4,854 research outputs found
Implementing TOPbase/Iron Project: Continuous Absorption from Fe II
We discuss implementation of TOPbase and Iron Project opacities for stellar
spectral codes. We use a technique employed by Peach, where a
Boltzmann-averaged cross section is calculated for selected temperatures, and
the opacity obtained from double interpolation in temperature and wavelength.
It is straightforward to include {\it all} levels for which cross sections have
been calculated. Boltzmann-averaged cross sections for Fe II show a local
maximum between 1700 and 2000[A]. We suggest this feature arises from 3d^5 4snl
to 3d^5 4pnl transitions within Fe II. IUE spectra of iron-rich CP stars show
local minima in this region. Theoretical calculations of a representative
stellar continuum demonstrate that Fe II photoionization contributes
significantly to the observed minima.Comment: 6 pages, 5 figures. See
http://www.astro.lsa.umich.edu/users/cowley/paper2r for better definition
figure
New Mn II energy levels from STIS-HST spectrum of the HgMn star HD 175640
The NIST database lists several Mn II lines that were observed in the
laboratory but not classified. They cannot be used in spectrum synthesis
because their atomic line data are unknown. These lines are concentrated in the
2380-2700 A interval. We aimed to assign energy levels and log gf values to
these lines. Semi-empirical line data for Mn II computed by Kurucz were used to
synthesize the ultraviolet spectrum of the slow-rotating, HgMn star HD 175640.
The spectrum was compared with the high-resolution spectrum observed with the
HST-STIS equipment. A UVES spectrum covering the 3050-10000 A region was also
examined. We determined a total of 73 new energy levels, 58 from the STIS
spectrum of HD 175640 and another 15 from the UVES spectrum. The new energy
levels give rise to numerous new computed lines. We have identified more than
50% of the unclassified lines listed in the NIST database and have changed the
assignement of another 24 lines. An abundance analysis of the star HD 175640,
based on the comparison of observed and computed ultraviolet spectra in the
1250-3040 A interval, is the by-product of this study on Mn II.Comment: Paper accepted by Astronomy & Astrophysic
The Puzzling Spectrum of HD 94509
The spectral features of HD 94509 are highly unusual, adding an extreme to
the zoo of Be and shell stars. The shell dominates the spectrum, showing lines
typical for spectral types mid-A to early-F, while the presence of a late/mid
B-type central star is indicated by photospheric hydrogen line wings and helium
lines. Numerous metallic absorption lines have broad wings but taper to narrow
cores. They cannot be fit by Voigt profiles.
We aim to describe and illustrate unusual spectral features of this star, and
make rough calculations to estimate physical conditions and abundances in the
shell. Furthermore, the central star is characterized.
We assume mean conditions for the shell. An electron density estimate is made
from the Inglis-Teller formula. Excitation temperatures and column densities
for Fe I and Fe II are derived from curves of growth. The neutral H column
density is estimated from high Paschen members. The column densities are
compared with calculations made with the photoionization code Cloudy.
Atmospheric parameters of the central star are constrained employing non-LTE
spectrum synthesis.
Overall chemical abundances are close to solar. Column densities of the
dominant ions of several elements, as well as excitation temperatures and the
mean electron density are well accounted for by a simple model. Several
features, including the degree of ionization, are less well described.
HD 94509 is a Be star with a stable shell, close to the terminal-age main
sequence. The dynamical state of the shell and the unusually shaped, but
symmetric line profiles, require a separate study.Comment: 10 pages, 9 tables, 13 figures; accepted for publication by Astronomy
and Astrophysic
High resolution spectroscopy of HgMn stars: a time of surprises
We present the results of a high spectral resolution study of a few
spectroscopic binaries with HgMn primary stars. We detect for the first time in
the spectra of HgMn stars that for many elements the line profiles are variable
over the rotation period. The strongest profile variations are found for the
elements Pt, Hg, Sr, Y, Zr, Mn, Ga, He and Nd. The slight variability of He and
Y is also confirmed from the study of high resolution spectra of another HgMn
star, alpha And.Comment: 2 pages, 2 figures, to appear in "Precision Spectroscopy in
Astrophysics
Explosive Ballooning Flux Tubes in Tokamaks
Tokamak stability to, potentially explosive, `ballooning' displacements of
elliptical magnetic flux tubes is examined in large aspect ratio equilibrium.
Above a critical pressure gradient the energy stored in the plasma may be
lowered by finite (but not infinitesimal) displacements of such tubes
(metastability). Above a higher pressure gradient, the linear stability
boundary, such tubes are linearly and nonlinearly unstable. The flux tube
displacement can be of the order of the pressure gradient scale length. Plasma
transport from displaced flux tubes may result in rapid loss of confinement.Comment: 4 pages, 6 figure
Dynamical detection of three triple stellar systems in open clusters
We present a kinematic analysis of three triple stellar systems belonging to
two open clusters: CPD-60{\deg}961 and HD66137 in NGC2516, and HD315031 in
NGC6530. All three systems are hierarchical triples with a close binary bound
to a third body in a wider orbit, whose presence is detected through velocity
variations of the close binary barycentre. Orbital parameters are derived from
radial velocity curves. Absolute parameters for all stars are estimated
assuming cluster membership. Some dynamical and evolutionary aspects of these
systems are discussed, particularly the possible influence of Kozai cycles. The
two systems of NGC2516 have similar orbital configurations with inner periods
of 11.23 d and 8.70 d and outer periods of 9.79 yr and 9.24 yr. We report also
radial velocity measurements of the components of the visual binary
CPD-60{\deg}944 in NGC2516. Including results from previous works, this cluster
would harbor 5 hierarchical triples. The young system HD315031 has an inner
binary with a period of 1.37 d and a very eccentric (e=0.85) outer orbit with a
period of 483 d. Possible dynamical evolutionary scenarios are discussed.
Long-term radial velocity monitoring is highlighted as strategy for the
detection of subsystems with intermediate separations, which are hard to cover
with normal spectroscopic studies or visual techniques.Comment: 9 pages, 5 figures. Accepted by Monthly Notices of the Royal
Astronomical Societ
Lanthanides and other spectral oddities in a Centauri
Context: There is considerable interest in the helium variable a Cen as a
bridge between helium-weak and helium-strong CP stars. Aims: We investigate Ce
III and other possible lanthanides in the spectrum the of hottest chemically
peculiar (CP) star in which these elements have been found. A {Kr II line
appears within a broad absorption which we suggest may be due to a high-level
transition in C II. Methods: Wavelengths and equivalent widths are measured on
high-resolution UVES spectra, analyzed, and their phase-variations
investigated. Results: New, robust identifications of Ce III and Kr II are
demonstrated. Nd III is likely present. A broad absorption near 4619[A] is
present at all phases of a Cen, and in some other early B stars. Conclusions:
The presence of lanthanides in a Cen strengthens the view that this star is a
significant link between the cooler CP stars and the hotter helium-peculiar
stars. Broad absorptions in a Cen are not well explained.Comment: Research Note accepted by Astronomy and Astrophysics; 4 pages, 4
Figs. 2 Table
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