scholarly journals Leon Mestel. 5 August 1927 — 15 September 2017

2021 ◽  
Vol 70 ◽  
pp. 335-358
Author(s):  
Robert Connon Smith ◽  
Nigel Weiss
Keyword(s):  
Magnetic Fields ◽  
Observational Data ◽  
Galaxy Formation ◽  
White Dwarf ◽  
Royal Society ◽  
Theoretical Models ◽  
Great Demand ◽  
Wide Range

Leon Mestel was best known for his wide-ranging work on cosmic magnetism, but he also worked on an equally wide range of non-magnetic problems in astrophysics, from star and galaxy formation to white dwarf cooling. Despite his work being primarily theoretical and highly mathematical, he was always aware of all the relevant observational data that both needed to be explained and also provided constraints for his theoretical models. He was internationally recognized as an authority on the influence of magnetic fields in astronomy, receiving a number of significant honours. He also had a deserved reputation for scrupulous honesty and integrity in his work. His life's work culminated in the publication of two editions of a magisterial monograph on stellar magnetism. He collaborated widely, influenced many other researchers and was in great demand as a conference speaker. He was also a conscientious academic, taking his full part in departmental teaching and administration, and a delightful companion and friend to all who knew him. He was well known for his many stories and jokes, which were widely enjoyed. His outside interests included being a long-standing member of the Editorial Board of this publication, Biographical Memoirs of Fellows of the Royal Society .

scholarly journals The Intermediate Polars

1983 ◽  
Vol 72 ◽  
pp. 155-172
Author(s):  
Brian Warner
Keyword(s):  
Magnetic Fields ◽  
White Dwarf ◽  
Variable Star ◽  
Theoretical Models ◽  
Cataclysmic Variable ◽  
General Role ◽  
X Ray ◽  
Low Mass ◽  
Intermediate Polars ◽  
X Ray Binaries

Until 1976, cataclysmic variable star research proceeded with few requirements for the inclusion of magnetic fields in theoretical models. Although models for low-mass X-ray binaries stressed the importance of magnetic fields (Lamb et at. 1973) and there was an increasing number of known magnetic single white dwarfs (Angel 1977), and a magnetised white dwarf had been one of the models proposed to explain the rapid oscillations in DQ Her (Herbst et al. 1974, Katz 1975), there was no anticipation of the more general role that magnetic fields now seem destined to play. The two major reviews of the time (Robinson 1976, Warner 1976) scarcely considered the presence of magnetic fields.

scholarly journals Waves and Oscillations in Magnetic Fluxtubes

1990 ◽  
Vol 138 ◽  
pp. 229-249
Author(s):  
M.P. Ryutova
Keyword(s):  
Magnetic Fields ◽  
Observational Data ◽  
Solar Atmosphere ◽  
Theoretical Models ◽  
Gas Flows ◽  
Solar Magnetic Fields ◽  
Filamentary Structure ◽  
Associated Gas ◽  
Structure And Dynamics ◽  
Theoretical Investigations

According to observational data solar magnetic fields have a pronounced filamentary structure. Theoretical investigations of plasmas containing structured magnetic fields, including the study of the properties of these structures and their interactions with associated gas flows, are of great importance for our understanding of the basic processes in the solar atmosphere, whose structure and dynamics are dominated by magnetic fields. In the present review theoretical models of thin magnetic fluxtubes and their behaviour in the ambient plasma are discussed.

Non-adiabatic interaction of ions with solar wind discontinuities

2020 ◽  
Author(s):  
Alexander Vinogradov ◽  
Anton Artemyev ◽  
Ivan Vasko ◽  
Alexei Vasiliev ◽  
Anatoly Petrukovich
Keyword(s):  
Solar Wind ◽  
Observational Data ◽  
Solar Wind Plasma ◽  
Theoretical Models ◽  
High Amplitude ◽  
Small Scale ◽  
Ion Scattering ◽  
Ion Temperature ◽  
Wide Range ◽  
Radial Evolution

<p>According to Helios, Ulysses, New Horizons measurements at a wide range of distances from the Sun, radial evolution of solar wind ion temperature significantly deviates from the adiabatic expansion model:  additional heating of the solar wind plasma is required to describe observational data. Solution of the solar wind heating problem is extremely important both for understanding the structure of the heliosphere and for adequately describing the atmospheres of distant stars. Solar wind magnetic field is turbulent and this turbulence is dominated by numerous small-scale high-amplitude coherent structures – such as quasi-1D discontinuities. Modern theoretical models predict that quasi-1D discontinuities can play important role in solar wind heating. We collected the statistics of MMS observations of thin quasi-1D discontinuities in the solar wind to reveal their characteristics. Analyzing observational data, we construct the discontinuity model and use it to consider non-adiabatic interaction of ions with solar wind discontinuities. We mainly focus on discontinuity roles in solar wind ion scattering and thermalization. This presentation shows how discontinuity configuration affects the scattering rates.</p>

scholarly journals Are white dwarf magnetic fields in close binaries generated during common-envelope evolution?

2019 ◽  
Vol 492 (1) ◽  
pp. 1523-1529 ◽  
Author(s):  
Diogo Belloni ◽  
Matthias R Schreiber
Keyword(s):  
Low Temperature ◽  
Magnetic Fields ◽  
Observational Data ◽  
White Dwarf ◽  
Close Binaries ◽  
Common Envelope ◽  
The Past ◽  
The Common ◽  
M Dwarf ◽  
Dynamo Process

ABSTRACT Understanding the origin of the magnetic fields in white dwarfs (WDs) has been a puzzle for decades. A scenario that has gained considerable attention in the past years assumes that such magnetic fields are generated through a dynamo process during common-envelope evolution. We performed binary population models using an up-to-date version of the bse code to confront the predictions of this model with observational results. We found that this hypothesis can explain only the observed distribution of WD magnetic fields in polars and pre-polars and the low-temperature WDs in pre-polars if it is re-scaled to fit the observational data. Furthermore, in its present version, the model fails to explain the absence of young, close detached WD+M-dwarf binaries harbouring hot magnetic WDs and predicts that the overwhelming majority of WDs in close binaries should be strongly magnetic, which is also in serious conflict with the observations. We conclude that either the common-envelope dynamo scenario needs to be substantially revised or a different mechanism is responsible for the generation of strong WD magnetic fields in close binaries.

scholarly journals Mapping large-scale-structure evolution over cosmic times

2021 ◽  
Author(s):  
Marta B. Silva ◽  
Ely D. Kovetz ◽  
Garrett K. Keating ◽  
Azadeh Moradinezhad Dizgah ◽  
Matthieu Bethermin ◽  
...  
Keyword(s):  
Galaxy Formation ◽  
Large Scale ◽  
High Redshift ◽  
Formation Rate ◽  
Far Infrared ◽  
Large Scale Structure ◽  
Scale Structure ◽  
Structure Evolution ◽  
Intensity Mapping ◽  
Wide Range

AbstractThis paper outlines the science case for line-intensity mapping with a space-borne instrument targeting the sub-millimeter (microwaves) to the far-infrared (FIR) wavelength range. Our goal is to observe and characterize the large-scale structure in the Universe from present times to the high redshift Epoch of Reionization. This is essential to constrain the cosmology of our Universe and form a better understanding of various mechanisms that drive galaxy formation and evolution. The proposed frequency range would make it possible to probe important metal cooling lines such as [CII] up to very high redshift as well as a large number of rotational lines of the CO molecule. These can be used to trace molecular gas and dust evolution and constrain the buildup in both the cosmic star formation rate density and the cosmic infrared background (CIB). Moreover, surveys at the highest frequencies will detect FIR lines which are used as diagnostics of galaxies and AGN. Tomography of these lines over a wide redshift range will enable invaluable measurements of the cosmic expansion history at epochs inaccessible to other methods, competitive constraints on the parameters of the standard model of cosmology, and numerous tests of dark matter, dark energy, modified gravity and inflation. To reach these goals, large-scale structure must be mapped over a wide range in frequency to trace its time evolution and the surveyed area needs to be very large to beat cosmic variance. Only a space-borne mission can properly meet these requirements.

scholarly journals No-z Model for Magnetic Fields of Different Astrophysical Objects and Stability of the Solutions

Data ◽  
2021 ◽  
Vol 6 (1) ◽  
pp. 4
Author(s):  
Evgeny Mikhailov ◽  
Daniela Boneva ◽  
Maria Pashentseva
Keyword(s):  
Magnetic Field ◽  
Magnetic Fields ◽  
Large Scale ◽  
Point Of View ◽  
Accretion Discs ◽  
Wide Range ◽  
Astrophysical Objects ◽  
Alpha Effect ◽  
The Stability ◽  
The Magnetic Field

A wide range of astrophysical objects, such as the Sun, galaxies, stars, planets, accretion discs etc., have large-scale magnetic fields. Their generation is often based on the dynamo mechanism, which is connected with joint action of the alpha-effect and differential rotation. They compete with the turbulent diffusion. If the dynamo is intensive enough, the magnetic field grows, else it decays. The magnetic field evolution is described by Steenbeck—Krause—Raedler equations, which are quite difficult to be solved. So, for different objects, specific two-dimensional models are used. As for thin discs (this shape corresponds to galaxies and accretion discs), usually, no-z approximation is used. Some of the partial derivatives are changed by the algebraic expressions, and the solenoidality condition is taken into account as well. The field generation is restricted by the equipartition value and saturates if the field becomes comparable with it. From the point of view of mathematical physics, they can be characterized as stable points of the equations. The field can come to these values monotonously or have oscillations. It depends on the type of the stability of these points, whether it is a node or focus. Here, we study the stability of such points and give examples for astrophysical applications.

scholarly journals Simulating the interstellar medium of galaxies with radiative transfer, non-equilibrium thermochemistry, and dust

2020 ◽  
Vol 499 (4) ◽  
pp. 5732-5748 ◽  
Author(s):  
Rahul Kannan ◽  
Federico Marinacci ◽  
Mark Vogelsberger ◽  
Laura V Sales ◽  
Paul Torrey ◽  
...  
Keyword(s):  
Interstellar Medium ◽  
Galaxy Formation ◽  
State Of The Art ◽  
Complex Function ◽  
Ionization State ◽  
Radiation Fields ◽  
Stellar Feedback ◽  
Wide Range ◽  
Dust Distribution ◽  
Non Equilibrium

ABSTRACT We present a novel framework to self-consistently model the effects of radiation fields, dust physics, and molecular chemistry (H2) in the interstellar medium (ISM) of galaxies. The model combines a state-of-the-art radiation hydrodynamics module with a H  and He  non-equilibrium thermochemistry module that accounts for H2 coupled to an empirical dust formation and destruction model, all integrated into the new stellar feedback framework SMUGGLE. We test this model on high-resolution isolated Milky-Way (MW) simulations. We show that the effect of radiation feedback on galactic star formation rates is quite modest in low gas surface density galaxies like the MW. The multiphase structure of the ISM, however, is highly dependent on the strength of the interstellar radiation field. We are also able to predict the distribution of H2, that allow us to match the molecular Kennicutt–Schmidt (KS) relation, without calibrating for it. We show that the dust distribution is a complex function of density, temperature, and ionization state of the gas. Our model is also able to match the observed dust temperature distribution in the ISM. Our state-of-the-art model is well-suited for performing next-generation cosmological galaxy formation simulations, which will be able to predict a wide range of resolved (∼10 pc) properties of galaxies.

scholarly journals Absorption by Cyclotron Harmonics in the Optical Spectrum of VV Puppis

1979 ◽  
Vol 53 ◽  
pp. 334-340 ◽  
Author(s):  
H.S. Stockman ◽  
James Liebert ◽  
Howard E. Bond
Keyword(s):  
White Dwarf ◽  
Optical Spectrum ◽  
Cyclotron Frequency ◽  
Theoretical Models ◽  
Doppler Broadening ◽  
Cyclotron Emission ◽  
Optical Continuum ◽  
Cyclotron Harmonics ◽  
Accretion Shock ◽  
Depth Effects

Most theoretical models of the AM Her variables (AM Her, AN UMa, W Pup and 2A0311-22) rely on strong cyclotron emission at the fundamental cyclotron frequency and higher harmonics to produce the observed, strongly-polarized optical continuum (e.g. Lamb and Masters 1979). The cyclotron lines, which presumably originate in the hot, isothermal accretion shock at the surface of the white dwarf (kT ≳ 10 keV, h/R* ≲ 0.1), should be blurred into a continuous spectrum by both optical depth effects and electron Doppler broadening. Thus the lack of even weak cyclotron features in the optical spectra of these objects is still compatible with a cyclotron origin.

scholarly journals From young massive star clusters to old globulars: long-term survival chances

2006 ◽  
Vol 2 (S237) ◽  
pp. 408-408
Author(s):  
Richard de Grijs
Keyword(s):  
Galaxy Formation ◽  
Massive Star ◽  
Cluster Formation ◽  
Star Clusters ◽  
Term Survival ◽  
Star Forming ◽  
Distant Galaxies ◽  
Wide Range ◽  
Galaxy Collisions

Young, massive star clusters (YMCs) are the most notable and significant end products of violent star-forming episodes triggered by galaxy collisions and close encounters. The question remains, however, whether or not at least a fraction of the compact YMCs seen in abundance in extragalactic starbursts, are potentially the progenitors of (≳10 Gyr) old globular cluster (GC)-type objects. If we could settle this issue convincingly, one way or the other, the implications of such a result would have far-reaching implications for a wide range of astrophysical questions, including our understanding of the process of galaxy formation and assembly, and the process and conditions required for star (cluster) formation. Because of the lack of a statistically significant sample of YMCs in the Local Group, however, we need to resort to either statistical arguments or to the painstaking approach of case-by-case studies of individual objects in more distant galaxies.

Dynamics and Performance of a Harmonically Excited Vertical Impact Damper

2008 ◽  
Vol 130 (2) ◽  
Author(s):  
Sanjiv Ramachandran ◽  
George Lesieutre
Keyword(s):  
Loss Factor ◽  
Vertical Direction ◽  
Theoretical Models ◽  
Coefficient Of Restitution ◽  
Numerical Continuation ◽  
Particle Impact ◽  
Impact Damper ◽  
High Loss ◽  
Wide Range ◽  
And Performance

Particle impact dampers (PIDs) have been shown to be effective in vibration damping. However, our understanding of such dampers is still limited, based on the theoretical models existing today. Predicting the performance of the PID is an important problem, which needs to be investigated more thoroughly. This research seeks to understand the dynamics of a PID as well as those parameters which govern its behavior. The system investigated is a particle impact damper with a ceiling, under the influence of gravity. The base is harmonically excited in the vertical direction. A two-dimensional discrete map is obtained, wherein the variables at one impact uniquely dictate the variables at the next impact. This map is solved using a numerical continuation procedure. Periodic impact motions and “irregular” motions are observed. The effects of various parameters such as the gap clearance, coefficient of restitution, and the base acceleration are analyzed. The dependence of the effective damping loss factor on these parameters is also studied. The loss factor results indicate peak damping for certain combinations of parameters. These combinations of parameters correspond to a region in parameter space where two-impacts-per-cycle motions are observed over a wide range of nondimensional base accelerations. The value of the nondimensional acceleration at which the onset of two-impacts-per-cycle solutions occurs depends on the nondimensional gap clearance and the coefficient of restitution. The range of nondimensional gap clearances over which two-impacts-per-cycle solutions are observed increases as the coefficient of restitution increases. In the regime of two-impacts-per-cycle solutions, the value of nondimensional base acceleration corresponding to onset of these solutions initially decreases and then increases with increasing nondimensional gap clearance. As the two-impacts-per-cycle solutions are associated with high loss factors that are relatively insensitive to changing conditions, they are of great interest to the designer.

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