242 research outputs found
Probing the Site for r-Process Nucleosyntheis with Abundances of Barium and Magnesium in Extremely Metal-Poor Stars
We suggest that if the astrophysical site for r-process nucleosynthesis in
the early Galaxy is confined to a narrow mass range of Type II supernova (SN
II) progenitors, with a lower mass limit of Mms = 20 Msun, a unique feature in
the observed distribution of [Ba/Mg] vs.[Mg/H] for extremely metal-poor stars
can be adequately reproduced. We associate this feature, a bifurcation of the
observed elemental ratios into two branches in the Mg abundance interval -2.7 <
[Mg/H] < -2.3, with two distinct processes. The first branch, which we call the
``y''-branch, is associated with the production of Ba and Mg from individual
massive supernovae. We conclude that SNe II with Mms = 20 Msun are the dominant
source of r-process nucleosynthesis in the early Galaxy. An SN-induced chemical
evolution model with this Mms-dependent Ba yield creates the y-branch,
reflecting the different nucleosynthesis yields of [Ba/Mg] for each SN II with
Mms > 20 Msun. The second branch, which we call the ``i''-branch, is associated
with the elemental abundance ratios of stars which were formed in the dense
shells of the interstellar medium swept up by SNe II with Mms < 20 Msun that do
not synthesize r-process elements, and applies to stars with observed Mg
abundances in the range [Mg/H] < -2.7. The Ba abundances in these stars reflect
those of the interstellar gas at the (later) time of their formation. The
existence of a [Ba/Mg] i-branch strongly suggests that SNe II which are
associated with stars of progenitor mass Mms < 20 Msun are infertile sources
for the production of r-process elements. We predict the existence of this
i-branch for other r-process elements, such as europium (Eu), to the extent
that their production site is in common with Ba.Comment: 6 pages including 3 figures, to appear in ApJ Letter
Jet-Induced Nucleosynthesis in Misaligned Microquasars
The jet axes and the orbital planes of microquasar systems are usually
assumed to be approximately perpendicular, eventhough this is not currently an
observational requirement. On the contrary, in one of the few systems where the
relative orientations are well-constrained, V4641Sgr, the jet axis is known to
lie not more than ~36 degrees from the binary plane. Such a jet, lying close to
the binary plane, and traveling at a significant fraction of the speed of light
may periodically impact the secondary star initiating nuclear reactions on its
surface. The integrated yield of such nuclear reactions over the age of the
binary system (less the radiative mass loss) will detectably alter the
elemental abundances of the companion star. This scenario may explain the
anomalously high Li enhancements (roughly ~20-200 times the sun's photospheric
value; or, equivalently, 0.1-1 times the average solar system value) seen in
the companions of some black-hole X-ray binary systems. (Such enhancements are
puzzling since Li nuclei are exceedingly fragile - being easily destroyed in
the interiors of stars - and Li would be expected to be depleted rather than
enhanced there.) Gamma-ray line signatures of the proposed process could
include the 2.22 MeV neutron capture line as well as the 0.478 MeV 7Li*
de-excitation line, both of which may be discernable with the INTEGRAL
satellite if produced in an optically thin region during a large outburst. For
very energetic jets, a relatively narrow neutral pion gamma-decay signature at
67.5 MeV could also be measurable with the GLAST satellite. We argue that about
10-20% of all microquasar systems ought to be sufficiently misaligned as to be
undergoing the proposed jet-secondary impacts.Comment: ApJ, accepted. Includes referee's suggestions and some minor
clarifications over previous versio
Atomic Diffusion and Mixing in Old Stars I. VLT/FLAMES-UVES Observations of Stars in NGC 6397
We present a homogeneous photometric and spectroscopic analysis of 18 stars
along the evolutionary sequence of the metal-poor globular cluster NGC 6397
([Fe/H] = -2), from the main-sequence turnoff point to red giants below the
bump. The spectroscopic stellar parameters, in particular stellar-parameter
differences between groups of stars, are in good agreement with broad-band and
Stroemgren photometry calibrated on the infrared-flux method. The spectroscopic
abundance analysis reveals, for the first time, systematic trends of iron
abundance with evolutionary stage. Iron is found to be 31% less abundant in the
turnoff-point stars than in the red giants. An abundance difference in lithium
is seen between the turnoff-point and warm subgiant stars. The impact of
potential systematic errors on these abundance trends (stellar parameters, the
hydrostatic and LTE approximations) is quantitatively evaluated and found not
to alter our conclusions significantly. Trends for various elements (Li, Mg,
Ca, Ti and Fe) are compared with stellar-structure models including the effects
of atomic diffusion and radiative acceleration. Such models are found to
describe the observed element-specific trends well, if extra (turbulent) mixing
just below the convection zone is introduced. It is concluded that atomic
diffusion and turbulent mixing are largely responsible for the sub-primordial
stellar lithium abundances of warm halo stars. Other consequences of atomic
diffusion in old metal-poor stars are also discussed.Comment: 20 pages (emulateapj), 11 figures, accepted for publication in Ap
Center-to-Limb Variation of Solar Line Profiles as a Test of NLTE Line Formation Calculations
We present new observations of the center-to-limb variation of spectral lines
in the quiet Sun. Our long-slit spectra are corrected for scattered light,
which amounts to 4-8 % of the continuum intensity, by comparison with a Fourier
transform spectrum of the disk center. We examine the effect of inelastic
collisions with neutral hydrogen in NLTE line formation calculations of the
oxygen infrared triplet, and the Na I 6160.8 A line. Adopting a classical
one-dimensional theoretical model atmosphere, we find that the sodium
transition, formed in higher layers, is much more effectively thermalized by
hydrogen collisions than the high-excitation oxygen lines. This result appears
as a simple consequence of the decrease of the ratio NH/Ne with depth in the
solar photosphere. The center-to-limb variation of the selected lines is
studied both under LTE and NLTE conditions. In the NLTE analysis, inelastic
collisions with hydrogen atoms are considered with a simple approximation or
neglected, in an attempt to test the validity of such approximation. For the
sodium line studied, the best agreement between theory and observation happens
when NLTE is considered and inelastic collisions with hydrogen are neglected in
the rate equations. The analysis of the oxygen triplet benefits from a very
detailed calculation using an LTE three-dimensional model atmosphere and NLTE
line formation. The chi**2 statistics favors including hydrogen collisions with
the approximation adopted, but the oxygen abundance derived in that case is
significantly higher than the value derived from OH infrared transitions.Comment: 10 pages, 8 figures, to appear in A&
Li - O anti-correlation in NGC 6752: evidence for Li-enriched polluting gas
Elemental correlations and anti-correlations are known to be present in
globular clusters (GCs) owing to pollution by CNO cycled gas. Because of its
fragility Li is destroyed at the temperature at which the CNO cycling occurs,
and this makes Li a crucial study for the nature of the contaminating stars. We
observed 112 un-evolved stars at the Turnoff of the NGC6752 cluster with FLAMES
at the VLT to investigate the presence and the extent of a Li-O correlation.
This correlation is expected if there is a simple pollution scenario. Li (670.8
nm) and O triplet (771 nm) abundances are derived in NLTE. All stars belong to
a very narrow region of the color-magnitude diagram, so they have similar
stellar parameters (Teff, log g). We find that O and Li correlate, with a high
statistical significance that confirms the early results for this cluster. At
first glance this is what is expected if a simple pollution of pristine gas
with CNO cycled gas (O-poor, Li-poor) occurred. The slope of the relationship,
however, is about 0.4, and differs from unity by over 7 Sigma. A slope of one
is the value predicted for a pure contamination model. We confirm an extended
Li-O correlation in non evolved stars of NGC 6752. At the same time the
characteristic of the correlation shows that a simple pollution scenario is not
sufficient to explain the observations. Within this scenario the contaminant
gas must have been enriched in Li. This would rule out massive stars as main
polluters, and favor the hypothesis that the polluting gas was enriched by
intermediate or high-mass AGB stars, unless the former can be shown to be able
to produce Li. According to our observations, the fraction of polluting gas
contained in the stars observed is a considerable fraction of the stellar mass
of the cluster.Comment: 8pages, 2 figures, accepted by A&A Lette
Stellar Iron Abundances: non-LTE Effects
We report new statistical equilibrium calculations for Fe I and Fe II in the
atmosphere of Late-Type stars. We used atomic models for Fe I and Fe II having
respectively 256 and 190 levels, as well as 2117 and 3443 radiative
transitions. Photoionization cross-sections are from the Iron Project. These
atomic models were used to investigate non-LTE effects in iron abundances of
Late-Type stars with different atmospheric parameters.
We found that most Fe I lines in metal-poor stars are formed in conditions
far from LTE. We derived metallicity corrections of about 0.3 dex with respect
to LTE values, for the case of stars with [Fe/H] ~ -3.0. Fe II is found not to
be affected by significant non-LTE effects. The main non-LTE effect invoked in
the case of Fe I is overionization by ultraviolet radiation, thus classical
ionization equilibrium is far to be satisfied. An important consequence is that
surface gravities derived by LTE analysis are in error and should be corrected
before final abundances corrections.
This apparently solves the observed discrepancy between spectroscopic surface
gravities derived by LTE analyses and those derived from Hipparcos parallaxes.
A table of non-LTE [Fe/H] and log g values for a sample of metal-poor late-type
stars is given.Comment: 22 pages, 9 figures, 1 table, ApJ style, accepte
The solar photospheric abundance of carbon.Analysis of atomic carbon lines with the CO5BOLD solar model
The use of hydrodynamical simulations, the selection of atomic data, and the
computation of deviations from local thermodynamical equilibrium for the
analysis of the solar spectra have implied a downward revision of the solar
metallicity. We are in the process of using the latest simulations computed
with the CO5BOLD code to reassess the solar chemical composition. We determine
the solar photospheric carbon abundance by using a radiation-hydrodynamical
CO5BOLD model, and compute the departures from local thermodynamical
equilibrium by using the Kiel code. We measure equivalent widths of atomic CI
lines on high resolution, high signal-to-noise ratio solar atlases. Deviations
from local thermodynamic equilibrium are computed in 1D with the Kiel code. Our
recommended value for the solar carbon abundance, relies on 98 independent
measurements of observed lines and is A(C)=8.50+-0.06, the quoted error is the
sum of statistical and systematic error. Combined with our recent results for
the solar oxygen and nitrogen abundances this implies a solar metallicity of
Z=0.0154 and Z/X=0.0211. Our analysis implies a solar carbon abundance which is
about 0.1 dex higher than what was found in previous analysis based on
different 3D hydrodynamical computations. The difference is partly driven by
our equivalent width measurements (we measure, on average, larger equivalent
widths with respect to the other work based on a 3D model), in part it is
likely due to the different properties of the hydrodynamical simulations and
the spectrum synthesis code. The solar metallicity we obtain from the CO5BOLD
analyses is in slightly better agreement with the constraints of
helioseismology than the previous 3D abundance results. (Abridged)Comment: Astronomy and Astrophysics, accepte
Spectroscopic abundance analysis of dwarfs in young open cluster IC 4665
We report a detailed spectroscopic abundance analysis for a sample of 18 F-K
dwarfs of the young open cluster IC 4665. Stellar parameters and element
abundances of Li, O, Mg, Si, Ca, Ti, Cr, Fe and Ni have been derived using the
spectroscopic synthesis tool SME (Spectroscopy Made Easy). Within the
measurement uncertainties the iron abundance is uniform with a standard
deviation of 0.04 dex. No correlation is found between the iron abundance and
the mass of the stellar convective zone, and between the Li abundance and the
Fe abundance. In other words, our results do not reveal any signature of
accretion and therefore do not support the scenario that stars with planets
(SWPs) acquire their on the average higher metallicity compared to field stars
via accretion of metal-rich planetary material. Instead the higher metallicity
of SWPs may simply reflect the fact that planet formation is more efficient in
high metallicity environs. However, since that many details of the planet
system formation processes remain poorly understood, further studies are needed
for a final settlement of the problem of the high metallicity of SWPs.
The standard deviation of [Fe/H] deduced from our observations, taken as an
upper limit on the metallicity dispersion amongst the IC 4665 member stars, has
been used to constrain proto-planetary disk evolution, terrestrial and giant
planets formation and evolution processes. Our results do not support the
possibility that the migration of gas giants and the circularization of
terrestrial planets' orbits are regulated by their interaction with a residual
population of planetesimals and dust particles.Comment: 18 pages, 6 figures, accepted for publication in Ap
On inelastic hydrogen atom collisions in stellar atmospheres
The influence of inelastic hydrogen atom collisions on non-LTE spectral line
formation has been, and remains to be, a significant source of uncertainty for
stellar abundance analyses, due to the difficulty in obtaining accurate data
for low-energy atomic collisions either experimentally or theoretically. For
lack of a better alternative, the classical "Drawin formula" is often used.
Over recent decades, our understanding of these collisions has improved
markedly, predominantly through a number of detailed quantum mechanical
calculations. In this paper, the Drawin formula is compared with the quantum
mechanical calculations both in terms of the underlying physics and the
resulting rate coefficients. It is shown that the Drawin formula does not
contain the essential physics behind direct excitation by H atom collisions,
the important physical mechanism being quantum mechanical in character.
Quantitatively, the Drawin formula compares poorly with the results of the
available quantum mechanical calculations, usually significantly overestimating
the collision rates by amounts that vary markedly between transitions.Comment: 9 pages, 6 figures, accepted for A&
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