1,992 research outputs found
Deep Spectroscopy in the Field of 3C 212
We present the results of longslit and multiaperture spectroscopy of faint
galaxies in the field of the z=1.049 quasar 3C212. We show that an apparently
aligned optical feature beyond the NW radio lobe has a redshift z=0.928, and a
similarly aligned feature just beyond the SE radio lobe has a redshift z=1.053,
quite close to that of the quasar. While the NW optical component is extremely
well aligned with the radio jet and has a morphology that is very similar to
that of the radio lobe lying 3" interior to it, the fact that we find three
other field galaxies with closely similar redshifts indicates that it is most
likely a member of an intervening group rather than an unusual example of true
alignment with the radio structure. In addition, we have found two galaxies
(besides the one near the SE radio lobe) having redshifts close to that of
3C212. We have firm or probable redshifts for 66 out of 82 galaxies we have
observed in this field. Comparison with the redshift distribution of faint
field galaxies indicates that a large fraction of the remaining 16 galaxies are
likely to be at redshifts >1.4. There are at least two low-redshift dwarf
starburst galaxies showing low metal abundance and high ionization in our
sample; such galaxies are likely to be relatively common in very faint samples,
and, under certain conditions, they could be confused with high-redshift
objects.Comment: 15 pages, 4 figures, one of which is a separate JPEG plate. To be
published in the April 1998 issue of The Astronomical Journa
Ferromagnetic GaâËâ Mnâ As produced by ion implantation and pulsed-laser melting
We demonstrate the formation of ferromagneticGaâËâMnâAsfilms by Mn ion implantation into GaAs followed by pulsed-laser melting. Irradiation with a single excimer laser pulse results in the epitaxial regrowth of the implanted layer with Mn substitutional fraction up to 80% and effective Curie temperature up to 29 K for samples with a maximum Mn concentration of xâ0.03. A remanent magnetization persisting above 85 K has been observed for samples with xâ0.10, in which 40% of the Mn resides on substitutional lattice sites. We find that the ferromagnetism in GaâËâMnâAs is rather robust to the presence of structural defects.The work at the Lawrence Berkeley National Laboratory
was supported by the Director, Office of Science, Office of
Basic Energy Sciences, Division of Materials Sciences and
Engineering, of the U.S. Department of Energy under Contract
No. DE-AC03-76SF00098. The work at Harvard was
supported by NASA Grant No. NAG8-1680. One of
the authors ~M.A.S.! acknowledges support from an NSF
Graduate Research Fellowship
Nanoscale density fluctuations in swift heavy ion irradiated amorphous SiO2
We report on the observation of nanoscale density fluctuations in 2 Îźm thick amorphous SiOâ layers irradiated with 185 MeV Au ions. At high fluences, in excess of approximately 5âĂâ10š² ions/cm², where the surface is completely covered by ion tracks, synchrotron small angle x-ray scattering measurements reveal the existence of a steady state of density fluctuations. In agreement with molecular dynamics simulations, this steady state is consistent with an ion track âannihilationâ process, where high-density regions generated in the periphery of new tracks fill in low-density regions located at the center of existing tracks.The authors acknowledge the Australian Research
Council and the Australian Synchrotron Research Program
for financial support and thank the staff at the ANU Heavy
Ion facility for their continued technical assistance. O.P.,
F.D., and K.N. acknowledge financial support from the
Academy of Finland under its Centre of Excellence program
as well as the OPNA project, and grants of computer
capacity from CSC
Formation and structural characterization of Ni nanoparticles embedded in SiOâ
Face-centered cubic Ni nanoparticles were formed in SiOâ by ion implantation and thermal annealing. Small-angle x-ray scattering in conjunction with transmission electron microscopy was used to determine the nanoparticle size as a function of annealing temperature, whereas the local atomic structure was measured with x-ray absorption spectroscopy. The influence of finite-size effects on the nanoparticle structural properties was readily apparent and included a decrease in coordination number and bond length and an increase in structural disorder for decreasing nanoparticle size. Such results are consistent with the non-negligible surface-to-volume ratio characteristic of nanoparticles. In addition, temperature-dependent x-ray absorption spectroscopy measurements showed the mean vibrational frequency (as obtained from the Einstein temperature) decreased with decreasing nanoparticle size. This reduction was attributed to the greater influence of the loosely bound, under-coordinated surface atoms prevailing over the effects of capillary pressure, the former enhancing the low frequency modes of the vibrational density of statesThis work was financially supported by the Australian
Synchrotron and the Australian Research Council with access to equipment provided by the Australian Nanofabrication
Facility
First Keck Nulling Observations of a Young Stellar Object: Probing the Circumstellar Environment of the Herbig Ae star MWC 325
We present the first N-band nulling plus K- and L-band V2 observations of a
young stellar object, MWC325, taken with the 85 m baseline Keck Interferometer.
The Keck nuller was designed for the study of faint dust signatures associated
with debris disks, but it also has a unique capability for studying the
temperature and density distribution of denser disks found around young stellar
objects. Interferometric observations of MWC 325 at K, L and N encompass a
factor of five in spectral range and thus, especially when spectrally dispersed
within each band, enable characterization of the structure of the inner disk
regions where planets form. Fitting our observations with geometric models such
as a uniform disk or a Gaussian disk show that the apparent size increases
monotonically with wavelength in the 2-12 um wavelength region, confirming the
widely held assumption based on radiative transfer models, now with spatially
resolved measurements over broad wavelength range, that disks are extended with
a temperature gradient. The effective size is a factor of about 1.3 and 2
larger in the L-band and N-band, respectively, compared to that in the K-band.
The existing interferometric measurements and the spectral energy distribution
can be reproduced by a flat disk or a weakly-shadowed nearly flat-disk model,
with only slight flaring in the outer regions of the disk, consisting of
representative "sub-micron" (0.1 um) and "micron" (2 um) grains of a 50:50
ratio of silicate and graphite. This is marked contrast with the disks
previously found in other Herbig Ae/Be stars suggesting a wide variety in the
disk properties among Herbig Ae/Be stars.Comment: Accepted for publication in the Ap
Nano-porosity in GaSb induced by swift heavy ion irradiation
Nano-porous structures form in GaSb after ion irradiation with 185 MeV Au ions. The porous layer formation is governed by the dominant electronic energy loss at this energy regime. The porous layer morphology differs significantly from that previously reported for low-energy, ion-irradiated GaSb. Prior to the onset of porosity, positron annihilation lifetime spectroscopy indicates the formation of small vacancy clusters in single ion impacts, while transmission electron microscopy reveals fragmentation of the GaSb into nanocrystallites embedded in an amorphous matrix. Following this fragmentation process, macroscopic porosity forms, presumably within the amorphous phase.The authors thank the Australian Research Council for
support and the staff at the ANU Heavy Ion Accelerator
Facility for their continued technical assistance. R.C.E. acknowledges the support
from the Office of Basic Energy Sciences of the U.S. DOE
(Grant No. DE-FG02-97ER45656)
Static Black Hole Solutions without Rotational Symmetry
We construct static black hole solutions that have no rotational symmetry.
These arise in theories, including the standard electroweak model, that include
charged vector mesons with mass . In such theories, a magnetically
charged Reissner-Nordstrom black hole with horizon radius less than a critical
value of the order of is classically unstable against the development
of a nonzero vector meson field just outside the horizon, indicating the
existence of static black hole solutions with vector meson hair. For the case
of unit magnetic charge, spherically symmetric solutions of this type have
previously been studied. For other values of the magnetic charge, general
arguments show that any new solution with hair cannot be spherically symmetric.
In this paper we develop and apply a perturbative scheme (which may have
applicability in other contexts) for constructing such solutions in the case
where the Reissner-Nordstrom solution is just barely unstable. For a few low
values of the magnetic charge the black holes retain a rotational symmetry
about a single axis, but this axial symmetry disappears for higher charges.
While the vector meson fields vanish exponentially fast at distances greater
than , the magnetic field and the metric have higher multipole
components that decrease only as powers of the distance from the black hole.Comment: 42 pages, phyzzx. 4 figures (PostScript, 1.7 MB when uncompressed)
available by email from the Authors on reques
Accretion Signatures from Massive Young Stellar Objects
High resolution (lambda / Delta-lambda = 50,000) K-band spectra of massive,
embedded, young stellar objects are presented. The present sample consists of
four massive young stars located in nascent clusters powering Galactic giant H
II regions. Emission in the 2.3 micron 2--0 vibrational--rotational bandhead of
CO is observed. A range of velocity broadened profiles seen in three of the
objects is consistent with the emission arising from a circumstellar disk seen
at various inclination angles. Br gamma spectra of the same spectral and
spatial resolution are also presented which support an accretion disk or torus
model for massive stars. In the fourth object, Br emission suggesting a
rotating torus is observed, but the CO profile is narrow, indicating that there
may be different CO emission mechanisms in massive stars and this is consistent
with earlier observations of the BN object and MWC 349. To--date, only young
massive stars of late O or early B types have been identified with clear
accretion disk signatures in such embedded clusters. Often such stars are found
in the presence of other more massive stars which are revealed by their
photospheric spectra but which exhibit no disk signatures. This suggests the
timescale for dissipating their disks is much faster than the less massive OB
stars or that the most massive stars do not form with accretion disks.Comment: 28 pages, 10 Figures, accepted for publication in the Astrophysical
Journa
Pt nanocrystals formed by ion implantation: a defect-mediated nucleation process
The influence of ion irradiation of SiOâ on the size of metalnanocrystals (NCs) formed by ion implantation has been investigated. Thin SiOâ films were irradiated with high-energy Ge ions then implanted with Pt ions. Without Geirradiation, the largest Pt NCs were observed beyond the Pt projected range. With irradiation, Ge-induced structural modification of the SiOâ layer yielded a decrease in Pt NC size with increasing Ge fluence at such depths. A defect-mediated NC nucleation mechanism is proposed and a simple yet effective means of modifying and controlling the Pt NC size is demonstrated.The authors thank the Australian Research Council for
financial support
Slowly Rotating Non-Abelian Black Holes
It is shown that the well-known non-Abelian static SU(2) black hole solutions
have rotating generalizations, provided that the hypothesis of linearization
stability is accepted. Surprisingly, this rotating branch has an asymptotically
Abelian gauge field with an electric charge that cannot vanish, although the
non-rotating limit is uncharged. We argue that this may be related to our
second finding, namely that there are no globally regular slowly rotating
excitations of the particle-like Bartnik-McKinnon solutions.Comment: 8 pages, LaTe
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