1,125 research outputs found
Low temperature heat capacity of Fe_{1-x}Ga_{x} alloys with large magneostriction
The low temperature heat capacity C_{p} of Fe_{1-x}Ga_{x} alloys with large
magnetostriction has been investigated. The data were analyzed in the standard
way using electron () and phonon () contributions. The
Debye temperature decreases approximately linearly with increasing
Ga concentration, consistent with previous resonant ultrasound measurements and
measured phonon dispersion curves. Calculations of from lattice
dynamical models and from measured elastic constants C_{11}, C_{12} and C_{44}
are in agreement with the measured data. The linear coefficient of electronic
specific heat remains relatively constant as the Ga concentration
increases, despite the fact that the magnetoelastic coupling increases. Band
structure calculations show that this is due to the compensation of majority
and minority spin states at the Fermi level.Comment: 14 pages, 6 figure
Heterodyne Spectroscopy of the 63 m O I Line in M42
We have used a laser heterodyne spectrometer to resolve the emission line
profile of the 63 micron 3P1 - 3P2 fine-structure transition of O I at two
locations in M42. Comparison of the peak antenna temperature with that of the
158 micron C II fine-structure line shows that the gas kinetic temperature in
the photodissociation region near theta1C is 175 - 220 K, the density is
greater than 2x10 ^5 cm-3, and the hydrogen column density is about 1.5x10 ^22
cm-2. A somewhat lower temperature and column density are found in the IRc2
region, most likely reflecting the smaller UV flux. The observed width of the O
I line is 6.8 km/s (FWHM) at theta1C, which is slightly broadened over the
intrinsic linewidth by optical depth effects. No significant other differences
between the O I and C II line profiles are seen, which shows that the narrow
emission from both neutral atomic oxygen and ionized carbon comes from the PDR.
The O I data do not rule out the possibility of weak broad-velocity emission
from shock-excited gas at IRc2, but the C II data show no such effect, as
expected from non-ionizing shock models.Comment: 11 pages including 2 postscript figures, uses aaspp4.st
Physical Conditions in Orion's Veil
Orion's veil consists of several layers of largely neutral gas lying between
us and the main ionizing stars of the Orion nebula. It is visible in 21cm H I
absorption and in optical and UV absorption lines of H I and other species.
Toward the Trapezium, the veil has two remarkable properties, high magnetic
field (~100 microGauss) and a surprising lack of molecular hydrogen given its
total hydrogen column density. Here we compute photoionization models of the
veil to establish its gas density and its distance from the Trapezium. We use a
greatly improved model of the hydrogen molecule that determines level
populations in ~1e5 rotational/vibrational levels and provides improved
estimates of molecular hydrogen destruction via the Lyman-Werner bands. Our
best fit photoionization models place the veil 1-3 pc in front of the star at a
density of 1e3-1e4 cubic centimeters. Magnetic energy dominates the energy of
non-thermal motions in at least one of the 21cm H I velocity components.
Therefore, the veil is the first interstellar environment where magnetic
dominance appears to exist. We find that the low ratio of molecular to atomic
hydrogen (< 1e-4) is a consequence of high UV flux incident upon the veil due
to its proximity to the Trapezium stars and the absence of small grains in the
region.Comment: 45 pages, 20 figures, accepted for publication in Ap
Monitoring the Variable Interstellar Absorption toward HD 219188 with HST/STIS
We discuss the results of continued spectroscopic monitoring of the variable
intermediate-velocity (IV) absorption at v = -38 km/s toward HD 219188. After
reaching maxima in mid-2000, the column densities of both Na I and Ca II in
that IV component declined by factors >= 2 by the end of 2006. Comparisons
between HST/STIS echelle spectra obtained in 2001, 2003, and 2004 and HST/GHRS
echelle spectra obtained in 1994--1995 indicate the following: (1) The
absorption from the dominant species S II, O I, Si II, and Fe II is roughly
constant in all four sets of spectra -- suggesting that the total N(H) and the
(mild) depletions have not changed significantly over a period of nearly ten
years. (2) The column densities of the trace species C I (both ground and
excited fine-structure states) and of the excited state C II* all increased by
factors of 2--5 between 1995 and 2001 -- implying increases in the hydrogen
density n_H (from about 20 cm^{-3} to about 45 cm^{-3}) and in the electron
density n_e (by a factor >= 3) over that 6-year period. (3) The column
densities of C I and C II* -- and the corresponding inferred n_H and n_e --
then decreased slightly between 2001 and 2004. (4) The changes in C I and C II*
are very similar to those seen for Na I and Ca II. The relatively low total
N(H) and the modest n_H suggest that the -38 km/s cloud toward HD 219188 is not
a very dense knot or filament. Partial ionization of hydrogen appears to be
responsible for the enhanced abundances of Na I, C I, Ca II, and C II*. In this
case, the variations in those species appear to reflect differences in density
and ionization [and not N(H)] over scales of tens of AU.Comment: 33 pages, 6 figures, aastex, accepted to Ap
Physical Conditoins in Orion's Veil II: A Multi-Component Study of the Line of Sight Toward the Trapezium
Orion's Veil is an absorbing screen that lies along the line of sight to the
Orion H II region. It consists of two or more layers of gas that must lie
within a few parsecs of the Trapezium cluster. Our previous work considered the
Veil as a whole and found that the magnetic field dominates the energetics of
the gas in at least one component. Here we use high-resolution STIS UV spectra
that resolve the two velocity components in absorption and determine the
conditions in each. We derive a volume hydrogen density, 21 cm spin
temperature, turbulent velocity, and kinetic temperature, for each. We combine
these estimates with magnetic field measurements to find that magnetic energy
significantly dominates turbulent and thermal energies in one component, while
the other component is close to equipartition between turbulent and magnetic
energies. We observe molecular hydrogen absorption for highly excited v, J
levels that are photoexcited by the stellar continuum, and detect blueshifted S
III and P III. These ions must arise from ionized gas between the mostly
neutral portions of the Veil and the Trapezium and shields the Veil from
ionizing radiation. We find that this layer of ionized gas is also responsible
for He I absorption in the Veil, which resolves a 40-year-old debate on the
origin of He I absorption towards the Trapezium. Finally, we determine that the
ionized and mostly atomic layers of the Veil will collide in less than 85,000
years.Comment: 43 pages, 15 figures, to be published in Ap
Atomic Diagnostics of X-ray Irradiated Protoplanetary Disks
We study atomic line diagnostics of the inner regions of protoplanetary disks
with our model of X-ray irradiated disk atmospheres which was previously used
to predict observable levels of the NeII and NeIII fine-structure transitions
at 12.81 and 15.55mum. We extend the X-ray ionization theory to sulfur and
calculate the fraction of sulfur in S, S+, S2+ and sulfur molecules. For the
D'Alessio generic T Tauri star disk, we find that the SI fine-structure line at
25.55mum is below the detection level of the Spitzer Infrared Spectrometer
(IRS), in large part due to X-ray ionization of atomic S at the top of the
atmosphere and to its incorporation into molecules close to the mid-plane. We
predict that observable fluxes of the SII 6718/6732AA forbidden transitions are
produced in the upper atmosphere at somewhat shallower depths and smaller radii
than the neon fine-structure lines. This and other forbidden line transitions,
such as the OI 6300/6363AA and the CI 9826/9852AA lines, serve as complementary
diagnostics of X-ray irradiated disk atmospheres. We have also analyzed the
potential role of the low-excitation fine-structure lines of CI, CII, and OI,
which should be observable by SOFIA and Herschel.Comment: Accepted by Ap
Formation of Primordial Stars in a LCDM Universe
We study the formation of the first generation of stars in the standard cold
dark matter model, using a very high-resolution hydordynamic simulations. Our
simulation achieves a dynamic range of 10^{10} in length scale. With accurate
treatment of atomic and molecular physics, it allows us to study the
chemo-thermal evolution of primordial gas clouds to densities up to n =
10^{16}/cc without assuming any a priori equation of state; a six orders of
magnitudes improvement over previous three-dimensional calculations. All the
relevant atomic and molecular cooling and heating processes, including cooling
by collision-induced continuum emission, are implemented. For calculating
optically thick H2 cooling at high densities, we use the Sobolev method. To
examine possible gas fragmentation owing to thermal instability, we compute
explicitly the growth rate of isobaric perturbations. We show that the cloud
core does not fragment in either the low-density or high-density regimes. We
also show that the core remains stable against gravitational deformation and
fragmentation. We obtain an accurate gas mass accretion rate within a 10 Msun
innermost region around the protostar. The protostar is accreting the
surrounding hot gas at a rate of 0.001-0.01 Msun/yr. From these findings we
conclude that primordial stars formed in early minihalos are massive. We carry
out proto-stellar evolution calculations using the obtained accretion rate. The
resulting mass of the first star is M_ZAMS = 60-100 Msun, with the exact mass
dependent on the actual accretion rate.Comment: 27 pages, 13 embedded figures. Revised versio
Multichannel quantum-defect theory for ultracold atom-ion collisions
We develop an analytical model for ultracold atom-ion collisions using the
multichannel quantum-defect formalism. The model is based on the analytical
solutions of the r^-4 long-range potential and on the application of a frame
transformation between asymptotic and molecular bases. This approach allows the
description of the atom-ion interaction in the ultracold domain in terms of
three parameters only: the singlet and triplet scattering lengths, assumed to
be independent of the relative motion angular momentum, and the lead dispersion
coefficient of the asymptotic potential. We also introduce corrections to the
scattering lengths that improve the accuracy of our quantum-defect model for
higher order partial waves, a particularly important result for an accurate
description of shape and Feshbach resonances at finite temperature. The theory
is applied to the system composed of a 40Ca+ ion and a Na atom, and compared to
numerical coupled-channel calculations carried out using ab initio potentials.
For this particular system, we investigate the spectrum of bound states, the
rate of charge-transfer processes, and the collision rates in the presence of
magnetic Feshbach resonances at zero and finite temperature.Comment: 39 pages, 21 figure
Interactions and dynamics in Li+Li2 ultracold collisions
A potential energy surface for the lowest quartet electronic state (AâČ4) of lithium trimer is developed and used to study spin-polarized Li+Li2collisions at ultralow kinetic energies. The potential energy surface allows barrierless atom exchange reactions. Elastic and inelastic cross sections are calculated for collisions involving a variety of rovibrational states of Li2. Inelastic collisions are responsible for trap loss in molecule production experiments. Isotope effects and the sensitivity of the results to details of the potential energy surface are investigated. It is found that for vibrationally excited states, the cross sections are only quite weakly dependent on details of the potential energy surface
Polarons with a twist
We consider a polaron model where molecular \emph{rotations} are important.
Here, the usual hopping between neighboring sites is affected directly by the
electron-phonon interaction via a {\em twist-dependent} hopping amplitude. This
model may be of relevance for electronic transport in complex molecules and
polymers with torsional degrees of freedom, such as DNA, as well as in
molecular electronics experiments where molecular twist motion is significant.
We use a tight-binding representation and find that very different polaronic
properties are already exhibited by a two-site model -- these are due to the
nonlinearity of the restoring force of the twist excitations, and of the
electron-phonon interaction in the model. In the adiabatic regime, where
electrons move in a {\em low}-frequency field of twisting-phonons, the
effective splitting of the energy levels increases with coupling strength. The
bandwidth in a long chain shows a power-law suppression with coupling, unlike
the typical exponential dependence due to linear phonons.Comment: revtex4 source and one eps figur
- âŠ