941 research outputs found
Modeling the effect of small-scale magnetic turbulence on the X-ray properties of Pulsar Wind Nebulae
Pulsar Wind Nebulae (PWNe) constitute an ideal astrophysical environment to
test our current understanding of relativistic plasma processes. It is well
known that magnetic fields play a crucial role in their dynamics and emission
properties. At present, one of the main issues concerns the level of magnetic
turbulence present in these systems, which in the absence of space resolved
X-ray polarization measures cannot be directly constrained. In this work we
investigate, for the first time using simulated synchrotron maps, the effect of
a small scale fluctuating component of the magnetic field on the emission
properties in X-ray. We illustrate how to include the effects of a turbulent
component in standard emission models for PWNe, and which consequences are
expected in terms of net emissivity and depolarization, showing that the X-ray
surface brightness maps can provide already some rough constraints. We then
apply our analysis to the Crab and Vela nebulae and, by comparing our model
with Chandra and Vela data, we found that the typical energies in the turbulent
component of the magnetic field are about 1.5 to 3 times the one in the ordered
field.Comment: 9 pages, 8 figures, accepted for publication in MNRA
Rotating disks in high-mass young stellar objects
We report on the detection of four rotating massive disks in two regions of
high-mass star formation. The disks are perpendicular to known bipolar outflows
and turn out to be unstable but long lived. We infer that accretion onto the
embedded (proto)stars must proceed through the disks with rates of ~10E-2
Msun/yr.Comment: 11 pages, 2 figures, 1 table; accepted for publication by ApJ
Study of water mass transfer dynamics in frescoes by dielectric spectroscopy
The knowledge of moisture content (MC) is essential for determining the state of preservation of various types of hand-work: from building materials, such as bricks and concrete, to objects of artistic value, in particular frescoes and mural paintings. In all the above, moisture is the primary source of damages, as it affects the durability of porous materials. Dielectric properties of porous materials are strongly affected by the presence of water, suggesting dielectric spectroscopy as a suitable
non-invasive diagnostic technique. The development of a quantitative relationship between MC and permittivity requires to investigate the dynamics of water mass transfer in porous media, and to determine its effect on the dielectric properties. In this paper a coupled mass transfer/dielectric problem is introduced and solved numerically, based on a finite element model. Results are compared to experimental dielectric measurement performed on plaster samples by the open coaxial method. The application of the dielectric technique to frescoes monitoring is proposed, showing the results obtained in an on-site study
Description of Anteon seramense (Hymenoptera. Dryinidae), a new species from Indonesia
Anteon seramense sp. nov. is described from Seram Island (Indonesia). Seram is an island
situated in a transition area between the Oriental and the Australian regions. Anteon seramense
can be distinguished from the related Australian species A. giluwense Olmi and
A. chelogynoides (Perkins) by the different sculpture of the scutum (granulated in A. seramense,
punctate and unsculptured among punctures in A. giluwense and A. chelogynoides)
and the different distal apex of the protarsal segment 5 (deeply hollow in A. seramense,
not hollow in A. giluwense and A. chelogynoides). Anteon seramense sp. nov. can be distinguished
from the related Oriental species A. heppneri Olmi and A. thai Olmi by the different
shape of the protarsal segment 5 (basal part slightly longer than distal part in A. seramense,
much longer in A. heppneri and A. thai)
The structure of molecular clumps around high-mass young stellar objects
We have used the IRAM 30-m and FCRAO 14-m telescopes to observe the molecular
clumps associated with 12 ultracompact (UC) HII regions in the J=6-5, 8-7 and
13-12 rotational transitions of methyl-acetylene (CH3C2H). Under the assumption
of LTE and optically thin emission, we have derived temperature estimates
ranging from 30 to 56 K. We estimate that the clumps have diameters of 0.2-1.6
pc, H_2 densities of 10^5-10^6 {cm^{-3}}, and masses of 10^2-2 10^4 M_\odot. We
compare these values with those obtained by other authors from different
molecular tracers and find that the H_2 density and the temperature inside the
clumps vary respectively like n_{H_2} ~ R^{-2.6} and T ~ R^{-0.5}, with R
distance from the centre. We also find that the virial masses of the clumps are
~3 times less than those derived from the CH3C2H column densities: we show that
a plausible explanation is that magnetic fields play an important role to
stabilise the clumps, which are on the verge of gravitational collapse.
Finally, we show that the CH3C2H line width increases for decreasing distance
from the clump centre: this effect is consistent with infall in the inner
regions of the clumps. We conclude that the clumps around UC HII regions are
likely to be transient (~10^(5) yr) entities, remnants of isothermal spheres
currently undergoing gravitational collapse: the high mass accretion rates
(~10^{-2} M_\odot yr^{-1}) lead to massive star formation at the centre of such
clumps.Comment: 15 pages, 11 figures, A & A in pres
Ionization toward the high-mass star-forming region NGC 6334 I
Context. Ionization plays a central role in the gas-phase chemistry of
molecular clouds. Since ions are coupled with magnetic fields, which can in
turn counteract gravitational collapse, it is of paramount importance to
measure their abundance in star-forming regions. Aims. We use spectral line
observations of the high-mass star-forming region NGC 6334 I to derive the
abundance of two of the most abundant molecular ions, HCO+ and N2H+, and
consequently, the cosmic ray ionization rate. In addition, the line profiles
provide information about the kinematics of this region. Methods. We present
high-resolution spectral line observations conducted with the HIFI instrument
on board the Herschel Space Observatory of the rotational transitions with Jup
> 5 of the molecular species C17O, C18O, HCO+, H13CO+, and N2H+. Results. The
HCO+ and N2H+ line profiles display a redshifted asymmetry consistent with a
region of expanding gas. We identify two emission components in the spectra,
each with a different excitation, associated with the envelope of NGC 6334 I.
The physical parameters obtained for the envelope are in agreement with
previous models of the radial structure of NGC 6334 I based on submillimeter
continuum observations. Based on our new Herschel/HIFI observations, combined
with the predictions from a chemical model, we derive a cosmic ray ionization
rate that is an order of magnitude higher than the canonical value of 10^(-17)
s-1. Conclusions. We find evidence of an expansion of the envelope surrounding
the hot core of NGC 6334 I, which is mainly driven by thermal pressure from the
hot ionized gas in the region. The ionization rate seems to be dominated by
cosmic rays originating from outside the source, although X-ray emission from
the NGC 6334 I core could contribute to the ionization in the inner part of the
envelope.Comment: This paper contains a total of 10 figures and 3 table
Reverberation of pulsar wind nebulae (III): Modelling of the plasma interface empowering a long term radiative evolution
The vast majority of Pulsar Wind Nebulae (PWNe) present in the Galaxy is
formed by middle-aged systems characterized by a strong interaction of the PWN
itself with the supernova remnant (SNR). Unfortunately, modelling these systems
can be quite complex and numerically expensive, due to the non-linearity of the
PWN-SNR evolution even in the simple 1D / one-zone case when the reverse shock
of the SNR reaches the PWN, and the two begin to interact (and reverberation
starts).
Here we introduce a new numerical technique that couples the numerical
efficiency of the one-zone thin shell approach with the reliability of a full
``lagrangian'' evolution, able to correctly reproduce the PWN-SNR interaction
during the reverberation and to consistently evolve the particle spectrum
beyond. Based on our previous findings, we show that our novel strategy
resolves many of the uncertainties present in previous approaches, as the
arbitrariness in the SNR structure, and ensure a robust evolution, compatible
with results that can be obtained with more complex 1D dynamical approaches.
Our approach enable us for the first time to provide reliable spectral models
of the later compression phases in the evolution of PWNe. While in general we
found that the compression is less extreme than that obtained without such
detailed dynamical considerations, leading to the formation of less structured
spectral energy distributions, we still find that a non negligible fraction of
PWNe might experience a super-efficient phase, with the optical and/or X-ray
luminosity exceeding the spin-down one.Comment: 12 pages, 2 tables, 5 figure
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