217 research outputs found
Gravitational-wave energy and other fluxes in ghost-free bigravity
One of the key ingredients for making binary waveform predictions in a
beyond-GR theory of gravity is understanding the energy and angular momentum
carried by gravitational waves and any other radiated fields. Identifying the
appropriate energy functional is unclear in Hassan-Rosen bigravity, a
ghost-free theory with one massive and one massless graviton. The difficulty
arises from the new degrees of freedom and length scales which are not present
in GR, rendering an Isaacson-style averaging calculation ambiguous. In this
article we compute the energy carried by gravitational waves in bigravity
starting from the action, using the canonical current formalism. The canonical
current agrees with other common energy calculations in GR, and is unambiguous
(modulo boundary terms), making it a convenient choice for quantifying the
energy of gravitational waves in bigravity or any diffeomorphism-invariant
theories of gravity. This calculation opens the door for future waveform
modeling in bigravity to correctly include backreaction due to emission of
gravitational waves.Comment: 18+4 pages, 2 figure
The Caenorhabditis elegans Synthetic Multivulva Genes Prevent Ras Pathway Activation by Tightly Repressing Global Ectopic Expression of lin-3 EGF
The Caenorhabditis elegans class A and B synthetic multivulva (synMuv) genes redundantly antagonize an EGF/Ras pathway to prevent ectopic vulval induction. We identify a class A synMuv mutation in the promoter of the lin-3 EGF gene, establishing that lin-3 is the key biological target of the class A synMuv genes in vulval development and that the repressive activities of the class A and B synMuv pathways are integrated at the level of lin-3 expression. Using FISH with single mRNA molecule resolution, we find that lin-3 EGF expression is tightly restricted to only a few tissues in wild-type animals, including the germline. In synMuv double mutants, lin-3 EGF is ectopically expressed at low levels throughout the animal. Our findings reveal that the widespread ectopic expression of a growth factor mRNA at concentrations much lower than that in the normal domain of expression can abnormally activate the Ras pathway and alter cell fates. These results suggest hypotheses for the mechanistic basis of the functional redundancy between the tumor-suppressor-like class A and B synMuv genes: the class A synMuv genes either directly or indirectly specifically repress ectopic lin-3 expression; while the class B synMuv genes might function similarly, but alternatively might act to repress lin-3 as a consequence of their role in preventing cells from adopting a germline-like fate. Analogous genes in mammals might function as tumor suppressors by preventing broad ectopic expression of EGF-like ligands.National Institutes of Health (U.S.) (grant GM24663)National Institutes of Health (U.S.). Pioneer Award (1DP1OD003936
The masses of hot subdwarfs
Masses are a fundamental parameter, but they are not well known for most hot
subdwarfs. In general, the mass of a hot subdwarf is derived with
asteroseismology or dynamical methods, for which it is often difficult to
obtain the necessary data from observations. We intend to find an approach to
deriving the masses of hot subdwarfs from observational data in the literature.
We presented full evolutionary calculations for hot subdwarfs in a wide mass
range (0.33 to 1.4 ) for a Population I metallicity of
=0.02, and obtained a relation between and , where , , and are the most probable
mass, effective temperature, and gravity. This relation is used to study the
masses of some observed hot subdwarfs. We proposed a method of determining the
masses of hot subdwarfs. Using this method, we studied the masses of hot
subdwarfs from the ESO supernova Ia progenitor survey and Hamburg quasar
survey. The study shows that most of subdwarf B stars have masses between 0.42
and 0.54 , whilst most sdO stars are in the range 0.40 0.55
. Comparing our study to the theoretical mass distributions of Han et
al. (2003), we found that sdO stars with mass less than 0.5
may evolve from sdB stars, whilst most high-mass( 0.5 ) sdO stars
result from mergers directly.Comment: 5 pages, 6 figures, accepted for publication in A&A Letter
Modelling the formation of double white dwarfs
We investigate the formation of the ten double-lined double white dwarfs that
have been observed so far. A detailed stellar evolution code is used to
calculate grids of single-star and binary models and we use these to
reconstruct possible evolutionary scenarios. We apply various criteria to
select the acceptable solutions from these scenarios. We confirm the conclusion
of Nelemans et al. (2000) that formation via conservative mass transfer and a
common envelope with spiral-in based on energy balance or via two such
spiralins cannot explain the formation of all observed systems. We investigate
three different prescriptions of envelope ejection due to dynamical mass loss
with angular-momentum balance and show that they can explain the observed
masses and orbital periods well. Next, we demand that the age difference of our
model is comparable to the observed cooling-age difference and show that this
puts a strong constraint on the model solutions. However, the scenario in which
the primary loses its envelope in an isotropic wind and the secondary transfers
its envelope, which is then re-emitted isotropically, can explain the observed
age differences as well. One of these solutions explains the DB-nature of the
oldest white dwarf in PG1115+116 along the evolutionary scenario proposed by
Maxted et al. (2002a), in which the helium core of the primary becomes exposed
due to envelope ejection, evolves into a giant phase and loses its
hydrogen-rich outer layers.Comment: 20 pages, 17 figures, 6 tables, accepted for publication in Astronomy
and Astrophysics. See http://www.astro.uu.nl/~sluys/publications/ for
high-resolution versions of Figs. 15 and 1
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