13,004 research outputs found
The structure and fate of white dwarf merger remnants
We present a large parameter study where we investigate the structure of
white dwarf (WD) merger remnants after the dynamical phase. A wide range of WD
masses and compositions are explored and we also probe the effect of different
initial conditions. We investigated the degree of mixing between the WDs, the
conditions for detonations as well as the amount of gas ejected. We find that
systems with lower mass ratios have more total angular momentum and as a result
more mass is flung out in a tidal tail. Nuclear burning can affect the amount
of mass ejected. Many WD binaries that contain a helium-rich WD achieve the
conditions to trigger a detonation. In contrast, for carbon-oxygen transferring
systems only the most massive mergers with a total mass above ~2.1 solar masses
detonate. Even systems with lower mass may detonate long after the merger if
the remnant remains above the Chandrasekhar mass and carbon is ignited at the
centre. Finally, our findings are discussed in the context of several possible
observed astrophysical events and stellar systems, such as hot subdwarfs, R
Coronae Borealis stars, single massive white dwarfs, supernovae of type Ia and
other transient events. A large database containing 225 white dwarf merger
remnants is made available via a dedicated web page.Comment: 23 pages, 15 figures, submitted to MNRAS. A database containing 225
WD merger remnants is available for download at
http://www.hs.uni-hamburg.de/DE/Ins/Per/Dan/wdwd_remnants.htm
Extended MSSM Neutralinos as the Source of the PAMELA Positron Excess
We consider a scenario within the Minimal Supersymmetric Standard Model
extended by a singlet chiral superfield, in which neutralino dark matter
annihilates to light singlet-like Higgs bosons, which proceed to decay to
either electron-positron or muon-antimuon pairs. Unlike neutralino
annihilations in the MSSM, this model can provide a good fit to the PAMELA
cosmic ray positron fraction excess. Furthermore, the singlet-like scalar Higgs
can induce a large Sommerfeld enhancement and provide an annihilation rate
sufficient to accommodate the observed positron excess
High spatial resolution studies of galaxies in the far IR: Observations with the KAO, and the promise of SOFIA
NASA, in collaboration with the West German Science Ministry (BMFT), plans a larger airborne telescope as a successor to the Kuipper Airborne Observatory (KAO) that will achieve these goals. The Stratospheric Observatory for Infrared Astronomy (SOFIA) is entering the final stages of Phase B review with targeted new start early in the next decade. SOFIA is a 2.7 m diameter telescope that is carried in a Boeing 747SP. In addition to having 3 times the spatial resolution of the KAO, and 10 times the light gathering power, it will incorporate improvements over the KAO in lower optical emissivity and better telescope tracking stability. The thin primary mirror will equilibrate quickly to ambient temperature at an altitude which, accompanied by airflow improvements across the telescope cavity, will result in better image quality. The sensitivity of SOFIA will allow us to see a large number of typical bright galactic HII regions in local group galaxies. The spatial resolution of 8 seconds (full width half maximum Airy disk) at 100 microns will allow these regions to be measured independently, if they are distributed similarly to those in our own galaxy. At this spatial resolution, the disks of normal galaxies will be easily resolved out to distances of several hundred Mpc. This portion of space includes many of the superluminous galaxies discovered by the Infrared Astronomy Satellite (IRAS), and this spatial scale is relevant for studies of the morphology of regions of interaction among the majority of these galaxies that are members of colliding pairs
Cooperative Stimulation of Dendritic Cells by Cryptococcus neoformans Mannoproteins and CpG Oligodeoxynucleotides
While mannosylation targets antigens to mannose receptors on dendritic cells (DC), the resultant immune response is suboptimal. We hypothesized that the addition of toll-like receptor (TLR) ligands would enhance the DC response to mannosylated antigens. Cryptococcus neoformans mannoproteins (MP) synergized with CpG-containing oligodeoxynucleotides to stimulate enhanced production of proinflammatory cytokines and chemokines from murine conventional and plasmacytoid DC. Synergistic stimulation required the interaction of mannose residues on MP with the macrophage mannose receptor (MR), CD206. Moreover, synergy with MP was observed with other TLR ligands, including tripalmitoylated lipopeptide (Pam3CSK4), polyinosine-polycytidylic acid (pI:C), and imiquimod. Finally, CpG enhanced MP-specific MHC II-restricted CD4+ T-cell responses by a mechanism dependent upon DC expression of CD206 and TLR9. These data suggest a rationale for vaccination strategies that combine mannosylated antigens with TLR ligands and imply that immune responses to naturally mannosylated antigens on pathogens may be greatly augmented if TLR and MR are cooperatively stimulated.National Institutes of Health (RO1 AI25780, RO1 AI37532, K08 AI 53542
Prospects For Detecting Dark Matter With Neutrino Telescopes In Light Of Recent Results From Direct Detection Experiments
Direct detection dark matter experiments, lead by the CDMS collaboration,
have placed increasingly stronger constraints on the cross sections for elastic
scattering of WIMPs on nucleons. These results impact the prospects for the
indirect detection of dark matter using neutrino telescopes. With this in mind,
we revisit the prospects for detecting neutrinos produced by the annihilation
of WIMPs in the Sun. We find that the latest bounds do not seriously limit the
models most accessible to next generation kilometer-scale neutrino telescopes
such as IceCube. This is largely due to the fact that models with significant
spin-dependent couplings to protons are the least constrained and, at the same
time, the most promising because of the efficient capture of WIMPs in the Sun.
We identify models where dark matter particles are beyond the reach of any
planned direct detection experiments while within reach of neutrino telescopes.
In summary, we find that, even when contemplating recent direct detection
results, neutrino telescopes still have the opportunity to play an important as
well as complementary role in the search for particle dark matter.Comment: 13 pages, 6 figure
High spatial resolution 100 micron observations of the M83 bar
A program of high spatial resolution far-infrared observations of galaxies using the Kuiper Airborne Observatory (KAO), was conducted to better understand the role of star formation, the general interstellar radiation field, and non-thermal activity in powering the prodigious far-infrared luminosities seen in spiral and interacting galaxies. Here, researchers present observations of the central region of the well-known barred spiral M83 (NGC 5236). The resultant channel 3 scans for M83 and IRC + 10216, after co-addition and smoothing, are shown. These data show that M83 is extended at 100 microns compared to a point source. A simple Gaussian deconvolution of the M83 data with the point source profile from IRC+10216 gives a full width half maximum (FWHM) of about 19 seconds for M83. By comparison with IRC+10216, researchers obtain a flux for the unresolved component in M83 of about 110 Jy. This is about 1/6 the total flux for M83 (Rice et al. 1988) and about 1/2 the PSC flux. The M83 and IRC+10216 profiles in the cross-scan direction (SE-NW) were also compared, and show that M83 is extended in this direction as well, with a width of about 18 seconds. A comparison of the different channel profiles for M83 and IRC+10216 shows that there is an asymmetry in the M83 data, in that the maximum in the profiles shifts from southeast to northwest as channel number increases. This corresponds to the extension in the bar seen in the CO data. Thus the far-infrared emission in the central region of M83 tends to trace the CO bar. The new 100 micron data is also compared with previous H alpha observations from the literature, to determine how well the far-infrared traces the stellar structure, the star formation as measured by H alpha, and the optical colors
Supersymmetric reduced models with a symmetry based on Filippov algebra
Generalizations of the reduced model of super Yang-Mills theory obtained by
replacing the Lie algebra structure to Filippov -algebra structures are
studied. Conditions for the reduced model actions to be supersymmetric are
examined. These models are related with what we call \{cal N}_{min}=2 super
-brane actions.Comment: v3: In the previous versions we overlooked that Eq.(3.9) holds more
generally, and missed some supersymmetric actions. Those are now included and
modifications including a slight change in the title were made accordingly.
1+18 page
Exclusive rare B -> K*e+e- decays at low recoil: controlling the long-distance effects
We present a model-independent description of the exclusive rare decays B->
K* e+e- in the low recoil region (large lepton invariant mass q^2 ~ m_b^2). In
this region the long-distance effects from quark loops can be computed with the
help of an operator product expansion in 1/Q, with Q={m_b, \sqrt{q^2}}.
Nonperturbative effects up to and including terms suppressed by Lambda/Q and
mc^2/mb^2 relative to the short-distance amplitude can be included in a
model-independent way. Based on these results, we propose an improved method
for determining the CKM matrix element |V{ub}| from a combination of rare and
semileptonic B and D decays near the zero recoil point. The residual
theoretical uncertainty from long distance effects in this |V{ub}|
determination comes from terms in the OPE of order alpha_s(Q)\Lambda/mb,
alpha_s^2(Q), mc^4/mb^4$ and duality violations and is estimated to be below
10%.Comment: 21 pages RevTex, 2 figures; v3: extensive numerical changes in the
NLL analysis, with improved stability under scale dependence. Typos fixed,
version to appear in Phys.Rev.
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