2,003 research outputs found
On Tits' Centre Conjecture for Fixed Point Subcomplexes
We give a short and uniform proof of a special case of Tits' Centre
Conjecture using a theorem of J-P. Serre and a result from our earlier work. We
consider fixed point subcomplexes of the building of a
connected reductive algebraic group , where is a subgroup of .Comment: 4 pages; minor changes, to appear in C. R. Acad. Sci. Paris Ser. I
Mat
Collapse of a Molecular Cloud Core to Stellar Densities: The First Three-Dimensional Calculations
We present results from the first three-dimensional calculations ever to
follow the collapse of a molecular cloud core (~ 10^{-18} g cm^{-3}) to stellar
densities (> 0.01 g cm^{-3}). The calculations resolve structures over 7 orders
of magnitude in spatial extent (~ 5000 AU - 0.1 R_\odot), and over 17 orders of
magnitude in density contrast. With these calculations, we consider whether
fragmentation to form a close binary stellar system can occur during the second
collapse phase. We find that, if the quasistatic core that forms before the
second collapse phase is dynamically unstable to the growth of non-axisymmetric
perturbations, the angular momentum extracted from the central regions of the
core, via gravitational torques, is sufficient to prevent fragmentation and the
formation of a close binary during the subsequent second collapse.Comment: ApJ Letters, in press (will appear in Nov 20 issue; available from
the ApJ Rapid Release web page). 7 pages, incl. 5 figures. Also available at
http://www.mpia-hd.mpg.de/theory/bat
Complete Reducibility and Commuting Subgroups
Let G be a reductive linear algebraic group over an algebraically closed
field of characteristic p. We study J.-P. Serre's notion of G-complete
reducibility for subgroups of G. In particular, for a subgroup H and a normal
subgroup N of H, we look at the relationship between G-complete reducibility of
N and of H, and show that these properties are equivalent if H/N is linearly
reductive, generalizing a result of Serre. We also study the case when H = MN
with M a G-completely reducible subgroup of G which normalizes N. We show that
if G is connected, N and M are connected commuting G-completely reducible
subgroups of G, and p is good for G, then H = MN is also G-completely
reducible.Comment: 21 pages; to appear in J. Reine Angew. Math. final for
The effect of magnetic fields on star cluster formation
We examine the effect of magnetic fields on star cluster formation by
performing simulations following the self-gravitating collapse of a turbulent
molecular cloud to form stars in ideal MHD. The collapse of the cloud is
computed for global mass-to-flux ratios of infinity, 20, 10, 5 and 3, that is
using both weak and strong magnetic fields. Whilst even at very low strengths
the magnetic field is able to significantly influence the star formation
process, for magnetic fields with plasma beta < 1 the results are substantially
different to the hydrodynamic case. In these cases we find large-scale
magnetically-supported voids imprinted in the cloud structure; anisotropic
turbulent motions and column density structure aligned with the magnetic field
lines, both of which have recently been observed in the Taurus molecular cloud.
We also find strongly suppressed accretion in the magnetised runs, leading to
up to a 75% reduction in the amount of mass converted into stars over the
course of the calculations and a more quiescent mode of star formation. There
is also some indication that the relative formation efficiency of brown dwarfs
is lower in the strongly magnetised runs due to the reduction in the importance
of protostellar ejections.Comment: 16 pages, 9 figures, 8 very pretty movies, MNRAS, accepted. Version
with high-res figures + movies available from
http://www.astro.ex.ac.uk/people/dprice/pubs/mcluster/index.htm
Formation of the First Supermassive Black Holes
We consider the physical conditions under which supermassive black holes
could have formed inside the first galaxies. Our SPH simulations indicate that
metal-free galaxies with a virial temperature ~10^4 K and with suppressed H2
formation (due to an intergalactic UV background) tend to form a binary black
hole system which contains a substantial fraction (>10%) of the total baryonic
mass of the host galaxy. Fragmentation into stars is suppressed without
substantial H2 cooling. Our simulations follow the condensation of ~5x10^6
M_sun around the two centers of the binary down to a scale of < 0.1pc. Low-spin
galaxies form a single black hole instead. These early black holes lead to
quasar activity before the epoch of reionization. Primordial black hole
binaries lead to the emission of gravitational radiation at redshifts z>10 that
would be detectable by LISA.Comment: 11 pages, 9 figures, revised version, ApJ in press (October 10, 2003
Maj. Gen. William B. Bate to Col. B. Ray (14 May 1864)
Report of Confederate soldiers repelling 2nd Union assault, made in two lineshttps://egrove.olemiss.edu/ciwar_milrec/1030/thumbnail.jp
Valproic acid and its congener propylisopropylacetic acid reduced the amount of soluble amyloid-ß oligomers released from 7PA2 cells
The amyloid hypothesis of Alzheimer's disease suggests that synaptic degeneration and pathology is caused by the accumulation of amyloid-β (Aβ) peptides derived from the amyloid precursor protein (APP). Subsequently, soluble Aβ oligomers cause the loss of synaptic proteins from neurons, a histopathological feature of Alzheimer's disease that correlates with the degree of dementia. In this study, the production of toxic forms of Aβ was examined in vitro using 7PA2 cells stably transfected with human APP. We show that conditioned media from 7PA2 cells containing Aβ oligomers caused synapse degeneration as measured by the loss of synaptic proteins, including synaptophysin and cysteine-string protein, from cultured neurons. Critically, conditioned media from 7PA2 cells treated with valproic acid (2-propylpentanoic acid (VPA)) or propylisopropylacetic acid (PIA) did not cause synapse damage. Treatment with VPA or PIA did not significantly affect total Aβ42 concentrations; rather these drugs selectively reduced the concentrations of Aβ42 oligomers in conditioned media. In contrast, treatment significantly increased the concentrations of Aβ42 monomers in conditioned media. VPA or PIA treatment reduced the concentrations of APP within lipid rafts, membrane compartments associated with Aβ production. These effects of VPA and PIA were reversed by the addition of platelet-activating factor, a bioactive phospholipid produced following activation of phospholipase A2, an enzyme sensitive to VPA and PIA. Collectively these data suggest that VPA and PIA reduce Aβ oligomers through inhibition of phospholipase A2 and suggest a novel therapeutic approach to Alzheimer's treatment
Intranasal Inhalation of Oxytocin Improves Face Processing in Developmental Prosopagnosia
Developmental prosopagnosia (DP) is characterised by a severe, lifelong impairment in face recognition. Little work has attempted to improve face processing in these individuals, but intriguingly, recent evidence suggests oxytocin can improve face processing in both healthy participants and individuals with autism. This study examined whether oxytocin could also improve face processing in individuals with DP. Ten adults with the condition and 10 matched controls were tested using a randomized placebo-controlled double-blind within-subject experimental design (AB-BA). Each participant took part in two testing sessions where they inhaled 24IU of oxytocin or placebo spray and completed two face processing tests: one assessing face memory and the other face perception. Results showed main effects of both participant group and treatment condition in both face processing tests, but the two did not interact. Specifically, the performance of DP participants was significantly lower than control performance under both oxytocin and placebo conditions, but oxytocin improved processing to a similar extent in both groups
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