Found 7 talks width keyword polarization

gFFJJVLAsoE-thumbnail
Thursday June 3, 2021
Dr. Javier Redondo
Universidad de Zaragoza

Abstract

We introduce the strong CP problem and the existence of the Axion as a possible solution. 

We discuss the possibility that axions are the dark matter of the Universe and the possible ways to

detect it or disprove it using: direct laboratory experiments as well as astrophysical and cosmological

arguments. 

zlN7_aJghM8-thumbnail
Tuesday May 18, 2021
Prof. ºAke Nordlund
Niels Bohr Institute, University of Copenhaguen

Abstract

(This seminar is organized by the IAU G5 commission on stellar and planetary atmospheres) 

Task-based computing is a method where computational problems are split
   into a large number of semi-independent tasks (cf.
   2018MNRAS.477..624N). The method is a general one, with application not
   limited to traditional grid-based simulations; it can be applied with
   advantages also to particle-based and hybrid simulations, which involve
   both particles and fields. The main advantages emerge when doing
   simulations of very complex and / or multi-scale systems, where the
   cost of updating is very unevenly distributed in space, with perhaps
   large volumes with very low update cost and small but important regions
   with large update costs.

   Possible applications in the context of stellar atmospheres include
   modelling that covers large scales, such as whole active regions on the
   Sun or even the entire Sun, while at the same time allows resolving
   small-scale details in the photosphere, chromosphere, and corona. In
   the context of planetary atmospheres, models of pebble-accreting hot
   primordial atmospheres that cover all scales, from the surfaces of
   Mars- and Earth-size embryos to the scale heights of the surrounding
   protoplanetary disks, have already been computed (2018MNRAS.479.5136P,
   2019MNRAS.482L.107P), and one can envision a number of applications
   where the task-based computing advantage is leveraged, for example to
   selectively do the detailed chemistry necessary to treat atmospheres
   saturated with evaporated solids, or to do complex cloud chemistry
   combined with 3-D radiative transfer.

   In the talk I will give a quick overview of the principles behind
   task-based computing, and then use both already published and still
   on-going work to illustrate how this may be used in practice. I will
   finish by discussing how these methods could be applied with great
   advantage to problems such as non-equilibrium ionization, non-LTE
   radiative transfer, and partial redistribution diagnostics of spectral
   lines.


-AuHzwDCpt0-thumbnail
Wednesday April 9, 2014
Dr. Nicolas Fabas
IAC

Abstract

To study the extended atmosphere of evolved stars such as Mira-type variables, spectropolarimetry is an innovative tool. For many kinds of stars, it has been used to measure global magnetic fields through circular polarization and the Zeeman effect. However, linear polarization has seldom been used in the past years even though phenomena such as scattering and the Hanle effect can definitely be studied as well, as it is done in solar physics. In this presentation, I am going to describe original results coming from a spectropolarimetric survey of Mira stars with NARVAL@TBL. Such results concern spectral lines like the Balmer lines of hydrogen and calcium lines. More specifically, I will focus on linear polarization and link this polarization to the propagation of the hypersonic radiative shock wave which is typical of Miras' atmospheres. In general, these environements are very dynamical and scattering in an aspherical atmosphere and velocity gradients can induce a strong linear polarization, likely to be further affected by weak magnetic fields. This analysis is very inspired of what is already done with solar spectra. In addition to that, I am going to present exclusive results about the first detection of a surface magnetic field in a Mira star and explain how the shock wave can impact this field. This work is likely to lead to collaborations with other disciplines such as interferometry (geometry of the scattering environement and characterization of the shock) and radio-astronomy (study of the polarization of masers).


NZSguiqvHUM-thumbnail
Thursday November 10, 2011
Dr. Ricardo Genova-Santos, Mr. Carlos Lopez Caraballo
Instituto de Astrofísica de Canarias, Spain

Abstract

The anomalous microwave emission (AME) is an additional diffuse foreground component, originated by an emission mechanism in the ISM different from the well-known synchrotron, free-free and thermal dust emissions. It was first discovered at the end of the nineties as a correlated signal between microwave CMB maps and infrared maps tracing the dust emission. Ever since several detections have been found in individual clouds in our Galaxy. This emission is an important contaminant for current and future CMB experiments, and therefore its characterization (both in temperature and in polarization) and understanding is mandatory. So far different theoretical models have been proposed to explain the physical mechanism that give rise to this emission. In this talk we will review these models and will present the current observational status of the AME, with particular emphasis on some recent studies that have been performed by our group in the IAC in the Perseus molecular complex and in the Pleiades reflection nebula.


eKc-Y0S0IkM-thumbnail
Tuesday October 4, 2011
Dr. Ivan Agudo
Instituto Astrofisica de Andalucia, Spain

Abstract

Relativistic jets in AGN in general, and in blazars in particular, are the most energetic and among the most powerful astrophysical objects known so far. Their relativistic nature provides them the ability to emit profusely in all spectral ranges from radio wavelengths to gamma-rays, as well as abrupt variability in all time scales (from hours to years). Since the birth of gamma-ray astronomy, locating the origin of gamma-ray emission has been a fundamental problem for the knowledge of the emission processes involved. Deep and densely time sampled monitoring programs with the Fermi Gamma-ray Space Telescope and several other facilities at most of the available spectral ranges (including polarization measurements where possible) are starting to shed light for the case of blazars. After a short review of the status of the problem, some of the latest results locating the GeV emission in the jets of some blazars, at >10 parsec from the central AGN engine, will be presented together with their implications about the gamma-ray emission mechanisms involved


3S2MWrNnrsA-thumbnail
Thursday April 7, 2011
Dr. María Jesús Martínez
Instituto de Astrofísica de Canarias, Spain

Abstract

This question is important because a large fraction of planetary nebulae (about 80%) are bipolar or elliptical rather than spherically symmetric. Modern theories invoke magnetic fields, among other causes, to explain the rich variety of aspherical components observed in PNe, as ejected matter is trapped along magnetic field lines. But, until recently, this idea was mostly a theoretical claim. Jordan et al. (2005) report the detection of kG magnetic fields in the central star of two non-spherical PNe, namely NGC1360 and LSS1362. We find that, contrary to that work, the magnetic field is null within errors for both stars. Then, a direct evidence of magnetic fields on the central stars of PNe is still missing — either the magnetic field is much weaker (< 600 G) than previously reported, or more complex (thus leading to cancellations), or both. The role of magnetic fields shaping PNe is still an open question.


UnBKTuL18ZM-thumbnail
Thursday November 11, 2010
Dr. Jaime de la Cruz Rodríguez
The Institute for Solar Physics of the Royal Swedish Academy of Science.

Abstract

Fibrils are thin elongated features visible in the solar chromosphere in and around magnetized regions. Because of their visual appearance they have been traditionally considered a tracer of the magnetic field lines. In this work we challenge that notion for the first time by comparing their orientation to that of the magnetic field, obtained via high-resolution spectro-polarimetric observations of Ca II lines. The short answer to the question posed in the title is that mostly yes, but not always.

« Newer Older »