255,153 research outputs found
X-ray spectroscopy of galaxy clusters: beyond the CIE modeling
X-ray spectra of galaxy clusters are dominated by the thermal emission from
the hot intracluster medium. In some cases, besides the thermal component,
spectral models require additional components associated, e.g., with resonant
scattering and charge exchange. The latter produces mostly underluminous fine
spectral features. Detection of the extra components therefore requires high
spectral resolution. The upcoming X-ray missions will provide such high
resolution, and will allow spectroscopic diagnostics of clusters beyond the
current simple thermal modeling. A representative science case is resonant
scattering, which produces spectral distortions of the emission lines from the
dominant thermal component. Accounting for the resonant scattering is essential
for accurate abundance and gas motion measurements of the ICM. The high
resolution spectroscopy might also reveal/corroborate a number of new spectral
components, including the excitation by non-thermal electrons, the deviation
from ionization equilibrium, and charge exchange from surface of cold gas
clouds in clusters. Apart from detecting new features, future high resolution
spectroscopy will also enable a much better measurement of the thermal
component. Accurate atomic database and appropriate modeling of the thermal
spectrum are therefore needed for interpreting the data.Comment: published in Space Science Review
Detailed and simplified non-equilibrium helium ionization in the solar atmosphere
Helium ionization plays an important role in the energy balance of the upper
chromosphere and transition region. Helium spectral lines are also often used
as diagnostics of these regions. We carry out 1D radiation-hydrodynamics
simulations of the solar atmosphere and find that the helium ionization is
mostly set by photoionization and direct collisional ionization, counteracted
by radiative recombination cascades. By introducing an additional recombination
rate mimicking the recombination cascades, we construct a simplified 3 level
helium model atom consisting of only the ground states. This model atom is
suitable for modeling non-equilibrium helium ionization in 3D numerical models.
We perform a brief investigation of the formation of the He I 10830 and He II
304 spectral lines. Both lines show non-equilibrium features that are not
recovered with statistical equilibrium models, and caution should therefore be
exercised when such models are used as a basis in the interpretation of
observations.Comment: 11 pages, 9 figures. Accepted for publication in Ap
Radiative transfer in cylindrical threads with incident radiation. VI. A hydrogen plus helium system
Context: Spectral lines of helium are commonly observed on the Sun. These observations contain important information about physical conditions and He/H abundance variations within solar outer structures.
Aims: The modeling of chromospheric and coronal loop-like structures visible in hydrogen and helium lines requires the use of appropriate diagnostic tools based on NLTE radiative tranfer in cylindrical geometry.
Methods: We use iterative numerical methods to solve the equations of NLTE radiative transfer and statistical equilibrium of atomic level populations. These equations are solved alternatively for hydrogen and helium atoms, using cylindrical coordinates and prescribed solar incident radiation. Electron density is determined by the ionization equilibria of both atoms. Two-dimensional effects are included.
Results: The mechanisms of formation of the principal helium lines are analyzed and the sources of emission inside the cylinder are located. The variations of spectral line intensities with temperature, pressure, and helium abundance, are studied.
Conclusions: The simultaneous computation of hydrogen and helium lines, performed by the new numerical code, allows the construction of loop models including an extended range of temperatures
Modeling X-ray Emission Line Profiles from Massive Star Winds - A Review
The Chandra and XMM-Newton X-ray telescopes have led to numerous advances in
the study and understanding of astrophysical X-ray sources. Particularly
important has been the much increased spectral resolution of modern X-ray
instrumentation. Wind-broadened emission lines have been spectroscopically
resolved for many massive stars. This contribution reviews approaches to the
modeling of X-ray emission line profile shapes from single stars, including
smooth winds, winds with clumping, optically thin versus thick lines, and the
effect of a radius-dependent photoabsorption coefficient.Comment: to appear in Advances in Space Researc
Observations and modeling of H_2 fluorescence with partial frequency redistribution in giant planet atmospheres
Partial frequency redistribution (PRD), describing the formation of the line
profile, has negligible observational effects for optical depths smaller than
~10^3, at the resolving power of most current instruments. However, when the
spectral resolution is sufficiently high, PRD modeling becomes essential in
interpreting the line shapes and determining the total line fluxes. We
demonstrate the effects of PRD on the H_2 line profiles observed at high
spectral resolution by the Far-Ultraviolet Spectroscopic Explorer (FUSE) in the
atmospheres of Jupiter and Saturn. In these spectra, the asymmetric shapes of
the lines in the Lyman (v"- 6) progression pumped by the solar Ly-beta are
explained by coherent scattering of the photons in the line wings. We introduce
a simple computational approximation to mitigate the numerical difficulties of
radiative transfer with PRD, and show that it reproduces the exact radiative
transfer solution to better than 10%. The lines predicted by our radiative
transfer model with PRD, including the H_2 density and temperature distribution
as a function of height in the atmosphere, are in agreement with the line
profiles observed by FUSE. We discuss the observational consequences of PRD,
and show that this computational method also allows us to include PRD in
modeling the continuum pumped H_2 fluorescence, treating about 4000 lines
simultaneously.Comment: 17 pages, accepted for publication in Ap
Molecular line survey of the high-mass star-forming region NGC 6334I with Herschel/HIFI and the SMA
We aim at deriving the molecular abundances and temperatures of the hot
molecular cores in the high-mass star-forming region NGC 6334I and consequently
deriving their physical and astrochemical conditions. In the framework of the
Herschel guaranteed time key program CHESS, NGC 6334I is investigated by using
HIFI aboard the Herschel Space Observatory. A spectral line survey is carried
out in the frequency range 480-1907 GHz, and auxiliary interferometric data
from the SMA in the 230 GHz band provide spatial information for disentangling
the different physical components contributing to the HIFI spectrum. The
spectral lines are identified with the aid of former surveys and spectral line
catalogs. The observed spectrum is then compared to a simulated synthetic
spectrum with XCLASS, assuming local thermal equilibrium, and best fit
parameters are derived using the model optimization package MAGIX. A total of
46 molecules are identified, with 31 isotopologues, resulting in about 4300
emission and absorption lines. High- energy levels of the dominant emitter
methanol and vibrationally excited HCN are detected. The number of unidentified
lines remains low with 75, or less than 2 percent of the lines detected. The
modeling suggests that several spectral features need two or more components to
be fitted properly. Other components could be assigned to cold foreground
clouds or to outflows, most visible in the SiO emission. A chemical variation
between the two embedded hot cores is found, with more N-bearing molecules
identified in SMA1 and O-bearing molecules in SMA2. Spectral line surveys give
powerful insights into the study of the interstellar medium. Different
molecules trace different physical conditions like the inner hot core, the
envelope, the outflows or the cold foreground clouds. The derived molecular
abundances provide further constraints for astrochemical models.Comment: 30 pages including appendix, 49 figures, accepted for publication in
Astronomy and Astrophysic
The Correlation between X-Ray Line Ionization and Optical Spectral Types of the OB Stars
Marked correlations are reported between the ionization of the X-ray line
spectra of normal OB stars, as observed by the Chandra X-Ray Observatory, and
their optical spectral types. These correlations include the progressive
weakening of the higher ionization relative to the lower ionization X-ray lines
with advancing spectral type, and the similarly decreasing intensity ratios of
the H-like to He-like lines of the alpha ions. These relationships were not
predicted by models, nor have they been clearly evident in astrophysical
studies of a few objects; rather, they have emerged from morphological analysis
of an adequate (albeit still small) sample, from which known peculiar objects
such as magnetic stars and very rapid rotators have been isolated to reveal the
normal trends. This process is analogous to that which first demonstrated the
strong relationships between the UV wind profiles and the optical spectral
types of normal OB stars, which likely bear a physical as well as a historical
connection to the present X-ray results. Since the optical spectral types are
calibrated in terms of fundamental stellar parameters, it follows that the
winds and X-ray spectra are determined by the latter. These observations
provide strong guidance for further astrophysical modeling of these phenomena.Comment: 19 pages, 7 figures, 2 tables; ApJ accepte
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