16,060 research outputs found
Magnetic fields of intermediate mass T Tauri stars
Aims. In this paper, we aim to measure the strength of the surface magnetic
fields for a sample of five intermediate mass T Tauri stars and one low mass T
Tauri star from late-F to mid-K spectral types. While magnetic fields of T
Tauri stars at the low mass range have been extensively characterized, our work
complements previous studies towards the intermediate mass range; this
complementary study is key to evaluate how magnetic fields evolve during the
transition from a convective to a radiative core.
Methods. We studied the Zeeman broadening of magnetically sensitive spectral
lines in the H-band spectra obtained with the CRIRES high-resolution
near-infrared spectrometer. These data are modelled using magnetic spectral
synthesis and model atmospheres. Additional constraints on non-magnetic line
broadening mechanisms are obtained from modelling molecular lines in the K band
or atomic lines in the optical wavelength region.
Results. We detect and measure mean surface magnetic fields for five of the
six stars in our sample: CHXR 28, COUP 107, V2062 Oph, V1149 Sco, and Par 2441.
Magnetic field strengths inferred from the most magnetically sensitive
diagnostic line range from 0.8 to 1.8 kG. We also estimate a magnetic field
strength of 1.9 kG for COUP 107 from an alternative diagnostic. The magnetic
field on YLW 19 is the weakest in our sample and is marginally detected, with a
strength of 0.8 kG.
Conclusions. We populate an uncharted area of the pre-main-sequence HR
diagram with mean magnetic field measurements from high-resolution
near-infrared spectra. Our sample of intermediate mass T Tauri stars in general
exhibits weaker magnetic fields than their lower mass counterparts. Our
measurements will be used in combination with other spectropolarimetric studies
of intermediate mass and lower mass T Tauri stars to provide input into
pre-main-sequence stellar evolutionary models.Comment: 8 pages, 8 figures, accepted for publication in Astronomy and
Astrophysic
Comparative study of radio pulses from simulated hadron-, electron-, and neutrino-initiated showers in ice in the GeV-PeV range
High energy particle showers produce coherent Cherenkov radio emission in
dense, radio-transparent media such as cold ice. Using PYTHIA and GEANT
simulation tools, we make a comparative study among electromagnetic (EM) and
hadronic showers initiated by single particles and neutrino showers initiated
by multiple particles produced at the neutrino-nucleon event vertex. We include
all the physics processes and do a complete 3-D simulation up to 100 TeV for
all showers and to 1 PeV for electron and neutrino induced showers. We
calculate the radio pulses for energies between 100 GeV and 1 PeV and find
hadron showers, and consequently neutrino showers, are not as efficient below 1
PeV at producing radio pulses as the electromagnetic showers. The agreement
improves as energy increases, however, and by a PeV and above the difference
disappears. By looking at the 3-D structure of the showers in time, we show
that the hadronic showers are not as compact as the EM showers and hence the
radiation is not as coherent as EM shower emission at the same frequency. We
show that the ratio of emitted pulse strength to shower tracklength is a
function only of a single, coherence parameter, independent of species and
energy of initiating particle.Comment: a few comments added, to bo published in PRD Nov. issue, 10 pages, 3
figures in tex file, 3 jpg figures in separate files, and 1 tabl
Asymptotic Behaviour of the Proper Length and Volume of the Schwarzschild Singularity
Though popular presentations give the Schwarzschild singularity as a point it
is known that it is spacelike and not timelike. Thus it has a "length" and is
not a "point". In fact, its length must necessarily be infinite. It has been
proved that the proper length of the Qadir-Wheeler suture model goes to
infinity [1], while its proper volume shrinks to zero, and the asymptotic
behaviour of the length and volume have been calculated. That model consists of
two Friedmann sections connected by a Schwarzschild "suture". The question
arises whether a similar analysis could provide the asymptotic behaviour of the
Schwarzschild black hole near the singularity. It is proved here that, unlike
the behaviour for the suture model, for the Schwarzschild essential singularity
and , where
is the mean extrinsic curvature, or the York time.Comment: 13 pages, 1 figur
The Coronal Structure of AB Doradus
We perform a numerical simulation of the corona of the young, rapidly
rotating K0 dwarf AB Doradus using a global MHD model. The model is driven by a
surface map of the radial magnetic field constructed using Zeeman-Doppler
Imaging. We find that the global structure of the stellar corona is dominated
by strong azimuthal tangling of the magnetic field due to the rapid rotation.
The MHD solution enables us to calculate realistic Alfv\'en surfaces and we can
therefore estimate the stellar mass loss rate and angular momentum loss rate
without making undue theoretical simplifications. We consider three cases,
parametrized by the base density of the corona, that span the range of possible
solutions for the system. We find that overall, the mass and angular-momentum
loss rates are higher than in the solar case; the mass loss rates are 10 to 500
times higher, and the angular momentum loss rate can be up to
higher than present day solar values. Our simulations show that this model can
be use to constrain the wide parameter space of stellar systems. It also shows
that an MHD approach can provide more information about the physical system
over the commonly used potential field extrapolation.Comment: 13 pages, 7 figure
Complex magnetic topology and strong differential rotation on the low-mass T Tauri star V2247 Oph
From observations collected with the ESPaDOnS spectropolarimeter at the
Canada-France-Hawaii Telescope, we report the detection of Zeeman signatures on
the low-mass classical TTauri star (cTTS) V2247Oph. Profile distortions and
circular polarisation signatures detected in photospheric lines can be
interpreted as caused by cool spots and magnetic regions at the surface of the
star. The large-scale field is of moderate strength and highly complex;
moreover, both the spot distribution and the magnetic field show significant
variability on a timescale of only one week, as a likely result of strong
differential rotation. Both properties make V2247Oph very different from the
(more massive) prototypical cTTS BPTau; we speculate that this difference
reflects the lower mass of V2247Oph.
During our observations, V2247Oph was in a low-accretion state, with emission
lines showing only weak levels of circular polarisation; we nevertheless find
that excess emission apparently concentrates in a mid-latitude region of strong
radial field, suggesting that it is the footpoint of an accretion funnel.
The weaker and more complex field that we report on V2247Oph may share
similarities with those of very-low-mass late-M dwarfs and potentially explain
why low-mass cTTSs rotate on average faster than intermediate mass ones. These
surprising results need confirmation from new independent data sets on V2247Oph
and other similar low-mass cTTSs.Comment: MNRAS (in press) - 12 pages, 9 figure
UVMag: stellar formation, evolution, structure and environment with space UV and visible spectropolarimetry
Important insights into the formation, structure, evolution and environment
of all types of stars can be obtained through the measurement of their winds
and possible magnetospheres. However, this has hardly been done up to now
mainly because of the lack of UV instrumentation available for long periods of
time. To reach this aim, we have designed UVMag, an M-size space mission
equipped with a high-resolution spectropolarimeter working in the UV and
visible spectral range. The UV domain is crucial in stellar physics as it is
very rich in atomic and molecular lines and contains most of the flux of hot
stars. Moreover, covering the UV and visible spectral domains at the same time
will allow us to study the star and its environment simultaneously. Adding
polarimetric power to the spectrograph will multiply tenfold the capabilities
of extracting information on stellar magnetospheres, winds, disks, and magnetic
fields. Examples of science objectives that can be reached with UVMag are
presented for pre-main sequence, main sequence and evolved stars. They will
cast new light onto stellar physics by addressing many exciting and important
questions. UVMag is currently undergoing a Research and Technology study and
will be proposed at the forthcoming ESA call for M-size missions. This
spectropolarimeter could also be installed on a large UV and visible
observatory (e.g. NASA's LUVOIR project) within a suite of instruments.Comment: Accepted in ApSS's special volume on UV astronom
Tuberculosis diagnosis and treatment practices of private physicians in Karachi, Pakistan
In a densely populated urban area of Karachi, Pakistan, a questionnaire survey was made of the knowledge and practices of 120 private general practitioners about the diagnosis and treatment of tuberculosis (TB). The majority knew that cough, fever and weight loss were the main symptoms of TB, but less than half knew that blood in sputum, poor appetite and chest pain were associated with the disease. Only 58.3% of physicians used sputum microscopy for diagnosing TB and 35.0% used it as a follow-up test. Only 41.7% treated TB patients themselves, the remaining referring their patients to specialists. Around 73.3% of the doctors were aware of the 4 first-line anti-TB drugs. Efforts to improve the knowledge of private practitioners, and strategies to enhance public-private collaboration forTB control in urban areas are urgently required
Multi-wavelength observing of a forming solar-like star
V2129 Oph is a 1.35 solar mass classical T Tauri star, known to possess a
strong and complex magnetic field. By extrapolating from an observationally
derived magnetic surface map, obtained through Zeeman-Doppler imaging, models
of V2129 Oph's corona have been constructed, and used to make predictions
regarding the global X-ray emission measure, the amount of modulation of X-ray
emission, and the density of accretion shocks. In late June 2009 we will under
take an ambitious multi-wavelength, multi-observing site, and near
contemporaneous campaign, combining spectroscopic optical, nIR, UV, X-ray,
spectropolarimetric and photometric monitoring. This will allow the validity of
the 3D field topologies derived via field extrapolation to be determined.Comment: 4 pages, proceedings of the 3rd MSSL workshop on High Resolution
X-ray Spectroscopy: towards IX
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