737 research outputs found
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
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
The nebulae around LBVs: a multiwavelength approach
We present first results of our study of a sample of Galactic LBV, aimed to
contribute to a better understanding of the LBV phenomenon, by recovering the
mass-loss history of the central object from the analysis of its associated
nebula. Mass-loss properties have been derived by a synergistic use of
different techniques, at different wavelengths, to obtain high-resolution,
multi-wavelength maps, tracing the different emitting components coexisting in
the stellar ejecta: the ionized/neutral gas and the dust. Evidence for
asymmetric mass-loss and observational evidence of possible mutual interaction
between gas and dust components have been observed by the comparison of mid-IR
(Spitzer/IRAC, VLT/VISIR) and radio (VLA) images of the nebulae, while
important information on the gas and dust composition have been derived from
Spitzer/IRS spectra.Comment: 5 pages, 4 figures. To appear in proceedings of 39th Liege
International Astrophysical Colloquium: The multi-wavelength view of Hot,
Massive Star
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
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