764 research outputs found
Effective order strong stability preserving Runge–Kutta methods
We apply the concept of effective order to strong stability preserving (SSP) explicit Runge–Kutta methods. Relative to classical Runge–Kutta methods, effective order methods are designed to satisfy a relaxed set of order conditions, but yield higher order accuracy when composed with special starting and stopping methods. The relaxed order conditions allow for greater freedom in the design of effective order methods. We show that this allows the construction of four-stage SSP methods with effective order four (such methods cannot have classical order four). However, we also prove that effective order five methods—like classical order five methods—require the use of non-positive weights and so cannot be SSP. By numerical optimization, we construct explicit SSP Runge–Kutta methods up to effective order four and establish the optimality of many of them. Numerical experiments demonstrate the validity of these methods in practice
The Molecular Structure of Cyclopropene, C_3C_4
Cyclopropene, C_3H_4, has been investigated by the electron diffraction method. The cyclic structure is confirmed and the following interatomic distances and angles are found: C–C, 1.525±0.02 A; C=C, 1.286±0.04 A; 〈C–H〉, 1.087±0.04 A; ∠H–C–H, 118° (assumed); ∠C=C–H, 152±12°
An Atlas of Computed Equivalent Widths of Quasar Broad Emission Lines
We present graphically the results of several thousand photoionization
calculations of broad emission line clouds in quasars, spanning seven orders of
magnitude in hydrogen ionizing flux and particle density. The equivalent widths
of 42 quasar emission lines are presented as contours in the particle density -
ionizing flux plane for a typical incident continuum shape, solar chemical
abundances, and cloud column density of . Results are
similarly given for a small subset of emission lines for two other column
densities ( and ), five other incident
continuum shapes, and a gas metallicity of 5 \Zsun. These graphs should prove
useful in the analysis of quasar emission line data and in the detailed
modeling of quasar broad emission line regions. The digital results of these
emission line grids and many more are available over the Internet.Comment: 16 pages, LaTeX (AASTeX aaspp4.sty); to appear in the 1997 ApJS: full
contents of the 9 photoionization grids presented in this paper may be found
at http://www.pa.uky.edu/~korista/grids/grids.htm
The Observability of Metal Lines Associated with the Lyman-alpha Forest
We develop a prescription for characterizing the strengths of metal lines
associated with Lyman-alpha forest absorbers (LYFAs) of a given neutral
hydrogen column density N_HI and metallicity [Fe/H]. This Line Observability
Index (LOX) is line-specific and translates, for weak lines, into a measure of
the equivalent width. It can be evaluated quickly for thousands of transitions
within the framework of a given model of the Lyman-alpha forest, providing a
ranking of the lines in terms of their strengths and enabling model builders to
select the lines that should be detectable in observed spectra of a given
resolution and signal-to-noise ratio. We compute the LOX for a large number of
elements and transitions in two cosmological models of the Lyman-alpha forest
at z=3 derived from a hydrodynamic simulation of structure formation, and we
discuss how the LOX depends on redshift and on model parameters such as the
mean baryonic density and radiation field. We find that the OVI (1032,1038)
doublet is the best probe of the metallicity in low column density LYFAs N_{HI}
\approx 10^{14.5} cm^{-2}). Metallicities down to [O/H] \sim -3 ([Fe/H] \sim
-3.5 with the assumed [O/Fe] ratio) yield OVI absorption features that should
be detectable in current high-quality spectra, provided that the expected
position of the OVI feature is not contaminated by HI absorption. The strongest
transitions in lower ionisation states of oxygen are OV(630), OIV(788), and
OIII(833), and are likely to be detected with next generation UV instruments.
Of the lines with rest wavelengths \lambda_r > 1216, which can potentially be
observed redwards of the \lya forest, the CIV(1548,1551) doublet is expected to
dominate in all LYFAs, regardless of the value of N_HI.Comment: Substantially revised version: larger line database, additional
cosmological model analyzed. Accepted for Ap
The D/H Ratio in the Interstellar Medium toward the White Dwarf PG0038+199
We determine the D/H ratio in the interstellar medium toward the DO white
dwarf PG0038+199 using spectra from the Far Ultraviolet Spectroscopic Explorer
(FUSE), with ground-based support from Keck HIRES. We employ curve of growth,
apparent optical depth and profile fitting techniques to measure column
densities and limits of many other species (H2, NaI, CI, CII, CIII, NI, NII,
OI, SiII, PII, SIII, ArI and FeII) which allow us to determine related ratios
such as D/O, D/N and the H2 fraction. Our efforts are concentrated on measuring
gas-phase D/H, which is key to understanding Galactic chemical evolution and
comparing it to predictions from Big Bang nucleosynthesis. We find column
densities log N(HI) = 20.41+-0.08, log N(DI)=15.75+-0.08 and log N(H2) =
19.33+-0.04, yielding a molecular hydrogen fraction of 0.14+-0.02 (2 sigma
errors), with an excitation temperature of 143+-5K. The high HI column density
implies that PG0038+199 lies outside of the Local Bubble; we estimate its
distance to be 297 (+164,-104)pc (1 sigma). D/[HI+2H2] toward PG0038+199 is
1.91(+0.52,-0.42) e-5 (2 sigma). There is no evidence of component structure on
the scale of Delta v > 8 km/s based on NaI, but there is marginal evidence for
structure on smaller scales. The D/H value is high compared to the majority of
recent D/H measurements, but consistent with the values for two other
measurements at similar distances. D/O is in agreement with other distant
measurements. The scatter in D/H values beyond ~100pc remains a challenge for
Galactic chemical evolution.Comment: 59 pages, 7 tables, 18 figures (1 standalone), accepted by ApJ v2
minor typos correcte
Strong stability preserving explicit Runge-Kutta methods of maximal effective order
We apply the concept of effective order to strong stability preserving (SSP)
explicit Runge-Kutta methods. Relative to classical Runge-Kutta methods,
methods with an effective order of accuracy are designed to satisfy a relaxed
set of order conditions, but yield higher order accuracy when composed with
special starting and stopping methods. We show that this allows the
construction of four-stage SSP methods with effective order four (such methods
cannot have classical order four). However, we also prove that effective order
five methods - like classical order five methods - require the use of
non-positive weights and so cannot be SSP. By numerical optimization, we
construct explicit SSP Runge-Kutta methods up to effective order four and
establish the optimality of many of them. Numerical experiments demonstrate the
validity of these methods in practice.Comment: 17 pages, 3 figures, 8 table
Ultraviolet observations of the X-ray photoionized wind of Cygnus X-1 during X-ray soft/high state
(Shortened) Ultraviolet observations of the black hole X-ray binary Cygnus
X-1 were obtained using the STIS on HSTubble. We detect P Cygni line features
show strong, broad absorption components when the X-ray source is behind the
companion star and noticeably weaker absorption when the X-ray source is
between us and the companion star. We fit the P Cygni profiles using the SEI
method applied to a spherically symmetric stellar wind subject to X-ray
photoionization from the black hole. The Si IV doublet provides the most
reliable estimates of the parameters of the wind and X-ray illumination. The
velocity increases with radius according to
, with and
km s.The microturbulent velocity was
km s. Our fit implies a ratio of X-ray luminosity to wind mass-loss rate
of L, measured at = 4.8. Our
models determine parameters that may be used to estimate the accretion rate
onto the black hole and independently predict the X-ray luminosity. Our
predicted L matches that determined by contemporaneous RXTE ASM remarkably
well, but is a factor of 3 lower than the rate according to
Bondi-Hoyle-Littleton spherical wind accretion. We suggest that some of the
energy of accretion may go into powering a jet.Comment: 34 pages, 21 figures, 4 tables, accepted for publication in Ap
The Molecular Structure of Cyclopropene, C_3C_4
Cyclopropene, C_3H_4, has been investigated by the electron diffraction method. The cyclic structure is confirmed and the following interatomic distances and angles are found: C–C, 1.525±0.02 A; C=C, 1.286±0.04 A; 〈C–H〉, 1.087±0.04 A; ∠H–C–H, 118° (assumed); ∠C=C–H, 152±12°
On the Absorption of X-rays in the Interstellar Medium
We present an improved model for the absorption of X-rays in the ISM intended
for use with data from future X-ray missions with larger effective areas and
increased energy resolution such as Chandra and XMM, in the energy range above
100eV. Compared to previous work, our formalism includes recent updates to the
photoionization cross section and revised abundances of the interstellar
medium, as well as a treatment of interstellar grains and the H2molecule. We
review the theoretical and observational motivations behind these updates and
provide a subroutine for the X-ray spectral analysis program XSPEC that
incorporates our model.Comment: ApJ, in press, for associated software see
http://astro.uni-tuebingen.de/nh
A hydrodynamic study of the circumstellar envelope of alpha Scorpii
Context: Both the absolute mass-loss rates and the mechanisms that drive the
mass loss of late-type supergiants are still not well known. Binaries such as
alpha Sco provide the most detailed empirical information about the winds of
these stars.
Aims: The goal was to improve the binary technique for the determination of
the mass-loss rate of alpha Sco A by including a realistic density distribution
and velocity field from hydrodynamic and plasma simulations.
Methods: We performed 3D hydrodynamic simulations of the circumstellar
envelope of alpha Sco in combination with plasma simulations accounting for the
heating, ionization, and excitation of the wind by the radiation of alpha Sco
B. These simulations served as the basis for an examination of circumstellar
absorption lines in the spectrum of alpha Sco B as well as of emission lines
from the Antares nebula.
Results: The present model of the extended envelope of alpha Sco reproduces
some of the structures that were observed in the circumstellar absorption lines
in the spectrum of alpha Sco B. Our theoretical density and velocity
distributions of the outflow deviate considerably from a spherically expanding
model, which was used in previous studies. This results in a higher mass-loss
rate of (2 +/- 0.5) x 10^-6 M_sun/yr. The hot H II region around the secondary
star induces an additional acceleration of the wind at large distances from the
primary, which is seen in absorption lines of Ti II and Cr II at -30 km/s.Comment: 12 pages, 14 figures, accepted for publication in A&
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