363,095 research outputs found
Stability of three-dimensional relativistic jets: implications for jet collimation
The stable propagation of jets in FRII sources is remarkable if one takes
into account that large-scale jets are subjected to potentially highly
disruptive three-dimensional (3D) Kelvin-Helmholtz instabilities. Numerical
simulations can address this problem and help clarify the causes of this
remarkable stability. Following previous studies of the stability of
relativistic flows in two dimensions (2D), it is our aim to test and extend the
conclusions of such works to three dimensions. We present numerical simulations
for the study of the stability properties of 3D, sheared, relativistic flows.
This work uses a fully parallelized code Ratpenat that solves equations of
relativistic hydrodynamics in 3D. The results of the present simulations
confirm those in 2D. We conclude that the growth of resonant modes in sheared
relativistic flows could be important in explaining the long-term collimation
of extragalactic jets.Comment: Accepted for publication in A&
The Evolution of Blue Stragglers Formed Via Stellar Collisions
We have used the results of recent smoothed particle hydrodynamic simulations
of colliding stars to create models appropriate for input into a stellar
evolution code. In evolving these models, we find that little or no surface
convection occurs, precluding angular momentum loss via a magnetically-driven
stellar wind as a viable mechanism for slowing rapidly rotating blue stragglers
which have been formed by collisions. Angular momentum transfer to either a
circumstellar disk (possibly collisional ejecta) or a nearby companion are
plausible mechanisms for explaining the observed low rotation velocities of
blue stragglers. Under the assumption that the blue stragglers seen in NGC 6397
and 47 Tuc have been created solely by collisions, we find that the majority of
these blue stragglers cannot have been highly mixed by convection or meridional
circulation currents at anytime during their evolution. Also, on the basis of
the agreement between the predictions of our non-rotating models and the
observed blue straggler distribution, the evolution of blue stragglers is
apparently not dominated by the effects of rotation.Comment: 36 pages, including 1 table and 7 postscript figures (LaTeX2e). Also
avaliable at http://astrowww.phys.uvic.ca/~ouellet/ . Accepted for
publication in A
The planetary system host HR\,8799: On its Bootis nature
HR\,8799 is a Bootis, Doradus star hosting a planetary
system and a debris disk with two rings. This makes this system a very
interesting target for asteroseismic studies. This work is devoted to the
determination of the internal metallicity of this star, linked with its
Bootis nature (i.e., solar surface abundances of light elements, and
subsolar surface abundances of heavy elements), taking advantage of its
Doradus pulsations. This is the most accurate way to obtain this
information, and this is the first time such a study is performed for a
planetary-system-host star. We have used the equilibrium code CESAM and the
non-adiabatic pulsational code GraCo. We have applied the Frequency Ratio
Method (FRM) and the Time Dependent Convection theory (TDC) to estimate the
mode identification, the Brunt-Va\"is\"al\"a frequency integral and the mode
instability, making the selection of the possible models. When the
non-seismological constraints (i.e its position in the HR diagram) are used,
the solar abundance models are discarded. This result contradicts one of the
main hypothesis for explaining the Bootis nature, namely the
accretion/diffusion of gas by a star with solar abundance. Therefore, according
to these results, a revision of this hypothesis is needed. The inclusion of
accurate internal chemical mixing processes seems to be necessary to explain
the peculiar abundances observed in the surface of stars with internal subsolar
metallicities. The use of the asteroseismological constraints, like those
provided by the FRM or the instability analysis, provides a very accurate
determination of the physical characteristics of HR 8799. However, a dependence
of the results on the inclination angle still remains. The determination of
this angle, more accurate multicolour photometric observations, and high
resolution spectroscopy can definitively fix the mass and metallicity of this
star.Comment: 11 pages, 10 figures. Accepted for publication in MNRA
X-ray spectra from magnetar candidates - III. Fitting SGRs/AXPs soft X-ray emission with non-relativistic Monte Carlo models
Within the magnetar scenario, the "twisted magnetosphere" model appears very
promising in explaining the persistent X-ray emission from the Soft Gamma
Repeaters and the Anomalous X-ray Pulsars (SGRs and AXPs). In the first two
papers of the series, we have presented a 3D Monte Carlo code for solving
radiation transport as soft, thermal photons emitted by the star surface are
resonantly upscattered by the magnetospheric particles. A spectral model
archive has been generated and implemented in XSPEC. Here we report on the
systematic application of our spectral model to different XMM-Newton and
Integral observations of SGRs and AXPs. We find that the synthetic spectra
provide a very good fit to the data for the nearly all the source (and source
states) we have analyzed.Comment: 13 Pages, 7 Figures, 4 Tables, accepted for publication in MNRA
Coronal cooling and its signatures in the rapid aperiodic variability of Galactic black-hole candidates
The most popular models for the complex phase and time lags in the rapid
aperiodic variability of Galactic X-ray binaries are based Comptonization of
soft seed photons in a hot corona, where small-scale flares are induced by
flares of the soft seed photon input (presumably from a cold accretion disc).
However, in their original version, these models have neglected the additional
cooling of the coronal plasma due to the increased soft seed photon input, and
assumed a static coronal temperature structure. In this paper, our
Monte-Carlo/Fokker-Planck code for time-dependent radiation transfer and
electron energetics is used to simulate the self-consistent coronal response to
the various flaring scenarios that have been suggested to explain phase and
time lags observed in some Galactic X-ray binaries. It is found that the
predictions of models involving slab-coronal geometries are drastically
different from those deduced under the assumption of a static corona. However,
with the inclusion of coronal cooling they may even be more successful than in
their original version in explaining some of the observed phase and time lag
features. The predictions of the model of inward-drifting density perturbations
in an ADAF-like, two-temperature flow also differ from the static-corona case
previously investigated, but may be consistent with the alternating phase lags
seen in GRS 1915+105 and XTE J1550-564. Models based on flares of a cool disc
around a hot, inner two-temperature flow may be ruled out for most objects
where significant Fourier-frequency-dependent phase and time lags have been
observed.Comment: 23 pages, including 8 figures and 2 tables; accepted for publication
in ApJ; extended discussion w.r.t. original versio
The contact binary VW Cephei revisited: surface activity and period variation
Context. Despite the fact that VW Cephei is one of the well-studied contact
binaries in the literature, there is no fully consistent model available that
can explain every observed property of this system.
Aims. Our motivation is to obtain new spectra along with photometric
measurements, to analyze what kind of changes may have happened in the system
in the past two decades, and to propose new ideas for explaining them.
Methods. For the period analysis we determined 10 new times of minima from
our light curves, and constructed a new OC diagram of the system. Radial
velocities of the components were determined using the cross-correlation
technique. The light curves and radial velocities were modelled simultaneously
with the PHOEBE code. All observed spectra were compared to synthetic spectra
and equivalent widths of the H line were measured on their differences.
Results. We have re-determined the physical parameters of the system
according to our new light curve and spectral models. We confirm that the
primary component is more active than the secondary, and there is a correlation
between spottedness and the chromospheric activity. We propose that flip-flop
phenomenon occurring on the primary component could be a possible explanation
of the observed nature of the activity. To explain the period variation of VW
Cep, we test two previously suggested scenarios: presence of a fourth body in
the system, and the Applegate-mechanism caused by periodic magnetic activity.
We conclude that although none of these mechanisms can be ruled out entirely,
the available data suggest that mass transfer with a slowly decreasing rate
gives the most likely explanation for the period variation of VW Cep.Comment: 13 pages, 18 figures, 9 tables, accepted for publication in Astronomy
and Astrophysic
The disappearance and reformation of the accretion disc during a low state of FO Aquarii
FO Aquarii, an asynchronous magnetic cataclysmic variable (intermediate
polar) went into a low-state in 2016, from which it slowly and steadily
recovered without showing dwarf nova outbursts. This requires explanation since
in a low-state, the mass-transfer rate is in principle too low for the disc to
be fully ionized and the disc should be subject to the standard thermal and
viscous instability observed in dwarf novae. We investigate the conditions
under which an accretion disc in an intermediate polar could exhibit a
luminosity drop of 2 magnitudes in the optical band without showing outbursts.
We use our numerical code for the time evolution of accretion discs, including
other light sources from the system (primary, secondary, hot spot). We show
that although it is marginally possible for the accretion disc in the low-state
to stay on the hot stable branch, the required mass-transfer rate in the normal
state would then have to be extremely high, of the order of 10 gs
or even larger. This would make the system so intrinsically bright that its
distance should be much larger than allowed by all estimates. We show that
observations of FO Aqr are well accounted for by the same mechanism that we
have suggested as explaining the absence of outbursts during low states of VY
Scl stars: during the decay, the magnetospheric radius exceeds the
circularization radius, so that the disc disappears before it enters the
instability strip for dwarf nova outbursts. Our results are unaffected, and
even reinforced, if accretion proceeds both via the accretion disc and directly
via the stream during some intermediate stages; the detailed process through
which the disc disappears still needs investigations.Comment: 6 pages, 5 figures. Accepted for publication in A&
Afghan EFL Lecturersâ Perceptions of Code-Switching
Code-switching is explained as switching between two languages at the same time while conversing in the same discourse. The aim of this study is to explore the perceptions of Afghan EFL lecturers toward code- switching. This study used a qualitative research approach in which the data was collected via semi-structured interviews with five EFL lecturers. The lecturers were interviewed to explore their perceptions and reasons for code-switching, and the extent to which they practice code-switching in their classrooms. The results revealed that the lecturers in the study had positive perceptions of code-switching and believed it as a beneficial strategy in facilitating the learning process. However, they also conveyed that an excessive practice of code-switching should be avoided. The study results also disclosed that the lecturers practiced code-switching for various functions such as explaining difficult concepts (translation), explaining complex grammatical points, clarifications, and giving instructions. Finally, the paper provides recommendation for further research
The challenges, uncertainties and opportunities of bioaerosol dispersion modelling from open composting facilities
Bioaerosols are ubiquitous organic particles that comprise viruses, bacteria and coarser fractions of organic matter. Known to adversely affect human health, the impact of bioaerosols on a population often manifests as outbreaks of illnesses such as Legionnaires Disease and Q fever, although the concentrations and environmental conditions in which these impacts occur are not well understood. Bioaerosol concentrations vary from source to source, but specific human activities such as water treatment, intensive agriculture and composting facilitate the generation of bioaerosol concentrations many times higher than natural background levels. Bioaerosols are not considered âtraditionalâ pollutants in the same way as PM10, PM2.5, and gases such as NO2, and consequently dispersion models do not include a bespoke method for their assessment. As identified in previous studies, priority areas for improving the robustness of these dispersion models include: 1) the development of bespoke monitoring studies designed to generate accurate modelling input data; 2) the publication of a robust emissions inventory; 3) a code of practice to provide guidelines for consistent bioaerosol modelling practices; and 4) a greater understanding of background bioaerosol emissions. The aim of this research project, funded by the Natural Environmental Research Council (NERC), is to address these key areas through a better understanding of the generation, concentration and potential dispersion of bioaerosols from intensive agricultural and biowaste facilities, using case studies developed at specific locations within the UK. The objective is to further refine existing bioaerosol monitoring and modelling guidelines to provide a more robust framework for regulating authorities and site operators. This contribution outlines the gaps that hinder robust dispersion modelling, and describes the on-site bioaerosol data collection methods used in the study, explaining how they might be used to close these gaps. Examples of bioaerosol dispersion modelled using ADMS 5 are presented and discussed
The Growth & Migration of Jovian Planets in Evolving Protostellar Disks with Dead Zones
The growth of Jovian mass planets during migration in their protoplanetary
disks is one of the most important problems that needs to be solved in light of
observations of the exosolar planets. Studies of the migration of planets in
standard gas disk models routinely show that migration is too fast to form
Jovian planets, and that such migrating planetary cores generally plunge into
the central stars in less than a Myr. In previous work, we have shown that a
poorly ionized, less viscous region in a protoplanetary disk called a dead zone
slows down the migration of fixed-mass planets. In this paper, we extend our
numerical calculations to include dead zone evolution along with the disk, as
well as planet formation via accretion of rocky and gaseous materials. Using
our symplectic-integrator-gas dynamics code, we find that dead zones, even in
evolving disks wherein migrating planets grow by accretion, still play a
fundamental role in saving planetary systems. We demonstrate that Jovian
planets form within 2.5 Myr for disks that are ten times more massive than a
minimum mass solar nebula (MMSN) with an opacity reduction and without slowing
down migration artificially. Our simulations indicate that protoplanetary disks
with an initial mass comparable to the MMSN only produce Neptunian mass
planets. We also find that planet migration does not help core accretion as
much in the oligarchic planetesimal accretion scenario as it was expected in
the runaway accretion scenario. Therefore we expect that an opacity reduction
(or some other mechanisms) is needed to solve the formation timescale problem
even for migrating protoplanets, as long as we consider the oligarchic growth.
We also point out a possible role of a dead zone in explaining long-lived,
strongly accreting gas disks.Comment: 16 pages, 15 figures, accepted for publication in Ap
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