1,724 research outputs found
Are collisions with neutral hydrogen important for modelling the Second Solar Spectrum of Ti I and Ca II ?
The physical interpretation of scattering line polarization offers a novel
diagnostic window for exploring the thermal and magnetic structure of the quiet
regions of the solar atmosphere. Here we evaluate the impact of isotropic
collisions with neutral hydrogen atoms on the scattering polarization signals
of the 13 lines of multiplet 42 of Ti I and on those of the K line and of the
IR triplet of Ca II, with emphasis on the collisional transfer rates between
nearby J-levels. To this end, we calculate the linear polarization produced by
scattering processes considering realistic multilevel models and solving the
statistical equilibrium equations for the multipolar components of the atomic
density matrix. We confirm that the lower levels of the 13 lines of multiplet
42 of Ti I are completely depolarized by elastic collisions. We find that
upper-level collisional depolarization turns out to have an unnoticeable impact
on the emergent linear polarization amplitudes, except for the {\lambda 4536
line for which it is possible to notice a rather small depolarization caused by
the collisional transfer rates. Concerning the Ca II lines, we show that the
collisional rates play no role on the polarization of the upper level of the K
line, while they have a rather small depolarizing effect on the atomic
polarization of the metastable lower levels of the Ca II IR triplet.Comment: Accepted for publication in Astronomy and Astrophysic
Estimating the magnetic field strength from magnetograms
A properly calibrated longitudinal magnetograph is an instrument that
measures circular polarization and gives an estimation of the magnetic flux
density in each observed resolution element. This usually constitutes a lower
bound of the field strength in the resolution element, given that it can be
made arbitrarily large as long as it occupies a proportionally smaller area of
the resolution element and/or becomes more transversal to the observer and
still produce the same magnetic signal. Yet, we know that arbitrarily stronger
fields are less likely --hG fields are more probable than kG fields, with
fields above several kG virtually absent-- and we may even have partial
information about its angular distribution. Based on a set of sensible
considerations, we derive simple formulae based on a Bayesian analysis to give
an improved estimation of the magnetic field strength for magnetographs.Comment: 8 pages, 7 figures, accepted for publication in A&
Detecting non-locality in multipartite quantum systems with two-body correlation functions
Bell inequalities define experimentally observable quantities to detect
non-locality. In general, they involve correlation functions of all the
parties. Unfortunately, these measurements are hard to implement for systems
consisting of many constituents, where only few-body correlation functions are
accessible. Here we demonstrate that higher-order correlation functions are not
necessary to certify nonlocality in multipartite quantum states by constructing
Bell inequalities from one- and two-body correlation functions for an arbitrary
number of parties. The obtained inequalities are violated by some of the Dicke
states, which arise naturally in many-body physics as the ground states of the
two-body Lipkin-Meshkov-Glick Hamiltonian.Comment: 10 pages, 2 figures, 1 tabl
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
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