1,828 research outputs found
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 RÃa de Ribadeo
[Resumen] El estudio sistemático de los sedimentos que forman parte de los fondos marinos de la rÃa de Ribadeo ha sido, hasta nuestros dÃas, 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 sÃntesis precisa de conocimientos sobre esta problemática oceanográfica, al menos sà 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 (lÃmite galaico-astur)
[Resumen] La variabilidad en las condiciones de sedimentación, en el aspecto morfológico y en las caracterÃsticas hidrodinámicas que ofrecen los diferen tes parajes de la RÃa 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 sÃ.[Abstract] The variability in the conditions of sedimentation, in its morfological aspect and in its hydrodinamic characteristics which the different zones of the RÃa 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
() over a large area of the () plane using 21.2 eV and 32 eV
photons in two distinct polarizations from an optimally doped single crystal of
BiSrCaCuO (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
- …