2,509 research outputs found
3D Radiative Hydrodynamics for Disk Stability Simulations: A Proposed Testing Standard and New Results
Recent three-dimensional radiative hydrodynamics simulations of
protoplanetary disks report disparate disk behaviors, and these differences
involve the importance of convection to disk cooling, the dependence of disk
cooling on metallicity, and the stability of disks against fragmentation and
clump formation. To guarantee trustworthy results, a radiative physics
algorithm must demonstrate the capability to handle both the high and low
optical depth regimes. We develop a test suite that can be used to demonstrate
an algorithm's ability to relax to known analytic flux and temperature
distributions, to follow a contracting slab, and to inhibit or permit
convection appropriately. We then show that the radiative algorithm employed by
Meji\'a (2004) and Boley et al. (2006) and the algorithm employed by Cai et al.
(2006) and Cai et al. (2007, in prep.) pass these tests with reasonable
accuracy. In addition, we discuss a new algorithm that couples flux-limited
diffusion with vertical rays, we apply the test suite, and we discuss the
results of evolving the Boley et al. (2006) disk with this new routine.
Although the outcome is significantly different in detail with the new
algorithm, we obtain the same qualitative answers. Our disk does not cool fast
due to convection, and it is stable to fragmentation. We find an effective
. In addition, transport is dominated by low-order
modes.Comment: Submitted to Ap
Imprints of Nuclear Symmetry Energy on Properties of Neutron Stars
Significant progress has been made in recent years in constraining the
density dependence of nuclear symmetry energy using terrestrial nuclear
laboratory data. Around and below the nuclear matter saturation density, the
experimental constraints start to merge in a relatively narrow region. At
supra-saturation densities, there are, however, still large uncertainties.
After summarizing the latest experimental constraints on the density dependence
of nuclear symmetry energy, we highlight a few recent studies examining
imprints of nuclear symmetry energy on the binding energy, energy release
during hadron-quark phase transitions as well as the -mode frequency and
damping time of gravitational wave emission of neutron stars.Comment: 10 pages. Invited talk given in the Nuclear Astrophysics session of
INPC2010, July 4-9, 2010, Vancouver, Canada; Journal of Physics: Conference
Series (2011
Very Red and Extremely Red Galaxies in the Fields of z ~ 1.5 Radio-Loud Quasars
We previously identified an excess of mostly red galaxies around 31 RLQs at
z=1-2. These fields have an ERO (extremely red object, R-K>6) density 2.7 times
higher than the field. Assuming the EROs are passively evolved galaxies at the
quasar redshifts, they have characteristic luminosities of only ~L^*. We also
present new observations of four z~1.54 RLQ fields: (1) Wide-field J & Ks data
confirm an Abell richness ~2 excess within 140" of Q0835+580 but an excess only
within 50" of Q1126+101. (2) In 3 fields we present deep narrow-band redshifted
H-alpha observations. We detect five candidate galaxies at the quasar
redshifts, a surface density 2.5x higher than the field. (3) SCUBA sub-mm
observations of 3 fields detect 2 quasars and 2 galaxies with SEDs best fit as
highly reddened galaxies at the quasar z. (4) H-band adaptive optics (AO)
imaging is used to estimate redshifts for 2 red, bulge-dominated galaxies using
the Kormendy relation. Both have structural redshifts foreground to the quasar,
but these are not confirmed by photometric redshifts, possibly because their
optical photometry is corrupted by scattered light from the AO guidestar. (5)
We use quantitative SED fits to constrain the photometric redshifts z_ph for
some galaxies. Most galaxies near Q0835+580 are consistent with being at its
redshift, including a candidate very old passively evolving galaxy. Many very &
extremely red objects have z_ph z_q, and dust reddening is required to fit most
of them, including many objects whose fits also require relatively old stellar
populations. Large reddenings of E(B-V)~0.6 are required to fit four J-K
selected EROs, though all but one of them have best-fit z_ph>z_q. These objects
may represent a population of dusty high-z galaxies underrepresented in
optically selected samples. (Abridged)Comment: Missing object 1126.424 added to Table 4; title changed to save
people the apparent trouble of reading the abstract. 38 pages, 16 figures, 2
in color; all-PostScript figure version available from
http://astro.princeton.edu/~pathall/tp3.ps.g
The I=2 pipi S-wave Scattering Phase Shift from Lattice QCD
The pi+pi+ s-wave scattering phase-shift is determined below the inelastic
threshold using Lattice QCD. Calculations were performed at a pion mass of
m_pi~390 MeV with an anisotropic n_f=2+1 clover fermion discretization in four
lattice volumes, with spatial extent L~2.0, 2.5, 3.0 and 3.9 fm, and with a
lattice spacing of b_s~0.123 fm in the spatial direction and b_t b_s/3.5 in the
time direction. The phase-shift is determined from the energy-eigenvalues of
pi+pi+ systems with both zero and non-zero total momentum in the lattice volume
using Luscher's method. Our calculations are precise enough to allow for a
determination of the threshold scattering parameters, the scattering length a,
the effective range r, and the shape-parameter P, in this channel and to
examine the prediction of two-flavor chiral perturbation theory: m_pi^2 a r =
3+O(m_pi^2/Lambda_chi^2). Chiral perturbation theory is used, with the Lattice
QCD results as input, to predict the scattering phase-shift (and threshold
parameters) at the physical pion mass. Our results are consistent with
determinations from the Roy equations and with the existing experimental phase
shift data.Comment: 22 pages, 16 figure
Evidence for a Bound H-dibaryon from Lattice QCD
We present evidence for the existence of a bound H dibaryon, an I = 0 , J = 0 , s = â 2 state with valence quark structure u u d d s s , at a pion mass of m Ï âŒ 389 â â MeV . Using the results of lattice QCD calculations performed on four ensembles of anisotropic clover gauge-field configurations, with spatial extents of L ⌠2.0 , 2.5, 3.0, and 3.9 fm at a spatial lattice spacing of b s ⌠0.123 â â fm , we find an H dibaryon bound by B H â = 16.6 ± 2.1 ± 4.6 â â MeV at a pion mass of m Ï âŒ 389 â â MeV
The Thermal Regulation of Gravitational Instabilities in Protoplanetary Disks III. Simulations with Radiative Cooling and Realistic Opacities
This paper presents a fully three-dimensional radiative hydrodymanics
simulation with realistic opacities for a gravitationally unstable 0.07 Msun
disk around a 0.5 Msun star. We address the following aspects of disk
evolution: the strength of gravitational instabilities under realistic cooling,
mass transport in the disk that arises from GIs, comparisons between the
gravitational and Reynolds stresses measured in the disk and those expected in
an alpha-disk, and comparisons between the SED derived for the disk and SEDs
derived from observationally determined parameters. The mass transport in this
disk is dominated by global modes, and the cooling times are too long to permit
fragmentation for all radii. Moreover, our results suggest a plausible
explanation for the FU Ori outburst phenomenon.Comment: 45 pages, 17 figures; submitted to Ap
CRISPR-Cas9 screens in human cells and primary neurons identify modifiers of C9ORF72 dipeptide-repeat-protein toxicity.
Hexanucleotide-repeat expansions in the C9ORF72 gene are the most common cause of amyotrophic lateral sclerosis and frontotemporal dementia (c9ALS/FTD). The nucleotide-repeat expansions are translated into dipeptide-repeat (DPR) proteins, which are aggregation prone and may contribute to neurodegeneration. We used the CRISPR-Cas9 system to perform genome-wide gene-knockout screens for suppressors and enhancers of C9ORF72 DPR toxicity in human cells. We validated hits by performing secondary CRISPR-Cas9 screens in primary mouse neurons. We uncovered potent modifiers of DPR toxicity whose gene products function in nucleocytoplasmic transport, the endoplasmic reticulum (ER), proteasome, RNA-processing pathways, and chromatin modification. One modifier, TMX2, modulated the ER-stress signature elicited by C9ORF72 DPRs in neurons and improved survival of human induced motor neurons from patients with C9ORF72 ALS. Together, our results demonstrate the promise of CRISPR-Cas9 screens in defining mechanisms of neurodegenerative diseases
Alkali Metal Bismuth(III) Chloride Double Salts
Evaporative co-crystallization of MCl (M = Na, K, Rb, Cs) with BiOCl in aqueous HCl produces double salts: MxBiyCl(x+3y)·zH2O. The sodium salt, Na2BiCl5·5H2O (monoclinic P21/c, a = 8.6983(7) Ă
, b = 21.7779(17) Ă
, c = 7.1831(6) Ă
, ÎČ = 103.0540(10)°, V = 1325.54(19) Ă
3, Z = 4) is composed of zigzag chains of ÎŒ2-Cl-cis-linked (BiCl5)n2nâ chains. Edge-sharing chains of NaCln(OH2)6ân octahedra (n = 0, 2, 3) are linked through ÎŒ3-Cl to Bi. The potassium salt, K7Bi3Cl16 (trigonal Râ3c, a = 12.7053(9) Ă
, b = 12.7053(9) Ă
, c = 99.794(7) Ă
, V = 13,951(2) Ă
3, Z = 18) contains (Bi2Cl10)4â edge-sharing dimers of octahedra and simple (BiCl6)3â octahedra. The K+ ions are 5- to 8-coordinate and the chlorides are 3-, 4-, or 5-coordinate. The rubidium salt, Rb3BiCl6·0.5H2O (orthorhombic Pnma, a = 12.6778(10) Ă
, b = 25.326(2) Ă
, c = 8.1498(7) Ă
, V = 2616.8(4) Ă
3, Z = 8) contains (BiCl6)3â octahedra. The Rb+ ions are 6-, 8-, and 9-coordinate, and the chlorides are 4- or 5-coordinate. Two cesium salts were formed: Cs3BiCl6 (orthorhombic Pbcm, a = 8.2463(9) Ă
, b = 12.9980(15) Ă
, c = 26.481(3) Ă
, V = 2838.4(6) Ă
3, Z = 8) being comprised of (BiCl6)3â octahedra, 8-coordinate Cs+, and 3-, 4-, and 5-coordinate Clâ. In Cs3Bi2Cl9 (orthorhombic Pnma, a = 18.4615(15) Ă
, b = 7.5752(6) Ă
, c = 13.0807(11) Ă
, V = 1818.87(11) Ă
3, Z = 4) Bi octahedra are linked by ÎŒ2-bridged Cl into edge-sharing Bi4 squares which form zigzag (Bi2Cl9)n3nâ ladders. The 12-coordinate Cs+ ions bridge the ladders, and the Clâ ions are 5- and 6-coordinate. Four of the double salts are weakly photoluminescent at 78 K, each showing a series of three excitation peaks near 295, 340, and 380 nm and a broad emission near 440 nm
Comprehensive Bayesian analysis of FRB-like bursts from SGR 1935+2154 observed by CHIME/FRB
The bright millisecond-duration radio burst from the Galactic magnetar SGR
1935+2154 in 2020 April was a landmark event, demonstrating that at least some
fast radio burst (FRB) sources could be magnetars. The two-component burst was
temporally coincident with peaks observed within a contemporaneous short X-ray
burst envelope, marking the first instance where FRB-like bursts were observed
to coincide with X-ray counterparts. In this study, we detail five new radio
burst detections from SGR 1935+2154, observed by the CHIME/FRB instrument
between October 2020 and December 2022. We develop a fast and efficient
Bayesian inference pipeline that incorporates state-of-the-art Markov chain
Monte Carlo techniques and use it to model the intensity data of these bursts
under a flexible burst model. We revisit the 2020 April burst and corroborate
that both the radio sub-components lead the corresponding peaks in their
high-energy counterparts. For a burst observed in 2022 October, we find that
our estimated radio pulse arrival time is contemporaneous with a short X-ray
burst detected by GECAM and HEBS, and Konus-Wind and is consistent with the
arrival time of a radio burst detected by GBT. We present flux and fluence
estimates for all five bursts, employing an improved estimator for bursts
detected in the side-lobes. We also present upper limits on radio emission for
X-ray emission sources which were within CHIME/FRB's field-of-view at trigger
time. Finally, we present our exposure and sensitivity analysis and estimate
the Poisson rate for FRB-like events from SGR 1935+2154 to be
events/day above a fluence of
during the interval from 28 August 2018 to 1 December 2022, although we note
this was measured during a time of great X-ray activity from the source.Comment: 22 pages, 6 figures, 4 tables. To be submitted to Ap
Measurement of the production of a W boson in association with a charm quark in pp collisions at âs = 7 TeV with the ATLAS detector
The production of a W boson in association with a single charm quark is studied using 4.6 fbâ1 of pp collision data at sâ = 7 TeV collected with the ATLAS detector at the Large Hadron Collider. In events in which a W boson decays to an electron or muon, the charm quark is tagged either by its semileptonic decay to a muon or by the presence of a charmed meson. The integrated and differential cross sections as a function of the pseudorapidity of the lepton from the W-boson decay are measured. Results are compared to the predictions of next-to-leading-order QCD calculations obtained from various parton distribution function parameterisations. The ratio of the strange-to-down sea-quark distributions is determined to be 0.96+0.26â0.30 at Q 2 = 1.9 GeV2, which supports the hypothesis of an SU(3)-symmetric composition of the light-quark sea. Additionally, the cross-section ratio Ï(W + +cÂŻÂŻ)/Ï(W â + c) is compared to the predictions obtained using parton distribution function parameterisations with different assumptions about the sâsÂŻÂŻÂŻ quark asymmetry
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