5,947 research outputs found
Measuring Ages and Elemental Abundances from Unresolved Stellar Populations: Fe, Mg, C, N, and Ca
We present a method for determining mean light-weighted ages and abundances
of Fe, Mg, C, N, and Ca, from medium resolution spectroscopy of unresolved
stellar populations. The method, pioneered by Schiavon (2007), is implemented
in a publicly available code called EZ_Ages. The method and error estimation
are described, and the results tested for accuracy and consistency, by
application to integrated spectra of well-known Galactic globular and open
clusters. Ages and abundances from integrated light analysis agree with studies
of resolved stars to within +/-0.1 dex for most clusters, and to within +/-0.2
dex for nearly all cases. The results are robust to the choice of Lick indices
used in the fitting to within +/-0.1 dex, except for a few systematic
deviations which are clearly categorized. The realism of our error estimates is
checked through comparison with detailed Monte Carlo simulations. Finally, we
apply EZ_Ages to the sample of galaxies presented in Thomas et al. (2005) and
compare our derived values of age, [Fe/H], and [alpha/Fe] to their analysis. We
find that [alpha/Fe] is very consistent between the two analyses, that ages are
consistent for old (Age > 10 Gyr) populations, but show modest systematic
differences at younger ages, and that [Fe/H] is fairly consistent, with small
systematic differences related to the age systematics. Overall, EZ_Ages
provides accurate estimates of fundamental parameters from medium resolution
spectra of unresolved stellar populations in the old and intermediate-age
regime, for the first time allowing quantitative estimates of the abundances of
C, N, and Ca in these unresolved systems. The EZ_Ages code can be downloaded at
http://www.ucolick.org/~graves/EZ_Ages.htmlComment: Accepted to ApJ
Electrical current-driven pinhole formation and insulator-metal transition in tunnel junctions
Current Induced Resistance Switching (CIS) was recently observed in thin
tunnel junctions (TJs) with ferromagnetic (FM) electrodes and attributed to
electromigration of metallic atoms in nanoconstrictions in the insulating
barrier. The CIS effect is here studied in TJs with two thin (20 \AA)
non-magnetic (NM) Ta electrodes inserted above and below the insulating
barrier. We observe resistance (R) switching for positive applied electrical
current (flowing from the bottom to the top lead), characterized by a
continuous resistance decrease and associated with current-driven displacement
of metallic ions from the bottom electrode into the barrier (thin barrier
state). For negative currents, displaced ions return into their initial
positions in the electrode and the electrical resistance gradually increases
(thick barrier state). We measured the temperature (T) dependence of the
electrical resistance of both thin- and thick-barrier states ( and R
respectively). Experiments showed a weaker R(T) variation when the tunnel
junction is in the state, associated with a smaller tunnel contribution.
By applying large enough electrical currents we induced large irreversible
R-decreases in the studied TJs, associated with barrier degradation. We then
monitored the evolution of the R(T) dependence for different stages of barrier
degradation. In particular, we observed a smooth transition from tunnel- to
metallic-dominated transport. The initial degradation-stages are related to
irreversible barrier thickness decreases (without the formation of pinholes).
Only for later barrier degradation stages do we have the appearance of metallic
paths between the two electrodes that, however, do not lead to metallic
dominated transport for small enough pinhole radius.Comment: 10 pages, 3 figure
Geometrical Magnetic Frustration in Rare Earth Chalcogenide Spinels
We have characterized the magnetic and structural properties of the CdLn2Se4
(Ln = Dy, Ho), and CdLn2S4 (Ln = Ho, Er, Tm, Yb) spinels. We observe all
compounds to be normal spinels, possessing a geometrically frustrated
sublattice of lanthanide atoms with no observable structural disorder. Fits to
the high temperature magnetic susceptibilities indicate these materials to have
effective antiferromagnetic interactions, with Curie-Weiss temperatures theta ~
-10 K, except CdYb2S4 for which theta ~ -40 K. The absence of magnetic long
range order or glassiness above T = 1.8 K strongly suggests that these
materials are a new venue in which to study the effects of strong geometrical
frustration, potentially as rich in new physical phenomena as that of the
pyrochlore oxides.Comment: 17 pages, 5 figures, submitted to Phys Rev B; added acknowledgement
Realization of Rectangular Artificial Spin Ice and Direct Observation of High Energy Topology
In this letter, we have constructed and experimentally investigated
frustrated arrays of dipoles forming two-dimensional artificial spin ices with
different lattice parameters (rectangular arrays with horizontal and vertical
lattice spacings denoted by and respectively). Arrays with three
different ratios , and are
studied. Theoretical calculations of low-energy demagnetized configurations for
these same parameters are also presented. Experimental data for demagnetized
samples confirm most of the theoretical results. However, the highest energy
topology (doubly-charged monopoles) does not emerge in our theoretical model,
while they are seen in experiments for large enough . Our results also
insinuate that magnetic monopoles may be almost free in rectangular lattices
with a critical ratio , supporting previous
theoretical predictions
Spectroscopic ages and metallicities of stellar populations: validation of full spectrum fitting
Fitting whole spectra at intermediate spectral resolution (R = 1000 -- 3000),
to derive physical properties of stellar populations, appears as an optimized
alternative to methods based on spectrophotometric indices: it uses all the
redundant information contained in the signal. This paper addresses the
validation of the method and it investigates the quality of the population
models together with the reliability of the fitting procedures. We are using
two algorithms: STECKMAP, a non-parametric regularized program and NBURSTS a
parametric non-linear minimization. We compare three spectral synthesis models
for single stellar populations: Pegase-HR, Galaxev (BC03) and Vazdekis/Miles,
and we analyse spectra of Galactic clusters whose populations are known from
studies of color-magnitude diagrams (CMD) and spectroscopy of individual stars.
We find that: (1) The quality of the models critically depends on the stellar
library they use. Pegase-HR and Vazdekis/Miles are consistent, while the
comparison between Pegase-HR and BC03 shows some systematics reflecting the
limitations of the stellar library (STELIB) used to generate the latter models;
(2) The two fitting programs are consistent; (3) For globular clusters and M67
spectra, the method restitutes metallicities in agreement with spectroscopy of
stars within 0.14 dex; (4) The spectroscopic ages are very sensitive to the
presence of a blue horizontal branch (BHB) or of blue stragglers. A BHB
morphology results in a young SSP-equivalent age. Fitting a free amount of blue
stars in addition to the SSP model to mimic the BHB improves and stabilizes the
fit and restores ages in agreement with CMDs studies. This method is
potentially able to disentangle age or BHB effects in extragalactic clusters.Comment: accepted in MNRAS; Full version available at
http://www-obs.univ-lyon1.fr/labo/perso/prugniel/mina/koleva.pd
Object-oriented Programming Laws for Annotated Java Programs
Object-oriented programming laws have been proposed in the context of
languages that are not combined with a behavioral interface specification
language (BISL). The strong dependence between source-code and interface
specifications may cause a number of difficulties when transforming programs.
In this paper we introduce a set of programming laws for object-oriented
languages like Java combined with the Java Modeling Language (JML). The set of
laws deals with object-oriented features taking into account their
specifications. Some laws deal only with features of the specification
language. These laws constitute a set of small transformations for the
development of more elaborate ones like refactorings
Maximum gravitational-wave energy emissible in magnetar flares
Recent searches of gravitational-wave (GW) data raise the question of what
maximum GW energies could be emitted during gamma-ray flares of highly
magnetized neutron stars (magnetars). The highest energies (\sim 10^{49} erg)
predicted so far come from a model [K. Ioka, Mon. Not. Roy. Astron. Soc. 327,
639 (2001)] in which the internal magnetic field of a magnetar experiences a
global reconfiguration, changing the hydromagnetic equilibrium structure of the
star and tapping the gravitational potential energy without changing the
magnetic potential energy. The largest energies in this model assume very
special conditions, including a large change in moment of inertia (which was
observed in at most one flare), a very high internal magnetic field, and a very
soft equation of state. Here we show that energies of 10^{48}-10^{49} erg are
possible under more generic conditions by tapping the magnetic energy, and we
note that similar energies may also be available through cracking of exotic
solid cores. Current observational limits on gravitational waves from magnetar
fundamental modes are just reaching these energies and will beat them in the
era of advanced interferometers.Comment: 16 pages, 5 figures, 1 tabl
The compound Poisson limit ruling periodic extreme behaviour of non-uniformly hyperbolic dynamics
We prove that the distributional limit of the normalised number of returns to
small neighbourhoods of periodic points of non-uniformly hyperbolic dynamical
systems is compound Poisson. The returns to small balls around a fixed point in
the phase space correspond to the occurrence of rare events, or exceedances of
high thresholds, so that there is a connection between the laws of Return Times
Statistics and Extreme Value Laws. The fact that the fixed point in the phase
space is a repelling periodic point implies that there is a tendency for the
exceedances to appear in clusters whose average sizes is given by the Extremal
Index, which depends on the expansion of the system at the periodic point.
We recall that for generic points, the exceedances, in the limit, are
singular and occur at Poisson times. However, around periodic points, the
picture is different: the respective point processes of exceedances converge to
a compound Poisson process, so instead of single exceedances, we have entire
clusters of exceedances occurring at Poisson times with a geometric
distribution ruling its multiplicity.
The systems to which our results apply include: general piecewise expanding
maps of the interval (Rychlik maps), maps with indifferent fixed points
(Manneville-Pomeau maps) and Benedicks-Carleson quadratic maps.Comment: To appear in Communications in Mathematical Physic
On the Iron content of NGC 1978 in the LMC: a metal rich, chemically homogeneous cluster
We present a detailed abundance analysis of giant stars in NGC 1978, a
massive, intermediate-age stellar cluster in the Large Magellanic Cloud,
characterized by a high ellipticity and suspected to have a metallicity spread.
We analyzed 11 giants, all cluster members, by using high resolution spectra
acquired with the UVES/FLAMES spectrograph at the ESO-Very Large Telescope. We
find an iron content of [Fe/H]=-0.38 dex with very low (0.07 dex) dispersion,
and a mean heliocentric radial velocity Vr=293.1 (with an error of 0.9 km/s)
and a velocity dispersion (3.1 km/s), thus excluding the presence of a
significant metallicity, as well as velocity, spread within the cluster.Comment: 14 pages, 4 figures, accepted for publication by Apj
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