89 research outputs found
Wavelengths and Energy Levels of the Upper Levels of Singly Ionized Nickel (Ni ii) from 3<i>d</i><sup>8</sup>(<sup>3</sup><i>F</i>)5<i>f</i> to 3d<sup>8</sup>(<sup>3</sup><i>F)</i>9<i>s</i>
Using high-resolution spectra of Ni ii recorded using Fourier transform (FT) spectroscopy of continuous, nickel-helium hollow cathode discharge sources in the region 143-5555 nm (1800-70,000 cm−1, the analysis of 1016 Ni ii lines confirmed and optimized 206 previously reported energy levels of the (3 F) parent term, from 3d 8(3 F)5f to 3d 8(3 F)9s, lying between 122,060 and 138,563 cm−1. The uncertainties of these levels have been improved by at least an order of magnitude compared with their previously reported values. With the increased resolution and spectral range of the FT measurements, compared to previously published grating spectra, we were able to extend our analysis to identify and establish 33 new energy levels of Ni ii, which are reported here for the first time. Eigenvector compositions of all revised and newly established energy levels were calculated using the orthogonal operator method. In addition, an improved ionization energy of 146,541.35 ± 0.15 cm−1 for Ni ii, using highly excited levels of the 3d 8(3 F)5g, 3d 8(3 F)6g, and 3d 8(3 F)6h configurations, has been derived.</p
A Comprehensive X-ray Absorption Model for Atomic Oxygen
An analytical formula is developed to represent accurately the
photoabsorption cross section of O I for all energies of interest in X-ray
spectral modeling. In the vicinity of the Kedge, a Rydberg series expression is
used to fit R-matrix results, including important orbital relaxation effects,
that accurately predict the absorption oscillator strengths below threshold and
merge consistently and continuously to the above-threshold cross section.
Further minor adjustments are made to the threshold energies in order to
reliably align the atomic Rydberg resonances after consideration of both
experimental and observed line positions. At energies far below or above the
K-edge region, the formulation is based on both outer- and inner-shell direct
photoionization, including significant shake-up and shake-off processes that
result in photoionization-excitation and double photoionization contributions
to the total cross section. The ultimate purpose for developing a definitive
model for oxygen absorption is to resolve standing discrepancies between the
astronomically observed and laboratory measured line positions, and between the
inferred atomic and molecular oxygen abundances in the interstellar medium from
XSTAR and SPEX spectral models
New Ritz wavelengths and transition probabilities for parity-forbidden, singly ionized nickel [Ni II] lines of astrophysical interest
We report accurate Ritz wavelengths for parity-forbidden [Ni II] transitions, derived from energy levels determined using high-resolution Fourier transform spectroscopy. Transitions between the 18 lowest Ni II energy levels of even-parity produced Ritz wavelengths for 126 parity-forbidden lines. Uncertainties for the Ritz wavelengths derived in this work are up to two orders of magnitude lower than previously published values. Transition probabilities were calculated using the semi-empirical orthogonal operator method, with uncertainties ranging from approximately 1 per cent for strong M1 lines and up to 10 per cent for weak E2 lines. Accurate forbidden line wavelengths and transition probabilities, particularly for lines in the infrared, are important in the analyses of low-density astrophysical plasmas, such as supernova remnants, planetary nebulae, and active galactic nuclei
Keck Hires Observations of the QSO First J104459.6+365605: Evidence for a Large Scale Outflow
This paper presents an analysis of a Keck HIRES spectrum of the QSO FIRST
J104459.6+365605. The line of sight towards the QSO contains two clusters of
outflowing clouds that give rise to broad blue shifted absorption lines. The
outflow velocities of the clouds range from -200 to -1200 km/s and from -3400
to -5200 km/s, respectively. The width of the individual absorption lines
ranges from 50 to more than 1000 km/s. The most prominent absorption lines are
those of Mg II, Mg I, and Fe II. The low ionization absorption lines occur at
the same velocities as the most saturated Mg II lines, showing that the Fe II,
Mg I and Mg II line forming regions must be closely associated. Many absorption
lines from excited states of Fe II are present, allowing a determination of the
population of several low lying energy levels. From this we determine an
electron density in the Fe II line forming regions of 4000 per cubic cm.
Modelling the ionization state of the absorbing gas with this value of the
electron density as a constraint, we find that the distance between the Fe II
and Mg I line forming region and the continuum source is of order 700 parsec.
From the correspondence in velocity between the Fe II, Mg I and Mg II lines
we infer that the Mg II lines must be formed at the same distance. The Mg II
absorption fulfills the criteria for Broad Absorption Lines defined by Weymann
et al. (1991). This large distance is surprising, since BALs are generally
thought to be formed in outflows at a much smaller distance from the nucleus.Comment: 34 pages, 11 figures. Accepted by The Astrophysical Journa
Magnesium and silicon in interstellar dust: an X-ray overview
The dense Galactic environment is a large reservoir of interstellar dust.
Therefore, this region represents a perfect laboratory to study the properties
of the cosmic dust grains. X-rays are the most direct way to detect the
interaction of light with dust present in these dense environments. The
interaction between the radiation and the interstellar matter imprints specific
absorption features in the X-ray spectrum. We study them with the aim of
defining the chemical composition, the crystallinity and structure of the dust
grains which populate the inner regions of the Galaxy. We investigate the
magnesium and the silicon K-edges detected in the Chandra/HETG spectra of eight
bright X-ray binaries, distributed in the neighbourhood of the Galactic centre.
We model the two spectral features using accurate extinction cross sections of
silicates, that we have measured at the synchrotron facility Soleil, France.
Near the Galactic centre magnesium and silicon show abundances similar to the
solar ones and they are highly depleted from the gas phase
( and ). We find that amorphous
olivine with a composition of is the most representative
compound along all lines of sight according to our fits. The contribution of
Mg-rich silicates and quartz is low (less than ). On average we observe a
percentage of crystalline dust equal to . For the extragalactic source
LMC X-1, we find a preference for forsterite, a magnesium-rich olivine. Along
this line of sight we also observe an underabundance of silicon .Comment: 16 pages, 7 figures, recommended for publication in Astronomy and
Astrophysic
Model atmospheres of chemically peculiar stars: Self-consistent empirical stratified model of HD24712
High-resolution spectra of some chemically peculiar stars clearly demonstrate
the presence of strong abundance gradients in their atmospheres. However, these
inhomogeneities are usually ignored in the standard scheme of model atmosphere
calculations, braking the consistency between model structure and
spectroscopically derived abundance pattern. In this paper we present first
empirical self-consistent stellar atmosphere model of roAp star HD24712, with
stratification of chemical elements included, and which is derived directly
from the observed profiles of spectral lines without time-consuming simulations
of physical mechanisms responsible for these anomalies. We used the LLmodels
stellar model atmosphere code and DDAFIT minimization tool for analysis of
chemical elements stratification and construction of self-consistent
atmospheric model. Empirical determination of Pr and Nd stratification in the
atmosphere of HD24712 is based on NLTE line formation for Prii/iii and Ndii/iii
with the use of the DETAIL code. Based on iterative procedure of stratification
analysis and subsequent re-calculation of model atmosphere structure we
constructed a self-consistent model of HD24712, i.e. the model which
temperature-pressure structure is consistent with results of stratification
analysis. It is shown that stratification of chemical elements leads to the
considerable changes in model structure as to compare with non-stratified
homogeneous case. We find that accumulation of REE elements allows for the
inverse temperature gradient to be present in upper atmosphere of the star with
the maximum temperature increase of about 600K.Comment: Comments: Accepted by A&A, 16 pages, 10 figures, 3 table
Calibration and in orbit performance of the reflection grating spectrometer onboard XMM-Newton
Context: XMM-Newton was launched on 10 December 1999 and has been operational
since early 2000. One of the instruments onboard XMM-Newton is the reflection
grating spectrometer (RGS). Two identical RGS instruments are available, with
each RGS combining a reflection grating assembly (RGA) and a camera with CCDs
to record the spectra. Aims: We describe the calibration and in-orbit
performance of the RGS instrument. By combining the preflight calibration with
appropriate inflight calibration data including the changes in detector
performance over time, we aim at profound knowledge about the accuracy in the
calibration. This will be crucial for any correct scientific interpretation of
spectral features for a wide variety of objects. Methods: Ground calibrations
alone are not able to fully characterize the instrument. Dedicated inflight
measurements and constant monitoring are essential for a full understanding of
the instrument and the variations of the instrument response over time.
Physical models of the instrument are tuned to agree with calibration
measurements and are the basis from which the actual instrument response can be
interpolated over the full parameter space. Results: Uncertainties in the
instrument response have been reduced to < 10% for the effective area and < 6
mA for the wavelength scale (in the range from 8 A to 34 A. The remaining
systematic uncertainty in the detection of weak absorption features has been
estimated to be 1.5%. Conclusions: Based on a large set of inflight calibration
data and comparison with other instruments onboard XMM-Newton, the calibration
accuracy of the RGS instrument has been improved considerably over the
preflight calibrations.Comment: Accepted for publication in Astronomy and Astrophysics, Astronomical
instrumentation sectio
Interstellar Iron and Silicon Depletions in Translucent Sight Lines
We report interstellar FeII and SiII column densities toward six translucent
sight lines (A_V >~ 1) observed with the Space Telescope Imaging Spectrograph
(STIS). The abundances were determined from the absorption of SiII] at 2335
Angstroms, and several weak Fe transitions including the first reported
detections of the 2234 Angstrom line. We derive an empirical f-value for the
FeII 2234 Angstrom transition of log(f lambda) = -1.54 +/- 0.05. The observed
sight lines sample a variety of extinction characteristics as indicated by
their R_V values, which range from 2.6 - 5.8. The dust-phase abundances of both
Si and Fe are positively correlated with the small-grain population (effective
radii smaller than a few hundred micron) toward the targets. The physical
conditions along the sight lines suggest that this relationship may be due to
differences in the survival of small particles in some interstellar
environments. The chemical composition of the small grains could either
resemble dust mantles or be silicate rich.Comment: accepted for publication in Ap
A self-consistent empirical model atmosphere, abundance and stratification analysis of the benchmark roAp star alpha Circini
Chemically peculiar (CP) stars are unique natural laboratories for
investigation of the microscopic diffusion processes of chemical elements. The
element segregation under the influence of gravity and radiation pressure leads
to the appearance of strong abundance gradients in the atmospheres of CP stars.
Consequently, the atmospheric temperature-pressure structure of these objects
could deviate significantly from the atmospheres of normal stars with
homogeneous abundances. In this study we performed a self-consistent, empirical
model atmosphere study of the brightest rapidly oscillating Ap star alpha Cir.
We account for chemical stratification in the model atmosphere calculations and
assess the importance of non-uniformed vertical element distribution on the
model structure, energy distribution and hydrogen line profiles. Based on an
iterative procedure of the chemical abundance analysis of 52 ions of 35
elements, stratification modeling of 4 elements (Si, Ca, Cr and Fe) and
subsequent re-calculations of the atmospheric structure, we derived a new model
atmosphere of alpha Cir, which is consistent with the inferred atmospheric
chemistry of the star. We find Teff=7500 K, logg=4.1, and demonstrate that
chemical stratification has a noticeable impact on the model structure and
modifies the formation of the hydrogen Balmer lines. Our spectroscopically
determined Teff of alpha Cir agrees with the fundamental effective temperature
of this star. This shows that temperatures inferred in detailed spectroscopic
analyses of cool magnetic CP stars are not affected by a large systematic bias.Comment: 14 pages, 11 figures; accepted for publication in Astronomy &
Astrophysic
- …