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A hot-wire method based thermal conductivity measurement apparatus for teaching purposes

S Alvarado, E Marín, A G Juárez, A Calderón and R Ivanov — 2012-05-13T23:00:00Z

The implementation of an automated system based on the hot-wire technique is described for the measurement of the thermal conductivity of liquids using equipment easily available in modern physics laboratories at high schools and universities (basically a precision current source and a voltage meter, a data acquisition card, a personal computer and a high purity platinum wire). The wire, which is immersed in the investigated sample, is heated by passing a constant electrical current through it, and its temperature evolution, Δ T , is measured as a function of time, t , for several values of the current. A straightforward methodology is then used for data processing in order to obtain the liquid thermal conductivity. The start point is the well known linear relationship between Δ T and ln( t ) predicted for long heating times by a model based on a solution of the heat conduction equation for an infinite lineal heat source embedded in an infinite medium into w...

Multiple-planet Scattering and the Origin of Hot Jupiters

C. Beaugé and D. Nesvorný — 2012-05-13T23:00:00Z

Doppler and transit observations of exoplanets show a pile-up of Jupiter-size planets in orbits with a 3 day period. A fraction of these hot Jupiters have retrograde orbits with respect to the parent star's rotation, as evidenced by the measurements of the Rossiter-McLaughlin effect. To explain these observations we performed a series of numerical integrations of planet scattering followed by the tidal circularization and migration of planets that evolved into highly eccentric orbits. We considered planetary systems having three and four planets initially placed in successive mean-motion resonances, although the angles were taken randomly to ensure orbital instability in short timescales. The simulations included the tidal and relativistic effects, and precession due to stellar oblateness. Our results show the formation of two distinct populations of hot Jupiters. The inner population (Population I) is characterized by semimajor axis a < 0.03 AU and mainly formed in th...

Bubbles and Knots in the Kinematical Structure of the Bipolar Planetary Nebula NGC 2818

Roberto Vázquez — 2012-05-13T23:00:00Z

High-resolution Hubble Space Telescope archive imaging and high-dispersion spectroscopy are used to study the complex morphological and kinematical structure of the planetary nebula, NGC 2818. We analyze narrowband Hα, [O III ], [N II ], [S II ], and He II images, addressing important morphological features. Ground-based long-slit echelle spectra were obtained crossing NGC 2818 at five different positions to precisely determine kinematical features in the structure of the nebula. A distance of 2.5 kpc was used to determine physical scales. Constructing models to fit the data with modern computational tools, we find NGC 2818 is composed of (1) a non-uniform bipolar structure with a semimajor axis of 0.92 pc (75''), possibly deformed by the stellar wind, (2) a 0.17 pc (14'') diameter central region, which is potentially the remnant of an equatorial enhancement, and (3) a great n...

Secular Changes in Eta Carinae's Wind 1998–2011

2012-05-06T23:00:00Z

Stellar wind-emission features in the spectrum of eta Carinae have decreased by factors of 1.5-3 relative to the continuum within the last 10 years. We investigate a large data set from several instruments (STIS, GMOS, UVES) obtained between 1998 and 2011 and analyze the progression of spectral changes in direct view of the star, in the reflected polar-on spectra at FOS4, and at the Weigelt knots. We find that the spectral changes occurred gradually on a timescale of about 10 years and that they are dependent on the viewing angle. The line strengths declined most in our direct view of the star. About a decade ago, broad stellar wind-emission features were much stronger in our line-of-sight view of the star than at FOS4. After the 2009 event, the wind-emission line strengths are now very similar at both locations. High-excitation He I and N II absorption lines in direct view of the star strengthened gradually. The terminal velocit...

A Rotating Molecular Jet from a Perseus Protostar

Gerardo Pech, Luis A. Zapata, Laurent Loinard, and Luis F. Rodríguez — 2012-05-06T23:00:00Z

We present the 12 CO(2-1) line and 1.4 mm continuum archival observations, made with the Submillimeter Array, of the outflow HH 797 located in the IC 348 cluster in Perseus. The continuum emission is associated with a circumstellar disk surrounding the class 0 object IC 348-MMS/SMM2, a very young solar analog. The line emission, on the other hand, delineates a collimated outflow and reveals velocity asymmetries about the flow axis over the entire length of the flow. The amplitude of velocity differences is of the order of 2 km s –1 over distances of about 1000 AU, and we interpret them as evidence for jet rotation—although we also discuss alternative possibilities. A comparison with theoretical models suggests that the magnetic field lines threading the protostellar jet might be anchored to the disk of a radius of about 20 AU.

Unstable fields in Kerr spacetimes

Gustavo Dotti, Reinaldo J Gleiser and Ignacio F Ranea-Sandoval — 2012-04-15T23:00:00Z

We show that both the interior region ##IMG## [http://ej.iop.org/images/0264-9381/29/9/095017/cqg414914ieqn1.gif] {$r M 2 Kerr naked singularity admit unstable solutions of the Teukolsky equation for any value of the spin weight. For every harmonic number, there is at least one axially symmetric mode that grows exponentially in time and decays properly in the radial directions. These can be used as Debye potentials to generate solutions for the scalar, Weyl spinor, Maxwell and linearized gravity field equations on these backgrounds, satisfying appropriate spatial boundary conditions and growing exponentially in time, as shown in detail for the Maxwell case. It is suggested that the existence of the unstable modes is related to the so-called time machine region, where the axial Killing vector field is timelike, and the Teukolsky equation, restricted to axially symmet...

Modelling, simulation and construction of a dc/dc boost power converter: a school experimental system

2012-03-28T23:00:00Z

We introduce a dc/dc boost power converter as a didactic prototype intended to support courses on electric circuit analysis experimentally. The corresponding mathematical model is obtained, the converter is designed and an experimental setup is described, constructed and tested. Simplicity of construction as well as low cost of components renders the feasible introduction of this equipment in undergraduate laboratories.

Testing plastic deformations of materials in the introductory undergraduate mechanics laboratory

C M Romo-Kröger — 2012-03-22T00:00:00Z

Normally, a mechanics laboratory at the undergraduate level includes an experiment to verify compliance with Hooke's law in materials, such as a steel spring and an elastic rubber band. Stress–strain curves are found for these elements. Compression in elastic bands is practically impossible to achieve due to flaccidity. A typical experiment for the complete loading–unloading cycle is to subject a tubular object to torsion. This paper suggests simple experiments for studying properties concerning elasticity and plasticity in elements of common use, subjected to stretching or compression, and also torsion reinforcing. The experiments use plastic binders, rubber bands and metal springs under a moderate load. This paper discusses an experiment with an original device to measure torsion deformations as a function of applied torques, which permitted construction of the hysteresis cycle for a rubber hose and various tubes. Another experiment was designed to define the temporal recovery ...

Confusing aspects in the calculation of the electrostatic potential of an infinite line of charge

J L Jiménez, I Campos and J A E Roa-Neri — 2012-02-28T00:00:00Z

In this work we discuss the trick of eliminating infinite potential of reference arguing that it corresponds to a constant of integration, in the problem of determining the electrostatic potential of an infinite line of charge with uniform density, and show how the problem must be tackled properly. The usual procedure is confusing for most students of undergraduate courses, basic or intermediate.

Proper Motions of the HH 110/270 System

P. Kajdič, B. Reipurth, A. C. Raga, J. Bally, and J. Walawender — 2012-03-26T23:00:00Z

We present a study of the HH 110/270 system based on three sets of optical images obtained with the ESO New Technology Telescope, the Subaru Telescope, and the Hubble Space Telescope ( HST ). The ground-based observations are made in the Hα and [S II ] emission lines and the HST observations are made in the Hα line only. Ground-based observations reveal the existence of nine knots, which have not been previously discussed and offer some important insight into the HH 110/270 history. We perform a kinematic study of the HH 110/270 system and an analysis of its emission properties. We measure proper motions of all the knots in the system. Four of the newly identified knots belong to the HH 270 jet. Their positions indicate that the jet's axis changed its direction in the past. We speculate that similar changes may have occurred many times in the past and this could be part of the reason for the unusual structure of the HH 110 jet. The H...