5,847 research outputs found
Magnetic topology and surface differential rotation on the K1 subgiant of the RS CVn system HR 1099
We present here spectropolarimetric observations of the RS CVn system HR 1099
(V711 Tau) secured from 1998 February to 2002 January with the
spectropolarimeter MuSiCoS at the Telescope Bernard Lyot (Observatoire du Pic
du Midi, France). We apply Zeeman-Doppler Imaging and reconstruct brightness
and magnetic surface topologies of the K1 primary subgiant of the system, at
five different epochs. We confirm the presence of large, axisymmetric regions
where the magnetic field is mainly azimuthal, providing further support to the
hypothesis that dynamo processes may be distributed throughout the whole
convective zone in this star. We study the short-term evolution of surface
structures from a comparison of our images with observations secured at
close-by epochs by Donati et al. (2003) at the Anglo-Australian Telescope. We
conclude that the small-scale brightness and magnetic patterns undergo major
changes within a timescale of 4 to 6 weeks, while the largest structures remain
stable over several years. We report the detection of a weak surface
differential rotation (both from brightness and magnetic tracers) indicating
that the equator rotates faster than the pole with a difference in rotation
rate between the pole and the equator about 4 times smaller than that of the
Sun. This result suggests that tidal forces also impact the global dynamic
equilibrium of convective zones in cool active stars.Comment: accepted by MNRA
The magnetic fields of forming solar-like stars
Magnetic fields play a crucial role at all stages of the formation of low
mass stars and planetary systems. In the final stages, in particular, they
control the kinematics of in-falling gas from circumstellar discs, and the
launching and collimation of spectacular outflows. The magnetic coupling with
the disc is thought to influence the rotational evolution of the star, while
magnetised stellar winds control the braking of more evolved stars and may
influence the migration of planets. Magnetic reconnection events trigger
energetic flares which irradiate circumstellar discs with high energy particles
that influence the disc chemistry and set the initial conditions for planet
formation. However, it is only in the past few years that the current
generation of optical spectropolarimeters have allowed the magnetic fields of
forming solar-like stars to be probed in unprecedented detail. In order to do
justice to the recent extensive observational programs new theoretical models
are being developed that incorporate magnetic fields with an observed degree of
complexity. In this review we draw together disparate results from the
classical electromagnetism, molecular physics/chemistry, and the geophysics
literature, and demonstrate how they can be adapted to construct models of the
large scale magnetospheres of stars and planets. We conclude by examining how
the incorporation of multipolar magnetic fields into new theoretical models
will drive future progress in the field through the elucidation of several
observational conundrums.Comment: 55 pages, review article accepted for publication in Reports on
Progress in Physics. Astro-ph version includes additional appendice
Rotationally Modulated X-ray Emission from T Tauri Stars
We have modelled the rotational modulation of X-ray emission from T Tauri
stars assuming that they have isothermal, magnetically confined coronae. By
extrapolating surface magnetograms we find that T Tauri coronae are compact and
clumpy, such that rotational modulation arises from X-ray emitting regions
being eclipsed as the star rotates. Emitting regions are close to the stellar
surface and inhomogeneously distributed about the star. However some regions of
the stellar surface, which contain wind bearing open field lines, are dark in
X-rays. From simulated X-ray light curves, obtained using stellar parameters
from the Chandra Orion Ultradeep Project, we calculate X-ray periods and make
comparisons with optically determined rotation periods. We find that X-ray
periods are typically equal to, or are half of, the optical periods. Further,
we find that X-ray periods are dependent upon the stellar inclination, but that
the ratio of X-ray to optical period is independent of stellar mass and radius.Comment: 10 pages, 8 figures, accepted for publication in MNRA
Components Qualification for a Possible use in the Mu2e Calorimeter Waveform Digitizers
The Mu2e experiment at Fermilab searches for the charged flavor violating
conversion of a muon into an electron in the Coulomb field of a nucleus. The
detector consists of a straw tube tracker and a CSI crystal electromagnetic
calorimeter, both housed in a superconducting solenoid. Both the front-end and
the digital electronics, located inside the cryostat, will be operated in
vacuum under a 1 T magnetic field, having to sustain the high flux of neutrons
and ionizing particles coming from the muons stopping target. These harsh
experimental conditions make the design of the calorimeter waveform digitizer
quite challenging. All the selected commercial devices must be tested
individually and qualified for radiation hardness and operation in high
magnetic field. At the moment the expected particles flux and spectra at the
digitizers location are not completely simulated and we are using initial rough
estimates to select the components for the first prototype. We are gaining
experience in the qualification procedures using the selected components but
the choice will be frozen only when dose and neutron flux simulations will be
completed. The experimental results of the first qualification campaign are
presented.Comment: TWEPP 2016 - Topical Workshop on Electronics for Particle Physics,
26-30 September 2016, Karlsruhe Institute of Technology (KIT
Magnetometry of the classical T Tauri star GQ Lup: non-stationary dynamos & spin evolution of young Suns
We report here results of spectropolarimetric observations of the classical T
Tauri star (cTTS) GQ Lup carried out with ESPaDOnS at the Canada-France-Hawaii
Telescope (CFHT) in the framework of the "Magnetic Protostars and Planets"
(MaPP) programme, and obtained at 2 different epochs (2009 July & 2011 June).
From these observations, we first infer that GQ Lup has a photospheric
temperature of 4,300+-50\^A K and a rotation period of 8.4+-0.3 d; it implies
that it is a 1.05+-0.07 Msun star viewed at an inclination of ~30deg, with an
age of 2-5 Myr, a radius of 1.7+-0.2 Rsun, and has just started to develop a
radiative core.
Large Zeeman signatures are clearly detected at all times, both in
photospheric lines & in accretion-powered emission lines, probing longitudinal
fields of up to 6 kG and hence making GQ Lup the cTTS with the strongest
large-scale fields known as of today. Rotational modulation of Zeeman
signatures is clearly different between our 2 runs, demonstrating that
large-scale fields of cTTSs are evolving with time and are likely produced by
non-stationary dynamo processes.
Using tomographic imaging, we reconstruct maps of the large-scale field, of
the photospheric brightness & of the accretion-powered emission of GQ Lup. We
find that the magnetic topology is mostly poloidal & axisymmetric; moreover,
the octupolar component of the large-scale field (of strength 2.4 & 1.6 kG in
2009 & 2011) dominates the dipolar component (of strength ~1 kG) by a factor of
~2, consistent with the fact that GQ Lup is no longer fully-convective.
GQ Lup also features dominantly poleward magnetospheric accretion at both
epochs. The large-scale dipole of GQ Lup is however not strong enough to
disrupt the surrounding accretion disc further than about half-way to the
corotation radius, suggesting that GQ Lup should rapidly spin up like other
similar partly-convective cTTSs (abridged).Comment: MNRAS, in press (17 pages, 10 figures, 1 table
Modeling X-ray emission from stellar coronae
By extrapolating from observationally derived surface magnetograms of
low-mass stars we construct models of their coronal magnetic fields and compare
the 3D field geometry with axial multipoles. AB Dor, which has a radiative
core, has a very complex field, whereas V374 Peg, which is completely
convective, has a simple dipolar field. We calculate global X-ray emission
measures assuming that the plasma trapped along the coronal loops is in
hydrostatic equilibrium and compare the differences between assuming isothermal
coronae, or by considering a loop temperature profiles. Our preliminary results
suggest that the non-isothermal model works well for the complex field of AB
Dor, but not for the simple field of V374 Peg.Comment: 4 pages, proceedings of Cool Stars 15, St Andrews, July 2008, to be
published in the Conference Proceedings Series of the American Institute of
Physic
Dynamic, self consistent electro-thermal simulation of power microwave devices including the effect of surface metallizations
We present an efficient simulation technique to account for the thermal spreading effects of surface metallizations in the self-consistent dynamic electro-thermal analysis of power microwave devices. Electro-thermal self-consistency is achieved by solving the coupled nonlinear system made of a temperature dependent device electrical model, and of an approximate description of the device thermal behavior through a thermal impedance matrix. The numerical solution is pursued in the frequency domain by the Harmonic Balance technique. The approach is applied to the thermal stability analysis of power AlGaAs/GaAs HBTs and the results show that metallizations have a significant impact on the occurrence of the device thermal collapse
Alfven Wave-Driven Supernova Explosion
We investigate the role of Alfven waves in the core-collapse supernova (SN)
explosion. We assume that Alfven waves are generated by convections inside a
proto-neutron star (PNS) and emitted from its surface. Then these waves
propagate outwards, dissipate via nonlinear processes, and heat up matter
around a stalled prompt shock. To quantitatively assess the importance of this
process for the revival of the stalled shock, we perform 1D time-dependent
hydrodynamical simulations, taking into account the heating via the dissipation
of Alfven waves that propagate radially outwards along open flux tubes. We show
that the shock revival occurs if the surface field strength is larger than
~2e15 G and if the amplitude of velocity fluctuation at the PNS surface is
larger than 20% of the local sound speed. Interestingly, the Alfven wave
mechanism is self-regulating in the sense that the explosion energy is not very
sensitive to the surface field strength and initial amplitude of Alfven waves
as long as they are larger than the threshold values given above.Comment: 7 pages, 3 figures embedded, submitted to Ap
Not all remote workers are similar: Technology acceptance, remote work beliefs, and wellbeing of remote workers during the second wave of the covid-19 pandemic
Although a large part of the worldâs workforce engaged in mandatory Work from Home during the COVID-19 pandemic, the experience was not the same for everyone. This study explores whether different groups of employees, based on their work and organizational characteristics (i.e., organizational size, number of days per week working from home, working in team) and personal characteristics (i.e., remote work experience, having children at home), express different beliefs about working remotely, acceptance of the technology necessary to Work from Home, and well-being. A study was conducted with 163 Italian workers who answered an online questionnaire from November 2020 to January 2021. A cluster analysis revealed that work, organizational, and personal variables distinguish five different types of workers. ANOVA statistics showed that remote workers from big companies who worked remotely several days a week, had experience (because they worked remotely before the national lockdowns), and worked in a team, had more positive beliefs about working remotely, higher technology acceptance, and better coping strategies, compared to the other groups of workers. Practical implications to support institutional and organizational decision-makers and HR managers to promote remote work and employee well-being are presented
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