84,320 research outputs found
NLTE study of scandium in the Sun
We investigate the formation of neutral and singly ionized scandium lines in
the solar photospheres. The research is aimed derive solar (Sc) values for scandium lines, which will later be used in
differential abundance analyses of metal-poor stars. Extensive statistical
equilibrium calculations were carried out for a model atom, which comprises 92
terms for \ion{Sc}{i} and 79 for \ion{Sc}{ii}. Photoionization cross-sections
are assumed to be hydrogenic. Synthetic line profiles calculated from the level
populations according to the NLTE departure coefficients were compared with the
observed solar spectral atlas. Hyperfine structure (HFS) broadening is taken
into account. The statistical equilibrium of scandium is dominated by a strong
underpopulation of \ion{Sc}{i} caused by missing strong lines. It is nearly
unaffected by the variation in interaction parameters and only marginally
sensitive to the choice of the solar atmospheric model. Abundance
determinations using the ODF model lead to a solar Sc abundance of between
and 3.13, depending on the choice of values.
The long known difference between photospheric and meteoritic scandium
abundances is confirmed for the experimental -values.Comment: 10 pages, 6 figures, A&A accepte
Electrical transport in the ferromagnetic state of manganites: Small-polaron metallic conduction at low temperatures
We report measurements of the resistivity in the ferromagnetic state of
epitaxial thin films of La_{1-x}Ca_{x}MnO_{3} and the low temperature specific
heat of a polycrystalline La_{0.8}Ca_{0.2}MnO_{3}. The resistivity below 100 K
can be well fitted by \rho - \rho_{o} = E \omega_{s}/sinh^{2}(\hbar
\omega_{s}/2k_{B}T) with \hbar \omega_{s}/k_{B} \simeq 100 K and E being a
constant. Such behavior is consistent with small-polaron coherent motion which
involves a relaxation due to a soft optical phonon mode. The specific heat data
also suggest the existence of such a phonon mode. The present results thus
provide evidence for small-polaron metallic conduction in the ferromagnetic
state of manganites.Comment: 4 pages, 4 figures, submitted to PR
The wedding of modified dynamics and non-exotic dark matter in galaxy clusters
We summarize the status of Modified Newtonian Dynamics (MOND) in galaxy
clusters. The observed acceleration is typically larger than the acceleration
threshold of MOND in the central regions, implying that some dark matter is
necessary to explain the mass discrepancy there. A plausible resolution of this
issue is that the unseen mass in MOND is in the form of ordinary neutrinos with
masses just below the experimentally detectable limit. In particular, we show
that the lensing mass reconstructions of the clusters 1E0657-56 (the bullet
cluster) and Cl0024+17 (the ring) do not pose a new challenge to this scenario.
However, the mass discrepancy for cool X-ray emitting groups, in which
neutrinos cannot cluster, pose a more serious problem, meaning that dark
baryons could present a more satisfactory solution to the problem of unseen
mass in MOND clusters.Comment: to appear in World Scientific, proceedings of DARK 200
A Model for Abundances in Metal-Poor Stars
It is argued that the abundances of r-process related elements in stars with
-3<[Fe/H]<-1 can be explained by the contributions of three sources. The
sources are: the first generations of very massive (>100 solar masses) stars
that are formed from Big Bang debris and are distinct from SNII, and two types
of SNII, the H and L events, which can occur only at [Fe/H]>-3. The H events
are of high frequency and produce dominantly heavy (A>130) r-elements but no Fe
(presumably leaving behind black holes). The L events are of low frequency and
produce Fe and dominantly light (A<130) r-elements (essentially none above Ba).
By using the observed abundances in two ultra-metal-poor stars and the solar
r-abundances, the initial or prompt inventory of elements produced by the first
generations of very massive stars and the yields of H and L events can be
determined. The abundances of a large number of elements in a star can then be
calculated from the model by using only the observed Eu and Fe abundances. To
match the model results and the observational data for stars with -3<[Fe/H]<-1
requires that the solar r-abundances for Sr, Y, Zr, and Ba must be
significantly increased from the standard values. Whether the solar
r-components of these elements used here to obtain a fit to the stellar data
can be reconciled with those obtained from solar abundances by subtracting the
s-components calculated from models is not clear.Comment: 47 pages, 19 figures, to appear in Ap
Can an observer really catch up with light
Given a null geodesic with a point in
conjugate to along , there will be a variation of
which will give a time-like curve from to . This is
a well-known theory proved in the famous book\cite{2}. In the paper we prove
that the time-like curves coming from the above-mentioned variation have a
proper acceleration which approaches infinity as the time-like curve approaches
the null geodesic. This means no observer can be infinitesimally near the light
and begin at the same point with the light and finally catch the light. Only
separated from the light path finitely, does the observer can begin at the same
point with the light and finally catch the light.Comment: 6 pages, no figures, submited to Physical Review
Radio Variability of Sagittarius A* - A 106 Day Cycle
We report the presence of a 106-day cycle in the radio variability of Sgr A*
based on an analysis of data observed with the Very Large Array (VLA) over the
past 20 years. The pulsed signal is most clearly seen at 1.3 cm with a ratio of
cycle frequency to frequency width f/Delta_f= 2.2+/-0.3. The periodic signal is
also clearly observed at 2 cm. At 3.6 cm the detection of a periodic signal is
marginal. No significant periodicity is detected at both 6 and 20 cm. Since the
sampling function is irregular we performed a number of tests to insure that
the observed periodicity is not the result of noise. Similar results were found
for a maximum entropy method and periodogram with CLEAN method. The probability
of false detection for several different noise distributions is less than 5%
based on Monte Carlo tests. The radio properties of the pulsed component at 1.3
cm are spectral index alpha ~ 1.0+/- 0.1 (for S nu^alpha), amplitude Delta
S=0.42 +/- 0.04 Jy and characteristic time scale Delta t_FWHM ~ 25 +/- 5 days.
The lack of VLBI detection of a secondary component suggests that the
variability occurs within Sgr A* on a scale of ~5 AU, suggesting an instability
of the accretion disk.Comment: 14 Pages, 3 figures. ApJ Lett 2000 accepte
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