307 research outputs found
Accretion disk coronae of Intermediate Polar Cataclysmic Variables - 3D MagnetoHydro-Dynamic modeling and thermal X-ray emission
IPCVs contain a magnetic, rotating white dwarf surrounded by a magnetically
truncated accretion disk. To explain their strong flickering X-ray emission,
accretion has been successfully taken into account. Nevertheless, observations
suggest that accretion phenomena could not be the only process behind it. An
intense flaring activity occurring on the surface of the disk may generate a
corona, contribute to the thermal X-ray emission and influence the system
stability. Our purposes are: investigating the formation of an extended corona
above the accretion disk, due to an intense flaring activity occurring on the
disk surface; studying its effects on the disk and stellar magnetosphere;
assessing its contribution to the observed X-ray flux. We have developed a 3D
MHD model of a IPCV. The model takes into account gravity, disk viscosity,
thermal conduction, radiative losses and coronal flare heating. To perform a
parameter space exploration, several system conditions have been considered,
with different magnetic field intensity and disk density values. From the
results of the evolution of the model, we have synthesized the thermal X-ray
emission. The simulations show the formation of an extended corona, linking
disk and star. The flaring activity is capable of strongly influencing the disk
configuration and its stability, effectively deforming the magnetic field
lines. Hot plasma evaporation phenomena occur in the layer immediately above
the disk. The flaring activity gives rise to a thermal X-ray emission in both
the [0.1-2.0] keV and the [2.0-10] keV bands. An intense coronal activity
occurring on the disk surface of an IPCV can affect the structure of the disk
depending noticeably on the density of the disk and the magnetic field of the
central object. Moreover, the synthesis of the thermal X-ray fluxes shows that
this flaring activity may contribute to the observed thermal X-ray emission
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
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
KATRIN Sensitivity to Sterile Neutrino Mass in the Shadow of Lightest Neutrino Mass
The presence of light sterile neutrinos would strongly modify the energy
spectrum of the Tritium \beta-electrons. We perform an analysis of the KATRIN
experiment's sensitivity by scanning almost all the allowed region of neutrino
mass-squared difference and mixing angles of the 3+1 scenario. We consider the
effect of the unknown absolute mass scale of active neutrinos on the
sensitivity of KATRIN to the sterile neutrino mass. We show that after 3 years
of data-taking, the KATRIN experiment can be sensitive to mixing angles as
small as sin^2 (2\theta_s) ~ 10^-2. Particularly we show that for small mixing
angles, sin^2 (2\theta_s) < 0.1, the KATRIN experiment can gives the strongest
limit on active-sterile mass-squared difference.Comment: 4 pages, 2 figures, matches the published versio
Impacts of fragmented accretion streams onto Classical T Tauri Stars: UV and X-ray emission lines
Context. The accretion process in Classical T Tauri Stars (CTTSs) can be
studied through the analysis of some UV and X-ray emission lines which trace
hot gas flows and act as diagnostics of the post-shock downfalling plasma. In
the UV band, where higher spectral resolution is available, these lines are
characterized by rather complex profiles whose origin is still not clear.
Aims. We investigate the origin of UV and X-ray emission at impact regions of
density structured (fragmented) accretion streams.We study if and how the
stream fragmentation and the resulting structure of the post-shock region
determine the observed profiles of UV and X-ray emission lines.
Methods. We model the impact of an accretion stream consisting of a series of
dense blobs onto the chromosphere of a CTTS through 2D MHD simulations. We
explore different levels of stream fragmentation and accretion rates. From the
model results, we synthesize C IV (1550 {\AA}) and OVIII (18.97 {\AA}) line
profiles.
Results. The impacts of accreting blobs onto the stellar chromosphere produce
reverse shocks propagating through the blobs and shocked upflows. These
upflows, in turn, hit and shock the subsequent downfalling fragments. As a
result, several plasma components differing for the downfalling velocity,
density, and temperature are present altoghether. The profiles of C IV doublet
are characterized by two main components: one narrow and redshifted to speed
50 km s and the other broader and consisting of subcomponents
with redshift to speed in the range 200 400 km s. The profiles
of OVIII lines appear more symmetric than C IV and are redshifted to speed
150 km s.
Conclusions. Our model predicts profiles of C IV line remarkably similar to
those observed and explains their origin in a natural way as due to stream
fragmentation.Comment: 11 pages, 10 figure
The flaring and quiescent components of the solar corona
The solar corona is a template to understand stellar activity. The Sun is a
moderately active star, and its corona differs from active stars: active
stellar coronae have a double-peaked EM(T) with the hot peak at 8-20 MK, while
the non flaring solar corona has one peak at 1-2 MK. We study the average
contribution of flares to the solar EM(T) to investigate indirectly the
hypothesis that the hot peak of the EM(T) of active stellar coronae is due to a
large number of unresolved solar-like flares, and to infer properties on the
flare distribution from nano- to macro-flares. We measure the disk-integrated
time-averaged emission measure, EM_F(T), of an unbiased sample of solar flares
analyzing uninterrupted GOES/XRS light curves over time intervals of one month.
We obtain the EM_Q(T) of quiescent corona for the same time intervals from the
Yohkoh/SXT data. To investigate how EM_F(T) and EM_Q(T) vary with the solar
cycle, we evaluate them at different phases of the cycle (from Dec. 1991 to
Apr. 1998). Irrespective of the solar cycle phase, EM_F(T) appears like a peak
of the distribution significantly larger than the values of EM_Q(T) for T~5-10
MK. As a result the time-averaged EM(T) of the whole solar corona is
double-peaked, with the hot peak, due to time-averaged flares, located at
temperature similar of that of active stars, but less enhanced. The EM_F(T)
shape supports the hypothesis that the hot EM(T) peak of active coronae is due
to unresolved solar-like flares. If this is the case, quiescent and flare
components should follow different scaling laws for increasing stellar
activity. In the assumption that the heating of the corona is entirely due to
flares, from nano- to macro-flares, then either the flare distribution or the
confined plasma response to flares, or both, are bimodal.Comment: 8 pages, 7 postscript figures, accepted for publication in Astronomy
and Astrophysic
Redshifted X-rays from the material accreting onto TW Hya: evidence of a low-latitude accretion spot
High resolution spectroscopy, providing constraints on plasma motions and
temperatures, is a powerful means to investigate the structure of accretion
streams in CTTS. In particular, the accretion shock region, where the accreting
material is heated to temperatures of a few MK as it continues its inward bulk
motion, can be probed by X-ray spectroscopy. To attempt to detect for the first
time the motion of this X-ray-emitting post-shock material, we searched for a
Doppler shift in the deep Chandra/HETGS observation of the CTTS TW Hya. This
test should unveil the nature of this X-ray emitting plasma component in CTTS,
and constrain the accretion stream geometry. We searched for a Doppler shift in
the X-ray emission from TW Hya with two different methods, by measuring the
position of a selected sample of emission lines, and by fitting the whole TW
Hya X-ray spectrum, allowing the line-of-sight velocity to vary. We found that
the plasma at T~2-4 MK has a line-of-sight velocity of 38.3+/-5.1 km/s with
respect to the stellar photosphere. This result definitively confirms that this
X-ray-emitting material originates in the post-shock region, at the base of the
accretion stream, and not in coronal structures. The comparison of the observed
velocity along the line of sight, 38.3+/-5.1 km/s, with the inferred intrinsic
velocity of the post shock of TW Hya, v_post~110-120 km/s, indicates that the
footpoints of the accretion streams on TW Hya are located at low latitudes on
the stellar surface. Our results indicate that complex magnetic field
geometries, such as that of TW Hya, permit low-latitude accretion spots.
Moreover, since on TW Hya the redshift of the soft X-ray emission is very
similar to that of the narrow component of the CIV resonance doublet at 1550
Ang, as found by Ardila et al. (2013), then the plasma at 2-4 MK and that at
0.1 MK likely originate in the same post-shock regions.Comment: Accepted for publication in Astronomy & Astrophysics; 2nd version
after language editor corrections; 16 pages, 8 figures, 6 table
The Sun as an X-Ray Star. IV. The Contribution of Different Regions of the Corona to Its X-Ray Spectrum
We study X-ray-synthesized spectra of solar regions as templates to interpret analogous stellar spectra. We define three classes of coronal structures of different brightness, low (background quiet corona), medium (active regions), and high (active region cores), and determine their contribution to the solar X-ray emission measure versus temperature, EM(T), luminosity, and spectrum. This study defines the extent of the solar analogy quantitatively and accurately. To this end, we have selected a large sample of full-disk Yohkoh soft X-ray telescope observations taken between the maximum and the minimum of solar cycle 22, obtaining the contribution of each class to the whole Sun's EM(T). From the EM(T) distributions, we synthesize the X-ray spectra of the Sun and of the single classes of solar coronal regions as they would be collected with the ROSAT Position Sensitive Proportional Counter (PSPC) and ASCA Solid-State Imaging Spectrometer. We find that the Sun during the cycle fits well in the stellar scenario as a low-activity star. The ROSAT PSPC hardness ratio (HR) and surface X-ray flux, FPSPC, both increase going from the background corona to the active regions and the cores of the active regions, and range between the values of low and intermediate activity stars. We suggest that the coronae of these stars may be explained as the effect of structures similar to those present on the Sun and that the various levels of X-ray luminosity, HR, and FPSPC are achieved by changing the surface coverage of the different classes of coronal regions
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