134 research outputs found
3D hydrodynamical CO5BOLD model atmospheres of red giant stars: I. Atmospheric structure of a giant located near the RGB tip
We investigate the character and role of convection in the atmosphere of a
prototypical red giant located close to the red giant branch (RGB) tip with
atmospheric parameters, Teff=3660K, log(g)=1.0, [M/H]=0.0. Differential
analysis of the atmospheric structures is performed using the 3D hydrodynamical
and 1D classical atmosphere models calculated with the CO5BOLD and LHD codes,
respectively. All models share identical atmospheric parameters, elemental
composition, opacities and equation-of-state. We find that the atmosphere of
this particular red giant consists of two rather distinct regions: the lower
atmosphere dominated by convective motions and the upper atmosphere dominated
by wave activity. Convective motions form a prominent granulation pattern with
an intensity contrast (~18%) which is larger than in the solar models (~15%).
The upper atmosphere is frequently traversed by fast shock waves, with vertical
and horizontal velocities of up to Mach ~2.5 and ~6.0, respectively. The
typical diameter of the granules amounts to ~5Gm which translates into ~400
granules covering the whole stellar surface. The turbulent pressure in the
giant model contributes up to ~35% to the total (i.e., gas plus turbulent)
pressure which shows that it cannot be neglected in stellar atmosphere and
evolutionary modeling. However, there exists no combination of the
mixing-length parameter and turbulent pressure that would allow to
satisfactorily reproduce the 3D temperature-pressure profile with 1D atmosphere
models based on a standard formulation of mixing-length theory.Comment: 13 pages, 18 figures, accepted for publication in A&
Galactic globular cluster 47 Tucanae: new ties between the chemical and dynamical evolution of globular clusters?
It is generally accepted today that Galactic globular clusters (GGCs) consist
of at least two generations of stars that are different in their chemical
composition and perhaps age. However, knowledge about the kinematical
properties of these stellar generations, which may provide important
information for constraining evolutionary scenarios of the GGCs, is still
limited. We therefore study the connections between chemical and kinematical
properties of different stellar generations in the Galactic globular cluster 47
Tuc. To achieve this goal, we used abundances of Li, O, and Na determined in
101 main sequence turn-off (TO) stars with the aid of 3D hydrodynamical model
atmospheres and NLTE abundance analysis methodology. We divided our sample TO
stars into three groups according to their position in the [Li/Na]-[Na/O] plane
to study their spatial distribution and kinematical properties. We find that
there are statistically significant radial dependencies of lithium and oxygen
abundances, A(Li) and A(O), as well as that of [Li/Na] abundance ratio. Our
results show that first-generation stars are less centrally concentrated and
dynamically hotter than stars belonging to subsequent generations. We also find
a significant correlation between the velocity dispersion and O and Na
abundance, and between the velocity dispersion and the [Na/O] abundance ratio.Comment: 4 pages, 6 figures, accepted for publication in A&
Convection and observable properties of late-type giants
We show that contrary to what is expected from 1D stationary model
atmospheres, 3D hydrodynamical modeling predicts a considerable influence of
convection on the spectral properties of late-type giants. This is due to the
fact that convection overshoots into the formally stable outer atmospheric
layers producing a notable granulation pattern in the 3D hydrodynamical models,
which has a direct influence on the observable spectra and colors. Within the
framework of standard 1D model atmospheres the average thermal stratification
of the 3D hydro model can not be reproduced with any reasonable choice of the
mixing length parameter and formulation of the turbulent pressure. The
differences in individual photometric colors -- in terms of 3D versus 1D --
reach up to ~0.2 mag, or \Delta Teff~70K. We discuss the impact of full 3D
hydrodynamical models on the interpretation of observable properties of
late-type giants, briefly mentioning problems and challenges which need to be
solved for bringing these models to a routine use within the astronomical
community in 5-10 years from now.Comment: 4 pages, 3 figures. Proceedings of the IAU Symposium 232 "The
Scientific Requirements for Extremely Large Telescopes", eds. P. Whitelock,
B. Leibundgut, and M. Dennefel
ISO observations of the planetary nebula Lindsay 305 in the Small Magellanic Cloud
We present ISO (Infrared Space Observatory) observations of the planetary
nebula Lindsay 305 (L 305) in the Small Magellanic Cloud. L 305 is very
prominent in the ISOCAM frames at 6.75 and 11.5 m, although it is under
the detection limit at 4.5 m. The obtained spectral energy distribution
shows a strong mid-IR excess, which, depending on the amount of energy radiated
at wavelengths longer than 11.5 m, may be as large as . However, since an accurate estimate of the total nebular luminosity
is not available up to date, the evolutionary status of L 305 can not yet be
constrained precisely.Comment: 4 pages, 2 figures, to appear in the Publications of the Astronomical
Society of Japa
Beyond the Galaxy with Gaia: evolutionary histories of galaxies in the Local Group
Gaia will play an important role in providing information about star
formation histories, merging events, intergalactic streams etc., for nearby
galaxies of the Local Group. One of the most crucial contributions will be
proper motions, especially for stars in the outermost parts of the galaxies,
obtainable for stellar populations to ~150 kpc with RGB stars. Together with
radial velocities for the brightest giants <80 kpc, this will provide
membership information for individual stars and global kinematical picture of
the most nearby galaxies, including the Magellanic Clouds (MCs). Gaia will also
provide photometric metallicities (\sigma([M/H])<0.3) for individual giants
and/or supergiants in dwarf galaxies to ~200 kpc. MSTO ages will be possible
for the youngest stellar populations in the most nearby galaxies (e.g., MCs),
whereas stars on RGB/AGB may provide age estimates for populations to ~150 kpc.
Gaia will allow to study the outermost parts of the galaxies, which (because of
their large spatial extent) are difficult to assess from the ground. Apart from
allowing to clarify the structure and evolution of the dwarf galaxies, this
will also make it possible to investigate galactic tidal debris, thus providing
additional details for the global picture of formation and evolution of the
Milky Way Galaxy.Comment: Proceedings of "The Three Dimensional Universe With Gaia", Paris,
October 4-7, 2004, 4 pages, 3 figure
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