Archivi tag: magnetic fields

Magnetism and Variability in O stars

For more than 30 years, spectroscopic observations from space have shown that wind variability in massive OB stars is a widespread phenomenon. This variability is not strictly periodic, but cyclic (like sunspots) with a dominant quasi period that scales with the estimated rotation period. The underlying cause or trigger of this variability is not known. The major time-variable wind features likely find their origin close to, or at the surface and have been suggested to be connected to non-radial pulsations or bright magnetic star spots. Continua a leggere Magnetism and Variability in O stars

Physics and Evolution of Magnetic and Related Stars

The Special Astrophysical Observatory of the Russian Academy of Sciences will organize a traditional International conference “Physics and Evolution of Magnetic and Related stars” during the period from August 25 to August 31, 2014. Continua a leggere Physics and Evolution of Magnetic and Related Stars

The Submillimeter Array: First Decade of Discovery

 

The Submillimeter Array was the first observatory capable of imaging and spectroscopy at sub-arcsecond angular resolution at submillimeter wavelengths. The SMA has made many fundamental contributions to astronomy from the study of nearby protoplanetary disks to distant, high-redshift submillimeter galaxies. The SMA has been a pioneer in developing submillimeter instrumentation, particularly wideband receivers and electronics. The ten years since the dedication of the SMA in 2003 have been a decade of discovery in submillimeter astronomy.  Continua a leggere The Submillimeter Array: First Decade of Discovery

Third BCool Meeting

According to dynamo models, the variable magnetic field of the Sun is the consequence of the interplay between two main ingredients. The first ingredient is the radial and latitudinal differential rotation that succeeds at generating a large-scale toroidal magnetic field from an initial poloidal field. The second ingredient is still a matter of debate, with models invoking either the cyclonic convection in the convection zone or the transport of decaying active regions by meridional circulation as possible processes to regenerate the poloidal magnetic component. When acting together, both effects succeed at building continuously a large-scale magnetic field that oscillates with time, giving rise to the 22 yr period of the solar cycle. Despite considerable progress in this field since the very first solar dynamo models, there are still many aspects of solar magnetism that the current models cannot reproduce or did not thoroughly explore.

Our understanding of the solar dynamo can benefit from the observation of solar-type stars, where dynamo types marginal or inactive in the Sun can be observed, either because these analogues of the Sun are caught by chance in an unfrequented activity state (similar, e.g., to the Maunder minimum) or because their physical properties (in particular their mass and rotation rate) differ sufficiently from the Sun’s to lead to a different dynamo output. Using spectropolarimetric observations, the magnetic fields of cool stars can now be directly characterized from the polarized signatures they produce in spectral lines, and the associated field geometries can be reconstructed using tomographic imaging techniques, like Zeeman-Doppler-Imaging.

More info: http://bcool2014.sciencesconf.org/

Ionising processes in atmospheric environments of planets, Brown Dwarfs, and M-dwarfs

The atmospheres of planets and very low-mass (VLM) stars are cold enough that clouds form and affect the local chemistry and the spectral appearance. Prominent planetary examples are the giant gas planet HD189733b and the super-Earth GJ1214b in which hazes block the view onto the gaseous atmosphere in the optical and mid-IR spectral ranges. Similar effects need to be expected in VLM stars, which in addition, are suggested to have strong magnetic fields. These magnetic fields add to the mystery of VLM stars as their rotational braking is  much less efficient than expected pointing to an inefficient magnetic field coupling.  How do atmospheric ionisation processes and processes causing magnetic field interaction differ in extraterrestrial environments compared to the solar system? Which role do magnetic-field modulated cosmic rays play?

Ionisation processes have been studied in well-defined areas of astro and geophysics. The study of the interaction of the atmospheric environment with the object’s magnetic field can allow for a  mutual benefit between these scientific communities. This meeting therefore invites astrophysics, geologists and meteorologists to exchange and discuss their views on ionisation, charge separation and discharge processes.

Physical Processes in the Interstellar Medium

The ISM represents a fascinating laboratory to study the physics of highly attenuated gases, chemical processes and atomic, molecular and solid state physics under extreme conditions and numerous other questions of natural sciences. The physics of the ISM plays a crucial role in many areas of astronomy. Galaxy formation and evolution, the formation of stars, cosmic nucleosynthesis, the origin of large complex, prebiotic molecules and the abundance, structure and growth of dust grains which constitute the fundamental building blocks of planets, all these processes are intimately coupled to the physics of the ISM. New observations with powerful telescopes have revealed that the ISM is a turbulent, multiphase gas, filled with structures on all resolvable spatial scales. This has lead to a paradigm shift in our understanding of the ISM, where the old equilibrium model is being replaced by a highly dynamical picture of strongly coupled, interacting and turbulently mixed gas phases that are far from equilibrium and that are continuously stirred by processes that are not well understood. We enter an era where for the first time enough information is available to gain a deep and comprehensive physical understanding of the ISM and the dynamical processes that govern its evolution.

Many physical processes in the ISM have been studied in isolation and under idealized conditions. It is however their nonlinear coupling that fully characterizes the structure and evolution of the multi-phase, dynamically evolving ISM. Therefore, the first funding period of the ISM-SPP (ism-spp.de) is dedicated to the investigation of the interplay between various processes in the ISM. In this conference, we aim at (i) summarizing the recent progress and isolating the open questions as well as (ii) bringing together experts from all three research pillars (laboratory studies, observations as well as theory and computations) in order to form a consistent picture of relevant physical processes in the ISM.

In this first conference in a series organised by the DFG priority program 1573 “The Physics of the Interstellar Medium” (http://www.ism-spp.de) we will concentrate on the following questions:

  • Which are the drivers of turbulence in the ISM and how does turbulence affect the morphology and the energy of the ISM on different scales?
  • What is the intrinsic structure of molecular clouds?
  • How do molecular clouds form and evolve?
  • How do interstellar dust grains and molecules form and evolve in the ISM and how do they affect physical processes in the ISM?
  • How do stars interact with and shape the multi-phase ISM?
  • How are the processes in the ISM affected by magnetic fields?
  • How are cosmic rays accelerated in the ISM, and how do cosmic rays affect interstellar structure?

Ultra strong magnetic fields in neutron stars

Una serie di simulazioni numeriche mostra per la prima volta che la presenza di instabilità nel nucleo delle stelle di neutroni può determinare la formazione di campi magnetici giganteschi che possono a loro volta causare violente e drammatiche esplosioni stellari mai osservate nell’Universo.

An ultra-dense (“hypermassive”) neutron star is formed when two neutron stars in a binary system finally merge. Its short life ends with the catastrophic collapse to a black hole, possibly powering a short gamma-ray burst, one of the brightest explosions observed in the Universe. Short gamma-ray bursts as observed with satellites like XMM Newton, Fermi or Swift release within a second the same amount of energy as our Galaxy in one year. It has been speculated for a long time that enormous magnetic field strengths, possibly higher than what has been observed in any known astrophysical system, are a key ingredient in explaining such emission.

Scientists at the Max Planck Institute for Gravitational Physics (Albert Einstein Institute/AEI) have now succeeded in simulating a mechanism which could produce such strong magnetic fields prior to the collapse to a black hole.

How can such ultra-high magnetic fields, stronger than ten or hundred million billion times the Earth’s magnetic field, be generated from the much lower initial neutron star magnetic fields? This could be explained by a phenomenon that can be triggered in a differentially rotating plasma in the presence of magnetic fields: neighbouring plasma layers, which rotate at different speeds, “rub against each other”, eventually setting the plasma into turbulent motion. In this process called magnetorotational instability magnetic fields can be strongly amplified. This mechanism is known to play an important role in many astrophysical systems such as accretion disks and core-collapse supernovae. It had been speculated for a long time that magnetohydrodynamic instabilities in the interior of hypermassive neutron stars could bring about the necessary magnetic field amplification. The actual demonstration that this is possible has only now been achieved with the present numerical simulations. The scientists of the Gravitational Wave Modelling Group at the AEI simulated a hypermassive neutron star with an initially ordered (“poloidal”) magnetic field, whose structure is subsequently made more complex by the star’s rotation. Since the star is dynamically unstable, it eventually collapses to a black hole surrounded by a cloud of matter, until the latter is swallowed by the black hole.

These simulations have unambiguously shown the presence of an exponentially rapid amplification mechanism in the stellar interior, the magnetorotational instability. This mechanism has so far remained essentially unexplored under the extreme conditions of ultra-strong gravity as found in the interior of hypermassive neutron stars.

This is because the physical conditions in the interior of these stars are extremely challenging. The discovery is interesting for at least two reasons. First, it shows for the first time unambiguously the development of the magnetorotational instability in the framework of Einstein’s theory of general relativity, in which there exist no analytical criteria to date to predict the instability. Second, this discovery can have a profound astrophysical impact, supporting the idea that ultra strong magnetic fields can be the key ingredient in explaining the huge amount of energy released by short gamma-ray bursts.

MPI: The largest magnetic fields in the universe

arXiv: Magnetorotational instability in relativistic hypermassive neutron stars

The innermost regions of relativistic jets and their magnetic fields

The study of relativistic jets in active galactic nuclei and other sources involving the accretion onto compact objects, like gamma-ray bursts and microquasars, has gained added interest thanks to the recent simultaneous multi spectral-range observations and the advances in the theoretical and numerical modeling. During June 10-14th, 2013, the Instituto de Astrofísica de Andalucía-CSIC in Granada, Spain, will host a meeting (poster) aimed to discuss the recent results in the study of relativistic jets. The meeting will focus on the study of the innermost regions of AGN jets to obtain a better understanding of the jet formation mechanisms and determine the origin and location of the high energy emission, as well as the role played by the magnetic field.

Topics to be discussed include:

  • Jet formation
  • Black hole, accretion disk, jet connection
  • Multi-spectral-range emission
  • Magnetic fields and polarization
  • Jet dynamics and stability
  • Unification models, microphysics, particle acceleration
  • Relativistic stellar jets

Magnetic Fields in the Universe IV: From Laboratory and Stars to the Primordial Structures

Magnetic Fields in the Universe IV: From Laboratory and Stars to the Primordial Structures – This is the webpage for the fourth edition of the “Magnetic Fields in the Universe: from Laboratory and Stars to Primordial Structures”.  Continua a leggere Magnetic Fields in the Universe IV: From Laboratory and Stars to the Primordial Structures