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

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

ATCA observations of the very young Planetary Nebula SAO 244567

The radio emission from the youngest known Planetary Nebula, SAO 244567, has been mapped at 1384, 2368, 4800, 8640, 16832 and 18752 MHz by using the Australian Telescope Compact Array (ATCA). These observations constitute the first detailed radio study of this very interesting object, as they allow us to obtain the overall radio morphology of the source and to compute, for the first time, the radio spectrum up to the millimetre range. The radio emission is consistent with free-free from a wind-like shell, which is also the region where most of the [O iii] emission comes from as revealed by Hubble Space Telescope (HST) images. However, two other possible models, a uniform sphere and a uniform thin shell, can also reproduce the radio-observed quantities. Physical parameters of the radio nebula and of the central star were derived, for all the three proposed scenarios, all consistent with SAO 244567 being a very young Planetary Nebula still embedded in the dusty remnant of the asymptotic giant branch phase. The optically thin radio flux density appears to decrease when compared to data from the literature. The apparent variability of the radio emission, probably related to the evolution of the central object, needs further investigation. © 2008 RAS

Probing the magnetosphere of the M8.5 dwarf TVLM 513-46546 by modelling its auroral radio emission. Hint of star exoplanet interaction?

In this paper, we simulate the cyclic circularly polarized pulses of the ultracool dwarf TVLM 513-46546, observed with the Very Large Array at 4.88 and 8.44 GHz on 2006 May, by using a three-dimensional model of the auroral radio emission from the stellar magnetosphere. During this epoch, the radio light curves are characterized by two pulses left-hand polarized at 4.88 GHz, and one doubly peaked (of opposite polarizations) pulse at 8.44 GHz. To take into account the possible deviation from the dipolar symmetry of the stellar magnetic-field topology, the model described in this paper is also able to simulate the auroral radio emission from 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 geometry of the magnetosphere. Through the analysis of the TVLM 513-46546 auroral radio emission, we derive some indications on the magnetospheric field topology that is able to simultaneously reproduce the timing and patterns of the auroral pulses measured at 4.88 and 8.44 GHz. Each set of model solutions simulates two auroral pulses (singly or doubly peaked) per period. To explain the presence of only one 8.44 GHz pulse per period, we analyse the case of auroral radio emission limited only to a magnetospheric sector activated by an external body, like the case of the interaction of Jupiter with its moons

Millimeter observations of planetary nebulae. A contribution to the Planck pre-launch catalogue

Aims: We present 43 GHz (7 mm) observations of a sample of radio-bright Planetary nebulae aimed to obtain, together with far-IR measurements (IRAS), reliable estimates of the fluxes emitted in the millimetre and sub-millimetre band, and, therefore, to test their detectability by the forthcoming ESA PLANCK mission. This spectral range, even though very important to constrain the physics of circumstellar environments, is still far from being completely exploited. Methods: The new millimetre 43 GHz observations were obtained by using the 32 m INAF-IRA Noto Radiotelescope. To estimate the millimetre and sub-millimetre fluxes, we extrapolated and summed the ionized gas (free-free radio emission) and dust (thermal emission) contributions in this frequency range. By comparison of the derived flux densities to the predicted sensitivity we investigate the possible detection of such source for all the channels of PLANCK Results: We conclude that almost 80% of our sample will be detected by PLANCK, with a 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 poorer 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 emissions, if present, are negligible

Radio detection of nebulae around four luminous blue variable stars in the Large Magellanic Cloud

The nebulae associated with four luminous blue variables (LBVs) in the Large Magellanic Cloud (LMC) have been observed at 5.5 and 9GHz using the Australia Telescope Compact Array, and radio emission has been detected for first time in sources R127, R143, S61 and S119. The radio maps of the nebulae have an angular resolution of ∼1.5arcsec and a sensitivity of 1.5-3.0 × 10-2mJybeam-1 and show a very similar morphology to that observed in Hα. This similarity permits us to assume that the Hα emission is not affected by strong intrinsic extinction due to dust within the nebulae. We estimate the masses of ionized gas in the LBV nebulae and find values consistent with those measured in Galactic LBVs. © 2012 The Authors Monthly Notices of the Royal Astronomical Society © 2012 RAS

Synergy SKA - CTA: Supernova remnants as cosmic accelerators

Supernova remnants (SNRs) are one of the most important sites where particles are accelerated with high efficiency and in a wide range of energies, becoming an important component of cosmic rays. A good test for this hypothesis will be possible using the data collected by next-generation radio and gamma-ray observatories, like the Square Kilometre Array (SKA) and the Cherenkov Telescope Array (CTA). Radio emission is fundamental to explore the SNR environment and to shed light on the physical processes involved in particle acceleration, providing direct links to high-energy physics. Two cases of SNRs recently studied in radio are presented, showing the importance of high-resolution radio images. An overview of SKA and its precursors is given with our ongoing preparation work. In particular, we present the EMU survey and the pathfinder project SCORPIO. Finally a direct view of the tight connection between SKA and CTA future studies of SNRs is provided

Photometric monitoring of Luminous Blue Variables

We present some preliminary results from our program of intensive near-infrared photometric monitoring ofa sample of confirmed and candidate Luminous Blue Variables (LBVs) conducted from 2008 to 2010. Clear long-term variability has been observed for Wray 17-96 and V481 Sct, with overall brightness variation greater than 1 mag in the J band. Other sources, such as LBV 1806-20 showed detectable variability with amplitudes of few tenths of a magnitude with a time-scale of about 60 days

High-resolution Very Large Array observations of 18 MIPSGAL bubbles

We present radio observations of 18 MIPSGAL bubbles performed at 5 GHz (6 cm) with the Karl G. Jansky Very Large Array in configuration B and BnA. The observations were aimed at understanding what kind of information high-resolution and high-sensitivity radio maps can supply on the circum-stellar envelopes of different kinds of evolved stars and what their comparison with infrared images with similar resolution can tell us. We found that the 18 bubbles can be grouped into five categories according to their radio morphology. The three bubbles presenting a central point source in the radio images all correspond to luminous blue variable star candidates. 11 bubbles show an elliptical shape and the total lack of a central object in the radio, and are likely associated with planetary nebulae. Under this assumption, we derive their distance, their ionized mass and their distribution on the Galactic plane. We discuss the possibility that the MIPSGAL bubbles catalogue (428 objects) may contain a large fraction of all Galactic planetary nebulae located at a distance between 1.4 kpc and 6.9 kpc and lying in the MIPSGAL field of view. Among the remaining bubbles, we identify also an H II region and a proto-planetary nebula candidate