pinecone/core-2020-05-10-deduplication huggingface dataset:
Comparisons and Connections between Mean Field Dynamo Theory and
  Accretion Disc Theory

The origin of large scale magnetic fields in astrophysical rotators, and the
conversion of gravitational energy into radiation near stars and compact
objects via accretion have been subjects of active research for a half century.
Magnetohydrodynamic turbulence makes both problems highly nonlinear, so both
subjects have benefitted from numerical simulations.However, understanding the
key principles and practical modeling of observations warrants testable
semi-analytic mean field theories that distill the essential physics. Mean
field dynamo (MFD) theory and alpha-viscosity accretion disc theory exemplify
this pursuit. That the latter is a mean field theory is not always made
explicit but the combination of turbulence and global symmetry imply such. The
more commonly explicit presentation of assumptions in 20th century textbook
MFDT has exposed it to arguably more widespread criticism than incurred by 20th
century alpha-accretion theory despite complementary weaknesses. In the 21st
century however, MFDT has experienced a breakthrough with a dynamical
saturation theory that consistently agrees with simulations. Such has not yet
occurred in accretion disc theory, though progress is emerging. Ironically
however, for accretion engines, MFDT and accretion theory are presently two
artificially uncoupled pieces of what should be a single coupled theory. Large
scale fields and accretion flows are dynamically intertwined because large
scale fields likely play a key role in angular momentum transport. I discuss
and synthesize aspects of recent progress in MFDT and accretion disc theory to
suggest why the two likely conspire in a unified theory.Comment: (added references and a brief associated discussion of the sign of
  the electromotive force in disc simulation) 9 pages, to appear in Astron.
  Nachr., Vol. 331, Issue 1, 201
Magnetohydrodynamic turbulence in a strongly magnetised plasma

I present a review of incompressible magnetohydrodynamic (MHD) turbulence in
a strongly magnetised plasma. The approach is phenomenological even where a
more rigorous theory is available, so that a reader armed with paper, pencil
and some determination may be able to work through most of the physics. The
focus is on the inertial-range spectra for very large (fluid and magnetic)
Reynolds numbers. These theories of the inertial-range are built on two
important facts: (i) Kraichnan's insight that the turbulent cascades are a
result of nonlinear interactions between oppositely directed wavepackets of
Elsasser fields; (ii) these oppositely directed wavepackets do not exchange
energy, but contribute only to changing each other's spatial structures. I
begin with a description and critique of the Iroshnikov-Kraichnan theory, and
explore the fundamental departures necessitated by the anisotropic nature of
the turbulence. Derivations of the inertial-range spectra of four regimes of
MHD turbulence -- the balanced weak, balanced strong, imbalanced weak and the
imbalanced strong cascades -- are then presented. The need for studying the
spectra of imbalanced turbulence when the waves on the outer scale have a short
correlation time is briefly discussed.Comment: invited review, submitted to Astronomische Nachrichte
Equatorial magnetic helicity flux in simulations with different gauges

We use direct numerical simulations of forced MHD turbulence with a forcing
function that produces two different signs of kinetic helicity in the upper and
lower parts of the domain. We show that the mean flux of magnetic helicity from
the small-scale field between the two parts of the domain can be described by a
Fickian diffusion law with a diffusion coefficient that is approximately
independent of the magnetic Reynolds number and about one third of the
estimated turbulent magnetic diffusivity. The data suggest that the turbulent
diffusive magnetic helicity flux can only be expected to alleviate catastrophic
quenching at Reynolds numbers of more than several thousands. We further
calculate the magnetic helicity density and its flux in the domain for three
different gauges. We consider the Weyl gauge, in which the electrostatic
potential vanishes, the pseudo-Lorenz gauge, where the speed of light is
replaced by the sound speed, and the `resistive gauge' in which the Laplacian
of the magnetic vector potential acts as resistive term. We find that, in the
statistically steady state, the time-averaged magnetic helicity density and the
magnetic helicity flux are the same in all three gauges.Comment: 6 pages 5 figure
Large-scale magnetic flux concentrations from turbulent stresses

In this study we provide the first numerical demonstration of the effects of
turbulence on the mean Lorentz force and the resulting formation of large-scale
magnetic structures. Using three-dimensional direct numerical simulations (DNS)
of forced turbulence we show that an imposed mean magnetic field leads to a
decrease of the turbulent hydromagnetic pressure and tension. This phenomenon
is quantified by determining the relevant functions that relate the sum of the
turbulent Reynolds and Maxwell stresses with the Maxwell stress of the mean
magnetic field. Using such a parameterization, we show by means of
two-dimensional and three-dimensional mean-field numerical modelling that an
isentropic density stratified layer becomes unstable in the presence of a
uniform imposed magnetic field. This large-scale instability results in the
formation of loop-like magnetic structures which are concentrated at the top of
the stratified layer. In three dimensions these structures resemble the
appearance of bipolar magnetic regions in the Sun. The results of DNS and
mean-field numerical modelling are in good agreement with theoretical
predictions. We discuss our model in the context of a distributed solar dynamo
where active regions and sunspots might be rather shallow phenomena.Comment: 9 pages, 10 figures, Astron. Nachr. (submitted
Magnetic fields in the early universe

We give a pedagogical introduction to two aspects of magnetic fields in the
early universe. We first focus on how to formulate electrodynamics in curved
space time, defining appropriate magnetic and electric fields and writing
Maxwell equations in terms of these fields. We then specialize to the case of
magnetohydrodynamics in the expanding universe. We emphasize the usefulness of
tetrads in this context. We then review the generation of magnetic fields
during the inflationary era, deriving in detail the predicted magnetic and
electric spectra for some models. We discuss potential problems arising from
back reaction effects and from the large variation of the coupling constants
required for such field generation.Comment: 11 pages, Review for Astronomische Nachrichten, fixed 2 typo
Mean electromotive force proportional to mean flow in mhd turbulence

In mean-field magnetohydrodynamics the mean electromotive force due to
velocity and magnetic field fluctuations plays a crucial role. In general it
consists of two parts, one independent of and another one proportional to the
mean magnetic field. The first part may be nonzero only in the presence of mhd
turbulence, maintained, e.g., by small-scale dynamo action. It corresponds to a
battery, which lets a mean magnetic field grow from zero to a finite value. The
second part, which covers, e.g., the alpha effect, is important for large-scale
dynamos. Only a few examples of the aforementioned first part of mean
electromotive force have been discussed so far. It is shown that a mean
electromotive force proportional to the mean fluid velocity, but independent of
the mean magnetic field, may occur in an originally homogeneous isotropic mhd
turbulence if there are nonzero correlations of velocity and electric current
fluctuations or, what is equivalent, of vorticity and magnetic field
fluctuations. This goes beyond the Yoshizawa effect, which consists in the
occurrence of mean electromotive forces proportional to the mean vorticity or
to the angular velocity defining the Coriolis force in a rotating frame and
depends on the cross-helicity defined by the velocity and magnetic field
fluctuations. Contributions to the mean electromotive force due to
inhomogeneity of the turbulence are also considered. Possible consequences of
the above and related findings for the generation of magnetic fields in cosmic
bodies are discussed.Comment: 7 pages, 1 figure, Astron. Nachr. (submitted
The Outbursts of Classical and Recurrent Novae

In this review, I present our current state of knowledge regarding both
Classical Nova and Recurrent Nova systems. Two particular objects (V1974 Cyg
and RS Oph) are chosen to illustrate the range of phenomena that may be
associated with their outbursts. The wider importance of nova research is
emphasised and some of the most pressing unsolved problems are summarised.Comment: 8 pages, 4 figures. Review paper presented at the May 2009 XMM-Newton
  Workshop on Super-Soft X-ray Sources. Published in Astronomische Nachrichten,
  Vol 331, p160 (2010). Revised version: Corrected minor typographical errors
  to P_orb vs P_rot on p2 and estimated Galactic nova rate on p
X-ray observations of classical novae. Theoretical implications

Detection of X-rays from classical novae, both in outburst and post-outburst,
provides unique and crucial information about the explosion mechanism. Soft
X-rays reveal the hot white dwarf photosphere, whenever hydrogen (H) nuclear
burning is still on and expanding envelope is transparent enough, whereas
harder X-rays give information about the ejecta and/or the accretion flow in
the reborn cataclysmic variable. The duration of the supersoft X-ray emission
phase is related to the turn-off of the classical nova, i.e., of the H-burning
on top of the white dwarf core. A review of X-ray observations is presented,
with a special emphasis on the implications for the duration of post-outburst
steady H-burning and its theoretical explanation. The particular case of
recurrent novae (both the "standard" objects and the recently discovered ones)
is also reviewed, in terms of theoretical feasibility of short recurrence
periods, as well as regarding implications for scenarios of type Ia supernovae.Comment: 6 pages, review paper accepted for publication in Astronomische
  Nachrichte
Expanding atmosphere models for SSS spectra of novae

Super Soft Source (SSS) spectra are powered by nuclear burning on the surface
of a white dwarf. The released energy causes a radiatively-driven wind that
leads to a radially extended atmosphere around the white dwarf. Significant
blue shifts in photospheric absorption lines are found in the spectra of novae
during their SSS phase, being an evidence of continued mass loss in this phase.
We present spherically symmetric PHOENIX models that account for the expansion
of the ejecta. A comparison to a plane parallel, hydrostatic atmosphere model
demonstrates that the mass loss can have a significant impact on the model
spectra. The dynamic model yields less pronounced absorption edges, and harder
X-ray spectra are the result. Therefore, lower effective temperatures are
needed to explain the observed spectra. Although both types of models are yet
to be fine-tuned in order to accurately determine best fit parameters, the
implications on the chemical abundances are going in opposite directions. With
the expanding models the requirement for strong depletion of the crucial
elements that cause these edges is now avoidable.Comment: 4 pages, accepted for A
Beginning of the Super-Soft Phase of the Classical Nova V2491 Cygni

We present the results of soft X-ray studies of the classical nova V2491
Cygni using the Suzaku observatory. On day 29 after outburst, a soft X-ray
component with a peak at ~0.5 keV has appeared, which is tantalising evidence
for the beginning of the super-soft X-ray emission phase. We show that an
absorbed blackbody model can describe the observed spectra, yielding a
temperature of 57 eV, neutral hydrogen column density of 2x10^{21} cm^{-2}, and
a bolometric luminosity of ~10^{36} erg s^{-1}. However, at the same time, we
also found a good fit with an absorbed thin-thermal plasma model, yielding a
temperature of 0.1 keV, neutral hydrogen column density of 4x10^{21} cm^{-2},
and a volume emission measure of ~10^{58} cm^{-3}. Owing to low spectral
resolution and low signal-to-noise ratio below 0.6 keV, the statistical
parameter uncertainties are large, but the ambiguity of the two very different
models demonstrates that the systematic errors are the main point of concern.
The thin-thermal plasma model implies that the soft emission originates from
optically thin ejecta, while the blackbody model suggests that we are seeing
optically thick emission from the white dwarf.Comment: Accepted for publication on Astronomische Nachrichten (4 pages, 2
  figures
X-ray emission from optical novae in M 31

The first supersoft source (SSS) identification with an optical nova in M 31
was based on ROSAT observations. Twenty additional X-ray counterparts (mostly
identified as SSS by their hardness ratios) were detected using archival ROSAT,
XMM-Newton and Chandra observations obtained before July 2002. Based on these
results optical novae seem to constitute the major class of SSS in M 31. An
analysis of archival Chandra HRC-I and ACIS-I observations obtained from July
2004 to February 2005 demonstrated that M 31 nova SSS states lasted from months
to about 10 years. Several novae showed short X-ray outbursts starting within
50 d after the optical outburst and lasting only two to three months. The
fraction of novae detected in soft X-rays within a year after the optical
outburst was more than 30%. Ongoing optical nova monitoring programs, optical
spectral follow-up and an up-to-date nova catalogue are essential for the X-ray
work. Re-analysis of archival nova data to improve positions and find
additional nova candidates are urgently needed for secure recurrent nova
identifications. Dedicated XMM-Newton/Chandra monitoring programs for X-ray
emission from optical novae covering the center area of M 31 continue to
provide interesting new results (e.g. coherent 1105s pulsations in the SSS
counterpart of nova M31N 2007-12b). The SSS light curves of novae allow us -
together with optical information - to estimate the mass of the white dwarf, of
the ejecta and the burned mass in the outburst. Observations of the central
area of M 31 allow us - in contrast to observations in the Galaxy - to monitor
many novae simultaneously and proved to be prone to find many interesting SSS
and nova types.Comment: 6 pages, 4 figures, accepted for publication in Aston.Nach
Supersoft sources in M 31: Comparing the XMM-Newton Deep Survey, ROSAT
  and Chandra catalogues

To investigate the transient nature of supersoft sources (SSSs) in M 31, we
compared SSS candidates of the XMM-Newton Deep Survey, ROSAT PSPC surveys and
the Chandra catalogues in the same field. We found 40 SSSs in the XMM-Newton
observations. While 12 of the XMM-Newton sources were brighter than the
limiting flux of the ROSAT PSPC survey, only two were detected with ROSAT ~10
yr earlier. Five correlate with recent optical novae which explains why they
were not detected by ROSAT. The remaining 28 XMM-Newton SSSs have fluxes below
the ROSAT detection threshold. Nevertheless we found one correlation with a
ROSAT source, which had significantly larger fluxes than during the XMM-Newton
observations. Ten of the XMM-Newton SSSs were detected by Chandra with <1-~6 yr
between the observations. Five were also classified as SSSs by Chandra. Of the
30 ROSAT SSSs three were confirmed with XMM-Newton, while for 11 sources other
classifications are suggested. Of the remaining 16 sources one correlates with
an optical nova. Of the 42 Chandra very-soft sources five are classified as
XMM-Newton SSSs, while for 22 we suggest other classifications. Of the
remaining 15 sources, nine are classified as transient by Chandra, one of them
correlates with an optical nova. These findings underlined the high variability
of the sources of this class and the connection between SSSs and optical novae.
Only three sources, were detected by all three missions as SSSs. Thus they are
visible for more than a decade, despite their variability.Comment: 6 pages, 1 figure, Proc. of workshop "SuperSoft X-ray Sources - New
  Developments", ESTEC/Villafranca, May 2009, accepted for publication in
  Aston.Nach
SSS in young stellar populations and the "prompt" component of Type Ia
  supernovae

We present the results of a search for UV and optical counterparts of the SSS
population in M31. We find that out of the 56 sources we included in our
search, 16 are associated with regions of ongoing or recent star formation. We
discuss two particularly interesting sources that are identified optically as
early type stars, one of which displayed long term X-ray evolution similar to
that observed in classical novae. We discuss the physical origin of supersoft
X-rays in these and the other SSS in young regions, and their possible link to
the so-called "prompt" component of the Type Ia supernova population.Comment: 5 pages, 3 figures; Proceedings of workshop "Supersoft X-ray Sources
  - New Developments", ESAC, May 2009; accepted for publication in
  Astronomische Nachrichte
The outbursts properties of AM CVn stars

We briefly summarize the observational properties of ultra-compact binaries
called AM CVn stars. We analize their outbursts originating from the
thermal-viscous instability in helium accretion disc. We present our
preliminary results in applying the model of Dwarf Novae outbursts to helium
discs. We can calculate models of outbursts of reasonable amplitude of 2 mag
with a constant alfa parameter throughout the calculation. Setting the mass
transfer rate close to its upper critical value produces model lightcurves that
resemble short superoutbursts.Comment: To be published in Astronomische Nachrichte
Finding the most variable stars in the Orion Belt with the All Sky
  Automated Survey

We look for high-amplitude variable young stars in the open clusters and
associations of the Orion Belt. We use public data from the ASAS-3 Photometric
V-band Catalogue of the All Sky Automated Survey, infrared photometry from the
2MASS and IRAS catalogues, proper motions, and the Aladin sky atlas to obtain a
list of the most variable stars in a survey area of side 5 deg centred on the
bright star Alnilam (eps Ori) in the centre of the Orion Belt. We identify 32
highly-variable stars, of which 16 had not been reported to vary before. They
are mostly variable young stars and candidates (16) and background giants (8),
but there are also field cataclysmic variables, contact binaries, and eclipsing
binary candidates. Of the young stars, which typically are active Herbig Ae/Be
and T Tauri stars with Halpha emission and infrared flux excess, we discover
four new variables and confirm the variability status of another two. Some of
them belong to the well known sigma Orionis cluster. Besides, six of the eight
giants are new variables, and three are new periodic variables.Comment: Astronomische Nachrichten, in pres
The young open cluster Trumpler 3

We present a photometric and spectroscopic study of the poorly investigated
open cluster Trumpler~3. Basic parameters such as the age of $70\pm10$ Myr, the
color excess $E(B-V)=0.30\pm0.02$ mag, the distance of $0.69\pm0.03$ kpc and
the limiting radius of 12' were redetermined and compared with previous
preliminary studies. The distance of $0.65\pm0.09$ kpc was determined
independently by spectral parallaxes. Simultaneously, our analysis allowed us
to estimate a total number of members to be $N_{\rm{tot}}=570\pm90$ and a total
mass of the cluster to be $M_{\rm{tot}}=270\pm40$ $\rm{M}_{\odot}$. We also
determined a state of cluster's dynamical evolution. We conclude that
Trumpler~3 is a young low-massive stellar ensemble with a typical mass function
slope, located near to the outer edge of the Galaxy's Orion Spur. As a result
of a wide-field search for short period variable stars, 24 variables were
discovered in the cluster's area. Only one of them -- a variable of the
$\gamma$-Dor type -- was found to be a likely cluster member
Four-colour photometry of EY Dra: a study of an ultra-fast rotating
  active dM1-2e star

We present more than 1000-day long photometry of EY Draconis in BV(RI)C
passbands. The changes in the light curve are caused by the spottedness of the
rotating surface. Modelling of the spotted surface shows that there are two
large active regions present on the star on the opposite hemispheres. The
evolution of the surface patterns suggests a flip-flop phenomenon. Using
Fourier analysis, we detect a rotation period of P_rot=0.45875d, and an
activity cycle with P~350d, similar to the 11-year long cycle of the Sun. This
cycle with its year-long period is the shortest one ever detected on active
stars. Two bright flares are also detected and analysed.Comment: 7 pages, 11 figures, 3 tables, accepted at AN, ephemeris correcte
Dissecting Galaxies with Quantitative Spectroscopy of the Brightest
  Stars in the Universe

Measuring distances to galaxies, determining their chemical composition,
investigating the nature of their stellar populations and the absorbing
properties of their interstellar medium are fundamental activities in modern
extragalactic astronomy helping to understand the evolution of galaxies and the
expanding universe. The optically brightest stars in the universe, blue
supergiants of spectral A and B, are unique tools for these purposes. With
absolute visual magnitudes up to M_V = -9.5 they are the ideal to obtain
accurate quantitative information about galaxies through the powerful modern
methods of quantitative stellar spectroscopy. The spectral analyis of
individual blue supergiant targets provides invaluable information about
chemical abundances and abundance gradients, which is more comprehensive than
the one obtained from HII regions, as it includes additional atomic species,
and which is also more accurate, since it avoids the systematic uncertainties
inherent in the strong line studies usually applied to the HII regions of
spiral galaxies beyond the Local Group. Simultaneously, the spectral analysis
yields stellar parameters and interstellar extinction for each individual
supergiant target, which provides an alternative very accurate way to determine
extragalactic distances through a newly developed method, called the
Flux-weighted Gravity - Luminosity Relationship (FGLR). With the present
generation of 10m-class telescopes these spectroscopic studies can reach out to
distances of 10 Mpc. The new generation of 30m-class will allow to extend this
work out to 30 Mpc, a substantial volume of the local universe.Comment: Karl Schwarzschild Lecture 2009. To appear in Astronomische
  Nachrichte
The Eastern Filament of W50

We present new spectral (FPI and long-slit) data on the Eastern optical
filament of the well known radionebula W50 associated with SS433. We find that
on sub-parsec scales different emission lines are emitted by different regions
with evidently different physical conditions. Kinematical properties of the
ionized gas show evidence for moderately high (V ~ 100 km/s) supersonic
motions. [OIII]5007 emission is found to be multi-component and differs from
lower-excitation [SII]6717 line both in spatial and kinematical properties.
Indirect evidence for very low characteristic densities of the gas (n ~
0.1cm^{-3}) is found. We propose radiative (possibly incomplete) shock waves in
low-density, moderately high metallicity gas as the most probable candidate for
the power source of the optical filament. Apparent nitrogen over-abundance is
better understood if the location of W50 in the Galaxy is taken into account.Comment: accepted to Astronomische Nachrichten; 9 pages, 4 figures, 2 table