Analytical maximum likelihood estimation of stellar magnetic fields

The polarised spectrum of stellar radiation encodes valuable information on the conditions of stellar atmospheres and the magnetic fields that permeate them. In this paper, we give explicit expressions to estimate the magnetic field vector and its associated error from the observed Stokes parameters. We study the solar case where specific intensities are observed and then the stellar case, where we receive the polarised flux. In this second case, we concentrate on the explicit expression for the case of a slow rotator with a dipolar magnetic field geometry. Moreover, we also give explicit formulae to retrieve the magnetic field vector from the LSD profiles without assuming mean values for the LSD artificial spectral line. The formulae have been obtained assuming that the spectral lines can be described in the weak field regime and using a maximum likelihood approach. The errors are recovered by means of the hermitian matrix. The bias of the estimators are analysed in depth.Comment: accepted for publication in MNRA

Analysis of Quiet-Sun Internetwork Magnetic Fields Based on Linear Polarization Signals

We present results from the analysis of Fe I 630 nm measurements of the quiet Sun taken with the spectropolarimeter of the Hinode satellite. Two data sets with noise levels of 1.2{\times}10-3 and 3{\times}10-4 are employed. We determine the distribution of field strengths and inclinations by inverting the two observations with a Milne-Eddington model atmosphere. The inversions show a predominance of weak, highly inclined fields. By means of several tests we conclude that these properties cannot be attributed to photon noise effects. To obtain the most accurate results, we focus on the 27.4% of the pixels in the second data set that have linear polarization amplitudes larger than 4.5 times the noise level. The vector magnetic field derived for these pixels is very precise because both circular and linear polarization signals are used simultaneously. The inferred field strength, inclination, and filling factor distributions agree with previous results, supporting the idea that internetwork fields are weak and very inclined, at least in about one quarter of the area occupied by the internetwork. These properties differ from those of network fields. The average magnetic flux density and the mean field strength derived from the 27.4% of the field of view with clear linear polarization signals are 16.3 Mx cm-2 and 220 G, respectively. The ratio between the average horizontal and vertical components of the field is approximately 3.1. The internetwork fields do not follow an isotropic distribution of orientations.Comment: To appear in APJ, Vol 749, 201

A search for magnetic fields on central stars in planetary nebulae

One of the possible mechanisms responsible for the panoply of shapes in planetary nebulae is the presence of magnetic fields that drive the ejection of ionized material during the proto-planetary nebula phase. Therefore, detecting magnetic fields in such objects is of key importance for understanding their dynamics. Still, magnetic fields have not been detected using polarimetry in the central stars of planetary nebulae. Circularly polarized light spectra have been obtained with the Focal Reducer and Low Dispersion Spectrograph at the Very Large Telescope of the European Southern Observatory and the Intermediate dispersion Spectrograph and Imaging System at the William Herschel Telescope. Nineteen planetary nebulae spanning very different morphology and evolutionary stages have been selected. Most of central stars have been observed at different rotation phases to point out evidence of magnetic variability. In this paper, we present the result of two observational campaigns aimed to detect and measure the magnetic field in the central stars of planetary nebulae on the basis of low resolution spectropolarimetry. In the limit of the adopted method, we can state that large scale fields of kG order are not hosted on the central star of planetary nebulae.Comment: Paper accepted to be published in Astronomy and Astrophysics on 20/01/201

Contribución al estudio de sedimentos de la Ría de Ribadeo

[Resumen] El estudio sistemático de los sedimentos que forman parte de los fondos marinos de la ría de Ribadeo ha sido, hasta nuestros días, poco conocido debido a que la informaci6n sobre el desarrollo de este ambiente oceanográfico es muy escasa; s6lo las formas litoral les motivadas por la acción marina -como acantilados, plataformas rocosas, playas y dunas y aquellas otras que se desarrollan en plantanales como "tesos" o bancos de arenas, han sido estudiadas con más detenimiento. Sin que intentemos con el exámen de un s6lo sondeo establecer una síntesis precisa de conocimientos sobre esta problemática oceanográfica, al menos sí es posible tratar algunos datos de caracter general relativos a la morfogénesis de las formaciones sedimentarias en profundidad.[Abstract] The systematic study of sediments that take place of the sea bottom of the Bay of Ribadeo was until now, little known due to the information about the growth in this oceanografic enviroment is very scarce; only the litoral shape caused by the marine action like cliffs rocky platforms, beaches, and sand duns- and those other wich develop in plants- such as "tesos" or tidal flats, have been study with more details. Whithout trying, with the unique examination of an only sounding, to stablish a precise synthesis of knowledge about this oceanographic problem; at least it is posible to try to morphogenesis of the sedimentary formations in the dee

Condiciones hidrodinámicas y de sedimentación en el estuario del Eo (límite galaico-astur)

[Resumen] La variabilidad en las condiciones de sedimentación, en el aspecto morfológico y en las características hidrodinámicas que ofrecen los diferen tes parajes de la Ría de Ribadeo, obligan a considerar como unidad independiente y con rasgos peculiares :propios el Estuario del Eo"; se trata de un amplio espacio donde coinciden los procesos continentales y de la red fluvial, con el juego de mareas; la asociación de ambos conjuntos morfogenéticos se traduce en accionamientos de fuertes contrastes entre las zonas marginales y centrales del estuario, muy particularmente en cuanto se refiere a los materiales que rellenan estos parajes. Como conclusión general hay que destacar que las condiciones de formación de los depósitos en cada uno de los conjuntos sedimentarios estudiados ~próximos a la costa, bancos de arena, etc.- son muy variables, no s610 independientemente sino también entre sí.[Abstract] The variability in the conditions of sedimentation, in its morfological aspect and in its hydrodinamic characteristics which the different zones of the Ría of Ribadeo offer, oblige us to consider as an independant unit and with its own peculiar features the Eo Estuary. It deals about a wide range where the continental processes and the river system coincide with the tide. The relation of both, morfogenetic joints is expressed in strong working contrasts between the marginal and central zones of the estuary, and in particular when referred to the materials that fill up these sites. As a general conclusion, we have to emphasize that the conditions in the formation of the deposits in each of the sedimentary joints studied –near the shores, the banks, etc.- are very changeable, not only independently, but also among themselves

How different Fermi surface maps emerge in photoemission from Bi2212

We report angle-resolved photoemission spectra (ARPES) from the Fermi energy ($E_F$) over a large area of the ($k_x,k_y$) plane using 21.2 eV and 32 eV photons in two distinct polarizations from an optimally doped single crystal of Bi$_2$Sr$_2$CaCu$_2$O$_{8+\delta}$ (Bi2212), together with extensive first-principles simulations of the ARPES intensities. The results display a wide-ranging level of accord between theory and experiment and clarify how myriad Fermi surface (FS) maps emerge in ARPES under various experimental conditions. The energy and polarization dependences of the ARPES matrix element help disentangle primary contributions to the spectrum due to the pristine lattice from those arising from modulations of the underlying tetragonal symmetry and provide a route for separating closely placed FS sheets in low dimensional materials.Comment: submitted to PR