82 research outputs found
Auroral Radio Emission from Stars: the case of CU Virginis
CU Virginis is a rapidly rotating Magnetic Chemically Peculiar star with at
present unique characteristics as radio emitter. The most intriguing one is the
presence of intense, 100% circularly polarized radiation ascribed to Cyclotron
Maser. Each time the star rotates, this highly beamed emission points two times
toward the Earth, like a pulsar. We observed CU Vir in April 2010 with the EVLA
in two bands centered at 1450 and 1850 MHz. We covered nearly the whole
rotational period, confirming the presence of the two pulses at a flux density
up to 20 mJy. Dynamical spectra, obtained with unprecedented spectral and
temporal sensitivity, allow us to clearly see the different time delays as a
function of the frequency. We interpret this behaviour as a propagation effect
of the radiation inside the stellar magnetosphere. The emerging scenario
suggests interesting similarities with the auroral radio emission from planets,
in particular with the Auroral Kilometric Radiation (AKR) from Earth, which
originates at few terrestrial radii above the magnetic poles and was only
recently discovered to be highly beamed. We conclude that the magnetospheres of
CU Vir, Earth and other planets, maybe also exoplanets, could have similar
geometrical and physical characteristics in the regions where the cyclotron
maser is generated. In addition, the pulses are perfect "markers" of the
rotation period. This has given us for the first time the possibility to
measure with extraordinary accuracy the spin down of a star on or near the main
sequence.Comment: 18 pages, 4 figures, Accepted to APJ Letter, EVLA special issu
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
A massive nebula around the Luminous Blue Variable star RMC143 revealed by ALMA
The luminous blue variable (LBV) RMC143 is located in the outskirts of the
30~Doradus complex, a region rich with interstellar material and hot luminous
stars. We report the sub-millimetre detection of its circumstellar
nebula with ALMA. The observed morphology in the sub-millimetre is different
than previously observed with HST and ATCA in the optical and centimetre
wavelength regimes. The spectral energy distribution (SED) of RMC143 suggests
that two emission mechanisms contribute to the sub-mm emission: optically thin
bremsstrahlung and dust. Both the extinction map and the SED are consistent
with a dusty massive nebula with a dust mass of
(assuming ). To date, RMC143 has the most
dusty LBV nebula observed in the Magellanic Clouds. We have also re-examined
the LBV classification of RMC143 based on VLT/X-shooter spectra obtained in
2015/16 and a review of the publication record. The radiative transfer code
CMFGEN is used to derive its fundamental stellar parameters. We find an
effective temperature of ~K, luminosity of log, and a relatively high mass-loss rate of ~yr. The luminosity is much lower than previously
thought, which implies that the current stellar mass of is
comparable to its nebular mass of (from an assumed
gas-to-dust ratio of 100), suggesting that the star has lost a large fraction
of its initial mass in past LBV eruptions or binary interactions. While the
star may have been hotter in the past, it is currently not hot enough to ionize
its circumstellar nebula. We propose that the nebula is ionized externally by
the hot stars in the 30~Doradus star-forming region.Comment: Paper accepted by A&A on 09/05/2019 and in proof stage. Second
comments by referee are included in this versio
Anomalous Microwave Emission in HII Regions: Is it Really Anomalous? The Case of RCW 49
The detection of an excess of emission at microwave frequencies with respect to the predicted free–free emission has been reported for several Galactic H ii regions. Here, we investigate the case of RCW 49, for which the Cosmic Background Imager tentatively (~3σ) detected Anomalous Microwave Emission (AME) at 31 GHz on angular scales of 7'. Using the Australia Telescope Compact Array, we carried out a multi-frequency (5, 19, and 34 GHz) continuum study of the region, complemented by observations of the H109α radio recombination line. The analysis shows that: (1) the spatial correlation between the microwave and IR emission persists on angular scales from 3farcm4 to 0farcs4, although the degree of the correlation slightly decreases at higher frequencies and on smaller angular scales; (2) the spectral indices between 1.4 and 5 GHz are globally in agreement with optically thin free–free emission, however, ~30% of these are positive and much greater than −0.1, consistent with a stellar wind scenario; and (3) no major evidence for inverted free–free radiation is found, indicating that this is likely not the cause of the Anomalous Emission in RCW 49. Although our results cannot rule out the spinning dust hypothesis to explain the tentative detection of AME in RCW 49, they emphasize the complexity of astronomical sources that are very well known and studied, such as H ii regions, and suggest that, at least in these objects, the reported excess of emission might be ascribed to alternative mechanisms such as stellar winds and shocks
- …