygotic stages, e.g. flowering plants and internally fertilising invertebrates, and consider how their sex allocation should respond to an environmental stress that reduces prospects of survival but does not affect all individuals equally, rather acting only on a subset of the population. Whereas dissemination of pollen and sperm can begin at sexual maturation, release of seeds and larvae is delayed by embryonic development. We find that the evolutionarily stable strategy for allocation between male and female functions will be critically dependent on the effect of stress on the trade-off between the costs of male and female reproduction, (i.e. of sperm and embryos). Thus, we identify evaluation of this factor as an important challenge to empiricists interested in the effects of stress on sex allocation. When only a small fraction of the population is stressed, we predict that stressed individuals will allocate their resources entirely to male function and unstressed individuals will increase their allocation to female function. Conversely, when the fraction of stress-affected individuals is high, stressed individuals should respond to this stressor by increasing investment in sperm and unstressed individuals should invest solely in embryos. A further prediction of the model is that we would not expect to find populations in the natural world where both stressed and unstressed individuals are both hermaphrodite
Supernova Remnant 1987A: Opening the Future by Reaching the Past
We report an up-turn in the soft X-ray light curve of supernova remnant (SNR)
1987A in late 2003 (~6200 days after the explosion), as observed with the
Chandra X-ray Observatory. Since early 2004, the rapid increase of the 0.5-2
keV band X-ray light curve can no longer be described by the exponential
density distribution model with which we successfully fitted the data between
1990 and 2003. Around day ~6200, we also find that the fractional contribution
to the observed soft X-ray flux from the decelerated shock begins to exceed
that of the fast shock and that the X-ray brightening becomes "global" rather
than "spotty". We interpret these results as evidence that the blast wave has
reached the main body of the dense circumstellar material all around the inner
ring. This interpretation is supported by other recent observations, including
a deceleration of the radial expansion of the X-ray remnant, a significant
up-turn in the mid-IR intensities, and the prevalence of the optical hot spots
around the entire inner ring, all of which occur at around day 6000. In
contrast to the soft X-ray light curve, the hard band (3-10 keV) X-ray light
curve increases at a much lower rate which is rather similar to the radio light
curve. The hard X-ray emission may thus originate from the reverse shock where
the radio emission is likely produced. Alternatively, the low increase rate of
the hard X-rays may simply be a result of the continuous softening of the
overall X-ray spectrum.Comment: AASTex preprint style 12 pages including 1 table and 4 figures,
Accepted by ApJ
Testing the standard fireball model of GRBs using late X-ray afterglows measured by Swift
We show that all X-ray decay curves of GRBs measured by Swift can be fitted
using one or two components both of which have exactly the same functional form
comprised of an early falling exponential phase followed by a power law decay.
The 1st component contains the prompt gamma-ray emission and the initial X-ray
decay. The 2nd component appears later, has a much longer duration and is
present for ~80% of GRBs. It most likely arises from the external shock which
eventually develops into the X-ray afterglow. In the remaining ~20% of GRBs the
initial X-ray decay of the 1st component fades more slowly than the 2nd and
dominates at late times to form an afterglow but it is not clear what the
origin of this emission is.
The temporal decay parameters and gamma/X-ray spectral indices derived for
107 GRBs are compared to the expectations of the standard fireball model
including a search for possible "jet breaks". For ~50% of GRBs the observed
afterglow is in accord with the model but for the rest the temporal and
spectral indices do not conform to the expected closure relations and are
suggestive of continued, late, energy injection. We identify a few possible jet
breaks but there are many examples where such breaks are predicted but are
absent.
The time, T_a, at which the exponential phase of the 2nd component changes to
a final powerlaw decay afterglow is correlated with the peak of the gamma-ray
spectrum, E_peak. This is analogous to the Ghirlanda relation, indicating that
this time is in some way related to optically observed break times measured for
pre-Swift bursts.Comment: submitted to Ap
A Comprehensive Analysis of Swift/XRT Data: III. Jet Break Candidates in X-ray and Optical Afterglow Lightcurves
The Swift/XRT data of 179 GRBs (from 050124 to 070129) and the optical
afterglow data of 57 pre- and post-Swift GRBs are analyzed, in order to
systematically investigate the jet-like breaks in the X-ray and optical
afterglow lightcurves. We find that not a single burst can be included in the
``Platinum'' sample, in which the data satisfy all the criteria of a jet break.
By releasing one or more requirements to define a jet break, some candidates of
various degrees could be identified. In the X-ray band, 42 out of the 103
well-sampled X-ray lightcurves have a decay slope of the post-break segment
>1.5 (``Bronze'' sample), and 27 of them also satisfy the closure relations of
the forward models (``Silver'' sample). The numbers of the ``Bronze'' and
``Silver'' candidates in the optical lightcurves are 27 and 23, respectively.
Thirteen bursts have well-sampled optical and X-ray lightcurves, but only seven
cases are consistent with an achromatic break, but even in these cases only one
band satisfies the closure relations (``Gold'' sample). The observed break time
in the XRT lightcurves is systematically earlier than that in the optical
bands. All these raise great concerns in interpreting the jet-like breaks as
jet breaks and further inferring GRB energetics from these breaks. By assuming
that these breaks are jet breaks, we perform a similar analysis as previous
work to calculate the jet opening angle (theta_j) and energetics (E_k) with the
``Silver'' and ``Gold'' jet break candidates. The derived E_K distribution
reveals a much larger scatter than the pre-Swift sample. A tentative
anti-correlation between theta_j and E_{K,iso} is found for both the pre-Swift
and Swift GRBs, indicating that the E_K could still be quasi-universal, if the
breaks in discussion are indeed jet breaks(abridge).Comment: 48 pages, including 5 tables and 8 figures. Accepted for publication
in ApJ. This is the third paper of a series. Paper I and II see
astro-ph/0612246 (ApJ, 2007, 666,1002) and arXiv:0705.1373 (ApJ, 2007, 669,
n2,in press
Many-Body Corrections to Charged-Current Neutrino Absorption Rates in Nuclear Matter
Including nucleon--nucleon correlations due to both Fermi statistics and
nuclear forces, we have developed a general formalism for calculating the
charged--current neutrino--nucleon absorption rates in nuclear matter. We find
that at one half nuclear density many--body effects alone suppress the rates by
a factor of two and that the suppression factors increase to 5 at
g cm. The associated increase in the neutrino--matter
mean--free--paths parallels that found for neutral--current interactions and
opens up interesting possibilities in the context of the delayed supernova
mechanism and protoneutron star cooling.Comment: 11 pages, APS REVTeX format, 1 PostScript figure, uuencoded
compressed, and tarred, submitted to Physical Review Letter
Nuclear Effects on Bremsstrahlung Neutrino Rates of Astrophysical Interest
We calculate in this work the rates for the neutrino pair production by
nucleon-nucleon bremsstrahlung taking into account the full contribution from a
nuclear one-pion-exchange potential. It is shown that if the temperatures are
low enough (), the integration over the nuclear part can be done
for the general case, ranging from the completely degenerate (D) to the
non-degenerate (ND) regime. We find that the inclusion of the full nuclear
contribution enhances the neutrino pair production by and
bremsstrahlung by a factor of about two in both the D and ND limits when
compared with previous calculations. This result may be relevant for the
physical conditions of interest in the semitransparent regions near the
neutrinosphere in type II supernovae, cooling of neutron stars and other
astrophysical situations.Comment: 11 pages, no figures, LaTex file. submitted to PR
An X-ray measurement of Titan's atmospheric extent from its transit of the Crab Nebula
Saturn's largest satellite, Titan, transited the Crab Nebula on 5 January
2003. We observed this astronomical event with the {\it Chandra} X-ray
Observatory. An ``occultation shadow'' has clearly been detected and is found
to be larger than the diameter of Titan's solid surface. The difference gives a
thickness for Titan's atmosphere of 880 60 km. This is the first
measurement of Titan's atmospheric extent at X-ray wavelengths. The value
measured is consistent with or slightly larger than those estimated from
earlier Voyager observations at other wavelengths. We discuss the possibility
of temporal variations in the thickness of Titan's atmosphere.Comment: 14 pages, 5 figures, AASTeX preprint. Accepted for publication in the
Astrophysical Journa
A Two-Dimensional MagnetoHydrodynamics Scheme for General Unstructured Grids
We report a new finite-difference scheme for two-dimensional
magnetohydrodynamics (MHD) simulations, with and without rotation, in
unstructured grids with quadrilateral cells. The new scheme is implemented
within the code VULCAN/2D, which already includes radiation-hydrodynamics in
various approximations and can be used with arbitrarily moving meshes (ALE).
The MHD scheme, which consists of cell-centered magnetic field variables,
preserves the nodal finite difference representation of div(\bB) by
construction, and therefore any initially divergence-free field remains
divergence-free through the simulation. In this paper, we describe the new
scheme in detail and present comparisons of VULCAN/2D results with those of the
code ZEUS/2D for several one-dimensional and two-dimensional test problems. The
code now enables two-dimensional simulations of the collapse and explosion of
the rotating, magnetic cores of massive stars. Moreover, it can be used to
simulate the very wide variety of astrophysical problems for which multi-D
radiation-magnetohydrodynamics (RMHD) is relevant.Comment: 22 pages, including 11 figures; Accepted to the Astrophysical
Journal. Higher resolution figures available at
http://zenith.as.arizona.edu/~burrows/mhd-code
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