2,832 research outputs found
Taming the -expansion with Large Spin Perturbation Theory
We apply analytic bootstrap techniques to the four-point correlator of
fundamental fields in the Wilson-Fisher model. In an -expansion
crossing symmetry fixes the double discontinuity of the correlator in terms of
CFT data at lower orders. Large spin perturbation theory, or equivalently the
recently proposed Froissart-Gribov inversion integral, then allows one to
reconstruct the CFT data of intermediate operators of any spin. We use this
method to compute the anomalous dimensions and OPE coefficients of leading
twist operators. To cubic order in the double discontinuity arises
solely from the identity operator and the scalar bilinear operator, making the
computation straightforward. At higher orders the double discontinuity receives
contributions from infinite towers of higher spin operators. At fourth order,
the structure of perturbation theory leads to a proposal in terms of functions
of certain degree of transcendentality, which can then be fixed by symmetries.
This leads to the full determination of the CFT data for leading twist
operators to fourth order.Comment: 16 pages. v2: Added discussion on low spin spectru
The circumstellar envelope of AFGL 4106
We present new imaging and spectroscopy of the post-red supergiant binary
AFGL 4106. Coronographic imaging in H-alpha reveals the shape and extent of the
ionized region in the circumstellar envelope (CSE). Echelle spectroscopy with
the slit covering almost the entire extent of the CSE is used to derive the
physical conditions in the ionized region and the optical depth of the dust
contained within the CSE.
The dust shell around AFGL 4106 is clumpy and mixed with ionized gas. H-alpha
and [N II] emission is brightest from a thin bow-shaped layer just outside of
the detached dust shell. On-going mass loss is traced by [Ca II] emission and
blue-shifted absorption in lines of low-ionization species. A simple model is
used to interpret the spatial distribution of the circumstellar extinction and
the dust emission in a consistent way.Comment: 10 pages, 11 figures. Accepted for publication in Astronomy &
Astrophysics Main Journa
Three-micron spectra of AGB stars and supergiants in nearby galaxies
The dependence of stellar molecular bands on the metallicity is studied using
infrared L-band spectra of AGB stars (both carbon-rich and oxygen-rich) and
M-type supergiants in the Large and Small Magellanic Clouds (LMC and SMC) and
in the Sagittarius Dwarf Spheroidal Galaxy. The spectra cover SiO bands for
oxygen-rich stars, and acetylene (C2H2), CH and HCN bands for carbon-rich AGB
stars. The equivalent width of acetylene is found to be high even at low
metallicity. The high C2H2 abundance can be explained with a high
carbon-to-oxygen (C/O) ratio for lower metallicity carbon stars. In contrast,
the HCN equivalent width is low: fewer than half of the extra-galactic carbon
stars show the 3.5micron HCN band, and only a few LMC stars show high HCN
equivalent width. HCN abundances are limited by both nitrogen and carbon
elemental abundances. The amount of synthesized nitrogen depends on the initial
mass, and stars with high luminosity (i.e. high initial mass) could have a high
HCN abundance. CH bands are found in both the extra-galactic and Galactic
carbon stars. None of the oxygen-rich LMC stars show SiO bands, except one
possible detection in a low quality spectrum. The limits on the equivalent
widths of the SiO bands are below the expectation of up to 30angstrom for LMC
metallicity. Several possible explanations are discussed. The observations
imply that LMC and SMC carbon stars could reach mass-loss rates as high as
their Galactic counterparts, because there are more carbon atoms available and
more carbonaceous dust can be formed. On the other hand, the lack of SiO
suggests less dust and lower mass-loss rates in low-metallicity oxygen-rich
stars. The effect on the ISM dust enrichment is discussed.Comment: accepted for A&
The M33 Variable Star Population Revealed by Spitzer
We analyze five epochs of Spitzer Space Telescope/Infrared Array Camera
(IRAC) observations of the nearby spiral galaxy M33. Each epoch covered nearly
a square degree at 3.6, 4.5, and 8.0 microns. The point source catalog from the
full dataset contains 37,650 stars. The stars have luminosities characteristic
of the asymptotic giant branch and can be separated into oxygen-rich and
carbon-rich populations by their [3.6] - [4.5] colors. The [3.6] - [8.0] colors
indicate that over 80% of the stars detected at 8.0 microns have dust shells.
Photometric comparison of epochs using conservative criteria yields a catalog
of 2,923 variable stars. These variables are most likely long-period variables
amidst an evolved stellar population. At least one-third of the identified
carbon stars are variable.Comment: Accepted for publication in ApJ. See published article for full
resolution figures and electronic table
Ice chemistry in massive Young Stellar Objects: the role of metallicity
We present the comparison of the three most important ice constituents
(water, CO and CO2) in the envelopes of massive Young Stellar Objects (YSOs),
in environments of different metallicities: the Galaxy, the Large Magellanic
Cloud (LMC) and, for the first time, the Small Magellanic Cloud (SMC). We
present observations of water, CO and CO2 ice in 4 SMC and 3 LMC YSOs (obtained
with Spitzer-IRS and VLT/ISAAC). While water and CO2 ice are detected in all
Magellanic YSOs, CO ice is not detected in the SMC objects. Both CO and CO2 ice
abundances are enhanced in the LMC when compared to high-luminosity Galactic
YSOs. Based on the fact that both species appear to be enhanced in a consistent
way, this effect is unlikely to be the result of enhanced CO2 production in
hotter YSO envelopes as previously thought. Instead we propose that this
results from a reduced water column density in the envelopes of LMC YSOs, a
direct consequence of both the stronger UV radiation field and the reduced
dust-to-gas ratio at lower metallicity. In the SMC the environmental conditions
are harsher, and we observe a reduction in CO2 column density. Furthermore, the
low gas-phase CO density and higher dust temperature in YSO envelopes in the
SMC seem to inhibit CO freeze-out. The scenario we propose can be tested with
further observations.Comment: accepted by MNRAS Letters; 5 pages, 3 figures, 1 tabl
An alternative to diagrams for the critical O(N) model: dimensions and structure constants to order
We apply the methods of modern analytic bootstrap to the critical
model in a expansion. At infinite the model possesses higher spin
symmetry which is weakly broken as we turn on . By studying consistency
conditions for the correlator of four fundamental fields we derive the CFT-data
for all the (broken) currents to order , and the CFT-data for the
non-singlet currents to order . To order our results are in
perfect agreement with those in the literature. To order we reproduce
known results for anomalous dimensions and obtain a variety of new results for
structure constants, including the global symmetry central charge to this
order.Comment: 32 pages + appendices, 2 figures. v2: Improved presentation + new
appendix considering mixed correlator. Version to appear in JHE
The remnant of SN1987A revealed at (sub-)mm wavelengths
Context: Supernova 1987A (SN1987A) exploded in the Large Magellanic Cloud
(LMC). Its proximity and rapid evolution makes it a unique case study of the
early phases in the development of a supernova remnant. One particular aspect
of interest is the possible formation of dust in SN1987A, as SNe could
contribute significantly to the dust seen at high redshifts. Aims: We explore
the properties of SN1987A and its circumburst medium as seen at mm and sub-mm
wavelengths, bridging the gap between extant radio and infrared (IR)
observations of respectively the synchrotron and dust emission. Methods:
SN1987A was observed with the Australia Telescope Compact Array (ATCA) at 3.2
mm in July 2005, and with the Atacama Pathfinder EXperiment (APEX) at 0.87 mm
in May 2007. We present the images and brightness measurements of SN1987A at
these wavelengths for the first time. Results: SN1987A is detected as an
unresolved point source of 11.2 +/- 2.0 mJy at 3.2 mm (5" beam) and 21 +/- 4
mJy at 0.87 mm (18" beam). These flux densities are in perfect agreement with
extrapolations of the powerlaw radio spectrum and modified-blackbody dust
emission, respectively. This places limits on the presence of free-free
emission, which is similar to the expected free-free emission from the ionized
ejecta from SN1987A. Adjacent, fainter emission is observed at 0.87 mm
extending ~0.5' towards the south-west. This could be the impact of the
supernova progenitor's wind when it was still a red supergiant upon a dense
medium. Conclusions: We have established a continuous spectral energy
distribution for the emission from SN1987A and its immediate surroundings,
linking the IR and radio data. This places limits on the contribution from
ionized plasma. Our sub-mm image reveals complexity in the distribution of cold
dust surrounding SN1987A, but leaves room for freshly synthesized dust in the
SN ejecta.Comment: Accepted for publication in Astronomy and Astrophysics Letters on 28
April 2011. A better quality figure 1 can be had from
http://www.astro.keele.ac.uk/~jacco/research/SN1987A087mm.ep
Conservation in two-particle self-consistent extensions of dynamical-mean-field-theory
Extensions of dynamical-mean-field-theory (DMFT) make use of quantum impurity
models as non-perturbative and exactly solvable reference systems which are
essential to treat the strong electronic correlations. Through the introduction
of retarded interactions on the impurity, these approximations can be made
two-particle self-consistent. This is of interest for the Hubbard model,
because it allows to suppress the antiferromagnetic phase transition in
two-dimensions in accordance with the Mermin-Wagner theorem, and to include the
effects of bosonic fluctuations. For a physically sound description of the
latter, the approximation should be conserving. In this paper we show that the
mutual requirements of two-particle self-consistency and conservation lead to
fundamental problems. For an approximation that is two-particle self-consistent
in the charge- and longitudinal spin channel, the double occupancy of the
lattice and the impurity are no longer consistent when computed from
single-particle properties. For the case of self-consistency in the charge- and
longitudinal as well as transversal spin channels, these requirements are even
mutually exclusive so that no conserving approximation can exist. We illustrate
these findings for a two-particle self-consistent and conserving DMFT
approximation.Comment: 17 pages, 9 figure
Spitzer SAGE-SMC Infrared Photometry of Massive Stars in the Small Magellanic Cloud
We present a catalog of 5324 massive stars in the Small Magellanic Cloud
(SMC), with accurate spectral types compiled from the literature, and a
photometric catalog for a subset of 3654 of these stars, with the goal of
exploring their infrared properties. The photometric catalog consists of stars
with infrared counterparts in the Spitzer, SAGE-SMC survey database, for which
we present uniform photometry from 0.3-24 um in the UBVIJHKs+IRAC+MIPS24 bands.
We compare the color magnitude diagrams and color-color diagrams to those of
the Large Magellanic Cloud (LMC), finding that the brightest infrared sources
in the SMC are also the red supergiants, supergiant B[e] (sgB[e]) stars,
luminous blue variables, and Wolf-Rayet stars, with the latter exhibiting less
infrared excess, the red supergiants being less dusty and the sgB[e] stars
being on average less luminous. Among the objects detected at 24 um are a few
very luminous hypergiants, 4 B-type stars with peculiar, flat spectral energy
distributions, and all 3 known luminous blue variables. We detect a distinct Be
star sequence, displaced to the red, and suggest a novel method of confirming
Be star candidates photometrically. We find a higher fraction of Oe and Be
stars among O and early-B stars in the SMC, respectively, when compared to the
LMC, and that the SMC Be stars occur at higher luminosities. We estimate
mass-loss rates for the red supergiants, confirming the correlation with
luminosity even at the metallicity of the SMC. Finally, we confirm the new
class of stars displaying composite A & F type spectra, the sgB[e] nature of
2dFS1804 and find the F0 supergiant 2dFS3528 to be a candidate luminous blue
variable with cold dust.Comment: 23 pages, 17 figures, 5 tables, accepted for publication in the
Astronomical Journa
KRW-beoordeling van macrofauna in kust- en overgangswateren met BEQI
Voor de beoordeling van de toestand van macrofauna in de Nederlandse kust- en overgangswateren zijn maatlatten ontwikkeld, gebaseerd op de Benthic Ecosystem Quality Index (BEQI). Deze methode gaat uit van een ecosysteembenadering en maakt niet alleen gebruik van informatie over de macrofauna zelf maar ook over zijn leefgebieden en voedselbronnen. Zo is het mogelijk directe en indirecte effecten van door de mens veroorzaakte druk te signaleren. De methode overstijgt de beoordeling van individuele meetpunten en maakt het mogelijk voor een waterlichaam als geheel op een betrouwbare manier vast te stellen of de toestand van de macrofauna afwijkt van een gekozen referenti
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