84,717 research outputs found
Extending the limits of globule detection -- ISOPHOT Serendipity Survey Observations of interstellar clouds
A faint MJysr bipolar globule was discovered with the
ISOPHOT 170 m Serendipity Survey (ISOSS). ISOSS J 20246+6541 is a cold
( K) FIR source without an IRAS pointsource counterpart.
In the Digitized Sky Survey B band it is seen as a 3\arcmin size bipolar
nebulosity with an average excess surface brightness of
mag/\arcsec . The CO column density distribution determined by
multi-isotopic, multi-level CO measurements with the IRAM-30m telescope agrees
well with the optical appearance. An average hydrogen column density of
cm was derived from both the FIR and CO data. Using a
kinematic distance estimate of 400 pc the NLTE modelling of the CO, HCO,
and CS measurements gives a peak density of cm. The
multiwavelength data characterise ISOSS 20246+6541 as a representative of a
class of globules which has not been discovered so far due to their small
angular size and low 100m brightness. A significant overabundance of
CO is found . This is likely due to
isotope selective chemical processes.Comment: 5 pages, 3 figure
A computational multiscale homogenization framework accounting for inertial effects: application to acoustic metamaterials modelling
A framework, based on an extended Hill–Mandel principle accounting for inertial effects (Multiscale Virtual Work principle), is developed for application to acoustic problems in the context of metamaterials modelling. The classical restrictions in the mean values of the micro-displacement fluctuations and their gradients are then accounted for in a saddle-point formulation of that variational principle in terms of Lagrange functionals. A physical interpretation of the involved Lagrange multipliers can then be readily obtained.
The framework is specifically tailored for modelling the phenomena involved in Locally Resonant Acoustic Metamaterials (LRAM). In this view, several additional hypotheses based on scale separation are used to split the fully coupled micro-macro set of equations into a quasi-static and an inertial system. These are then solved by considering a projection of the microscale equations into their natural modes, which allows for a low-cost computational treatment of the multiscale problem. On this basis, the issue of numerically capturing the local resonance phenomena in a FE
context is addressed. Objectivity of the obtained results in terms of the macroscopic Finite Element (FE) discretization is checked as well as accuracy of the homogenization procedure by comparing with direct numerical simulations (DNS). The appearance of local resonance band-gaps is then modelled for a homogeneous 2D problem and its extension to multi-layered macroscopic material is presented as an initial attempt towards acoustic metamaterial design for tailored band-gap attenuation.Peer ReviewedPostprint (author's final draft
A computational multiscale homogenization framework accounting for inertial effects: application to acoustic metamaterials modelling
A framework, based on an extended Hill–Mandel principle accounting for inertial effects (Multiscale Virtual Work principle), is developed for application to acoustic problems in the context of metamaterials modelling. The classical restrictions in the mean values of the micro-displacement fluctuations and their gradients are then accounted for in a saddle-point formulation of that variational principle in terms of Lagrange functionals. A physical interpretation of the involved Lagrange multipliers can then be readily obtained. The framework is specifically tailored for modelling the phenomena involved in Locally Resonant Acoustic Metamaterials (LRAM). In this view, several additional hypotheses based on scale separation are used to split the fully coupled micro-macro set of equations into a quasi-static and an inertial system. These are then solved by considering a projection of the microscale equations into their natural modes, which allows for a low-cost computational treatment of the multiscale problem. On this basis, the issue of numerically capturing the local resonance phenomena in a FE context is addressed. Objectivity of the obtained results in terms of the macroscopic Finite Element (FE) discretization is checked as well as accuracy of the homogenization procedure by comparing with direct numerical simulations (DNS). The appearance of local resonance band-gaps is then modelled for a homogeneous 2D problem and its extension to multi-layered macroscopic material is presented as an initial attempt towards acoustic metamaterial design for tailored band-gap attenuatio
The multi-faceted Type II-L supernova 2014G from pre-maximum to nebular phase
We present multi-band ultraviolet, optical, and near-infrared photometry,
along with visual-wavelength spectroscopy, of supernova (SN) 2014G in the
nearby galaxy NGC 3448 (25 Mpc). The early-phase spectra show strong emission
lines of the high ionisation species He II/N IV/C IV during the first 2-3 d
after explosion, traces of a metal-rich CSM probably due to pre-explosion mass
loss events. These disappear by day 9 and the spectral evolution then continues
matching that of normal Type II SNe. The post-maximum light curve declines at a
rate typical of Type II-L class. The extensive photometric coverage tracks the
drop from the photospheric stage and constrains the radioactive tail, with a
steeper decline rate than that expected from the Co decay if
-rays are fully trapped by the ejecta. We report the appearance of an
unusual feature on the blue-side of H after 100 d, which evolves to
appear as a flat spectral feature linking H and the O I doublet. This
may be due to interaction of the ejecta with a strongly asymmetric, and
possibly bipolar CSM. Finally, we report two deep spectra at ~190 and 340 d
after explosion, the latter being arguably one of the latest spectra for a Type
II-L SN. By modelling the spectral region around the Ca II, we find a
supersolar Ni/Fe production. The strength of the O I 6300,6363
doublet, compared with synthetic nebular spectra, suggests a progenitor with a
zero-age main-sequence mass between 15 and 19 M.Comment: 24 pages, 14 figure
A method for computing synchrotron and inverse-Compton emission from hydrodynamic simulations of supernova remnants
The observational signature of supernova remnants (SNRs) is very complex, in
terms of both their geometrical shape and their spectral properties, dominated
by non-thermal synchrotron and inverse-Compton scattering. We propose a
post-processing method to analyse the broad-band emission of SNRs based on
three-dimensional hydrodynamical simulations. From the hydrodynamical data, we
estimate the distribution of non-thermal electrons accelerated at the shock
wave and follow the subsequent evolution as they lose or gain energy by
adiabatic expansion or compression and emit energy by radiation. As a first
test case, we use a simulation of a bipolar supernova expanding into a cloudy
medium. We find that our method qualitatively reproduces the main observational
features of typical SNRs and produces fluxes that agree with observations to
within a factor of a few. allowing for further use in more extended sets of
models.Comment: 15 pages, 3 figures; accepted, HEDLA 2014 special issue of High
Energy Density Physic
Multi-D magnetohydrodynamic modelling of pulsar wind nebulae: recent progress and open questions
In the last decade, the relativistic magnetohydrodynamic (MHD) modelling of
pulsar wind nebulae, and of the Crab nebula in particular, has been highly
successful, with many of the observed dynamical and emission properties
reproduced down to the finest detail. Here, we critically discuss the results
of some of the most recent studies: namely the investigation of the origin of
the radio emitting particles and the quest for the acceleration sites of
particles of different energies along the termination shock, by using wisps
motion as a diagnostic tool; the study of the magnetic dissipation process in
high magnetization nebulae by means of new long-term three-dimensional
simulations of the pulsar wind nebula evolution; the investigation of the
relativistic tearing instability in thinning current sheets, leading to fast
reconnection events that might be at the origin of the Crab nebula gamma-ray
flares.Comment: 30 pages, 12 figure
Multi-frequency study of Local Group Supernova Remnants The curious case of the Large Magellanic Cloud SNR J0528-6714
Aims. Recent ATCA, XMM-Newton and MCELS observations of the Magellanic Clouds
(MCs) cover a number of new and known SNRs which are poorly studied, such as
SNR J0528-6714 . This particular SNR exhibits luminous radio-continuum
emission, but is one of the unusual and rare cases without detectable optical
and very faint X-ray emission (initially detected by ROSAT and listed as object
[HP99] 498). We used new multi-frequency radio-continuum surveys and new
optical observations at H{\alpha}, [S ii] and [O iii] wavelengths, in
combination with XMM-Newton X-ray data, to investigate the SNR properties and
to search for a physical explanation for the unusual appearance of this SNR.
Methods. We analysed the X-ray and Radio-Continuum spectra and present
multi-wavelength morphological studies of this SNR.
Results. We present the results of new moderate resolution ATCA observations
of SNR J0528-6714. We found that this object is a typical older SNR with a
radio spectral index of {\alpha}=-0.36 \pm 0.09 and a diameter of D=52.4 \pm
1.0 pc. Regions of moderate and somewhat irregular polarisation were detected
which are also indicative of an older SNR. Using a non-equilibrium ionisation
collisional plasma model to describe the X-ray spectrum, we find temperatures
kT of 0.26 keV for the remnant. The low temperature, low surface brightness,
and large extent of the remnant all indicate a relatively advanced age. The
near circular morphology indicates a Type Ia event.
Conclusions. Our study revealed one of the most unusual cases of SNRs in the
Local Group of galaxies - a luminous radio SNR without optical counterpart and,
at the same time, very faint X-ray emission. While it is not unusual to not
detect an SNR in the optical, the combination of faint X-ray and no optical
detection makes this SNR very unique.Comment: 6 pages, 5 figures, Accepted for publication in A&
- …