4,031 research outputs found
Coupled currents in cosmic strings
We first examine the microstructure of a cosmic string endowed with two
simple Abelian currents. This microstructure depends on two state parameters.
We then provide the macroscopic description of such a string and show that it
depends on an additional Lorentz-invariant state parameter that relates the two
currents. We find that in most of the parameter space, the two-current string
is essentially equivalent to the single current-carrying string, i.e., only one
field condenses onto the defect. In the regions where two currents are present,
we find that as far as stability is concerned, one can approximate the dynamics
with good accuracy using an analytic model based on either a logarithmic (on
the electric side, i.e., for timelike currents) or a rational (on the magnetic
side, i.e., for spacelike currents) worldsheet Lagrangian.Comment: 25 pages, 9 figure
An Effective Temperature Scale for Late M and L Dwarfs, from Resonance Absorption Lines of CsI and RbI
We present Keck HIRES spectra of 6 late-M dwarfs and 11 L dwarfs. Our goal is
to assign effective temperatures to the objects using detailed atmospheric
models and fine analysis of the alkali resonance absorption lines of CsI and
RbI. These yield mutually consistent results (+-150 K) when we use
``cleared-dust'' models, which account for the removal of refractory species
from the molecular states but do not include dust opacities. We find a tendency
for the RbI line to imply a slightly higher temperature, which we ascribe to an
incomplete treatment of the overlying molecular opacities. The final effective
temperatures we adopt are based on the CsI fits alone, though the RbI fits
support the CsI temperature sequence. This work, in combination with results
from the infrared, hints that dust in these atmospheres has settled out of the
high atmosphere but is present in the deep photosphere. We also derive radial
and rotational velocities for all the objects, finding that the previously
discovered trend of rapid rotation for very low mass objects is quite
pervasive. To improve on our analysis, there is a clear need for better
molecular line lists and a more detailed understanding of dust formation and
dynamics.Comment: 53 pages, including 20 figures and 2 Tables; accepted in Ap
Multiplicity of Nearby Free-floating Ultra-cool Dwarfs: a HST-WFPC2 search for companions
We present HST/WFPC2 observations of a sample of 134 ultra-cool objects
(spectral types later than M7) coming from the DENIS, 2MASS and SDSS surveys,
with distances estimated to range from 7 pc to 105 pc. Fifteen new ultra-cool
binary candidates are reported here. Eleven known binaries are confirmed and
orbital motion is detected in some of them. We estimate that the closest binary
systems in this sample have periods between 5 and 20 years, and thus dynamical
masses will be derived in the near future. For the calculation of binary
frequency we restrict ourselves to systems with distances less than 20 pc.
After correction of the binaries bias, we find a ratio of visual binaries (at
the HST limit of detection) of around 10%, and that ~15% of the 26 objects
within 20 parsecs are binary systems with separations between 1 and 8 A.U. The
observed frequency of ultra-cool binaries is similar than that of binaries with
G-type primaries in the separation range from 2.1 A.U. to 140 A.U. There is
also a clear deficit of ultra-cool binaries with separations greater than 15
A.U., and a possible tendency for the binaries to have mass ratios near unity.
Most systems have indeed visual and near-infrared brightness ratios between 1
and 0.3. We discuss our results in the framework of current scenarios for the
formation and evolution of free-floating brown dwarfs.Comment: 67 pages, 14 figures, Accepted for publication in AJ, September 2003.
First submission to AJ: august 2002, 5 submission
Shadow of a Colossus: A z=2.45 Galaxy Protocluster Detected in 3D Ly-a Forest Tomographic Mapping of the COSMOS Field
Using moderate-resolution optical spectra from 58 background Lyman-break
galaxies and quasars at within a area of the
COSMOS field ( projected area density or mean transverse separation), we reconstruct a 3D
tomographic map of the foreground Ly forest absorption at
with an effective smoothing scale of
comoving. Comparing with 61
coeval galaxies with spectroscopic redshifts in the same volume, we find that
the galaxy positions are clearly biased towards regions with enhanced IGM
absorption in the tomographic map. We find an extended IGM overdensity with
deep absorption troughs at associated with a recently-discovered
galaxy protocluster at the same redshift. Based on simulations matched to our
data, we estimate the enclosed dark matter mass within this IGM overdensity to
be , and
argue based on this mass and absorption strength that it will form at least one
galaxy cluster with , although its elongated nature suggests that
it will likely collapse into two separate clusters. We also point out a compact
overdensity of six MOSDEF galaxies at within a radius and , which does not appear
to have a large associated IGM overdensity. These results demonstrate the
potential of Ly forest tomography on larger volumes to study galaxy
properties as a function of environment, as well as revealing the large-scale
IGM overdensities associated with protoclusters and other features of
large-scale structure.Comment: To be submitted to ApJ. Figure 3 can be viewed on Youtube:
https://youtu.be/KeW1UJOPMY
High Energy Particles from Monopoles Connected by Strings
Monopole-antimonopole pairs connected by strings and monopole-string networks
with strings attached to each monopole can be formed at phase transitions
in the early universe. In such hybrid defects, monopoles accelerate under the
string tension and can reach ultrarelativistic Lorentz factors, .
We study the radiation of gauge quanta by accelerating monopoles. For monopoles
with a chromomagnetic charge, we also discuss the high-energy hadron production
through emission of virtual gluons and their subsequent fragmentation into
hadrons. The relevant parameter for gauge boson radiation is , where
is the boson mass and is the proper acceleration of the monopole. For , the gauge bosons can be considered as massless and the typical energy of
the emitted quanta is . In the opposite limit, , the
radiation power is exponentially suppressed and gauge quanta are emitted with a
typical energy in a narrow range .
Cosmological monopole-string networks can produce photons and hadrons of
extremely high energies. For a wide range of parameters these energies can be
much greater than the Planck scale.Comment: 28 pages, ReVTex, 5 postscript figures. Minor changes, some
references added. Submitted to Phys. Rev.
Spectroscopic identification of DENIS-selected brown dwarf candidates in the Upper Scorpius OB association
We present low-resolution (R=900) optical (576.1--1,051.1 nm) spectroscopic
observations of 40 candidate very low-mass members in the Upper Scorpius OB
association. These objects were selected using the , and photometry
available in the DENIS database. We have derived spectral types and we have
measured H and NaI doublet (at 818.3 and 819.5 nm) equivalent widths.
We assess the youth of the objects by comparing them to their older
counterparts of similar spectral type in the Pleiades cluster and the field.
Our analysis indicates that 28 of our targets are young very low-mass objects,
and thus they are strong candidate members of the OB association. The other 12
DENIS sources are foreground M dwarfs or background red giants. Our sample of
spectroscopic candidate members includes 18 objects with spectral types in the
range M6.5 and M9, which are likely young brown dwarfs. We classify these
candidates as accreting/non accreting using the scheme proposed by Barrado y
Navascu\'es & Mart\'\i n (2003). We find 5 substellar-mass candidate cluster
members that are still undergoing mass accretion, indicating that the timescale
for accretion onto brown dwarfs can be as long as 5 Myr in some cases.Comment: Accepted for publication in The Astronomical Journal, January 200
Small Fermi surface in the one-dimensional Kondo lattice model
We study the one-dimensional Kondo lattice model through the density matrix
renormalization group (DMRG). Our results for the spin correlation function
indicate the presence of a small Fermi surface in large portions of the phase
diagram, in contrast to some previous studies that used the same technique. We
argue that the discrepancy is due to the open boundary conditions, which
introduce strong charge perturbations that strongly affect the spin Friedel
oscillations.Comment: 5 pages, 7 figure
Dynamical Stability of Witten Rings
The dynamical stability of cosmic rings, or vortons, is investigated for the
particular equation of state given by the Witten bosonic model. It is found
that there exists a finite range of the state parameter for which the vorton
states are actually stable against dynamical perturbations. Inclusion of the
electromagnetic self action into the equation of state slightly shrinks the
stability region but otherwise yields no qualitative difference. If the Witten
bosonic model represents a good approximation for more realistic string models,
then the cosmological vorton excess problem can only be solved by assuming
either that strings are formed at low energy scales or that some quantum
instability may develop at a sufficient rate.Comment: 11 pages, LaTeX-ReVTeX (v.3), 2 figures available upon request, DAMTP
R-94/1
Variation of PM2.5 concentrations in relation to street washing activities
Several studies conducted in urban areas have pointed out that road dust resuspension contributes significantly to PM concentration levels. Street washing is one of the methods proposed to reduce resuspended road dust contributions to ambient PM concentrations. As resuspended particles are mainly found in the coarse mode, published studies investigating the effects of street washing have focused on PM10 size fraction. As the PM2.5 mass fraction of particles originating from mechanical abrasion processes may still be significant we conducted a study in order to evaluate the effects of street washing on the mitigation of resuspension of fine particles. The PM2.5 mass concentration data were examined and integrated with the occurrence of street washing activities. In addition, the effect of the meteorological variability, traffic flow and street washing activities, on ambient PM2.5 levels was valuated by means of a multivariate regression model. The results revealed that traffic low is the most important factor that controls PM2.5 hourly concentrations while street washing activities did not influence fine particle mass levels
Channel-Hot-Carrier degradation of strained MOSFETs : A device level and nanoscale combined approach
Strained MOSFETs with SiGe at the source/drain regions and different channel lengths have been studied at the nanoscale with a conductive atomic force microscope (CAFM) and at device level, before and after channel-hot-carrier (CHC) stress. The results show that although strained devices have a larger mobility, they are more sensitive to CHC stress. This effect has been observed to be larger in short channel devices. The higher susceptibility of strained MOSFETs to the stress has been related to a larger density of defects close to the diffusions, as suggested by CAFM data
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