3,839 research outputs found
Crossing conditions in coupled cluster theory
We derive the crossing conditions at conical intersections between electronic
states in coupled cluster theory, and show that if the coupled cluster Jacobian
matrix is nondefective, two (three) independent conditions are correctly placed
on the nuclear degrees of freedom for an inherently real (complex) Hamiltonian.
Calculations using coupled cluster theory on an
conical intersection in hypofluorous acid illustrate the nonphysical artifacts
associated with defects at accidental same-symmetry intersections. In
particular, the observed intersection seam is folded about a space of the
correct dimensionality, indicating that minor modifications to the theory are
required for it to provide a correct description of conical intersections in
general. We find that an accidental symmetry allowed
intersection in hydrogen sulfide is properly described, showing no artifacts as
well as linearity of the energy gap to first order in the branching plane.Comment: 9 pages and 4 figure
Running quark mass in two flavor QCD
We present first results for the step scaling function sigma_P of the
renormalization factor Z_P of the pseudoscalar density. The simulations are
performed within the framework of the Schroedinger functional with two flavors
of O(a) improved Wilson fermions. The knowledge of sigma_P is required to
compute the renormalization group invariant quark masses. We also study the
performance of a variant of the HMC algorithm using two pseudofermion fields.Comment: 3 pages, 2 figures, Lattice2002(spectrum
The Double-Lined Spectrum of LBV 1806-20
Despite much theoretical and observational progress, there is no known firm
upper limit to the masses of stars. Our understanding of the interplay between
the immense radiation pressure produced by massive stars in formation and the
opacity of infalling material is subject to theoretical uncertainties, and many
observational claims of ``the most massive star'' have failed the singularity
test. LBV 1806-20 is a particularly luminous object, L~10^6 Lsun, for which
some have claimed very high mass estimates (M_initial>200 Msun), based, in
part, on its similarity to the Pistol Star. We present high-resolution
near-infrared spectroscopy of LBV 1806-20, showing that it is possibly a binary
system with components separated in velocity by ~70 kms. If correct, then this
system is not the most massive star known, yet it is a massive binary system.
We argue that a binary, or merged, system is more consistent with the ages of
nearby stars in the LBV 1806-20 cluster. In addition, we find that the velocity
of V_LSR=36 kms is consistent with a distance of 11.8 kpc, a luminosity of
10^6.3 Lsun, and a system mass of ~130 Msun.Comment: ApJL, accepte
Non-perturbative renormalization of the axial current with improved Wilson quarks
We present a new normalization condition for the axial current, which is
derived from the PCAC relation with non-vanishing mass.
Using this condition reduces the O(r_0 m) corrections to the axial current
normalization constant Z_A for an easier chiral extrapolation in the cases,
where simulations at zero quark-mass are not possible. The method described
here also serves as a preparation for a determination of Z_A in the full
two-flavor theory.Comment: 3 pages, 3 figures, Lattice2003(improve
A precise determination of the charm quark's mass in quenched QCD
We present a lattice determination of the charm quark's mass, using the mass
of the D_s meson as experimental input. All errors are under control with the
exception of the quenched approximation. Setting the scale with F_K=160 MeV,
our final result for the renormalization group invariant (RGI) quark mass is
M_c = 1.654(45) GeV, which translates to m_c(m_c) =1.301(34) GeV for the
running mass in the MSbar scheme. A 6 percent increase of the RGI quark mass is
observed when the scale is set by the nucleon mass. This is a typical quenched
scale ambiguity, which is reduced to about 3 percent for m_c(m_c), and to 4
percent for the mass ratio M_c/M_s. In contrast, the mass splitting
m(Dstar_s)-m(D_s) changes from 117(11) MeV to 94(11) MeV, which is
significantly smaller than the experimental value of 144 MeV.Comment: 27 pages, 5 figure
Radial Velocities of Stars in the Galactic Center
We present results from K band slit scan observations of a ~20''x20'' region
of the Galactic center (GC) in two separate epochs more than five years apart.
The high resolution (R>=14,000) observations allow the most accurate radial
velocity and acceleration measurements of the stars in the central parsec of
the Galaxy. Detected stars can be divided into three groups based on the CO
absorption band heads at ~2.2935 microns and the He I lines at ~2.0581 microns
and ~2.112, 2.113 microns: cool, narrow-line hot and broad-line hot. The radial
velocities of the cool, late-type stars have approximately a symmetrical
distribution with its center at ~-7.8(+/-10.3) km/s and a standard deviation
~113.7(+/-10.3) km/s. Although our statistics are dominated by the brightest
stars, we estimate a central black hole mass of 3.9(+/-1.1) million solar
masses, consistent with current estimates from complete orbits of individual
stars. Our surface density profile and the velocity dispersion of the late type
stars support the existence of a low density region at the Galactic center
suggested by earlier observations. Many hot, early-type stars show radial
velocity changes higher than maximum values allowed by pure circular orbital
motions around a central massive object, suggesting that the motions of these
stars greatly deviate from circular orbital motions around the Galactic center.
The correlation between the radial velocities of the early type He I stars and
their declination offsets from Sagittarius A* suggests that a systematic
rotation is present for the early-type population. No figure rotation around
the Galactic center for the late type stars is supported by the new
observations.Comment: 61 pages, 18 figures, 7 tables; accepted for publication in
Astrophysical Journa
Radiation therapy combined with intracerebral administration of carboplatin for the treatment of brain tumors
Background: In this study we determined if treatment combining radiation therapy (RT) with intracerebral (i.c.) administration of carboplatin to F98 glioma bearing rats could improve survival over that previously reported by us with a 15 Gy dose (5 Gy × 3) of 6 MV photons.Methods: First, in order to reduce tumor interstitial pressure, a biodistribution study was carried out to determine if pretreatment with dexamethasone alone or in combination with mannitol and furosemide (DMF) would increase carboplatin uptake following convection enhanced delivery (CED). Next, therapy studies were carried out in rats that had received carboplatin either by CED over 30 min (20 μg) or by Alzet pumps over 7 d (84 μg), followed by RT using a LINAC to deliver either 20 Gy (5 Gy × 4) or 15 Gy (7.5 Gy × 2) dose at 6 or 24 hrs after drug administration. Finally, a study was carried out to determine if efficacy could be improved by decreasing the time interval between drug administration and RT.Results: Tumor carboplatin values for D and DMF-treated rats were 9.4 ±4.4 and 12.4 ±3.2 μg/g, respectively, which were not significantly different (P = 0.14). The best survival data were obtained by combining pump delivery with 5 Gy × 4 of X-irradiation with a mean survival time (MST) of 107.7 d and a 43% cure rate vs. 83.6 d with CED vs. 30-35 d for RT alone and 24.6 d for untreated controls. Treatment-related mortality was observed when RT was initiated 6 h after CED of carboplatin and RT was started 7 d after tumor implantation. Dividing carboplatin into two 10 μg doses and RT into two 7.5 Gy fractions, administered 24 hrs later, yielded survival data (MST 82.1 d with a 25% cure rate) equivalent to that previously reported with 5 Gy × 3 and 20 μg of carboplatin.Conclusions: Although the best survival data were obtained by pump delivery, CED was highly effective in combination with 20 Gy, or as previously reported, 15 Gy, and the latter would be preferable since it would produce less late tissue effects.peer-reviewe
The water abundance behind interstellar shocks: results from /PACS and /IRS observations of HO, CO, and H
We have investigated the water abundance in shock-heated molecular gas,
making use of measurements of far-infrared CO and HO line
emissions in combination with measurements of mid-IR H rotational
emissions. We present far-infrared line spectra obtained with 's PACS
instrument in range spectroscopy mode towards two positions in the protostellar
outflow NGC 2071 and one position each in the supernova remnants W28 and 3C391.
These spectra provide unequivocal detections, at one or more positions, of 12
rotational lines of water, 14 rotational lines of CO, 8 rotational lines of OH
(4 lambda doublets), and 7 fine-structure transitions of atoms or atomic ions.
We first used a simultaneous fit to the CO line fluxes, along with H
rotational line fluxes measured previously by , to constrain the
temperature and density distribution within the emitting gas; and we then
investigated the water abundances implied by the observed HO line fluxes.
The water line fluxes are in acceptable agreement with standard theoretical
models for nondissociative shocks that predict the complete vaporization of
grain mantles in shocks of velocity km/s, behind which the
characteristic gas temperature is K and the HO/CO ratio is 1.2Comment: 42 pages, 15 figures, accepted for publication in the Astrophysical
Journa
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