5,393 research outputs found
Recent advances on IMF research
Here I discuss recent work on brown dwarfs, massive stars and the IMF in
general. The stellar IMF can be well described by an invariant two-part power
law in present-day star-formation events within the Local Group of galaxies. It
is nearly identical in shape to the pre-stellar core mass function. The
majority of brown dwarfs follow a separate IMF. Evidence from globular clusters
and ultra-compact dwarf galaxies has emerged that IMFs may have been top heavy
depending on the star-formation rate density. The IGIMF then ranges from bottom
heavy at low galaxy-wide star formation rates to being top-heavy in
galaxy-scale star bursts.Comment: 6 pages, LaTeX, to appear in The Labyrinth of Star Formation, 18-22
June 2012, Crete, (eds.) D. Stamatellos, S. Goodwin, and D. Ward-Thompson,
Springer, in press; replaced version: very minor corrections plus the
addition of reference Smith & Lucey (2013) on the bottom-heavy IMF in
elliptical galaxie
Nonequilibrium Zaklan model on Apollonian Networks
The Zaklan model had been proposed and studied recently using the equilibrium
Ising model on Square Lattices (SL) by Zaklan et al (2008), near the critica
temperature of the Ising model presenting a well-defined phase transition; but
on normal and modified Apollonian networks (ANs), Andrade et al. (2005, 2009)
studied the equilibrium Ising model. They showed the equilibrium Ising model
not to present on ANs a phase transition of the type for the 2D Ising model.
Here, using agent-based Monte-Carlo simulations, we study the Zaklan model with
the well-known majority-vote model (MVM) with noise and apply it to tax evasion
on ANs, to show that differently from the Ising model the MVM on ANs presents a
well defined phase transition. To control the tax evasion in the economics
model proposed by Zaklan et al, MVM is applied in the neighborhood of the
critical noise to the Zaklan model. Here we show that the Zaklan model
is robust because this can be studied besides using equilibrium dynamics of
Ising model also through the nonequilibrium MVM and on various topologies
giving the same behavior regardless of dynamic or topology used here.Comment: 11 pages, 6 figures. arXiv admin note: substantial text overlap with
arXiv:1204.0386 and arXiv:0910.196
G_2 Domain Walls in M-theory
M-theory is considered in its low-energy limit on a G_2 manifold with
non-vanishing flux. Using the Killing spinor equations for linear flux, an
explicit set of first-order bosonic equations for supersymmetric solutions is
found. These solutions describe a warped product of a domain wall in
four-dimensional space-time and a deformed G_2 manifold. It is shown how these
domain walls arise from the perspective of the associated four-dimensional N=1
effective supergravity theories. We also discuss the inclusion of membrane and
M5-brane sources.Comment: 30 pages, Late
Strong disorder renormalization group study of aperiodic quantum Ising chains
We employ an adaptation of a strong-disorder renormalization-group technique
in order to analyze the ferro-paramagnetic quantum phase transition of Ising
chains with aperiodic but deterministic couplings under the action of a
transverse field. In the presence of marginal or relevant geometric
fluctuations induced by aperiodicity, for which the critical behavior is
expected to depart from the Onsager universality class, we derive analytical
and asymptotically exact expressions for various critical exponents (including
the correlation-length and the magnetization exponents, which are not easily
obtainable by other methods), and shed light onto the nature of the ground
state structures in the neighborhood of the critical point. The main results
obtained by this approach are confirmed by finite-size scaling analyses of
numerical calculations based on the free-fermion method
HTL Resummation of the Thermodynamic Potential
Starting from the Phi-derivable approximation scheme at leading-loop order,
the thermodynamical potential in a hot scalar theory, as well as in QED and
QCD, is expressed in terms of hard thermal loop propagators. This
nonperturbative approach is consistent with the leading-order perturbative
results, ultraviolet finite, and, for gauge theories, explicitly
gauge-invariant. For hot QCD it is argued that the resummed approximation is
applicable in the large-coupling regime, down to almost twice the transition
temperature.Comment: minor changes, to appear in PRD, 27 pages, 15 eps figure
The Pipe Nebula as seen with Herschel: Formation of filamentary structures by large-scale compression ?
A growing body of evidence indicates that the formation of filaments in
interstellar clouds is a key component of the star formation process. In this
paper, we present new Herschel PACS and SPIRE observations of the B59 and Stem
regions in the Pipe Nebula complex, revealing a rich, organized network of
filaments. The asymmetric column density profiles observed for several
filaments, along with the bow-like edge of B59, indicates that the Pipe Nebula
is being compressed from its western side, most likely by the winds from the
nearby Sco OB2 association. We suggest that this compressive flow has
contributed to the formation of some of the observed filamentary structures. In
B59, the only region of the entire Pipe complex showing star formation
activity, the same compressive flow has likely enhanced the initial column
density of the clump, allowing it to become globally gravitationally unstable.
Although more speculative, we propose that gravity has also been responsible
for shaping the converging filamentary pattern observed in B59. While the
question of the relative impact of large-scale compression and gravity remains
open in B59, large-scale compression appears to be a plausible mechanism for
the initial formation of filamentary structures in the rest of the complexComment: 9 pages, 9 figures, accepted for publication in A&
Dumping Syndrome and Postbariatric Hypoglycemia:Supporting Evidence for a Common Etiology
BACKGROUND: Dumping syndrome (DS) and postbariatric hypoglycemia (PBH) are frequent complications of bariatric surgery. Previously known as "early and late dumping," these complications have been separated due to differences in their onset and behaviors. OBJECTIVES: To investigate a potentially common etiology of DS and PBH using an analysis of a mixed meal test (MMT) study. SETTING: A large teaching hospital in the Netherlands. METHODS: From all patients who underwent bariatric surgery in 2008-2011, a random selection completed an MMT (n = 47). Patients scored complaints related to DS and PBH with a standardized questionnaire at several time intervals. The groups were divided into patients with (DS+; n = 22) and without (DS-; n = 25) an increase in DS symptoms after the start of the MMT. Glucose and gut hormone levels were compared. Hypoglycemia was defined as a blood glucose level below 3.3 mmol/L. RESULTS: The DS+ group had lower blood glucose values compared to the DS- group, which reached significance at 90 and 120 minutes (P < .05). For the DS+ group, glucagon-like peptide-1 (GLP-1), peptide YY (PYY), and satiety were higher at various time intervals (P < .05) compared to the DS- group. No differences were found for insulin and hunger score. GLP-1 and PYY were correlated with symptoms of DS. CONCLUSION: Patients with DS complaints had lower postprandial glucose values. GLP-1 and PYY values were elevated in the DS+ group early and late during the test. These hormones also correlated with DS. These findings support the hypothesis of a common etiology of DS and PBH and a role of GLP-1 and PYY in both complications
Sharpenings of Li's criterion for the Riemann Hypothesis
Exact and asymptotic formulae are displayed for the coefficients
used in Li's criterion for the Riemann Hypothesis. For we obtain
that if (and only if) the Hypothesis is true,
(with and explicitly given, also for the case of more general zeta or
-functions); whereas in the opposite case, has a non-tempered
oscillatory form.Comment: 10 pages, Math. Phys. Anal. Geom (2006, at press). V2: minor text
corrections and updated reference
Comparative study of the Martian suprathermal electron depletions based on Mars Global Surveyor, Mars Express and Mars Atmosphere and Volatile EvolutioN missions observations
Nightside suprathermal electron depletions have been observed at Mars by three spacecraft to date: Mars Global Surveyor, Mars Express, and the Mars Atmosphere and Volatile EvolutioN (MAVEN) mission. This spatial and temporal diversity of measurements allows us to propose here a comprehensive view of the Martian electron depletions through the first multispacecraft study of the phenomenon. We have analyzed data recorded by the three spacecraft from 1999 to 2015 in order to better understand the distribution of the electron depletions and their creation mechanisms. Three simple criteria adapted to each mission have been implemented to identify more than 134,500 electron depletions observed between 125 and 900 km altitude. The geographical distribution maps of the electron depletions detected by the three spacecraft confirm the strong link existing between electron depletions and crustal magnetic field at altitudes greater than ~170 km. At these altitudes, the distribution of electron depletions is strongly different in the two hemispheres, with a far greater chance to observe an electron depletion in the Southern Hemisphere, where the strongest crustal magnetic sources are located. However, the unique MAVEN observations reveal that below a transition region near 160–170 km altitude the distribution of electron depletions is the same in both hemispheres, with no particular dependence on crustal magnetic fields. This result supports the suggestion made by previous studies that these low-altitudes events are produced through electron absorption by atmospheric CO2
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