4,159 research outputs found
TRACE-derived temperature and emission measure profiles along long-lived coronal loops: the role of filamentation
In a recent letter (ApJ 517, L155) Lenz et al. have shown the evidence of
uniform temperature along steady long coronal loops observed by TRACE in two
different passbands (171 A and 195 A filters). We propose that such an evidence
can be explained by the sub-arcsecond structuring of the loops across the
magnetic field lines. In this perspective, we present a model of a bundle of
six thin parallel hydrostatic filaments with temperature stratification
dictated by detailed energy balance and with temperatures at their apex ranging
between 0.8 and 5 MK. If analyzed as a single loop, the bundle would appear
isothermal along most of its length.Comment: 9 pages, 4 figs, LaTeX text, PostScript figure
Mass Accretion Processes in Young Stellar Objects: Role of Intense Flaring Activity
According to the magnetospheric accretion scenario, young low-mass stars are
surrounded by circumstellar disks which they interact with through accretion of
mass. The accretion builds up the star to its final mass and is also believed
to power the mass outflows, which may in turn have a significant role in
removing the excess angular momentum from the star-disk system. Although the
process of mass accretion is a critical aspect of star formation, some of its
mechanisms are still to be fully understood. On the other hand, strong flaring
activity is a common feature of young stellar objects (YSOs). In the Sun, such
events give rise to perturbations of the interplanetary medium. Similar but
more energetic phenomena occur in YSOs and may influence the circumstellar
environment. In fact, a recent study has shown that an intense flaring activity
close to the disk may strongly perturb the stability of circumstellar disks,
thus inducing mass accretion episodes (Orlando et al. 2011). Here we review the
main results obtained in the field and the future perspectives.Comment: 4 pages, 2 Figures; accepted for publication on Acta Polytechnica
(Proceedings of the Frascati Workshop 2013
Thermal structure of hot non-flaring corona from Hinode/EIS
In previous studies a very hot plasma component has been diagnosed in solar
active regions through the images in three different narrow-band channels of
SDO/AIA. This diagnostic from EUV imaging data has also been supported by the
matching morphology of the emission in the hot Ca XVII line, as observed with
Hinode/EIS. This evidence is debated because of unknown distribution of the
emission measure along the line of sight. Here we investigate in detail the
thermal distribution of one of such regions using EUV spectroscopic data. In an
active region observed with SDO/AIA, Hinode/EIS and XRT, we select a subregion
with a very hot plasma component and another cooler one for comparison. The
average spectrum is extracted for both, and 14 intense lines are selected for
analysis, that probe the 5.5 < log T < 7 temperature range uniformly. From
these lines the emission measure distributions are reconstructed with the MCMC
method. Results are cross-checked with comparison of the two subregions, with a
different inversion method, with the morphology of the images, and with the
addition of fluxes measured with from narrow and broad-band imagers. We find
that, whereas the cool region has a flat and featureless distribution that
drops at temperature log T >= 6.3, the distribution of the hot region shows a
well-defined peak at log T = 6.6 and gradually decreasing trends on both sides,
thus supporting the very hot nature of the hot component diagnosed with
imagers. The other cross-checks are consistent with this result. This study
provides a completion of the analysis of active region components, and the
resulting scenario supports the presence of a minor very hot plasma component
in the core, with temperatures log T > 6.6.Comment: 12 pages, 8 figures, accepted for publicatio
Hydrodynamic modelling of ejecta shrapnel in the Vela supernova remnant
Many supernova remnants (SNRs) are characterized by a knotty ejecta
structure. The Vela SNR is an excellent example of remnant in which detached
clumps of ejecta are visible as X-ray emitting bullets that have been observed
and studied in great detail. We aim at modelling the evolution of ejecta
shrapnel in the Vela SNR, investigating the role of their initial parameters
(position and density) and addressing the effects of thermal conduction and
radiative losses. We performed a set of 2-D hydrodynamic simulations describing
the evolution of a density inhomogeneity in the ejecta profile. We explored
different initial setups. We found that the final position of the shrapnel is
very sensitive to its initial position within the ejecta, while the dependence
on the initial density contrast is weaker. Our model also shows that moderately
overdense knots can reproduce the detached features observed in the Vela SNR.
Efficient thermal conduction produces detectable effects by determining an
efficient mixing of the ejecta knot with the surrounding medium and shaping a
characteristic elongated morphology in the clump.Comment: Accepted for publication in Monthly Notices of the Royal Astronomical
Societ
How does collegiality survive managerially led universities? Evidence from a European Survey
Today's universities are, accordingly to Clark's entrepreneurial model, sustained by managerialism, whereas collegialism may remain in contrast or work in a different way. More recent literature suggests the clash such as the potential for coexistence between managerialism and collegialism. The study analyses data from a survey of 26 universities in 8 European countries, focusing on middle managers (MMs). The results show that at the level of the individual institutions, there are notable positive correlations between the presence of collegial and of managerial cultures. Multilevel regressions at institutional level are analysed, to ‘predict’ collegiality in light of the universities' managerial culture and other factors affecting organizational change: accountability; distribution of discretional power; funding; impact of quality assurance (QA) and evaluation. The results illustrate that in more managerial universities, collegial culture increases above all when MMs believe that distance-steering tools (QA and evaluation) have had positive impacts. We find that collegiality can indeed thrive, even when ‘managerially led’
A detailed study of the rise phase of a long duration X-ray flare in the young star TWA 11B
We analyzed a long duration flare observed in a serendipitous XMM-Newton
detection of the M star CD-39 7717B (TWA 11B), member of the young stellar
association TW Hya (~ 8 Myr). Only the rise phase (with a duration of ~ 35 ks)
and possibly the flare peak were observed. We took advantage of the high
count-rate of the X-ray source to carry out a detailed analysis of its spectrum
during the whole exposure. After a careful analysis, we interpreted the rise
phase as resulting from the ignition of a first group of loops (event A) which
triggered a subsequent two-ribbon flare (event B). Event A was analyzed using a
single-loop model, while a two-ribbon model was applied for event B. Loop
semi-lengths of ~ 4 R* were obtained. Such large structures had been previously
observed in very young stellar objects (~ 1 - 4 Myr). This is the first time
that they have been inferred in a slightly more evolved star. The fluorescent
iron emission line at 6.4 keV was detected during event B. Since TWA 11B seems
to have no disk, the most plausible explanation found for its presence in the
X-ray spectrum of this star is collisional - or photo- ionization. As far as we
are concerned, this is only the third clear detection of Fe photospheric
fluorescence in stars other than the Sun.Comment: Accepted for publication in ApJ. 15 pages, 9 figure
La valutazione e i suoi effetti sull'università: una comparazione fra Italia e Francia
The paper presents the results of a comparative study on four universities in France and in Italy, which investigates how evaluation helps to strengthen the university as autonomous and professional organization, able to formulating strategies and/or tactics of adaptation in relation to regulatory changes and context, overcoming the traditional loosely coupled feature. The hypothesis is that the academic organizations show different effects of resilience, linked to intended and unintended changes
X-Raying the Dark Side of Venus - Scatter from Venus Magnetotail?
This work analyzes the X-ray, EUV and UV emission apparently coming from the
Earth-facing (dark) side of Venus as observed with Hinode/XRT and SDO/AIA
during a transit across the solar disk occurred in 2012. We have measured
significant X-Ray, EUV and UV flux from Venus dark side. As a check we have
also analyzed a Mercury transit across the solar disk, observed with Hinode/XRT
in 2006. We have used the latest version of the Hinode/XRT Point Spread
Function (PSF) to deconvolve Venus and Mercury X-ray images, in order to remove
possible instrumental scattering. Even after deconvolution, the flux from Venus
shadow remains significant while in the case of Mercury it becomes negligible.
Since stray-light contamination affects the XRT Ti-poly filter data from the
Venus transit in 2012, we performed the same analysis with XRT Al-mesh filter
data, which is not affected by the light leak. Even the Al-mesh filter data
show residual flux. We have also found significant EUV (304 A, 193 A, 335 A)
and UV (1700 A) flux in Venus shadow, as measured with SDO/AIA. The EUV
emission from Venus dark side is reduced when appropriate deconvolution methods
are applied; the emission remains significant, however. The light curves of the
average flux of the shadow in the X-ray, EUV, and UV bands appear different as
Venus crosses the solar disk, but in any of them the flux is, at any time,
approximately proportional to the average flux in a ring surrounding Venus, and
therefore proportional to the average flux of the solar regions around Venus
obscuring disk line of sight. The proportionality factor depends on the band.
This phenomenon has no clear origin; we suggest it may be due to scatter
occurring in the very long magnetotail of Venus.Comment: This paper has been accepted in The Astrophysical Journa
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