18,310 research outputs found
Stellar populations in the Carina region: The Galactic plane at l = 291
Previous studies of the Carina region have revealed its complexity and
richness as well as a significant number of early-type stars. In many cases,
these studies only concentrated on the central region or were not homogeneous.
This latter aspect, in particular, is crucial because very different ages and
distances for key clusters have been claimed in recent years. The aim of this
work is to study in detail an area of the Galactic plane in Carina. We analyze
the properties of different stellar populations and focus on a sample of open
clusters and their population of YSOs and highly reddened early stars. We also
studied the stellar mass distribution in these clusters and the possible
scenario of their formation. Finally, we outline the Galactic spiral structure
in this direction. We obtained photometric data for six young open clusters
located in Carina at l = 291, and their adjacent stellar fields, which we
complemented with spectroscopic observations of a few selected targets. We also
culled additional information from the literature. Our results provide more
reliable estimates of distances, color excesses, masses, and ages of the
stellar populations in this direction. We estimate the basic parameters of the
studied clusters and find that they identify two overdensities of young stellar
populations. We find evidence of PMS populations inside them, with an apparent
coeval stellar formation in the most conspicuous clusters. We also discuss
apparent age and distance gradients in the direction NW-SE. We study the mass
distributions of several clusters in the region. They consistently show a
canonical IMF slope. We discover and characterise an abnormally reddened
massive stellar population. Spectroscopic observations of ten stars of this
latter population show that all selected targets were massive OB stars. Their
location is consistent with the position of the Car-Sag spiral arm.Comment: 15 pages, 13 figure
First clear evidence of quantum chaos in the bound states of an atomic nucleus
We study the spectral fluctuations of the Pb nucleus using the
complete experimental spectrum of 151 states up to excitation energies of
MeV recently identified at the Maier-Leibnitz-Laboratorium at Garching,
Germany. For natural parity states the results are very close to the
predictions of Random Matrix Theory (RMT) for the nearest-neighbor spacing
distribution. A quantitative estimate of the agreement is given by the Brody
parameter , which takes the value for regular systems and
for chaotic systems. We obtain which
is, to our knowledge, the closest value to chaos ever observed in experimental
bound states of nuclei. By contrast, the results for unnatural parity states
are far from RMT behavior. We interpret these results as a consequence of the
strength of the residual interaction in Pb, which, according to
experimental data, is much stronger for natural than for unnatural parity
states. In addition our results show that chaotic and non-chaotic nuclear
states coexist in the same energy region of the spectrum.Comment: 9 pages, 1 figur
Long-range ferromagnetism of Mn12 acetate single-molecule magnets under a transverse magnetic field
We use neutron diffraction to probe the magnetization components of a crystal
of Mn12 single-molecule magnets. Each of these molecules behaves, at low
temperatures, as a nanomagnet with spin S = 10 and strong anisotropy along the
crystallographic c axis. Application of a magnetic field perpendicular to c
induces quantum tunneling between opposite spin orientations, enabling the
spins to attain thermal equilibrium. Below approximately 0.9 K, intermolecular
interactions turn this equilibrium state into a ferromagnetically ordered
phase. However, long range ferromagnetic correlations nearly disappear for
fields larger 5.5 T, possibly suggesting the existence of a quantum critical
point.Comment: 4 pages, 4 figure
Justifications-on-demand as a device to promote shifts of attention associated with relational thinking in elementary arithmetic
Student responses to arithmetical questions that can be solved by using arithmetical structure can serve to reveal the extent and nature of relational, as opposed to computational thinking. Here, student responses to probes which require them to justify-on-demand are analysed using a conceptual framework which highlights distinctions between different forms of attention. We analyse a number of actions observed in students in terms of forms of attention and shifts between them: in the short-term (in the moment), medium-term (over several tasks), and long-term (over a year). The main factors conditioning studentsÂŽ attention and its movement are identified and some didactical consequences are proposed
100 MHz Amplitude and Polarization Modulated Optical Source for Free-Space Quantum Key Distribution at 850 nm
We report on an integrated photonic transmitter of up to 100 MHz repetition
rate, which emits pulses centered at 850 nm with arbitrary amplitude and
polarization. The source is suitable for free space quantum key distribution
applications. The whole transmitter, with the optical and electronic components
integrated, has reduced size and power consumption. In addition, the
optoelectronic components forming the transmitter can be space-qualified,
making it suitable for satellite and future space missions.Comment: 6 figures, 2 table
Shape of the spatial mode function of photons generated in noncollinear spontaneous parametric downconversion
We show experimentally how noncollinear geometries in spontaneous parametric
downconversion induce ellipticity of the shape of the spatial mode function.
The degree of ellipticity depends on the pump beam width, especially for highly
focused beams. We also discuss the ellipticity induced by the spectrum of the
pump beam
Influence of processing route on electrical and thermal conductivity of Al/SiC composites with bimodal particle distribution
Al/SiC composites with volume fractions of SiC between 0.55 and 0.71 were made from identical tapped and vibrated powder preforms by squeeze casting (SC) and by two different setups for gas pressure infiltration (GPI), one that allows short (1-2min) liquid metal/ceramic contact time (fast GPI) and the other that operates with rather long contact time, i.e., 10-15min, (slow GPI). Increased liquid metal-ceramic contact time is shown to be the key parameter for the resulting thermal and electrical conductivity in the Al/SiC composites for a given preform. While for the squeeze cast samples neither dissolution of the SiC nor formation of Al4C3 was observed, the gas pressure assisted infiltration led inevitably to a reduced electrical and thermal conductivity of the matrix due to partial decomposition of SiC leading to Si in the matrix. Concomitantly, formation of Al4C3 at the interface was observed in both sets of gas pressure infiltrated samples. Longer contact times lead to much higher levels of Si in the matrix and to more Al4C3 formation at the interface. The difference in thermal conductivity between the SC samples and the fast GPI samples could be rationalized by the reduced matrix thermal conductivity only. On the other hand, in order to rationalize the thermal conductivity of the slow GPI a reduction in the metal/ceramic interface thermal conductance due to excessive Al4C3-formation had to be invoked. The CTE of the composites generally tended to decrease with increasing volume fraction of SiC except for the samples in which a large expansive drift was observed during the CTE measurement by thermal cycles. Such drift was essentially observed in the SC samples with high volume fraction of SiC while it was much smaller for the GPI sample
Relationship between sport level competition and serving skill in female beach volleyball
El objetivo principal de este estudio fue analizar el saque empleado por las jugadoras de vĂłley playa femenino, para determinar si existen diferencias en funciĂłn del nivel de juego. Se analizaron 1.100 acciones de saque. Las variables del saque estudiadas fueron: tipo, profundidad de golpeo, lateralidad, trayectoria, destino, y tĂ©cnica de golpeo. Se realizĂł un estudio de confiabilidad, para determinar el grado de concordancia inter e intraobservadores, llegando a alcanzar una concordancia entre los observadores >0,80. Los resultados del anĂĄlisis correlacional mostraron una diferencia en el patrĂłn de saque utilizado tras el cruce de variables criterio estudiadas, mostrando el test de Chi-cuadrado de Pearson significaciĂłn en el cruce (p .80. The results obtained by the correlation analysis showed a difference in the pattern of the service employed after crossing the studied criterion variables. Pearsonâs Chi-square test showed a significance (p <.05). The serve pattern identified for international competitive level couples showed a greater difficulty on the execution and precision of the service, becoming the first element of attack; while for the national competitive level ones, the technical difficulty in execution and precision was lowe
Phenotypic reversion of Aspergillus nidulans morphological deteriorated variants in the presence of osmotic stabilisers
Strains of Aspergillus nidulans with chromosome duplications are unstable at mitosis. They produce sectors which are mainly of two types: improved sectors that result from partial or total loss of the duplication segment and deteriorated sectors having poor conidiation and dark brown mycelium. It is postulated that deteriorated variants carry additional duplications resulting from non-homologous sister-chromatid exchange within the duplicated segments. (Nga and Roper, 1968 Genetics 58: 193-209). Deteriorated sectors are unstable but can give more derivatives which probably are the result of transpositions of the tandem duplication segment to other regions of the genome (Azevedo and Roper, 1970 Genetical Research 16: 79-93). Crosses between these more stable deteriorated variants are not always successful due probably to incompatibility factors
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