361 research outputs found
Formation of the black-hole binary M33 X-7 via mass-exchange in a tight massive system
M33 X-7 is among the most massive X-Ray binary stellar systems known, hosting
a rapidly spinning 15.65 Msun black hole orbiting an underluminous 70 Msun Main
Sequence companion in a slightly eccentric 3.45 day orbit. Although
post-main-sequence mass transfer explains the masses and tight orbit, it leaves
unexplained the observed X-Ray luminosity, star's underluminosity, black hole's
spin, and eccentricity. A common envelope phase, or rotational mixing, could
explain the orbit, but the former would lead to a merger and the latter to an
overluminous companion. A merger would also ensue if mass transfer to the black
hole were invoked for its spin-up. Here we report that, if M33 X-7 started as a
primary of 85-99 Msun and a secondary of 28-32 Msun, in a 2.8-3.1 day orbit,
its observed properties can be consistently explained. In this model, the Main
Sequence primary transferred part of its envelope to the secondary and lost the
rest in a wind; it ended its life as a ~16 Msun He star with a Fe-Ni core which
collapsed to a black hole (with or without an accompanying supernova). The
release of binding energy and, possibly, collapse asymmetries "kicked" the
nascent black hole into an eccentric orbit. Wind accretion explains the X-Ray
luminosity, while the black hole spin can be natal.Comment: Manuscript: 18 pages, 2 tables, 2 figure. Supplementary Information:
34 pages, 6 figures. Advance Online Publication (AOP) on
http://www.nature.com/nature on October 20, 2010. To Appear in Nature on
November 4, 201
The Mechanisms of Codon Reassignments in Mitochondrial Genetic Codes
Many cases of non-standard genetic codes are known in mitochondrial genomes.
We carry out analysis of phylogeny and codon usage of organisms for which the
complete mitochondrial genome is available, and we determine the most likely
mechanism for codon reassignment in each case. Reassignment events can be
classified according to the gain-loss framework. The gain represents the
appearance of a new tRNA for the reassigned codon or the change of an existing
tRNA such that it gains the ability to pair with the codon. The loss represents
the deletion of a tRNA or the change in a tRNA so that it no longer translates
the codon. One possible mechanism is Codon Disappearance, where the codon
disappears from the genome prior to the gain and loss events. In the
alternative mechanisms the codon does not disappear. In the Unassigned Codon
mechanism, the loss occurs first, whereas in the Ambiguous Intermediate
mechanism, the gain occurs first. Codon usage analysis gives clear evidence of
cases where the codon disappeared at the point of the reassignment and also
cases where it did not disappear. Codon disappearance is the probable
explanation for stop to sense reassignments and a small number of reassignments
of sense codons. However, the majority of sense to sense reassignments cannot
be explained by codon disappearance. In the latter cases, by analysis of the
presence or absence of tRNAs in the genome and of the changes in tRNA
sequences, it is sometimes possible to distinguish between the Unassigned Codon
and Ambiguous Intermediate mechanisms. We emphasize that not all reassignments
follow the same scenario and that it is necessary to consider the details of
each case carefully.Comment: 53 pages (45 pages, including 4 figures + 8 pages of supplementary
information). To appear in J.Mol.Evo
Observed Consequences of Presupernova Instability in Very Massive Stars
This chapter concentrates on the deaths of very massive stars, the events
leading up to their deaths, and how mass loss affects the resulting death. The
previous three chapters emphasized the theory of wind mass loss, eruptions, and
core collapse physics, but here we emphasize mainly the observational
properties of the resulting death throes. Mass loss through winds, eruptions,
and interacting binaries largely determines the wide variety of different types
of supernovae that are observed, as well as the circumstellar environments into
which the supernova blast waves expand. Connecting these observed properties of
the explosions to the initial masses of their progenitor stars is, however, an
enduring challenge and is especially difficult for very massive stars.
Superluminous supernovae, pair instability supernovae, gamma ray bursts, and
"failed" supernovae are all end fates that have been proposed for very massive
stars, but the range of initial masses or other conditions leading to each of
these (if they actually occur) are still very certain. Extrapolating to infer
the role of very massive stars in the early universe is essentially
unencumbered by observational constraints and still quite dicey.Comment: 39 pages, 5 figures, to appear as chapter in the book "Very Massive
Stars in the Local Universe", ed. J. Vin
Eta Carinae and the Luminous Blue Variables
We evaluate the place of Eta Carinae amongst the class of luminous blue
variables (LBVs) and show that the LBV phenomenon is not restricted to
extremely luminous objects like Eta Car, but extends luminosities as low as
log(L/Lsun) = 5.4 - corresponding to initial masses ~25 Msun, and final masses
as low as ~10-15 Msun. We present a census of S Doradus variability, and
discuss basic LBV properties, their mass-loss behaviour, and whether at maximum
light they form pseudo-photospheres. We argue that those objects that exhibit
giant Eta Car-type eruptions are most likely related to the more common type of
S Doradus variability. Alternative atmospheric models as well as
sub-photospheric models for the instability are presented, but the true nature
of the LBV phenomenon remains as yet elusive. We end with a discussion on the
evolutionary status of LBVs - highlighting recent indications that some LBVs
may be in a direct pre-supernova state, in contradiction to the standard
paradigm for massive star evolution.Comment: 27 pages, 6 figures, Review Chapter in "Eta Carinae and the supernova
imposters" (eds R. Humphreys and K. Davidson) new version submitted to
Springe
Supernovae from massive stars
Massive stars, by which we mean those stars exploding as core collapse
supernovae, play a pivotal role in the evolution of the Universe. Therefore,
the understanding of their evolution and explosion is fundamental in many
branches of physics and astrophysics, among which, galaxy evolution,
nucleosynthesis, supernovae, neutron stars and pulsars, black holes, neutrinos
and gravitational waves. In this chapter, the author presents an overview of
the presupernova evolution of stars in the range between 13 and 120 , with initial metallicities between [Fe/H]=-3 and [Fe/H]=0 and initial
rotation velocities . Emphasis is placed upon those
evolutionary properties that determine the final fate of the star with special
attention to the interplay among mass loss, mixing and rotation. A general
picture of the evolution and outcome of a generation of massive stars, as a
function of the initial mass, metallicity and rotation velocity, is finally
outlined.Comment: Author version of a chapter for 'Handbook of Supernovae,' edited by
A. Alsabti and P. Murdin, Springer. 59 pages, 27 figure
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Silicon uptake by a pasture grass experiencing simulated grazing is greatest under elevated precipitation
Background
Grasses are hyper-accumulators of silicon (Si) and often up-regulate Si following herbivory. Positive correlations exist between Si and plant water content, yet the extent to which Si uptake responses can be mediated by changes in soil water availability has rarely been studied and never, to our knowledge, under field conditions. We used field-based rain-exclusion shelters to investigate how simulated grazing (shoot clipping) and altered rainfall patterns (drought and elevated precipitation, representing 50% and 150% of ambient precipitation levels, respectively) affected initial patterns of root- and shoot-Si uptake in a native Australian grass (Microlaena stipoides) in Si-supplemented and untreated soils.
Results
Si supplementation increased soil water retention under ambient and elevated precipitation but not under drought, although this had little effect on Si uptake and growth (tiller numbers or root biomass) of M. stipoides. Changes in rainfall patterns and clipping had strong individual effects on plant growth and Si uptake and storage, whereby clipping increased Si uptake by M. stipoides under all rainfall treatments but to the greatest extent under elevated precipitation. Moreover, above-ground–below-ground Si distribution only changed following elevated precipitation by decreasing the ratio of root:shoot Si concentrations.
Conclusions
Results highlight the importance of soil water availability for Si uptake and suggest a role for both active and passive Si transport mechanisms. Such manipulative field studies may provide a more realistic insight into how grasses initially respond to herbivory in terms of Si-based defence under different environmental conditions
Altered branching patterns of Purkinje cells in mouse model for cortical development disorder
Disrupted cortical cytoarchitecture in cerebellum is a typical pathology in reeler. Particularly interesting are structural problems at the cellular level: dendritic morphology has important functional implication in signal processing. Here we describe a combinatorial imaging method of synchrotron X-ray microtomography with Golgi staining, which can deliver 3-dimensional(3-D) micro-architectures of Purkinje cell(PC) dendrites, and give access to quantitative information in 3-D geometry. In reeler, we visualized in 3-D geometry the shape alterations of planar PC dendrites (i.e., abnormal 3-D arborization). Despite these alterations, the 3-D quantitative analysis of the branching patterns showed no significant changes of the 77 ± 8° branch angle, whereas the branch segment length strongly increased with large fluctuations, comparing to control. The 3-D fractal dimension of the PCs decreased from 1.723 to 1.254, indicating a significant reduction of dendritic complexity. This study provides insights into etiologies and further potential treatment options for lissencephaly and various neurodevelopmental disorders
Is Bacterial Persistence a Social Trait?
The ability of bacteria to evolve resistance to antibiotics has been much reported in recent years. It is less well-known that within populations of bacteria there are cells which are resistant due to a non-inherited phenotypic switch to a slow-growing state. Although such ‘persister’ cells are receiving increasing attention, the evolutionary forces involved have been relatively ignored. Persistence has a direct benefit to cells because it allows survival during catastrophes–a form of bet-hedging. However, persistence can also provide an indirect benefit to other individuals, because the reduced growth rate can reduce competition for limiting resources. This raises the possibility that persistence is a social trait, which can be influenced by kin selection. We develop a theoretical model to investigate the social consequences of persistence. We predict that selection for persistence is increased when: (a) cells are related (e.g. a single, clonal lineage); and (b) resources are scarce. Our model allows us to predict how the level of persistence should vary with time, across populations, in response to intervention strategies and the level of competition. More generally, our results clarify the links between persistence and other bet-hedging or social behaviours
Phylogenetic and Preliminary Phenotypic Analysis of Yeast PAQR Receptors: Potential Antifungal Targets
Proteins belonging to the Progestin and AdipoQ Receptor (PAQR) superfamily of membrane bound receptors are ubiquitously found in fungi. Nearly, all fungi possess two evolutionarily distinct paralogs of PAQR protein, which we have called the PQRA and PQRB subtypes. In the model fungus Saccharomyces cerevisiae, these subtypes are represented by the Izh2p and Izh3p proteins, respectively. S. cerevisiae also possesses two additional PQRA-type receptors called Izh1p and Izh4p that are restricted to other species within the “Saccharomyces complex”. Izh2p has been the subject of several recent investigations and is of particular interest because it regulates fungal growth in response to proteins produced by plants and, as such, represents a new paradigm for interspecies communication. We demonstrate that IZH2 and IZH3 gene dosage affects resistance to polyene antifungal drugs. Moreover, we provide additional evidence that Izh2p and Izh3p negatively regulate fungal filamentation. These data suggest that agonists of these receptors might make antifungal therapeutics, either by inhibiting fungal development or by sensitizing fungi to the toxic effects of current antifungal therapies. This is particularly relevant for pathogenic fungi such as Candida glabrata that are closely related to S. cerevisiae and contain the same complement of PAQR receptors
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