2,730 research outputs found
Detailed diagnostics of an X-ray flare in the single giant HR 9024
We analyze a 96 ks Chandra/HETGS observation of the single G-type giant HR
9024. The high flux allows us to examine spectral line and continuum
diagnostics at high temporal resolution, to derive plasma parameters. A
time-dependent 1D hydrodynamic model of a loop with half-length cm (), cross-section radius
cm, with a heat pulse of 15 ks and ~erg cm s
deposited at the loop footpoints, satisfactorily reproduces the observed
evolution of temperature and emission measure, derived from the analysis of the
strong continuum emission. For the first time we can compare predictions from
the hydrodynamic model with single spectral features, other than with global
spectral properties. We find that the model closely matches the observed line
emission, especially for the hot ( K) plasma emission of the FeXXV
complex at \AA. The model loop has and aspect
ratio as typically derived for flares observed in active stellar
coronae, suggesting that the underlying physics is the same for these very
dynamic and extreme phenomena in stellar coronae independently on stellar
parameters and evolutionary stage.Comment: 26 pages. Accepted for publication on the Astrophysical Journa
X-ray flares in Orion young stars. I. Flare characteristics
Pre-main sequence (PMS) stars are known to produce powerful X-ray flares
which resemble magnetic reconnection solar flares scaled by factors up to 10^4.
However, numerous puzzles are present including the structure of X-ray emitting
coronae and magnetospheres, effects of protoplanetary disks, and effects of
stellar rotation. To investigate these issues in detail, we examine 216 of the
brightest flares from 161 PMS stars observed in the Chandra Orion Ultradeep
Project (COUP). These constitute the largest homogeneous dataset of PMS, or
indeed stellar flares at any stellar age, ever acquired. Our effort is based on
a new flare spectral analysis technique that avoids nonlinear parametric
modeling. It can be applied to much weaker flares and is more sensitive than
standard methods. We provide a catalog with >30 derived flare properties and an
electronic atlas for this unique collection of stellar X-ray flares. The
current study (Paper I) examines the flare morphologies, and provides general
comparison of COUP flare characteristics with those of other active X-ray stars
and the Sun. Paper II will concentrate on relationships between flare behavior,
protoplanetary disks, and other stellar properties. Several results are
obtained. First, the COUP flares studied here are among the most powerful,
longest, and hottest stellar X-ray flares ever studied. Second, no significant
statistical differences in peak flare luminosity or temperature distributions
are found among different morphological flare classes, suggesting a common
underlying mechanism for all flares. Third, comparison with the general
solar-scaling laws indicates that COUP flares may not fit adequately proposed
power-temperature and duration-temperature solar-stellar fits. Fourth, COUP
super-hot flares are found to be brighter but shorter than ... ABRIDGEDComment: Accepted for publication in ApJ (07/11/08); 63 pages, 16 figures, 4
table
Geometry Diagnostics of a Stellar Flare from Fluorescent X-rays
We present evidence of Fe fluorescent emission in the Chandra HETGS spectrum
of the single G-type giant HR 9024 during a large flare. In analogy to solar
X-ray observations, we interpret the observed Fe K line as being
produced by illumination of the photosphere by ionizing coronal X-rays, in
which case, for a given Fe photospheric abundance, its intensity depends on the
height of the X-ray source. The HETGS observations, together with 3D Monte
Carlo calculations to model the fluorescence emission, are used to obtain a
direct geometric constraint on the scale height of the flaring coronal plasma.
We compute the Fe fluorescent emission induced by the emission of a single
flaring coronal loop which well reproduces the observed X-ray temporal and
spectral properties according to a detailed hydrodynamic modeling. The
predicted Fe fluorescent emission is in good agreement with the observed value
within observational uncertainties, pointing to a scale height \rstar. Comparison of the HR 9024 flare with that recently observed on II
Peg by Swift indicates the latter is consistent with excitation by X-ray
photoionization.Comment: accepted for publication on the Astrophysical Journal Letter
The Soft X-ray Lightcurves of Partially Eclipsed Stellar Flares
Most stellar flares' soft X-ray lightcurves possess a `typical' morphology,
which consists of a rapid rise followed by a slow exponential decay. However, a
study of 216 of the brightest flares on 161 pre-main sequence stars, observed
during the Chandra Orion-Ultradeep Project (COUP), showed that many flare
lightcurves depart from this typical morphology. While this can be attributed
to the superposition of multiple typical flares, we explore the possibility
that the time-variable eclipsing of flares by their host stars may also be an
important factor. We assume each flare is contained within a single, uniform
plasma density magnetic loop and specify the intrinsic variation of the flare's
emission measure with time. We consider rotational eclipse by the star itself,
but also by circumstellar discs and flare-associated prominences. Based on this
simple model, we generate a set of flares similar to those observed in the COUP
database. Many eclipses simply reduce the flare's maximum emission measure or
decay time. We conclude therefore that eclipses often pass undetected, but
usually have only a modest influence on the flare emission measure profile and
hence the derived loop lengths. We show that eclipsing can easily reproduce the
observed atypical flare morphologies. The number of atypical modelled flare
morphologies is however much less than that found in the COUP sample. The large
number of observed atypical flare morphologies, therefore, must be attributed
to other processes such as multiple flaring loops.Comment: 11 pages, 9 figure
Oral platelet gel supernatant plus supportive medical treatment versus supportive medical treatment in the management of radiation-induced oral mucositis: a matched explorative active control trial by propensity analysis
OBJECTIVES:: In this active control trial, the rate of radio-induced WHO grade 3/4 oral mucositis and the change in quality of life, assessed by OMWQ-HN, were measured in subjects with head and neck cancer treated by platelet gel supernatant (PGS) and supportive medical treatment versus subjects treated by supportive medical treatment alone. MATERIALS AND METHODS:: Eighty patients with nonmetastatic head and neck cancer underwent curative or adjuvant radiotherapy. All patients underwent supportive medical treatment and/or PGS at the beginning and during radiotherapy. Sixteen patients received PGS in association with supportive medical treatment. To obtain 2 groups virtually randomized for important clinical characteristics subjects were matched, by propensity analysis, with a group of subjects (64 patients) treated with supportive medical treatment alone. RESULTS:: Subjects treated with standard supportive treatment experienced significant higher WHO grade 3/4 toxicity (55%; 35/64) than subjects treated by PGS (13%; 3/16). The reduced toxicity found in PGS group paralleled with the evidence that they developed later symptoms with respect to controls. The Cox proportional hazard model indicated that patients treated with standard supportive medical treatment experienced 2.7-fold increase (hazard ratio=2.7; 95% confidence interval, 1.3-5.7) in the occurrence of WHO grade 3/4 toxicity. PGS group significantly experienced higher quality of life than control groups as measured by OMWQ-HN. A significant decrease in the opioid analgesics usage was found in the PGS group. CONCLUSIONS:: These preliminary data should be interpreted with caution and could serve as a framework around which to design future trials
Magneto-Acoustic Waves of Small Amplitude in Optically Thin Quasi-Isentropic Plasmas
The evolution of quasi-isentropic magnetohydrodynamic waves of small but
finite amplitude in an optically thin plasma is analyzed. The plasma is assumed
to be initially homogeneous, in thermal equilibrium and with a straight and
homogeneous magnetic field frozen in. Depending on the particular form of the
heating/cooling function, the plasma may act as a dissipative or active medium
for magnetoacoustic waves, while Alfven waves are not directly affected. An
evolutionary equation for fast and slow magnetoacoustic waves in the single
wave limit, has been derived and solved, allowing us to analyse the wave
modification by competition of weakly nonlinear and quasi-isentropic effects.
It was shown that the sign of the quasi-isentropic term determines the scenario
of the evolution, either dissipative or active. In the dissipative case, when
the plasma is first order isentropically stable the magnetoacoustic waves are
damped and the time for shock wave formation is delayed. However, in the active
case when the plasma is isentropically overstable, the wave amplitude grows,
the strength of the shock increases and the breaking time decreases. The
magnitude of the above effects depends upon the angle between the wave vector
and the magnetic field. For hot (T > 10^4 K) atomic plasmas with solar
abundances either in the interstellar medium or in the solar atmosphere, as
well as for the cold (T < 10^3 K) ISM molecular gas, the range of temperature
where the plasma is isentropically unstable and the corresponding time and
length-scale for wave breaking have been found.Comment: 14 pages, 10 figures. To appear in ApJ January 200
X-Ray flares in Orion Young Stars. II. Flares, Magnetospheres, and Protoplanetary Disks
We study the properties of powerful X-ray flares from 161 pre-main sequence
(PMS) stars observed with the Chandra X-ray Observatory in the Orion Nebula
region. Relationships between flare properties, protoplanetary disks and
accretion are examined in detail to test models of star-disk interactions at
the inner edge of the accretion disks. Previous studies had found no
differences in flaring between diskfree and accreting systems other than a
small overall diminution of X-ray luminosity in accreting systems. The most
important finding is that X-ray coronal extents in fast-rotating diskfree stars
can significantly exceed the Keplerian corotation radius, whereas X-ray loop
sizes in disky and accreting systems do not exceed the corotation radius. This
is consistent with models of star-disk magnetic interaction where the inner
disk truncates and confines the PMS stellar magnetosphere. We also find two
differences between flares in accreting and diskfree PMS stars. First, a
subclass of super-hot flares with peak plasma temperatures exceeding 100 MK are
preferentially present in accreting systems. Second, we tentatively find that
accreting stars produce flares with shorter durations. Both results may be
consequences of the distortion and destabilization of the stellar magnetosphere
by the interacting disk. Finally, we find no evidence that any flare types,
even slow-rise flat-top flares are produced in star-disk magnetic loops. All
are consistent with enhanced solar long-duration events with both footprints
anchored in the stellar surface.Comment: Accepted for publication in ApJ (07/17/08); 46 pages, 14 figures, 2
table
Heptagon-Containing Nanographene Embedded into [10]Cycloparaphenylene
We report the synthesis and characterization of a novel type of nanohoop, consisting of a cycloparaphenylene derivative incorporating a curved heptagon-containing π-extended polycyclic aromatic hydrocarbon (PAH) unit. We demonstrate that this new macrocycle behaves as a supramolecular receptor of curved π-systems such as fullerenes C60 and C70, with remarkably large binding constants (ca. 107 M−1), as estimated by fluorescence measurements. Nanosecond and femtosecond spectroscopic analysis show that these host-guest complexes are capable of quasi-instantaneous charge separation upon photoexcitation, due to the ultrafast charge transfer from the macrocycle to the complexed fullerene. These results demonstrate saddle-shaped PAHs with dibenzocycloheptatrienone motifs as structural components for new macrocycles displaying molecular receptor abilities and versatile photochemical responses with promising electron-donor properties in host-guest complexes
On the origin of 140 GHz emission from the 4 July 2012 solar flare
The sub-THz event observed on the 4 July 2012 with the Bauman Moscow State
Technical University Radio Telescope RT-7.5 at 93 and 140~GHz as well as
Kislovodsk and Mets\"ahovi radio telescopes, Radio Solar Telescope Network
(RSTN), GOES, RHESSI, and SDO orbital stations is analyzed. The spectral flux
between 93 and 140 GHz has been observed increasing with frequency. On the
basis of the SDO/AIA data the differential emission measure has been
calculated. It is shown that the thermal coronal plasma with the temperature
above 0.5~MK cannot be responsible for the observed sub-THz flare emission. The
non-thermal gyrosynchrotron mechanism can be responsible for the microwave
emission near ~GHz but the observed millimeter spectral characteristics are
likely to be produced by the thermal bremsstrahlung emission from plasma with a
temperature of about 0.1~MK.Comment: 18 pages, 6 figure
- …