60 research outputs found
On mesogranulation, network formation and supergranulation
We present arguments which show that in all likelihood mesogranulation is not
a true scale of solar convection but the combination of the effects of both
highly energetic granules, which give birth to strong positive divergences
(SPDs) among which we find exploders, and averaging effects of data processing.
The important role played by SPDs in horizontal velocity fields appears in the
spectra of these fields where the scale 4 Mm is most energetic; we
illustrate the effect of averaging with a one-dimensional toy model which shows
how two independent non-moving (but evolving) structures can be transformed
into a single moving structure when time and space resolution are degraded.
The role of SPDs in the formation of the photospheric network is shown by
computing the advection of floating corks by the granular flow. The coincidence
of the network bright points distribution and that of the corks is remarkable.
We conclude with the possibility that supergranulation is not a proper scale of
convection but the result of a large-scale instability of the granular flow,
which manifests itself through a correlation of the flows generated by SPDs.Comment: 10 pages, 11 figures, to appear in Astronomy and Astrophysic
Abundances and Kinematics of Field Halo and Disk Stars I: Observational Data and Abundance Analysis
We describe observations and abundance analysis of a high-resolution,
high-S/N survey of 168 stars, most of which are metal-poor dwarfs. We follow a
self-consistent LTE analysis technique to determine the stellar parameters and
abundances, and estimate the effects of random and systematic uncertainties on
the resulting abundances. Element-to-iron ratios are derived for key alpha,
odd, Fe-peak, r- and s-process elements. Effects of Non-LTE on the analysis of
Fe I lines are shown to be very small on the average. Spectroscopically
determined surface gravities are derived that are generally close to those
obtained from Hipparcos parallaxes.Comment: 41 pages, 7 Postscript figures. Accepted for publication in the A
A Comparison of Copper Abundances in Globular Cluster and Halo Field Giant Stars
We derive [Cu/Fe] for 117 giant stars in ten globular clusters (M3, M4, M5,
M10, M13, M15, M71, NGC 7006, NCG 288, and NGC 362) and find that globular
cluster Cu abundances appear to follow [Cu/Fe] trends found in the field. This
result is interesting in light of recent work which indicates that the globular
cluster Omega Centauri shows no trend in [Cu/Fe] with [Fe/H] over the abundance
range -2.0 <[Fe/H]< -0.8. Of particular interest are the two clusters M4 and
M5. While at a similar metallicity ([Fe/H] ~- 1.2), they differ greatly in some
elemental abundances: M4 is largely overabundant in Si, Ba, and La compared to
M5. We find that it is also overabundant in Cu with respect to M5, though this
overabundance is in accord with [Cu/Fe] ratios found in the field.Comment: 39 pages, 7 figures, to appear in April 2003 A
Lithium abundances of halo dwarfs based on excitation temperature. I. LTE
The discovery of the Spite plateau in the abundances of 7Li for metal-poor
stars led to the determination of an observationally deduced primordial lithium
abundance. However, after the success of the Wilkinson Microwave Anisotropy
Probe (WMAP) in determining the baryon density, OmegaBh^2, there was a
discrepancy between observationally determined and theoretically determined
abundances in the case of 7Li. One of the most important uncertain factors in
the calculation of the stellar 7Li abundance is the effective temperature,
Teff. We use sixteen metal-poor halo dwarfs to calculate new Teff values using
the excitation energy method. With this temperature scale we then calculate new
Li abundances for this group of stars in an attempt to resolve the 7Li
discrepancy. Using high signal-to-noise (S/N ~ 100) spectra of 16 metal-poor
halo dwarfs, obtained with the UCLES spectrograph on the AAT, measurements of
equivalent widths from a set of unblended FeI lines are made. These equivalent
widths are then used to calculate new Teff values with the use of the single
line radiative transfer program WIDTH6, where we have constrained the gravity
using either theoretical isochrones or the Hipparcos parallax, rather than the
ionization balance. The lithium abundances of the stars are calculated with
these temperatures. The physical parameters are derived for the 16 programme
stars, and two standards. These include Teff, log g, [Fe/H], microturbulence
and 7Li abundances. A comparison between the temperature scale of this work and
those adopted by others has been undertaken. We find good consistency with the
temperatures derived from the Halpha line by Asplund et al. (2006), but not
with the hotter scale of Melendez & Ramirez (2004).Comment: 13 pages, 9 figure
Non-LTE line formation for heavy elements in four very metal-poor stars
Stellar parameters and abundances of Na, Mg, Al, K, Ca, Sr, Ba, and Eu are
determined for four very metal-poor stars (-2.66 < [Fe/H] < -2.15) based on
non-LTE line formation and analysis of high-resolution (R ~60000 and 90000)
high signal-to-noise (S/N > 200) observed spectra. A model atom for H I is
presented. An effective temperature was obtained from the Balmer Halpha and
Hbeta line wing fits, the surface gravity from the Hipparcos parallax if
available and the non-LTE ionization balance between Ca I and Ca II. Based on
the hyperfine structure affecting the Ba II resonance line, the fractional
abundance of the odd isotopes of Ba was derived for HD 84937 and HD 122563 from
a requirement that Ba abundances from the resonance line and subordinate lines
of Ba II must be equal. For each star, non-LTE leads to a consistency of Teff
from two Balmer lines and to a higher temperature compared to the LTE case, by
up to 60 K. Non-LTE effects are important in spectroscopic determination of
surface gravity from Ca I/Ca II. For each star with a known trigonometric
gravity, non-LTE abundances from the lines of two ionization stages agree
within the error bars, while a difference in the LTE abundances consists of
0.23 dex to 0.40 dex for different stars. Departures from LTE are found to be
significant for the investigated atoms, and they strongly depend on stellar
parameters. For HD 84937, the Eu/Ba ratio is consistent with the relative solar
system r-process abundances, and the fraction of the odd isotopes of Ba, f_odd,
equals 0.43+-0.14. The latter can serve as a constraint on r-process models.
The lower Eu/Ba ratio and f_odd = 0.22+-0.15 found for HD 122563 suggest that
the s-process or the unknown process has contributed significantly to the Ba
abundance in this star.Comment: accepted for publication in A&A, November 16, 200
Oxygen Abundances in Metal-Poor Stars
We present oxygen abundances derived from both the permitted and forbidden
oxygen lines for 55 subgiants and giants with [Fe/H] values between -2.7 and
solar with the goal of understanding the discrepancy in the derived abundances.
A first attempt, using \teff{} values from photometric calibrations and surface
gravities from luminosities, obtained agreement between the indicators for
turn-off stars, but the disagreement was large for evolved stars. We find that
the difference in the oxygen abundances derived from the permitted and
forbidden lines is most strongly affected by \teff{}, and we derive a new
\teff{} scale based on forcing the two sets of lines to give the same oxygen
abundances. These new parameters, however, do not agree with other observables,
such as theoretical isochrones or Balmer-line profile based \teff{}
determinations. Our analysis finds that one-dimensional, LTE analyses (with
published NLTE corrections for the permitted lines) cannot fully resolve the
disagreement in the two indicators without adopting a temperature scale
incompatible with other temperature indicators. We also find no evidence of
circumstellar emission in the forbidden lines, removing such emission as a
possible cause for the discrepancy.Comment: 41 pages, 12 tables, 17 figures. Accepted for publication in Oct.
2003 Ap
Atomic diffusion and mixing in old stars II. Observations of stars in the globular cluster NGC 6397 with VLT/FLAMES-GIRAFFE
Evolutionary trends in the surface abundances of heavier elements have
recently been identified in the globular cluster NGC 6397 ([Fe/H]=-2),
indicating the operation of atomic diffusion in these stars. Such trends
constitute important constraints for the extent to which diffusion modifies the
internal structure and surface abundances of solar-type, metal-poor stars. We
perform an independent check of the reality and size of abundance variations
within this metal-poor globular cluster. Observational data covering a large
stellar sample, located between the cluster turn-off point and the base of the
red giant branch, are homogeneously analysed. The spectroscopic data were
obtained with the medium-high resolution spectrograph FLAMES/GIRAFFE on
VLT-UT2. We derive independent effective-temperature scales from profile
fitting of Balmer lines and by applying colour-temperature calibrations to
Str\"omgren uvby and broad-band BVI photometry. An automated spectral analysis
code is used together with a grid of MARCS model atmospheres to derive stellar
surface abundances of Mg, Ca, Ti, and Fe. We identify systematically higher
iron abundances for more evolved stars. The turn-off point stars are found to
have 0.13dex lower surface abundances of iron compared to the coolest, most
evolved stars in our sample. There is a strong indication of a similar trend in
magnesium, whereas calcium and titanium abundances are more homogeneous. Within
reasonable error limits, the obtained abundance trends are in agreement with
the predictions of stellar structure models including diffusive processes
(sedimentation, levitation), if additional turbulent mixing below the outer
convection zone is included.Comment: 11 pages + appendix 6 pages, 9 figures. Accepted for publication in
A&
The Chemical Composition of Carbon-Rich, Very Metal-Poor Stars: A New Class of Mildly Carbon-Rich Objects Without Excess of Neutron-Capture Elements
We report on an analysis of the chemical composition of five carbon-rich,
very metal-poor stars based on high-resolution spectra. One star, CS22948-027,
exhibits very large overabundances of carbon, nitrogen, and the neutron-capture
elements, as found in the previous study of Hill et al.. This result may be
interpreted as a consequence of mass transfer from a binary companion that
previously evolved through the asymptotic giant branch stage. By way of
contrast, the other four stars we investigate exhibit no overabundances of
barium ([Ba/Fe]<0), while three of them have mildly enhanced carbon and/or
nitrogen ([C+N]+1). We have been unable to determine accurate carbon and
nitrogen abundances for the remaining star (CS30312-100). These stars are
rather similar to the carbon-rich, neutron-capture-element-poor star
CS22957-027 discussed previously by Norris et al., though the carbon
overabundance in this object is significantly larger ([C/Fe]=+2.2). Our results
imply that these carbon-rich objects with ``normal'' neutron-capture element
abundances are not rare among very metal-deficient stars. One possible process
to explain this phenomenon is as a result of helium shell flashes near the base
of the AGB in very low-metallicity, low-mass (M~< 1M_sun) stars, as recently
proposed by Fujimoto et al..
The moderate carbon enhancements reported herein ([C/Fe]+1) are similar to
those reported in the famous r-process-enhanced star CS22892-052. We discuss
the possibility that the same process might be responsible for this similarity,
as well as the implication that a completely independent phenomenon was
responsible for the large r-process enhancement in CS22892-052.Comment: 53 pages, 8 figures, to appear in Ap
Abundances of 30 elements in 23 metal-poor stars
We report the abundances of 30 elements in 23 metal-poor ([Fe/H] <-1.7)
giants. These are based on 7774 equivalent widths and spectral synthesis of 229
additional lines. Hyperfine splitting is taken into account when appropriate.
Our choice of model atmospheres has the most influence on the accuracy of our
abundances. We consider the effect of different model atmospheres on our
results. In addition to the random errors in Teff, log g, and microturbulent
velocity, there are several sources of systematic error. These include using
Teff determined from FeI lines rather than colors, ignoring NLTE effects on the
FeI/FeII ionization balance, using models with solar [alpha/Fe] ratios and
using Kurucz models with overshooting. Of these, only the use of models with
solar [alpha/Fe] ratios had a negligible effect. However, while the absolute
abundances can change by > 0.10 dex, the relative abundances, especially
between closely allied atoms such as the rare earth group, often show only
small (<0.03 dex) changes. We found that some strong lines of FeI, MnI and CrI
consistently gave lower abundances by ~0.2 dex, a number larger than the quoted
errors in the gf values. After considering a model with depth-dependent
microturbulent velocity and a model with hotter temperatures in the upper
layers, we conclude that the latter did a better job of resolving the problem
and agreeing with observational evidence for the structure of stars. The error
analysis includes the effects of correlation of Teff, log g, and microturbulent
velocity errors, which is crucial for certain element ratios, such as [Mg/Fe].
The abundances presented here are being analyzed and discussed in a separate
series of papers.Comment: 27 pages, 9 figures, Table 2 included separately, to published in
ApJ
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