19 research outputs found
Discs in misaligned binary systems
We perform SPH simulations to study precession and changes in alignment
between the circumprimary disc and the binary orbit in misaligned binary
systems. We find that the precession process can be described by the rigid-disc
approximation, where the disc is considered as a rigid body interacting with
the binary companion only gravitationally. Precession also causes change in
alignment between the rotational axis of the disc and the spin axis of the
primary star. This type of alignment is of great important for explaining the
origin of spin-orbit misaligned planetary systems. However, we find that the
rigid-disc approximation fails to describe changes in alignment between the
disc and the binary orbit. This is because the alignment process is a
consequence of interactions that involve the fluidity of the disc, such as the
tidal interaction and the encounter interaction. Furthermore, simulation
results show that there are not only alignment processes, which bring the
components towards alignment, but also anti-alignment processes, which tend to
misalign the components. The alignment process dominates in systems with
misalignment angle near 90 degrees, while the anti-alignment process dominates
in systems with the misalignment angle near 0 or 180 degrees. This means that
highly misaligned systems will become more aligned but slightly misaligned
systems will become more misaligned.Comment: 15 pages, 16 figures, 1 table, accepted for publication in MNRA
The importance of episodic accretion for low-mass star formation
A star acquires much of its mass by accreting material from a disc. Accretion
is probably not continuous but episodic. We have developed a method to include
the effects of episodic accretion in simulations of star formation. Episodic
accretion results in bursts of radiative feedback, during which a protostar is
very luminous, and its surrounding disc is heated and stabilised. These bursts
typically last only a few hundred years. In contrast, the lulls between bursts
may last a few thousand years; during these lulls the luminosity of the
protostar is very low, and its disc cools and fragments. Thus, episodic
accretion enables the formation of low-mass stars, brown dwarfs and
planetary-mass objects by disc fragmentation. If episodic accretion is a common
phenomenon among young protostars, then the frequency and duration of accretion
bursts may be critical in determining the low-mass end of the stellar initial
mass function.Comment: To appear in the Astrophysical Journal. Press release available at:
http://www.astro.cf.ac.uk/pub/Dimitrios.Stamatellos/News/News.html Full
resolution paper available at http://stacks.iop.org/0004-637X/730/3
Radiation Driven Implosion and Triggered Star Formation
We present simulations of initially stable isothermal clouds exposed to
ionizing radiation from a discrete external source, and identify the conditions
that lead to radiatively driven implosion and star formation. We use the
Smoothed Particle Hydrodynamics code SEREN (Hubber et al. 2010) and the
HEALPix-based photoionization algorithm described in Bisbas et al. (2009). We
find that the incident ionizing flux is the critical parameter determining the
evolution: high fluxes simply disperse the cloud, whereas low fluxes trigger
star formation. We find a clear connection between the intensity of the
incident flux and the parameters of star formation.Comment: 4 pages, 2 figures, conference proceedings, IAU Symposium 270 (eds.
Alves, Elmegreen, Girart, Trimble
When is working memory important for arithmetic?: the impact of strategy and age
Our ability to perform arithmetic relies heavily on working memory, the manipulation and maintenance of information in mind. Previous research has found that in adults, procedural strategies, particularly counting, rely on working memory to a greater extent than retrieval strategies. During childhood there are changes in the types of strategies employed, as well as an increase in the accuracy and efficiency of strategy execution. As such it seems likely that the role of working memory in arithmetic may also change, however children and adults have never been directly compared. This study used traditional dual-task methodology, with the addition of a control load condition, to investigate the extent to which working memory requirements for different arithmetic strategies change with age between 9-11 years, 12-14 years and young adulthood. We showed that both children and adults employ working memory when solving arithmetic problems, no matter what strategy they choose. This study highlights the importance of considering working memory in understanding the difficulties that some children and adults have with mathematics, as well as the need to include working memory in theoretical models of mathematical cognition
Dynamic Evolution of Pathogenicity Revealed by Sequencing and Comparative Genomics of 19 Pseudomonas syringae Isolates
Closely related pathogens may differ dramatically in host range, but the molecular, genetic, and evolutionary basis for these differences remains unclear. In many Gram- negative bacteria, including the phytopathogen Pseudomonas syringae, type III effectors (TTEs) are essential for pathogenicity, instrumental in structuring host range, and exhibit wide diversity between strains. To capture the dynamic nature of virulence gene repertoires across P. syringae, we screened 11 diverse strains for novel TTE families and coupled this nearly saturating screen with the sequencing and assembly of 14 phylogenetically diverse isolates from a broad collection of diseased host plants. TTE repertoires vary dramatically in size and content across all P. syringae clades; surprisingly few TTEs are conserved and present in all strains. Those that are likely provide basal requirements for pathogenicity. We demonstrate that functional divergence within one conserved locus, hopM1, leads to dramatic differences in pathogenicity, and we demonstrate that phylogenetics-informed mutagenesis can be used to identify functionally critical residues of TTEs. The dynamism of the TTE repertoire is mirrored by diversity in pathways affecting the synthesis of secreted phytotoxins, highlighting the likely role of both types of virulence factors in determination of host range. We used these 14 draft genome sequences, plus five additional genome sequences previously reported, to identify the core genome for P. syringae and we compared this core to that of two closely related non-pathogenic pseudomonad species. These data revealed the recent acquisition of a 1 Mb megaplasmid by a sub-clade of cucumber pathogens. This megaplasmid encodes a type IV secretion system and a diverse set of unknown proteins, which dramatically increases both the genomic content of these strains and the pan-genome of the species
Forming isolated brown dwarfs by turbulent fragmentation
We use Smoothed Particle Hydrodynamics to explore the circumstances under which an isolated very low mass pre-stellar core can be formed by colliding turbulent flows and collapse to form a brown dwarf. Our simulations suggest that the flows need not be very fast, but do need to be very strongly convergent, i.e. the gas must flow in at comparable speeds from all sides, which seems rather unlikely. We therefore revisit the object Oph-B11, which André et al. have identified as a pre-stellar core with mass between ∼0.020 M⊙ and ∼0.030 M⊙. We re-analyse the observations using a Markov-chain Monte Carlo method that allows us (i) to include the uncertainties on the distance, temperature and dust mass opacity, and (ii) to consider different Bayesian prior distributions of the mass. We estimate that the posterior probability that Oph-B11 has a mass below the hydrogen-burning limit at ∼0.075 M⊙, is between 0.66 and 0.86 . We conclude that, if Oph-B11 is destined to collapse, it probably will form a brown dwarf. However, the flows required to trigger this appear to be so contrived that it is difficult to envisage this being the only way, or even a major way, of forming isolated brown dwarfs. Moreover, Oph-B11 could easily be a transient, bouncing, prolate core, seen end-on; there could, indeed should, be many such objects masquerading as very low mass pre-stellar cores