We present ALMA and ATCA observations of the luminous blue variable RMC 127. The radio maps show for the first time the core of the nebula and evidence that the nebula is strongly asymmetric with a ...Z-pattern shape. Hints of this morphology are also visible in the archival Hubble Space Telescope H image, which overall resembles the radio emission. The emission mechanism in the outer nebula is optically thin free-free in the radio. At high frequencies, a component of point-source emission appears at the position of the star, up to the ALMA frequencies. The rising flux density distribution ( S ∼ 0.78 0.05 ) of this object suggests thermal emission from the ionized stellar wind and indicates a departure from spherical symmetry with n e ( r ) ∝ r − 2 . We examine different scenarios to explain this excess of thermal emission from the wind and show that this can arise from a bipolar outflow, supporting the suggestion by other authors that the stellar wind of RMC 127 is aspherical. We fit the data with two collimated ionized wind models, and we find that the mass-loss rate can be a factor of two or more smaller than in the spherical case. We also fit the photometry obtained by IR space telescopes and deduce that the mid- to far-IR emission must arise from extended, cool ( ∼ 80 K ) dust within the outer ionized nebula. Finally, we discuss two possible scenarios for the nebular morphology: the canonical single-star expanding shell geometry and a precessing jet model assuming the presence of a companion star.
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 22.4 GHz. 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 × 10−4 M⊙ 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 × 106 cm−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.
The fast rotating star CU Virginis is a magnetic chemically peculiar star with an oblique dipolar magnetic field. The continuum radio emission has been interpreted as gyrosynchrotron emission arising ...from a thin magnetospheric layer. Previous radio observations at 1.4 GHz showed that a 100 per cent circular polarized and highly directive emission component overlaps to the continuum emission two times per rotation, when the magnetic axis lies in the plane of the sky. This sort of radio lighthouse has been proposed to be due to cyclotron maser emission generated above the magnetic pole and propagating perpendicularly to the magnetic axis. Observations carried out with the Australia Telescope Compact Array at 1.4 and 2.5 GHz one year after this discovery show that this radio emission is still present, meaning that the phenomenon responsible for this process is steady on a time-scale of years. The emitted radiation spans at least 1 GHz, being observed from 1.4 to 2.5 GHz. On the light of recent results on the physics of the magnetosphere of this star, the possibility of plasma radiation is ruled out. The characteristics of this radio lighthouse provide us a good marker of the rotation period, since the peaks are visible at particular rotational phases. After one year, they show a delay of about 15 min. This is interpreted as a new abrupt spinning down of the star. Among several possibilities, a quick emptying of the equatorial magnetic belt after reaching the maximum density can account for the magnitude of the breaking. The study of the coherent emission in stars like CU Vir, as well as in pre-main-sequence stars, can give important insight into the angular momentum evolution in young stars. This is a promising field of investigation that high-sensitivity radio interferometers such as Square Kilometre Array can exploit.
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-dot =(1.46{+-}0.15)x10{sup -6} M{sub sun} yr{sup -1}.
Abstract
The nebulae associated with four luminous blue variables (LBVs) in the Large Magellanic Cloud (LMC) have been observed at 5.5 and 9 GHz 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 ∼1.5 arcsec and a sensitivity of 1.5-3.0 × 10−2 mJy beam−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.
Aims. In order to fully understand the physical processes in the magnetospheres of the Magnetic Chemically Peculiar stars, we performed multi- frequency radio observations of CU Virginis. The radio ...emission of this kind of stars arises from the interaction between energetic electrons and magnetic field. Our analysis is used to test the physical scenario proposed for the radio emission from the MCP stars and to derive quantitative information about physical parameters not directly observable. Methods. The radio data were acquired with the VLA and cover the whole rotational period of CU Virginis. For each observed frequency the radio light curves of the total flux density and fraction of circular polarization were fitted using a three- dimensional MCP magnetospheric model simulating the stellar radio emission as a function of the magnetospheric physical parameters. Results. The observations show a clear correlation between the radio emission and the orientation of the magnetosphere of this oblique rotator. The radio emission is explained as the result of the acceleration of the wind particles in the current sheets just beyond the Alfven radius, that eventually return toward the star following the magnetic field and emitting radiation by gyrosyncrotron mechanisms. The accelerated electrons we probed with our simulations have a hard energetic spectrum ( N(E) \propto E and the acceleration process has an efficiency of about 10 super(-3). The Alfven radius we determined is in the range of 12{-}17 similar to R_\ast and, for a dipolar field of 3000 Gauss at the magnetic pole of the star, we determine a mass loss from the star of about 10 6M_{\odot} yr super(-1). In the inner magnetosphere, inside the Alfven radius, the confined stellar wind accumulates and reaches temperatures in the range of 10 super(5)-10 super(6) K, and a detectable X-ray emission is expected.
Context. The source MWC 930 is a star just ~2° above the Galactic plane whose nature is not clear and has not been studied in detail so far. While a post-asymptotic giant branch classification was ...proposed in the past, studies of its optical spectrum and photometry pointed toward strong variability, therefore, the object was reclassified as a luminous blue variable (LBV) candidate. Aims. The LBVs typically undergo phases of strong mass loss in the form of eruptions that can create shells of ejecta around the star. Our goal is to search for the presence of such a circumstellar nebula in MWC 930 and investigate its properties. Methods. To do so, we make use of space-based infrared data from our Spitzer campaign performed with the InfraRed Array Camera (IRAC) and the InfraRed Spectrograph (IRS), as well as data from optical and infrared (IR) surveys. Results. In our Spitzer images, we clearly detect an extended shell around MWC 930 at wavelengths longer than 5 μm. The mid-infrared spectrum is dominated by the central star and mostly shows forbidden lines of FeII with an underlying continuum that decreases with wavelengths up to ~15 μm and then inverts its slope, displaying a second peak around 60 μm, which is evidence of cold dust grains formed in a past eruption. By modeling the spectral energy distribution, we identify two central components, beside the star and the outer shell. These extra sources of radiation are interpreted as material close to the central star, which may be due to a recent ejection. Features of C-bearing molecules or grains are not detected.
During the month of 2009 December, the blazar 3C 454.3 became the brightest gamma-ray source in the sky, reaching a peak flux F {approx} 2000 x 10{sup -8} photons cm{sup -2} s{sup -1} for E > 100 ...MeV. Starting in 2009 November intensive multifrequency campaigns monitored the 3C 454 gamma-ray outburst. Here, we report on the results of a two-month campaign involving AGILE, INTEGRAL, Swift/XRT, Swift/BAT, and Rossi XTE for the high-energy observations and Swift/UVOT, KANATA, Goddard Robotic Telescope, and REM for the near-IR/optical/UV data. GASP/WEBT provided radio and additional optical data. We detected a long-term active emission phase lasting {approx}1 month at all wavelengths: in the gamma-ray band, peak emission was reached on 2009 December 2-3. Remarkably, this gamma-ray super-flare was not accompanied by correspondingly intense emission in the optical/UV band that reached a level substantially lower than the previous observations in 2007-2008. The lack of strong simultaneous optical brightening during the super-flare and the determination of the broadband spectral evolution severely constrain the theoretical modeling. We find that the pre- and post-flare broadband behavior can be explained by a one-zone model involving synchrotron self-Compton plus external Compton emission from an accretion disk and a broad-line region. However, the spectra of the 2009 December 2-3 super-flare and of the secondary peak emission on 2009 December 9 cannot be satisfactorily modeled by a simple one-zone model. An additional particle component is most likely active during these states.
We have observed the radio nebula surrounding the Galactic luminous blue variable candidate G79.29+0.46 with the Expanded Very Large Array (EVLA) at 6 cm. These new radio observations allow a ...morphological comparison between the radio emission, which traces the ionized gas component, and the mid-IR emission, a tracer of the dust component. The InfraRed Array Camera (8 Delta *mm) and the Multiband Imaging Photometer for Spitzer (24 Delta *mm and 70 Delta *mm) images have been reprocessed and compared with the EVLA map. We confirm the presence of a second shell at 24 Delta *mm and also provide evidence for its detection at 70 Delta *mm. The differences between the spatial morphology of the radio and mid-IR maps indicate the existence of two dust populations, the cooler one emitting mostly at longer wavelengths. Analysis of the two dusty, nested shells have provided us with an estimate of the characteristic timescales for shell ejection, providing important constraints for stellar evolutionary models. Finer details of the ionized gas distribution can be appreciated thanks to the improved quality of the new 6 cm image, most notably the highly structured texture of the nebula. Evidence of interaction between the nebula and the surrounding interstellar medium can be seen in the radio map, including brighter features that delineate regions where the shell structure is locally modified. In particular, the brighter filaments in the southwest region appear to frame the shocked southwestern clump reported from CO observations.
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