42,934 research outputs found
Isotopic Abundances of Carbon and Oxygen in Oxygen-Rich Giant Stars
16O/17O and 12C/13C ratios in 23 M giants are determined from high resolution
IR spectra observed with the KPNO FTS. The masses of our sample are estimated
with the use of the evolutionary tracks by Claret (2004), which could account
for only about half of our sample. The resulting rather large variation of
16O/17O in low mass stars is well consistent with the prediction of the
evolutionary models, but quite low 16O/17O ratios observed in some higher mass
stars cannot be explained with the model prediction. The observed 12C/13C
ratios are mostly around 10, in contradiction with the predict 12C/13C ratios
of about 20. Thus we confirm a long-standing 12C/13C puzzle, and it appears
that this dilemma may not be resolved yet even with extra mixing such as "cool
bottom processing" expected only in low mass stars.Comment: 3 pages, 3 figures, to appear in Proceedings of the IAU Symposium
No.239 "Convection in Astrophysics" eds. F. Kupka, I. W. Roxburgh & K. L.
Cha
Photoinduced insulator-metal transition in correlated electrons -- a Floquet analysis with the dynamical mean-field theory
In order to investigate photoinduced insulator-metal transitions observed in
correlated electron systems, we propose a new theoretical method, where we
combine a Floquet-matrix method for AC-driven systems with the dynamical
mean-field theory. The method can treat nonequilibrium steady states exactly
beyond the linear-response regime. We have applied the method to the
Falicov-Kimball model coupled to AC electric fields, and numerically obtained
the spectral function, the nonequilibrium distribution function and the
current-voltage characteristic. The results show that intense AC fields indeed
drive Mott-like insulating states into photoinduced metallic states in a
nonlinear way.Comment: 4 pages, 3 figures, Proceedings of LT2
The K giant star Arcturus: the hybrid nature of its infrared spectrum
We study infrared spectrum of Arcturus to clarify the nature of the cool
component of its atmosphere, referred to as the CO-mosphere, with the use of
the IR spectral atlas by Hinkle et al.(1995). The nature of CO lines shows an
abrupt change at logW/nu = -4.75, and the lines stronger than this limit can no
longer be analyzed by the classical line-formation theory. A more simple
manifestation of this fact is that the curves-of-growth (CG) of CO lines show
an unpredictable upturn at logW/nu = -4.75. Similar unusual behaviors of
empirical CG are confirmed in other red (super)giant stars, and it looks as if
the CG is a hybrid of at least two components of different origins. Although
strong lines of the CO fundamentals observed in Arcturus show strengthening
compared with the predicted photospheric spectrum, the weaker lines show slight
weakening, and we interpret these results as due to absorption/emission by the
molecular clouds formed in the extended atmosphere. Now do clouds exist in
stellar atmospheres? It is by no means easy to answer this question by
spectroscopic observations alone, but we find several arguments in favor of
such a possibility in Arcturus by analyzing the CO lines. In cooler
(super)giant stars in which CO lines show similar unusual behaviors as in
Arcturus, the presence of molecular clouds in the outer atmospheres was
demonstrated by direct observations with spatial interferometry. We suggest
that the formation of molecular clouds is a general feature in cool luminous
stars from early K to late M (super)giant stars.Comment: 18 pages, 15 Postscript figures, 1 Table in electronic form, Accepted
for publication in Astronomy and Astrophysic
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