42 research outputs found
Solar BaII 4554 A line as Doppler diagnostics: NLTE analysis in 3D hydrodynamical model
The aim of this paper is to analyse the validity of the Dopplergram and
lambda-meter techniques for the Doppler diagnostics of solar photospheric
velocities using the BaII 4554 A line. Both techniques are evaluated by means
of NLTE radiative transfer calculations of the BaII 4554 A line in a
three-dimensional hydrodynamical model of solar convection. We consider the
cases of spatially unsmeared profiles and the profiles smeared to the
resolution of ground-based observations. We find that: (i)
Speckle-reconstructed Dopplergram velocities reproduce the ``true'' velocities
well at heights around 300 km, except for intergranular lanes with strong
downflows where the velocity can be overestimated. (ii) The lambda-meter
velocities give a good representation of the ``true'' velocities through the
whole photosphere, both under the original and reduced spatial resolutions. The
velocities derived from the inner wing of smeared BaII 4554 A line profiles are
more reliable than those for the outer wing. Only under high spatial resolution
does the inner wing velocities calculated in intergranular regions give an
underestimate (or even a sign reversal) compared with the model velocities.
(iii) NLTE effects should be taken into account in modelling the BaII 4554 A
line profiles. Such effects are more pronounced in intergranular regions. Our
analysis supports the opinion that the Dopplergram technique applied to the
BaII 4554 A line is a valuable tool for the Doppler diagnostics of the middle
photosphere around 300 km. The \lambda-meter technique applied to this line
gives us a good opportunity to ``trace'' the non-thermal motions along the
whole photosphere up to the temperature minimum and lower chromosphere.Comment: accepted by Astronomy and Astrophysic
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