40 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
Auroral radio emission from low-mass stars
It is now a well-established fact that also very low mass stars harbor planetary systems. These stars represent the large majority of our nearby stars but, despite their proximity, their low optical luminosity makes it difficult to apply the usual methods for exoplanet search. An effective probe for the environment of these stars is the auroral radio emission. This kind of emission is well understood for those stars whose magnetic field can be approximated as a dipole. In these cases the radio emission has a peculiar signature in time and in polarization. The presence of a planet nearby the star triggers or perturbs this emission leading to a predictable modulation. We present the case study of the ultra-cool dwarf TVLM 513-46546, for which we take advantage of VLA observations at 4.9 and 8.4 GHz. We reproduce the cyclic circularly-polarized pulses of the star using a 3D model of the auroral radio emission from the stellar magnetosphere. To take into account the possible deviation from the dipolar symmetry, the model simulates a magnetosphere shaped like an offset-dipole. To reproduce the timing and pattern of the observed pulses we explored the space of parameters controlling the auroral beaming pattern and the magnetosphere geometry. Our model explains the observed anomalies of the radio emission at 8.4 GHz as a possible interaction of the star with an external body, like in the case of the interaction between Jupiter and Io
From pre- to young planetary nebulae: Radio continuum variability
Searching for variability, we have observed a sample of hot post-asymptotic giant branch stars and young planetary nebula candidates with the Very Large Array at 4.8, 8.4 and 22.4GHz. The sources had been previously detected in the radio continuum, which is a proof that the central stars have started ionizing their circumstellar envelopes, and an increase in radio flux with time can be expected as a result of the progression of the ionization front. Such a behaviour has been found in IRAS 18062+2410, whose radio modelling has allowed us to determine that its ionized mass has increased from 10-4 to 3.3 × 10-4M⊙ in 8 yr and its envelope has become optically thin at lower frequencies. Different temporal behaviours have been found for three other sources. IRAS 17423-1755 has shown a possibly periodic pattern and an inversion of its radio spectral index, as expected from a varying stellar wind. We estimate that the radio flux arises from a very compact region around the central star (∼1015 cm) with an electron density of 2 × 106cm-3. IRAS 22568+6141 and IRAS 17516-2525 have decreased their radio flux densities by about 10 per cent per year over 4 yr. While a linear increase in the flux density with time points to the progression of the ionization front in the envelope, decreases as well as quasi-periodic patterns may indicate the presence of unstable stellar winds/jets or thick dusty envelopes absorbing ionizing photons. © 2011 The Authors Monthly Notices of the Royal Astronomical Society © 2011 RAS
Spitzer, Very Large Telescope, and Very Large Array observations of the Galactic luminous blue variable candidate HD 168625
We present mid-IR and radio observations of the Galactic luminous blue variables (LBVs) candidate HD 168625 and its associated nebula.We obtained mid-IR spectroscopic observations using the Infrared Spectrograph on board the Spitzer Space Telescope, and performed mid-IR and radio imaging observations using VISIR on the Very Large Telescope and the Very Large Array with comparable angular resolution. Our spectroscopic observations detected spectral features attributable to polycyclic aromatic hydrocarbons (PAHs) and therefore indicate the presence of a photodissociation region (PDR) around the ionized nebula. This result increases the number of LBVs and LBV candidates where a PDR has been found, confirming the importance of such a component in the total mass-loss budget of the central object during this elusive phase of massive star evolution. We have analyzed and compared the mid-IR and radio maps, and derive several results concerning the associated nebula. There is evidence for grain distribution variations across the nebula, with a predominant contribution from bigger grains in the northern part of the nebula while PAH and smaller grains are more concentrated in the southern part. A compact radio component located where there is a lack of thermal dust grains corroborates the presence of a shock in the southern nebula, which could arise as a consequence of the interaction of a fast outflow with the slower, expanding dusty nebula. Such a shock would be a viable means for PAH production as well as for changes in the grain size distribution. Finally, from the detection of a central radio component probably associated with the wind from the central massive supergiant, we derive a current mass-loss rate of ?M = (1.46 ± 0.15) × 10 -6M⊙ yr-1. © 2010. The American Astronomical Society. All rights reserved