12,441 research outputs found
Modeling the magnetic field in the protostellar source NGC 1333 IRAS 4A
Magnetic fields are believed to play a crucial role in the process of star
formation. We compare high-angular resolution observations of the submillimeter
polarized emission of NGC 1333 IRAS 4A, tracing the magnetic field around a
low-mass protostar, with models of the collapse of magnetized molecular cloud
cores. Assuming a uniform dust alignment efficiency, we computed the Stokes
parameters and synthetic polarization maps from the model density and magnetic
field distribution by integrations along the line-of-sight and convolution with
the interferometric response. The synthetic maps are in good agreement with the
data. The best-fitting models were obtained for a protostellar mass of 0.8
solar masses, of age 9e4 yr, formed in a cloud with an initial mass-to-flux
ratio ~2 times the critical value. The magnetic field morphology in NGC 1333
IRAS 4A is consistent with the standard theoretical scenario for the formation
of solar-type stars, where well-ordered, large-scale, rather than turbulent,
magnetic fields control the evolution and collapse of the molecular cloud cores
from which stars form.Comment: 4 pages, 5 figures. Accepted by Astronomy and Astrophysic
The structure and stability of molecular cloud cores in external radiation fields
We have considered the thermal equilibrium in pre-protostellar cores in the
approximation where the dust temperature is independent of interactions with
the gas and where the gas is heated both by collisions with dust grains and
ionization by cosmic rays. We have then used these results to study the
stability of cores in the limit where thermal pressure dominates over magnetic
field and turbulence. We find that for cores with characteristics similar to
those observed, the gas and dust temperatures are coupled in the core interior.
As a consequence, the gas temperature like the dust temperature decreases
towards the center of these objects. The density structure computed taking into
account such deviations from isothermality are not greatly different from that
expected for an isothermal Bonnor-Ebert sphere. It is impossible in the
framework of these models to have a stable equilibrium core with mass above
about 5 solar masses and column density compatible with observed values. We
conclude from this that observed high mass cores are either supported by
magnetic field or turbulence or are already in a state of collapse. Lower mass
cores on the other hand have stable states and we conclude that the much
studied object B68 may be in a state of stable equilibrium if the internal gas
temperature is computed in self-consistent fashion. Finally we note that in
molecular clouds such as Ophiuchus and Orion with high radiation fields and
pressures, gas and dust temperatures are expected to be well coupled and hence
one expects temperatures to be relatively high as compared to low pressure
clouds like Taurus.Comment: 11 pages, 6 figures. Astronomy & Astrophysics, in pres
Low noise tunnel diode receivers for satellite application
Low noise tunnel diode receivers for satellite application
Dielectric Response of Periodic Systems from Quantum Monte Carlo Calculations
We present a novel approach that allows to calculate the dielectric response
of periodic systems in the quantum Monte Carlo formalism. We employ a many-body
generalization for the electric enthalpy functional, where the coupling with
the field is expressed via the Berry-phase formulation for the macroscopic
polarization. A self-consistent local Hamiltonian then determines the
ground-state wavefunction, allowing for accurate diffusion quantum Monte Carlo
calculations where the polarization's fixed point is estimated from the average
on an iterative sequence, sampled via forward-walking. This approach has been
validated for the case of an isolated hydrogen atom, and then applied to a
periodic system, to calculate the dielectric susceptibility of
molecular-hydrogen chains. The results found are in excellent agreement with
the best estimates obtained from the extrapolation of quantum-chemistry
calculations.Comment: 5 page 2figure
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Future Needs in Mast Cell Biology.
The pathophysiological roles of mast cells are still not fully understood, over 140 years since their description by Paul Ehrlich in 1878. Initial studies have attempted to identify distinct "subpopulations" of mast cells based on a relatively small number of biochemical characteristics. More recently, "subtypes" of mast cells have been described based on the analysis of transcriptomes of anatomically distinct mouse mast cell populations. Although mast cells can potently alter homeostasis, in certain circumstances, these cells can also contribute to the restoration of homeostasis. Both solid and hematologic tumors are associated with the accumulation of peritumoral and/or intratumoral mast cells, suggesting that these cells can help to promote and/or limit tumorigenesis. We suggest that at least two major subsets of mast cells, MC1 (meaning anti-tumorigenic) and MC2 (meaning pro-tumorigenic), and/or different mast cell mediators derived from otherwise similar cells, could play distinct or even opposite roles in tumorigenesis. Mast cells are also strategically located in the human myocardium, in atherosclerotic plaques, in close proximity to nerves and in the aortic valve. Recent studies have revealed evidence that cardiac mast cells can participate both in physiological and pathological processes in the heart. It seems likely that different subsets of mast cells, like those of cardiac macrophages, can exert distinct, even opposite, effects in different pathophysiological processes in the heart. In this chapter, we have commented on possible future needs of the ongoing efforts to identify the diverse functions of mast cells in health and disease
Cellular xenotransplantation of animal cells into people: benefits and risk
The main benefit of xenotransplantation is its potential to overcome the worldwide organ shortage experienced in allotransplantation. Allogeneic transplantation is the only successful therapy for several life-threatening diseases, with cell, tissue or organ donation only partially meeting the demand and many patients dying while waiting for treatment. With supply falling short of demand, it is foreseen that the use of porcine material may at some stage overcome the existing gap between organ availability and clinical need. Recently, pig islet cells have been utilised in clinical trials, with safety being demonstrated. Indeed, pig-derived cells present several advantages: i) porcine cells have a stable function and differentiation pattern and are not tumorigenic; ii) pig cells have been shown to meet the physiological needs in large animal models; iii) the source of pig cells can be scaled up to meet demands in a highly standardised manner, and with respect to animal welfare regulations; iv) ‘designated-pathogen-free’ (DPF) pig lines can be produced, which could result in a higher safety profile than allotransplantation itself; v) the risk of zoonosis, which was raised years ago as the major hurdle, has been recently circumvented and is actually viewed as a controlled risk; and vi) immune risks are being circumvented via the use of genetically modified donor animals and encapsulation of porcine cells, particularly for the treatment of diabetes. Overall, the benefit appears to outweigh potential risks with respect to cellular xenotransplantation and this is discussed further in this review
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