778 research outputs found
A three-dimensional model for the radio emission of magnetic chemically peculiar stars
In this paper we present a three-dimensional numerical model for the radio
emission of Magnetic Chemically Peculiar stars, on the hypothesis that
energetic electrons emit by the gyrosynchrotron mechanism. For this class of
radio stars, characterized by a mainly dipolar magnetic field whose axis is
tilted with respect to the rotational axis, the geometry of the magnetosphere
and its deformation due to the stellar rotation are determined. The radio
emitting region is determined by the physical conditions of the magnetosphere
and of the stellar wind. Free-free absorption by the thermal plasma trapped in
the inner magnetosphere is also considered. Several free parameters are
involved in the model, such as the size of the emitting region, the energy
spectrum and the number density of the emitting electrons, and the
characteristics of the plasma in the inner magnetosphere. By solving the
equation of radiative transfer, along a path parallel to the line of sight, the
radio brightness distribution and the total flux density as a function of
stellar rotation are computed. As the model is applied to simulate the observed
5 GHz lightcurves of HD37479 and HD37017, several possible magnetosphere
configurations are found. After simulations at other frequencies, in spite of
the large number of parameters involved in the modeling, two solutions in the
case of HD37479 and only one solution in the case of HD37017 match the observed
spectral indices. The results of our simulations agree with the magnetically
confined wind-shock model in a rotating magnetosphere. The X-ray emission from
the inner magnetosphere is also computed, and found to be consistent with the
observations.Comment: 15 pages, 10 figures, A&A in pres
Radio continuum properties of young planetary nebulae
We have selected a small sample of post-AGB stars in transition towards the
planetary nebula and present new Very Large Array multi-frequency high-angular
resolution radio observations of them. The multi-frequency data are used to
create and model the targets' radio continuum spectra, proving that these stars
started their evolution as very young planetary nebulae. In the optically thin
range, the slopes are compatible with the expected spectral index (-0.1). Two
targets (IRAS 18062+2410 and 17423-1755) seem to be optically thick even at
high frequency, as observed in a handful of other post-AGB stars in the
literature, while a third one (IRAS 20462+3416) shows a possible contribution
from cold dust. In IRAS 18062+2410, where we have three observations spanning a
period of four years, we detect an increase in its flux density, similar to
that observed in CRL 618.
High-angular resolution imaging shows bipolar structures that may be due to
circumstellar tori, although a different hypothesis (i.e., jets) could also
explain the observations. Further observations and monitoring of these sources
will enable us to test the current evolutionary models of planetary nebulae.Comment: 8 pages, 3 figures, accepted for publication in MNRA
VISIR/VLT and VLA joint imaging analysis of the circumstellar nebula around IRAS~18576+0341
High spatial and sensitivity images of the Luminous Blue Variable IRAS
18576+0341 were obtained using the mid infrared imager VISIR at the Very Large
Telescope and the Very Large Array interferometer. The resulting mid-infrared
continuum maps show a similar clumpy and approximately circular symmetric
nebula, which contrasts sharply with the asymmetry that characterizes the
ionized component of the envelope, as evidenced from the radio and [Ne II] line
images obtained with comparable spatial resolution. In particular, there is
excellent overall agreement between the 12.8 micron map and the radio images,
consistent with free-free emission from circumstellar ionized material
surrounding a central stellar wind. The color temperature and optical depth
maps obtained from mid-infrared images show only slight fluctuations,
suggesting quite uniform dust characteristics over the dust shell. We explore
various possibilities to understand the cause of the different morphology of
the dusty and gaseous component of the circumstellar envelope which are
compatible with the observations.Comment: 26 pages, 8 figures. Accepted for publication in The Astrophysical
Journa
Resolving the radio nebula around beta Lyrae
In this paper we present high spatial resolution radio images of the puzzling
binary system beta Lyrae obtained with MERLIN at 5 GHz. We find a nebula
surrounding the binary with a brightness temperature of 11000+-700K
approximately 40AU across. This definitively confirms the thermal origin of the
radio emission, which is consistent with emission from the wind of the B6-8II
component (mass loss of order of 10^-7 Msun per year), ionized by the radiation
field of the hotter companion. This nebula, surrounding the binary, is the
proof that beta Layrae evolved in a non-conservative way, i. e. not all the
mass lost by the primary is accretted by the secondary, and present
measurements indicate that almost 0.015Msun had been lost from the system since
the onset of the Roche lobe overflow phase. Moreover, the nebula is aligned
with the jet-like structures inferred from recent optical measurements,
indicating a possible connection among them.Comment: 5 pages, 2 figures. Accepted for publication in A&
3D-modelling of the stellar auroral radio emission
The electron cyclotron maser is the coherent emission process that gives rise
to the radio lighthouse effect observed in the hot magnetic chemically peculiar
star CU Virginis. It has also been proposed to explain the highly circularly
polarized radio pulses observed on some ultra cool dwarfs, with spectral type
earlier than M7. Such kind of coherent events resemble the auroral radio
emission from the magnetized planets of the solar system. In this paper, we
present a tridimensional model able to simulate the timing and profile of the
pulses emitted by those stars characterized by a dipolar magnetic field by
following the hypothesis of the laminar source model, used to explain the
beaming of the terrestrial auroral kilometric radiation. This model proves to
be a powerful tool to understand the auroral radio-emission phenomenon,
allowing us to derive some general conclusions about the effects of the model's
free parameters on the features of the coherent pulses, and to learn more about
the detectability of such kind of pulsed radio emission.Comment: 11 pages, 8 figures; accepted for publication in MNRA
Millimeter observations of Planetary Nebulae: a contribution to the Planck pre-launch catalogue
We present new millimetre 43 GHz observations of a sample of radio-bright
Planetary Nebulae. Such observations were carried out to have a good
determination of the high-frequency radio spectra of the sample in order to
evaluate, together with far-IR measurements (IRAS), the fluxes emitted by the
selected source in the millimetre and sub-millimetre band. This spectral range,
even very important to constraint the physics of circumstellar environment, is
still far to be completely exploited. To estimate the millimetre and
sub-millimetre fluxes, we extrapolated and summed together the ionized gas
(free-free radio emission) and dust (thermal emission) contributions at this
frequency range. By comparison of the derived flux densities to the foreseen
sensitivity we investigate the possible detection of such source for all the
channels of the forthcoming ESA's PLANCK mission. We conclude that almost 80%
of our sample will be detected by PLANCK, with the higher detection rate in the
higher frequency channels, where there is a good combination of brighter
intrinsic flux from the sources and reduced extended Galactic foregrounds
contamination despite a worst instrumental sensitivity. From the new 43 GHz,
combined with single-dish 5 GHz observations from the literature, we derive
radio spectral indexes, which are consistent with optically thin free-free
nebula. This result indicates that the high frequency radio spectrum of our
sample sources is dominated by thermal free-free and other emission, if
present, are negligible.Comment: Submitted to A&A, 6 pages, 3 figure
Searching for OH maser emission towards the MIPSGAL compact Galactic bubbles
We conducted radio observations searching for OH 18-cm maser emission from a
sample of 169 unclassified MIPSGAL compact Galactic bubbles. These sources are
thought to be the circumstellar envelopes of different kinds of evolved stars.
Our observations were aimed at shedding light on the nature of MIPSGAL bubbles,
since their characterisation is a fundamental aid for the development of
accurate physical models of stellar and Galaxy evolution. The maser emission is
observatively linked to the last stages of the life of low- and
intermediate-mass stars, which may constitute a significant fraction of the
MIPSGAL bubbles. In particular OH masers are usually observed towards post-AGB
stars. Our observations were performed with the Green Bank Telescope and, for
each source, produced spectra around the four OH 18-cm transitions. The
observations were compared with archive interferometer data in order to exclude
possible contamination from nearby sources. The main result is that the OH
maser emission is not a common feature among the MIPSGAL bubbles, with only one
certain detection. We conclude that among the MIPSGAL bubbles the post-AGB
stars could be very rare
In Reply: Rongeurs, neurosurgeons, and COVID-19: how do we protect health care personnel during neurosurgical operations in the midst of aerosol-generation from high-speed drills?
To the Editor:
We appreciated the manuscript by Chan et al,“Rongeurs, Neurosurgeons, and COVID-19: How Do We Protect Health Care Personnel During Neurosurgical Operations in the Midst of Aerosol-Generation From High-Speed Drills?” published April 2020 in Neurosurgery. We are grateful to the authors of the letter1 for their precise analysis and useful recommendations to fellow neurosurgeons and trainees to uphold the traditional craft of using hand drills and rongeurs in order to minimize aerosol generation from the neurosurgical-powered instruments during the COVID-19 pandemic. Nevertheless, we write in reply to present our surgical experience during the COVID-19 pandemic
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