127 research outputs found
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
Identification of three new proto-Planetary Nebulae exhibiting the unidentified feature at 21 um
Among its great findings, the IRAS mission showed the existence of an
unidentified mid-IR feature around 21 um. Since its discovery, this feature has
been detected in all C-rich proto-PNe of intermediate spectral type (A-G) and -
weakly - in a few PNe and AGB stars, but the nature of its carriers remains
unknown. In this paper, we show the detection of this feature in the spectra of
three new stars transiting from the AGB to the PN stage obtained with the
Spitzer Space Telescope. Following a recent suggestion, we try to model the
SEDs of our targets with amorphous carbon and FeO, which might be responsible
for the unidentified feature. The fit thus obtained is not completely
satisfactory, since the shape of the feature is not well matched. In the
attempt to relate the unidentified feature to other dust features, we retrieved
mid-IR spectra of all the 21-um sources currently known from ISO and Spitzer
on-line archives and noticed a correlation between the flux emitted in the
21-um feature and that emitted at 7 and 11 um (PAH bands and HAC broad
emission). Such a correlation may point to a common nature of the carriers.Comment: Accepted for publication in Ap
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
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