4,863 research outputs found
Clarification of the Bootstrap Percolation Paradox
We study the onset of the bootstrap percolation transition as a model of
generalized dynamical arrest. We develop a new importance-sampling procedure in
simulation, based on rare events around "holes", that enables us to access
bootstrap lengths beyond those previously studied. By framing a new theory in
terms of paths or processes that lead to emptying of the lattice we are able to
develop systematic corrections to the existing theory, and compare them to
simulations. Thereby, for the first time in the literature, it is possible to
obtain credible comparisons between theory and simulation in the accessible
density range.Comment: 4 pages with 3 figure
Improving The Electroluminescence Of [zn(salophen)(oh2)] In Polyfluorene-based Light-emitting Diode: The Role Of Energy Transfer And Charge Recombination
Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)Conselho Nacional de Desenvolvimento CientÃfico e Tecnológico (CNPq)Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)Conselho Nacional de Desenvolvimento CientÃfico e Tecnológico (CNPq)Combining two or more different compounds with advantageous properties has been a useful and straightforward strategy in achieving a new class of materials with improved physical properties. This has been especially true for electronic polymers, whose optoelectronic properties can be completely tuned, and even improved, when mixed with other polymeric materials, dye molecules and guest coordination compounds. Here, a light-emitting diode prepared with the conjugated polymer poly[(9,9-dioctylfluorenyl-2,7-diyl)-alt-co-(9,9-di-{5'-pentanyl}-fluorenyl-2,7-diyl)] (PFOFPen) as the host material and aquo[N,N'-bis(salicylidene)-o-phenylenediamine] zinc(II) ([Zn(salophen)(OH2)]) as the guest molecule was studied in terms of its photo and electroluminescence properties. The role of the Zn-II coordination compound as a guest in the electroluminescence is discussed as a strategy for the improvement of the electroluminescence performance of coordination compounds using conjugated polymers as matrices. An additional advantage of these composites is that they are solution processable, a low-cost and time efficient alternative to vacuum vapor deposition. Additionally, the photophysical processes involved in both electroluminescence and photoluminescence emissions are discussed because they are markedly different.272295302FAPESP [2013/16245-2]CNPq [470529/2012-1]National Institute of Organic Electronics (INEO) (MCT/CNPq/FAPESP)UNICAMP/FAEPEXFundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)Conselho Nacional de Desenvolvimento CientÃfico e Tecnológico (CNPq)Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)Conselho Nacional de Desenvolvimento CientÃfico e Tecnológico (CNPq
The Molecular Hydrogen Deficit in Gamma-Ray Burst Afterglows
Recent analysis of five gamma-ray burst (GRB) afterglow spectra reveal the
absence of molecular hydrogen absorption lines, a surprising result in light of
their large neutral hydrogen column densities and the detection of H in
similar, more local star-forming regions like 30 Doradus in the Large
Magellanic Cloud (LMC). Observational evidence further indicates that the bulk
of the neutral hydrogen column in these sight lines lies 100 pc beyond the
progenitor and that H was absent prior to the burst, suggesting that direct
flux from the star, FUV background fields, or both suppressed its formation. We
present one-dimensional radiation hydrodynamical models of GRB host galaxy
environments, including self-consistent radiative transfer of both ionizing and
Lyman-Werner photons, nine-species primordial chemistry with dust formation of
H, and dust extinction of UV photons. We find that a single GRB progenitor
is sufficient to ionize neutral hydrogen to distances of 50 - 100 pc but that a
galactic Lyman-Werner background is required to dissociate the molecular
hydrogen in the ambient ISM. Intensities of 0.1 - 100 times the Galactic mean
are necessary to destroy H in the cloud, depending on its density and
metallicity. The minimum radii at which neutral hydrogen will be found in
afterglow spectra is insensitive to the mass of the progenitor or the initial
mass function (IMF) of its cluster, if present.Comment: 12 pages, 7 figures, accepted for Ap
A proto brown dwarf candidate in Taurus
Aims. We search for brown dwarfs at the Class 0/I evolutionary stage, or proto brown dwarfs.
Methods. We present a multi wavelength study, ranging from optical at 0.8 μm to radio wavelengths at 6 cm, of a cool, very faint, and red multiple object, SSTB213 J041757, detected by Spitzer toward the Barnard 213 dark cloud, in Taurus.
Results. The SED of SSTB213 J041757 displays a clear excess at long wavelengths resembling that of a Class I object. The mid-IR source has two possible counterparts, A and B, in the near-IR and optical images, and the 350 μm observations detect clear extended emission, presumably from an envelope around the two sources. The position of A & B in the (Ic− J) versus (J − [3.6]) colour-colour diagram is consistent with them being Galactic sources and not extragalactic contaminants. A proper-motion study confirms this result for A, while it is inconclusive for B. The temperature and mass of the two possible central objects, according to COND evolutionary models, range between 1550−1750 K and 3−4 M_(Jupiter), and 950−1300 K and 1−2 M_(Jupiter), for A and B, respectively. The integrated SED provides bolometric temperatures and luminosities of 280 K and 0.0034 L_⊙, assuming that the emission at wavelengths > 5 μm is associated with component A, and 150 K and 0.0033 L_⊙, assuming that the emission at wavelengths > 5 μm is associated with component B, which would imply the SSTB213 J041757 object has a luminosity well below the luminosity of other very low luminosity objects discovered up to date.
Conclusions. With these characteristics, SSTB213 J041757 seems to be a promising, and perhaps double, proto brown dwarf candidate
A submillimeter search for pre- and proto-brown dwarfs in Chamaeleon II
Context. Chamaeleon II molecular cloud is an active star forming region that
offers an excellent opportunity for studying the formation of brown dwarfs in
the southern hemisphere. Aims. Our aims are to identify a population of pre-
and proto- brown dwarfs (5 sigma mass limit threshold of ~0.015 Msun) and
provide information on the formation mechanisms of substellar objects. Methods.
We performed high sensitivity observations at 870 microns using the LABOCA
bolometer at the APEX telescope towards an active star forming region in
Chamaeleon II. The data are complemented with an extensive multiwavelength
catalogue of sources from the optical to the far-infrared to study the nature
of the LABOCA detections. Results. We detect fifteen cores at 870 microns, and
eleven of them show masses in the substellar regime. The most intense objects
in the surveyed field correspond to the submillimeter counterparts of the well
known young stellar objects DK Cha and IRAS 12500-7658. We identify a possible
proto-brown dwarf candidate (ChaII-APEX-L) with IRAC emission at 3.6 and 4.5
microns. Conclusions. Our analysis indicates that most of the spatially
resolved cores are transient, and that the point-like starless cores in the
sub-stellar regime (with masses between 0.016 Msun and 0.066 Msun) could be
pre-brown dwarfs cores gravitationally unstable if they have radii smaller than
220 AU to 907 AU (1.2" to 5" at 178 pc) respectively for different masses. ALMA
observations will be the key to reveal the energetic state of these pre-brown
dwarfs candidates.Comment: 11 pages, 6 figure
Silicon isotopic abundance toward evolved stars and its application for presolar grains
Galactic chemical evolution (GCE) is important for understanding the
composition of the present-day interstellar medium (ISM) and of our solar
system. In this paper, we aim to track the GCE by using the 29Si/30Si ratios in
evolved stars and tentatively relate this to presolar grain composition. We
used the APEX telescope to detect thermal SiO isotopologue emission toward four
oxygen-rich M-type stars. Together with the data retrieved from the Herschel
science archive and from the literature, we were able to obtain the 29Si/30Si
ratios for a total of 15 evolved stars inferred from their optically thin 29SiO
and 30SiO emission. These stars cover a range of masses and ages, and because
they do not significantly alter 29Si/30Si during their lifetimes, they provide
excellent probes of the ISM metallicity (or 29Si/30Si ratio) as a function of
time. The 29Si/30Si ratios inferred from the thermal SiO emission tend to be
lower toward low-mass oxygen-rich stars (e.g., down to about unity for W Hya),
and close to an interstellar or solar value of 1.5 for the higher-mass carbon
star IRC+10216 and two red supergiants. There is a tentative correlation
between the 29Si/30Si ratios and the mass-loss rates of evolved stars, where we
take the mass-loss rate as a proxy for the initial stellar mass or current
stellar age. This is consistent with the different abundance ratios found in
presolar grains. We found that older objects (up to possibly 10 Gyr old) in our
sample trace a previous, lower 29Si/30Si value of about 1. Material with this
isotopic ratio is present in two subclasses of presolar grains, providing
independent evidence of the lower ratio. Therefore, the 29Si/30Si ratio derived
from the SiO emission of evolved stars is a useful diagnostic tool for the
study of the GCE and presolar grains.Comment: 7 pages, 4 figure
His bundle pacing guided by automated intrinsic morphology matching is feasible in patients with narrow QRS complexes
Pace mapping and visual comparison of the local pacing response with the intrinsic QRS morphology form the mainstay of His bundle pacing (HBP). We evaluated the performance of a surface lead morphology match algorithm for automated classification of the pacing response in patients with narrow intrinsic QRS undergoing electroanatomic mapping (EAM)-guided HBP. HBP was attempted in 43 patients. In 28 cases with narrow QRS, the EnSite AutoMap Module was used for automated assessment of the QRS morphology resulting from pace mapping in the His cloud area with either a diagnostic catheter or the His lead. An intrinsic morphology match score (IMS) was calculated for 1.546 QRS complexes and assessed regarding its accuracy and performance in classifying the individual pacing response as either selective HBP (S-HBP), nonselective HBP (NS-HBP) or right ventricular stimulation. Automated morphology comparison of 354 intrinsic beats with the individual reference determined a test accuracy of 99% (95% CI 98.96–99.04) and a precision of 97.99–99.5%. For His-lead stimulation, an IMS ≥ 89% identified S-HBP with a sensitivity, specificity and positive predictive value of 1.00 (0.99, 1.00) and a negative predictive value of 0.99 (0.98, 1.00). An IMS between 78 and < 89% indicated NS-HBP with a sensitivity and specificity of 1.00 (0.99, 1.00) and 0.99 (0.98, 1.00), respectively. IMS represents a new automated measure for standardized individual morphology classification in patients with normal QRS undergoing EAM-guided HBP. Clinical trial registration: NCT04416958
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