Speed of gravity and gravitomagnetism

A v_J/c correction to the Shapiro time delay seems verified by a 2002 Jovian observation by VLBI. In this Essay, this correction is interpreted as an effect of the aberration of light in an optically refractive medium which supplies an analog of Jupiter's gravity field rather than as a measurement of the speed of gravity, as it was first proposed by other authors. The variation of the index of refraction is induced by the Lorentz invariance of the weak gravitational field equations for Jupiter in a uniform translational slow motion with velocity v_J=13.5 km/s. The correction on time delay and deflection is due not to the Kerr (or Lense-Thirring) stationary gravitomagnetic field of Jupiter, but to its Schwarzschild gravitostatic field when measured from the barycenter of the solar system.Comment: 6 pags, final published version, Honorable Mention in the 2004 Essay Competition of the Gravity Research Foundation, GR

Conductances in normal and normal-superconductor structures

We study theoretically electronic transport through a normal metal-- superconductor (NS) interface and show that more than one conductance may be defined, depending on the pair of chemical potentials whose difference one chooses to relate linearly to the current. We argue that the situation is analogous to that found for purely normal transport, where different conductance formulae can be invoked. We revisit the problem of the "right" conductance formula in a simple language, and analyze its extension to the case of mesoscopic superconductivity. The well-known result that the standard conductance of a NS interface becomes 2 (in units of $2e^2/h$) in the transmissive limit, is viewed here in a different light. We show that it is not directly related to the presence of Andreev reflection, but rather to a particular choice of chemical potentials. This value of 2 is measurable because only one single-contact resistance is involved in a typical experimental setup, in contrast with the purely normal case, where two of them intervene. We introduce an alternative NS conductance that diverges in the transmissive limit due to the inability of Andreev reflection to generate a voltage drop. We illustrate numerically how different choices of chemical potential can yield widely differing I--V curves for a given NS interface.Comment: Minor changes have been introduced and several references have been added, 12 pages, submitted to special issue of ``Superlattices and Microstructures

THE PROBLEM OF ESTIMATING CAUSAL RELATIONS BY REGRESSING ACCOUNTING (SEMI) IDENTITIES

Inferences about the coefficient values of a model estimated with a linear regression cannot be made when both the dependent and the independent variable are part of an accounting (semi) identity. The coefficients will no longer indicate a causal relation as they must adapt to satisfy the identity. A good example is an investment-cash flow sensitivity model. Este trabajo habla de la imposibilidad de extraer conclusiones sobre el valor de los coeficientes de un modelo de regresión lineal que intenta estimar una relación causal, cuando tanto la variable dependiente como la variable independiente forman parte de una (semi) identidad contable. Los coeficientes no sirven para explicar la relación causal, ya que su valor se adaptará para cumplir la identidad. Como ejemplo ilustrativo se presenta el modelo de la sensibilidad de la inversión al cash-flow.Sensibilidad de la inversión al cash flow, identidades contables, semi-identidades contables Investment-cash flow sensitivities, Accounting identities, Accounting semi-identities

The oxygen abundance in the IFU era

Spatially-resolved information of gas-phase emission provided by integral field units (IFUs) are allowing us to perform a new generation of emission-line surveys, based on large samples of HII regions and full two-dimensional coverage. Here we present two highlights of our current studies employing this technique: 1) A statistical approach to the abundance gradients of spiral galaxies, which indicates an -universal- radial gradient for oxygen abundance; and 2) The discovery of a new scaling relation of HII regions in spiral galaxies, the "local" mass-metallicity relation of star-forming galaxies.Comment: 6 pages, to appear in Highlights of Spanish Astrophysics VII, Proceedings of the X Scientific Meeting of the Spanish Astronomical Society held on July 9-13, 2012, in Valencia, Spai