1,637 research outputs found
Radiation Pressure Feedback in Galaxies
We evaluate radiation pressure from starlight on dust as a feedback mechanism
in star-forming galaxies by comparing the luminosity and flux of star-forming
systems to the dust Eddington limit. The linear LFIR--L'HCN correlation
provides evidence that galaxies may be regulated by radiation pressure
feedback. We show that star-forming galaxies approach but do not dramatically
exceed Eddington, but many systems are significantly below Eddington, perhaps
due to the "intermittency" of star formation. Better constraints on the
dust-to-gas ratio and the CO- and HCN-to-H2 conversion factors are needed to
make a definitive assessment of radiation pressure as a feedback mechanism.Comment: To appear in "Conditions and impact of star formation: New results
with Herschel and beyond", Proceedings of the 5th Zermatt ISM symposium. 2
pages, 2 figure
Origin of G Magnetic Fields in the Central Engine of Gamma Ray Bursts
Various authors have suggested that the gamma-ray burst (GRB) central engine
is a rapidly rotating, strongly magnetized, G) compact
object. The strong magnetic field can accelerate and collimate the relativistic
flow and the rotation of the compact object can be the energy source of the
GRB. The major problem in this scenario is the difficulty of finding an
astrophysical mechanism for obtaining such intense fields. Whereas, in
principle, a neutron star could maintain such strong fields, it is difficult to
justify a scenario for their creation. If the compact object is a black hole,
the problem is more difficult since, according to general relativity it has "no
hair" (i.e., no magnetic field). Schuster, Blackett, Pauli, and others have
suggested that a rotating neutral body can create a magnetic field by
non-minimal gravitational-electromagnetic coupling (NMGEC). The
Schuster-Blackett form of NMGEC was obtained from the Mikhail and Wanas's
tetrad theory of gravitation (MW). We call the general theory NMGEC-MW.
We investigate here the possible origin of the intense magnetic fields G in GRBs by NMGEC-MW. Whereas these fields are difficult to
explain astrophysically, we find that they are easily explained by NMGEC-MW. It
not only explains the origin of the G fields when the
compact object is a neutron star, but also when it is a black hole.Comment: 9 pages, accepted for publication in JCA
How to SYN in seven easy steps
The calculation of expected spectral line strengths and profiles is a
powerful tool for the analysis of the solar atmosphere, and other stellar
atmospheres. We present here a recipe in seven easy steps for the development
of such spectral synthesis software.Comment: 4 pages, 1 figure, 1 tabl
Pattern formation in crystal growth under parabolic shear flow
Morphological instability of the solid-liquid interface occuring in a crystal
growing from an undercooled thin liquid being bounded on one side by a free
surface and flowing down inclined plane is investigated by a linear stability
analysis under shear flow. It is found that restoring forces due to gravity and
surface tension is important factor for stabilization of the solid-liquid
interface on long length scales. This is a new stabilizing effect different
from the Gibbs-Thomson effect. A particular long wavelength mode of about 1 cm
of wavy pattern observed on the surface of icicles covered with thin layer of
flowing water is obtained from the dispersion relation including the effect of
flow and restoring forces.Comment: 30 pages, 4 figure
Relativistic MHD Winds from Rotating Neutron Stars
We solve for the time-dependent dynamics of axisymmetric, general
relativistic magnetohydrodynamic winds from rotating neutron stars. The mass
loss rate is obtained self-consistently as a solution to the MHD equations,
subject to a finite thermal pressure at the stellar surface. We consider both
monopole and dipole magnetic field geometries and we explore the parameter
regime extending from low magnetization (low-sigma_o), almost thermally-driven
winds to high magnetization (high-sigma_o), relativistic Poynting-flux
dominated outflows. We compute the angular momentum and rotational energy loss
rates as a function of sigma_o and compare with analytic expectations from the
classical theory of pulsars and magnetized stellar winds. In the case of the
monopole, our high-sigma_o calculations asymptotically approach the analytic
force-free limit. If we define the spindown rate in terms of the open magnetic
flux, we similarly reproduce the spindown rate from recent force-free
calculations of the aligned dipole. However, even for sigma_o as high as ~20,
we find that the location of the Y-type point (r_Y), which specifies the radius
of the last closed field line in the equatorial plane, is not the radius of the
light cylinder R_L = c/omega (R = cylindrical radius), as has previously been
assumed in most estimates and force-free calculations. Instead, although the
Alfven radius at intermediate latitudes quickly approaches R_L as sigma_o
exceeds unity, r_Y remains significantly less than R_L. Because r_Y < R_L, our
calculated spindown rates thus exceed the classic ``vacuum dipole'' rate. We
discussthe implications of our results for models of rotation-powered pulsars
and magnetars, both in their observed states and in their hypothesized rapidly
rotating initial state.Comment: Accepted for publication in MNRAS. 19 pages, 21 figure
ESC NN-Potentials in Momentum Space. I. PS-PS Exchange Potentials
A momentum space representation is derived for the Nijmegen
Extended-Soft-Core (ESC) interactions. The partial wave projection of this
representation is carried through, in principle for Two-Meson-Exchange (TME) in
general. Explicit results for the momentum space partial wave NN-potentials
from PS-PS-Exchange are given.Comment: 23 pages, 2 PostScript figures, revtex
Improved Approximate String Matching and Regular Expression Matching on Ziv-Lempel Compressed Texts
We study the approximate string matching and regular expression matching
problem for the case when the text to be searched is compressed with the
Ziv-Lempel adaptive dictionary compression schemes. We present a time-space
trade-off that leads to algorithms improving the previously known complexities
for both problems. In particular, we significantly improve the space bounds,
which in practical applications are likely to be a bottleneck
Bleaching cotton in textile conservation: a closer look using atomic force microscopy
Aqueous bleaching may be used in textile conservation to improve the appearance of historic and culturally significant textiles. It is generally accepted amongst conservators that bleaching imparts damage. The aim of this research is to characterise the condition of cotton fibre's surface pre- and post-bleaching using atomic force microscopy (AFM). Unprocessed cotton calico (ârawâ cotton), scoured cotton, and a historic cotton dress shirt (circa. 1920) were bleached using three separate methods: NaBH4 for 15 min; H2O2/NaBO3 for 1 h; and H2O2/NaBO3 buffered to pH 8.4 for 1 h. AFM was used in tapping-mode to obtain height, amplitude, and phase images. AFM imaging was able to distinguish between the cuticle, primary walls, and secondary walls of the cotton fibres. The data shows that bleaching has the effect of softening and removing individual layers of the cotton structure. Unprocessed cotton calico and scoured cotton fared better against the impact of bleaching. This was in stark contrast to the historic shirt where the already damaged surface of cotton fibres underwent further degradation using both oxidative and reductive bleaching. In general, reductive bleaching was more aggressive on the fibre surface compared to oxidative bleaching. The use of AFM provides further evidence of the physical effects of bleaching on historic textiles, and cotton textiles more broadly, and it has the potential to influence the conservatorâs decision-making
Reconstructing the 3-D Trajectories of CMEs in the Inner Heliosphere
A method for the full three-dimensional (3-D) reconstruction of the
trajectories of coronal mass ejections (CMEs) using Solar TErrestrial RElations
Observatory (STEREO) data is presented. Four CMEs that were simultaneously
observed by the inner and outer coronagraphs (COR1 and 2) of the Ahead and
Behind STEREO satellites were analysed. These observations were used to derive
CME trajectories in 3-D out to ~15Rsun. The reconstructions using COR1/2 data
support a radial propagation model. Assuming pseudo-radial propagation at large
distances from the Sun (15-240Rsun), the CME positions were extrapolated into
the Heliospheric Imager (HI) field-of-view. We estimated the CME velocities in
the different fields-of-view. It was found that CMEs slower than the solar wind
were accelerated, while CMEs faster than the solar wind were decelerated, with
both tending to the solar wind velocity.Comment: 17 pages, 10 figures, 1 appendi
Nuclear Effects on Bremsstrahlung Neutrino Rates of Astrophysical Interest
We calculate in this work the rates for the neutrino pair production by
nucleon-nucleon bremsstrahlung taking into account the full contribution from a
nuclear one-pion-exchange potential. It is shown that if the temperatures are
low enough (), the integration over the nuclear part can be done
for the general case, ranging from the completely degenerate (D) to the
non-degenerate (ND) regime. We find that the inclusion of the full nuclear
contribution enhances the neutrino pair production by and
bremsstrahlung by a factor of about two in both the D and ND limits when
compared with previous calculations. This result may be relevant for the
physical conditions of interest in the semitransparent regions near the
neutrinosphere in type II supernovae, cooling of neutron stars and other
astrophysical situations.Comment: 11 pages, no figures, LaTex file. submitted to PR
- âŠ