29 research outputs found
Fundamental Parameters and Chemical Composition of Arcturus
We derive a self-consistent set of atmospheric parameters and abundances of
17 elements for the red giant star Arcturus: Teff = 4286+/-30 K, logg =
1.66+/-0.05, and [Fe/H] = -0.52+/-0.04. The effective temperature was
determined using model atmosphere fits to the observed spectral energy
distribution from the blue to the mid-infrared (0.44 to 10 um). The surface
gravity was calculated using the trigonometric parallax of the star and stellar
evolution models. A differential abundance analysis relative to the solar
spectrum allowed us to derive iron abundances from equivalent width
measurements of 37 FeI and 9 FeII lines, unblended in the spectra of both
Arcturus and the Sun; the [Fe/H] value adopted is derived from FeI lines. We
also determine the mass, radius, and age of Arcturus: M = 1.08+/-0.06 Msun, R =
25.4+/-0.2 Rsun, and t = 7.1(+1.5/-1.2) Gyr. Finally, abundances of the
following elements are measured from an equivalent width analysis of atomic
features: C, O, Na, Mg, Al, Si, K, Ca, Sc, Ti, V, Cr, Mn, Co, Ni, and Zn. We
find the chemical composition of Arcturus typical of that of a local thick-disk
star, consistent with its kinematics.Comment: ApJ, in pres
Formation of MnI lines in the solar atmosphere
We present a detailed NLTE analysis of 39 MnI lines in the solar spectrum.
The influence of NLTE effects on the line formation and element abundance is
investigated. Our goal is the derivation of solar log gfe values for manganese
lines, which will later be used in differential abundance analysis of
metal-poor stars. The method of spectrum synthesis is employed, which is based
on a solar model atmosphere with initially specified element abundances. A
manganese abundance of 5.47 dex is used with the theoretical line-blanketed
model atmosphere. Statistical equilibrium calculations are carried out for the
model atom, which comprises 245 and 213 levels for MnI and MnII, respectively.
Photoionization cross-sections are assumed hydrogenic. For line synthesis van
der Waals broadening is calculated according to Anstee & O'Mara's formalism. It
is shown that hyperfine structure of the Mn lines also has strong broadening
effects, and that manganese is prone to NLTE effects in the solar atmosphere.
The nature of the NLTE effects and the validity of the LTE approach are
discussed in detail. The role of photoionization and collisional interaction is
investigated.Comment: 17 pages, 27 figures, accepted for publication in A&
C, N and O abundances in red clump stars of the Milky Way
The Hipparcos orbiting observatory has revealed a large number of
helium-core-burning "clump" stars in the Galactic field. These low-mass stars
exhibit signatures of extra-mixing processes that require modeling beyond the
first dredge-up of standard models. The 12C/13C ratio is the most robust
diagnostic of deep mixing, because it is insensitive to the adopted stellar
parameters. In this work we present 12C/13C determinations in a sample of 34
Galactic clump stars as well as abundances of nitrogen, carbon and oxygen.
Abundances of carbon were studied using the C2 Swan (0,1) band head at 5635.5
A. The wavelength interval 7980-8130 A with strong CN features was analysed in
order to determine nitrogen abundances and 12C/13C isotope ratios. The oxygen
abundances were determined from the [O I] line at 6300 A. Compared with the Sun
and dwarf stars of the Galactic disk, mean abundances in the investigated clump
stars suggest that carbon is depleted by about 0.2 dex, nitrogen is enhanced by
0.2 dex and oxygen is close to abundances in dwarfs. Comparisons to
evolutionary models show that the stars fall into two groups: the one is of
first ascent giants with carbon isotope ratios altered according to the first
dredge-up prediction, and the other one is of helium-core-burning stars with
carbon isotope ratios altered by extra mixing. The stars investigated fall to
these groups in approximately equal numbers.Comment: 8 pages 6 figures Accepted for publication in MNRA
Stellar abundances and ages for metal-rich Milky Way globular clusters - Stellar parameters and elemental abundances for 9 HB stars in NGC6352
[ABRIDGED] Metal-rich globular clusters provide important tracers of the
formation of our Galaxy. Moreover, and not less important, they are very
important calibrators for the derivation of properties of extra-galactic
metal-rich stellar populations. Nonetheless, only a few of the metal-rich
globular clusters in the Milky Way have been studied using high-resolution
stellar spectra to derive elemental abundances. In this paper we present
elemental abundances for nine HB stars in the metal-rich globular cluster
NGC6352. The elemental abundances are based on high-resolution, high
signal-to-noise spectra obtained with VLT/UVES. The elemental abundances have
been derived using standard LTE calculations. We find that NGC6352 has [Fe/H]=
-0.55, is enhanced in the alpha-elements to about +0.2 dex for Ca, Si, and Ti
relative to Fe. For the iron-peak elements we find solar values. Based on the
spectroscopically derived stellar parameters we find that an E(B-V)=0.24 and
(m-M) roughly equal to 14.05 better fits the data than the nominal values. An
investigation of log(gf)-values for suitable FeI lines lead us to the
conclusion that the commonly used correction to the May et al.(1974) data
should not be employed. Note: only the postscript reproduces the finding chart
correctly.Comment: 24 pages (including on-line only table with all equivalent width
measurements), 12 figures, Accepted for publication in A&A. Note: only the
postscript reproduces the finding chart correctl
ISO-SWS calibration and the accurate modelling of cool-star atmospheres - II. General results
The fine calibration of the ISO-SWS detectors (Infrared Space Observatory -
Short Wavelength Spectrometer) has proven to be a delicate problem. We
therefore present a detailed spectroscopic study in the 2.38 -- 12 micron
wavelength range of a sample of 16 A0 -- M2 stars used for the calibration of
ISO-SWS. By investigating the discrepancies between the ISO-SWS data of these
sources, the theoretical predictions of their spectra, the high-resolution
FTS-KP (Kitt Peak) spectrum of Alpha Boo and the solar FTS-ATMOS (Atmospheric
Trace Molecule Spectroscopy) spectrum, both calibration problems and problems
in computing the theoretical models and the synthetic spectra are revealed. The
underlying reasons for these problems are sought for and the impact on the
further calibration of ISO-SWS and on the theoretical modelling is discussed
extensively.Comment: 17 pages, 19 figures; Astronomy and Astrophysics, in press; preprints
can be obtained by contacting [email protected] or via WWW on
http://www.ster.kuleuven.ac.be/~leen or via anonymous ftp on
ftp://ftp.ster.kuleuven.ac.be/dist/leen/latex/h3317 Appendix electronically
available (3 pages, 7 figures
Oxygen trends in the Galactic thin and thick disks
We present oxygen abundances for 72 F and G dwarf stars in the solar
neighbourhood. Using the kinematics of the stars we divide them into two
sub-samples with space velocities that are typical for the thick and thin
disks, respectively. The metallicities of the stars range from [Fe/H] = -0.9 to
+0.4 and we use the derived oxygen abundances of the stars to: (1) perform a
differential study of the oxygen trends in the thin and the thick disk; (2) to
follow the trend of oxygen in the thin disk to the highest metallicities. We
analyze the forbidden oxygen lines at 6300 {\AA} and 6363 {\AA} as well as the
(NLTE afflicted) triplet lines around 7774 {\AA}. For the forbidden line at
6300 A we have spectra of very high S/N (>400) and resolution (R=215 000). This
has enabled a very accurate modeling of the oxygen line and the blending Ni
lines. The high internal accuracy in our determination of the oxygen abundances
from this line is reflected in the very tight trends we find for oxygen
relative to iron........Comment: Accepted for publication in A&A, 16 page
The photospheric abundances of active binaries II. Atmospheric parameters and abundance patterns for 6 single-lined RS CVn systems
Photospheric parameters and abundances are presented for a sample of
single-lined chromospherically active binaries from a differential LTE analysis
of high-resolution spectra. Abundances have been derived for 13 chemical
species, including several key elements such as Li, Mg, and Ca. Two methods
have been used. The effective temperatures, surface gravities and
microturbulent velocities were first derived from a fully self-consistent
analysis of the spectra, whereby the temperature is determined from the
excitation equilibrium of the Fe I lines. The second approach relies on
temperatures derived from the (B-V) colour index. These two methods give
broadly consistent results for the stars in our sample, suggesting that the
neutral iron lines are formed under conditions close to LTE. We discuss the
reliability in the context of chromospherically active stars of various colour
indices used as temperature indicators, and conclude that the (V-R) and (V-I)
colours are likely to be significantly affected by activity processes.
Irrespective of the method used, our results indicate that the X-ray active
binaries studied are not as metal poor as previously claimed, but are at most
mildly iron-depleted relative to the Sun (-0.41<[Fe/H]<+0.11). A significant
overabundance of several chemical species is observed (e.g., the
alpha-synthezised elements). These abundance patterns are discussed in relation
to stellar activity.Comment: 20 pages, 14 figures, accepted for publication in A&
ISO-SWS calibration and the accurate modelling of cool-star atmospheres - IV. G9 to M2 stars
presented. The 2.38 -- 4.08 micron wavelength-range of band 1 of ISO-SWS
(Short-Wavelength Spectrometers on board of the Infrared Space Observatory) in
which many different molecules -- with their own dependence on each of the
stellar parameters -- are absorbing, enables us to estimate the effective
temperature, the gravity, the microturbulence, the metallicity, the
CNO-abundances, the 12C/13C-ratio and the angular diameter from the ISO-SWS
data. Using the Hipparcos' parallax, the radius, luminosity and
gravity-inferred mass are derived. The stellar parameters obtained are in good
agreement with other published values, though also some discrepancies with
values deduced by other authors are noted. For a few stars (Delta Dra, Xi Dra,
Alpha Tuc, H Sco and Alpha Cet) some parameters -- e.g. the CNO-abundances --
are derived for the first time. By examining the correspondence between
different ISO-SWS observations of the same object and between the ISO-SWS data
and the corresponding synthetic spectrum, it is shown that the relative
accuracy of ISO-SWS in band 1 (2.38 -- 4.0 micron) is better than 2% for these
high-flux sources. The high level of correspondence between observations and
theoretical predictions, together with a confrontation of the estimated
Teff(ISO) value with Teff-values derived from colours -- which demonstrates the
consistency between V-K, BC(K), Teff and the angular diameter derived from
optical or IR data -- proves that both the used MARCS models to derive the
stellar quantities and the flux calibration of the ISO-SWS detectors have
reached a high level of reliability.Comment: 19 pages, 15 figures; Astronomy and Astrophysics, in press; preprints
can be obtained by contacting [email protected] or via WWW on
http://www.ster.kuleuven.ac.be/~leen or via anonymous ftp on
ftp://ftp.ster.kuleuven.ac.be/dist/leen/latex/h3318 Appendix electronically
available (26 pages, 22 figures