2,711 research outputs found
Dynamics of Bulk vs. Nanoscale WS_2: Local Strain and Charging Effects
We measured the infrared vibrational properties of bulk and nanoparticle
WS in order to investigate the structure-property relations in these novel
materials. In addition to the symmetry-breaking effects of local strain,
nanoparticle curvature modifies the local charging environment of the bulk
material. Performing a charge analysis on the \emph{xy}-polarized E
vibrational mode, we find an approximate 1.5:1 intralayer charge difference
between the layered 2H material and inorganic fullerene-like (IF)
nanoparticles. This effective charge difference may impact the solid-state
lubrication properties of nanoscale metal dichalcogenides.Comment: 6 pages, 5 figure
Galactic Extinction from Colors and Counts of Field Galaxies in WFPC2 Frames: An Application to GRB 970228
We develop the ``simulated extinction method'' to measure average foreground
Galactic extinction from field galaxy number-counts and colors. The method
comprises simulating extinction in suitable reference fields by changing the
isophotal detection limit. This procedure takes into account selection effects,
in particular, the change in isophotal detection limit (and hence in isophotal
magnitude completeness limit) with extinction, and the galaxy color--magnitude
relation.
We present a first application of the method to the HST WFPC2 images of the
gamma-ray burster GRB 970228. Four different WFPC2 high-latitude fields,
including the HDF, are used as reference to measure the average extinction
towards the GRB in the F606W passband. From the counts, we derive an average
extinction of A_V = 0.5 mag, but the dispersion of 0.4 mag between the
estimates from the different reference fields is significantly larger than can
be accounted by Poisson plus clustering uncertainties. Although the counts
differ, the average colors of the field galaxies agree well. The extinction
implied by the average color difference between the GRB field and the reference
galaxies is A_V = 0.6 mag, with a dispersion in the estimated extinction from
the four reference fields of only 0.1 mag. All our estimates are in good
agreement with the value of 0.81\pm0.27 mag obtained by Burstein & Heiles, and
with the extinction of 0.78\pm0.12 measured by Schlegel et al. from maps of
dust IR emission. However, the discrepancy between the widely varying counts
and the very stable colors in these high-latitude fields is worth
investigating.Comment: 14 pages, 2 figures; submitted to the Astrophysical Journa
The Velocity Function of Galaxies
We present a galaxy circular velocity function, Psi(log v), derived from
existing luminosity functions and luminosity-velocity relations. Such a
velocity function is desirable for several reasons. First, it enables an
objective comparison of luminosity functions obtained in different bands and
for different galaxy morphologies, with a statistical correction for dust
extinction. In addition, the velocity function simplifies comparison of
observations with predictions from high-resolution cosmological N-body
simulations.
We derive velocity functions from five different data sets and find rough
agreement among them, but about a factor of 2 variation in amplitude. These
velocity functions are then compared with N-body simulations of a LCDM model
(corrected for baryonic infall) in order to demonstrate both the utility and
current limitations of this approach. The number density of dark matter halos
and the slope of the velocity function near v_*, the circular velocity
corresponding to an ~L_* spiral galaxy, are found to be comparable to that of
observed galaxies. The primary sources of uncertainty in construction of
Psi(log v) from observations and N-body simulations are discussed and
explanations are suggected to account for these discrepancies.Comment: Latex. 28 pages, 4 figures. Accepted by Ap
Supernova 1996L: evidence of a strong wind episode before the explosion
Observations of the type II SN 1996L reveal the presence of a slowly
expanding (V~700$ km/s) shell at ~ 10^(16) cm from the exploding star. Narrow
emission features are visible in the early spectra superposed on the normal SN
spectrum. Within about two months these features develop narrow symmetric
P-Cygni profiles. About 100 days after the explosion the light curve suddenly
flattens, the spectral lines broaden and the Halpha flux becomes larger than
what is expected from a purely radioactive model. These events are interpreted
as signatures of the onset of the interaction between the fast moving ejecta
and a slowly moving outer shell of matter ejected before the SN explosion. At
about 300 days the narrow lines disappear and the flux drops until the SN fades
away, suggesting that the interaction phase is over and that the shell has been
swept away. Simple calculations show that the superwind episode started 9 yr
before the SN explosion and lasted 6 yr, with an average dM/dt=10^(-3)
M_solar/yr. Even at very late epochs (up to day 335) the typical forbidden
lines of [OI], CaII], [FeII] remain undetected or very weak. Spectra after day
270 show relatively strong emission lines of HeI. These lines are narrower than
other emission lines coming from the SN ejecta, but broader than those from the
CSM. These high excitation lines are probably the result of non-thermal
excitation and ionization caused by the deposition of the gamma-rays emitted in
the decay of radioactive material mixed in the He layer.Comment: 8 pages, 6 figures, Latex, To appear in M.N.R.A.
A non-Markovian quantum trajectory approach to radiation into structured continuum
We present a non-Markovian quantum trajectory method for treating atoms
radiating into a reservoir with a non-flat density of states. The results of an
example numerical simulation of the case where the free space modes of the
reservoir are altered by the presence of a cavity are presented and compared
with those of an extended system approach
Observations of Lick Standard Stars Using the SCORPIO Multi-Slit Unit at the SAO 6-m Telescope
We present Lick line-index measurements of standard stars from the list of
Worthey. The spectra were taken with the multi-slit unit of the SCORPIO
spectrograph at the 6-m Special Astrophysical observatory telescope. We
describe in detail our method of analysis and explain the importance of using
the Lick index system for studying extragalactic globular clusters. Our results
show that the calibration of our instrumental system to the standard Lick
system can be performed with high confidence.Comment: 12 pages, 3 figure
Toy models of crossed Andreev reflection
We propose toy models of crossed Andreev reflection in multiterminal hybrid
structures containing out-of-equilibrium conductors. We apply the description
to two possible experiments: (i) to a device containing a large quantum dot
inserted in a crossed Andreev reflection circuit. (ii) To a device containing
an Aharonov-Bohm loop inserted in a crossed Andreev reflection circuit.Comment: 5 pages, 9 figures, minor modification
Globular Cluster and Galaxy Formation: M31, the Milky Way and Implications for Globular Cluster Systems of Spiral Galaxies
The globular cluster (GC) systems of the Milky Way and of our neighboring
spiral galaxy, M31, comprise 2 distinct entities, differing in 3 respects. 1.
M31 has young GCs, ages from ~100 Myr to 5 Gyr old, as well as old globular
clusters. No such young GCs are known in the Milky Way. 2. We confirm that the
oldest M31 GCs have much higher nitrogen abundances than do Galactic GCs at
equivalent metallicities. 3. Morrison et al. found M31 has a subcomponent of
GCs that follow closely the disk rotation curve of M31. Such a GC system in our
own Galaxy has yet to be found. These data are interpreted in terms of the
hierarchical-clustering-merging (HCM) paradigm for galaxy formation. We infer
that M31 has absorbed more of its dwarf systems than has the Milky Way. This
inference has 3 implications: 1. All spiral galaxies likely differ in their GC
properties, depending on how many companions each galaxy has, and when the
parent galaxy absorbs them. The the Milky Way ties down one end of this
spectrum, as almost all of its GCs were absorbed 10-12 Gyr ago. 2. It suggests
that young GCs are preferentially formed in the dwarf companions of parent
galaxies, and then absorbed by the parent galaxy during mergers. 3. Young GCs
seen in tidally-interacting galaxies might come from dwarf companions of these
galaxies, rather than be made a-new in the tidal interaction. There is no ready
explanation for the marked difference in nitrogen abundance for old M31 GCs
relative to the oldest Galactic GCs. The predictions made by Li & Burstein
regarding the origin of nitrogen abundance in globular clusters are consistent
with what is found for the old M31 GCs compared to that for the two 5 Gyr-old
M31 GCs.Comment: to be published in ApJ, Oct 2004; 13 pages of text, 2 tables, 7
postscript figure
The Distance to SN 1999em from the Expanding Photosphere Method
We present optical and IR spectroscopy of the first two months of evolution
of the Type II SN 1999em. We combine these data with high-quality optical/IR
photometry beginning only three days after shock breakout, in order to study
the performance of the ``Expanding Photosphere Method'' (EPM) in the
determination of distances. With this purpose we develop a technique to measure
accurate photospheric velocities by cross-correlating observed and model
spectra. The application of this technique to SN 1999em shows that we can reach
an average uncertainty of 11% in velocity from an individual spectrum. Our
analysis shows that EPM is quite robust to the effects of dust. In particular,
the distances derived from the VI filters change by only 7% when the adopted
visual extinction in the host galaxy is varied by 0.45 mag. The superb time
sampling of the BVIZJHK light-curves of SN 1999em permits us to study the
internal consistency of EPM and test the dilution factors computed from
atmosphere models for Type II plateau supernovae. We find that, in the first
week since explosion, the EPM distances are up to 50% lower than the average,
possibly due the presence of circumstellar material. Over the following 65
days, on the other hand, our tests lend strong credence to the atmosphere
models, and confirm previous claims that EPM can produce consistent distances
without having to craft specific models to each supernova. This is particularly
true for the VI filters which yield distances with an internal consistency of
4%. From the whole set of BVIZJHK photometry, we obtain an average distance of
7.5+/-0.5 Mpc, where the quoted uncertainty (7%) is a conservative estimate of
the internal precision of the method obtained from the analysis of the first 70
days of the supernova evolution.Comment: 68 pages, 15 tables, 22 figures, to appear in Ap
Observation of Collective Excitations of the Dilute 2D Electron System
We report inelastic light scattering measurements of dispersive spin and
charge density excitations in dilute 2D electron systems reaching densities
less than 10^{10} cm^{-2}. In the quantum Hall state at nu=2, roton critical
points in the spin inter--Landau level mode show a pronounced softening as r_s
is increased. Instead of a soft mode instability predicted by Hartree--Fock
calculations for r_s ~ 3.3, we find evidence of multiple rotons in the
dispersion of the softening spin excitations. Extrapolation of the data
indicates the possibility of an instability for r_s >~ 11.Comment: Submitted to Physical Review Letter
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