109 research outputs found
The 5th edition of the Roma-BZCAT. A short presentation
The 5th edition of the Roma-BZCAT Multifrequency Catalogue of Blazars is
available in a printed version and online at the ASDC website
(http://www.asdc.asi.it/bzcat); it is also in the NED database. It presents
several relevant changes with respect to the past editions which are briefly
described in this paper.Comment: 5 pages, 3 figures. Accepted for publication in Astrophysics and
Space Scienc
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
Orbital solutions for SB2 systems with a HgMn component
From a new set of spectroscopic observations we determined orbital parameters of six SB2 systems with one or both components being HgMn stars.We slightly refined the orbital periods for HD 32964, HD 173524, HD 174933 and HD 216494. Our results for HD 358 are in agreement with the previous literature studies. Regarding HD 33647, our orbital period is shorter than previous determinations. HD 173524 is a triple system. From the variations of the γ-velocity deduced from our and literature data, we refined the orbital period and we estimated the eccentricity of the third companion, for which we get e ≈ 0.13 and Porb = 36 ± 3 years. HD 191110 and HD 216494 seem to be the only synchronous SB2 within our sample. Following the hypothesis of rotational axes perpendicular to the orbital plane, we estimated the angle of the orbital plane to the line of sight. We identified in our spectra the signature of the Hβ of the third component of HD 216494 and we highlight a slight variation of the γ-velocity due to the presence of this component.Based on observations collected at the Stellar Station ``M. G. Fracastoro'' of the Catania Astrophysical Observatory, Italy.Appendix A is only available in electronic form at http://www.edpsciences.or
Chandra's X-ray study confirms that the magnetic standard Ap star KQ Vel hosts a neutron star companion
KQ Vel is a peculiar A0p star with a strong surface magnetic field of about
7.5 kG. It has a slow rotational period of nearly 8 years. Bailey et al. (2015)
detected a binary companion of uncertain nature, and suggested it could be a
neutron star or a black hole. In this Letter we analyze X-ray data obtained by
the Chandra telescope to ascertain information about the stellar magnetic field
and interaction between the star and its companion. We confirm previous X-ray
detection of KQ Vel with a relatively large X-ray luminosity of 2\times 10^{30}
erg/s. X-ray spectra suggest the presence of hot gas at > 20MK and, possibly,
of a non-thermal component. X-ray light curves are variable, but better quality
data are needed to determine periodicity if any. We interpret X-ray spectra as
a combination of two components: the non-thermal emission arising from the
aurora on the A0p star and the hot thermal plasma filling the extended shell
surrounding the "propelling" neutron star. We explore various alternatives, but
a hybrid model involving the stellar magnetosphere along with a hot shell
around the propelling neutron star seems most plausible. We speculate that KQ
Vel was originally a triple system, and the Ap star is a merger product. We
conclude that KQ Vel is an intermediate-mass binary consisting of a strongly
magnetic main sequence star and a neutron star.Comment: accepted to A&AL. 8 pages including the appendix where the formalism
describing the X-ray emission from a hot shell around a propelling neutron
star in settling accretion regime is presente
Bionanocomposite blown films: insights on the theological and mechanical behavior
In this work, bionanocomposites based on two different types of biopolymers belonging to the MaterBi® family and containing two kinds of modified nanoclays were compounded in a twin-screw extruder and then subjected to a film blowing process, aiming at obtaining sustainable films potentially suitable for packaging applications. The preliminary characterization of the extruded bionanocomposites allowed establishing some correlations between the obtained morphology and the material rheological and mechanical behavior. More specifically, the morphological analysis showed that, regardless of the type of biopolymeric matrix, a homogeneous nanofiller dispersion was achieved; furthermore, the established biopolymer/nanofiller interactions caused a restrain of the dynamics of the biopolymer chains, thus inducing a significant modification of the material rheological response, which involves the appearance of an apparent yield stress and
the amplification of the elastic feature of the viscoelastic behavior. Besides, the rheological characterization under non-isothermal elongational flow revealed a marginal effect of the embedded nanofillers on the biopolymers behavior, thus indicating their suitability for film blowing processing. Additionally, the processing behavior of the bionanocomposites was evaluated and compared to that of similar systems based on a low-density polyethylene matrix: this way, it was
possible to identify the most suitable materials for film blowing operations. Finally, the assessment of the mechanical properties of the produced blown films documented the potential exploitation of the selected materials for packaging applications, also at an industrial level
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