7,234 research outputs found
On the Abundance of Holmium in the Sun
The abundance of holmium (Z = 67) in the Sun remains uncertain. The
photospheric abundance, based on lines of Ho II, has been reported as +0.26 +/-
0.16 (on the usual scale where log(H) = 12.00), while the meteoretic value is
+0.51 +/- 0.02. Cowan code calculations have been undertaken to improve the
partition function for this ion by including important contributions from
unobserved levels arising from the (4f^{11}6p + 4f^{10}(5d + 6s)^{2}) group.
Based on 6994 computed energy levels, the partition function for Ho II is 67.41
for a temperature of 6000 K. This is approximately 1.5 times larger than the
value derived from the 49 published levels. The new partition function alone
leads to an increase in the solar abundance of Ho to log(Ho) = +0.43. This is
within 0.08 dex of the meteoretic abundance. Support for this result has been
obtained through LTE spectrum synthesis calculations of a previously
unidentified weak line at 3416.38 A in the solar spectrum. Attributing the
feature to Ho II, the observations may be fitted with log(Ho) = +0.53. This
calculation assumes log(gf) = 0.25 and is uncertain by at least 0.1 dex.Comment: 16 pages, 4 figures, accepted for publication in Solar Physic
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
Line identification studies using traditional techniques and wavelength coincidence statistics
Traditional line identification techniques result in the assignment of individual lines to an atomic or ionic species. These methods may be supplemented by wavelength coincidence statistics (WCS). The strength and weakness of these methods are discussed using spectra of a number of normal and peculiar B and A stars that have been studied independently by both methods. The present results support the overall findings of some earlier studies. WCS would be most useful in a first survey, before traditional methods have been applied. WCS can quickly make a global search for all species and in this way may enable identifications of an unexpected spectrum that could easily be omitted entirely from a traditional study. This is illustrated by O I. WCS is a subject to well known weakness of any statistical technique, for example, a predictable number of spurious results are to be expected. The danger of small number statistics are illustrated. WCS is at its best relative to traditional methods in finding a line-rich atomic species that is only weakly present in a complicated stellar spectrum
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