11,335 research outputs found
X-rays from Saturn: A study with XMM-Newton and Chandra over the years 2002-05
We present the results of the two most recent (2005) XMM-Newton observations
of Saturn together with the re-analysis of an earlier (2002) observation from
the XMM-Newton archive and of three Chandra observations in 2003 and 2004.
While the XMM-Newton telescope resolution does not enable us to resolve
spatially the contributions of the planet's disk and rings to the X-ray flux,
we can estimate their strengths and their evolution over the years from
spectral analysis, and compare them with those observed with Chandra. The
spectrum of the X-ray emission is well fitted by an optically thin coronal
model with an average temperature of 0.5 keV. The addition of a fluorescent
oxygen emission line at ~0.53 keV improves the fits significantly. In
accordance with earlier reports, we interpret the coronal component as emission
from the planetary disk, produced by the scattering of solar X-rays in Saturn's
upper atmosphere, and the line as originating from the Saturnian rings. The
strength of the disk X-ray emission is seen to decrease over the period 2002 -
2005, following the decay of solar activity towards the current minimum in the
solar cycle. By comparing the relative fluxes of the disk X-ray emission and
the oxygen line, we suggest that the line strength does not vary over the years
in the same fashion as the disk flux. We consider possible alternatives for the
origin of the line. The connection between solar activity and the strength of
Saturn's disk X-ray emission is investigated and compared with that of Jupiter.
We also discuss the apparent lack of X-ray aurorae on Saturn and conclude that
they are likely to lie below the sensitivity threshold of current Earth-bound
observatories. A similar comparison for Uranus and Neptune leads to the same
disappointing conclusion.Comment: 10 pages, 5 figures; to be published in 'Astronomy and Astrophysics
Carrier Transport in Magnesium Diboride: Role of Nano-inclusions
Anisotropic-gap and two-band effects smear out the superconducting transition
(Tc) in literature reported thermal conductivity of MgB2, where large
electronic contributions also suppress anomaly-manifestation in their
negligible phononic-parts. Present thermal transport results on scarcely
explored specimens featuring nano-inclusions exhibit a small but clear
Tc-signature, traced to relatively appreciable phononic conduction, and its
dominant electronic-scattering. The self-formed MgO as extended defects
strongly scatter the charge carriers and minutely the phonons with their
longer-mean-free-path near Tc. Conversely, near room temperature, the
shorter-dominant-wavelength phonon's transport is hugely affected by these
nanoparticles, undergoing ballistic to diffusive crossover and eventually
entering the Ioffe-Regel mobility threshold regime.Comment: 14 pages, 4 figures, 28 reference
The Role of Opacities in Stellar Pulsation
We examine the role of opacities in stellar pulsation with reference to
Cepheids and RR Lyraes, and examine the effect of augmented opacities on the
theoretical pulsation light curves in key temperature ranges. The temperature
ranges are provided by recent experimental and theoretical work that have
suggested that the iron opacities have been considerably underestimated. For
Cepheids, we find that the augmented opacities have noticeable effects in
certain period ranges (around ) even though there is a
degeneracy with mixing length. We also find significant effects in theoretical
models of B-star pulsators.Comment: 6 pages, 3 Figures, Proceeding for the "Workshop on Astrophysical
Opacities
Forbidden oxygen lines at various nucleocentric distances in comets
To study the formation of the [OI] lines - i.e., 5577 A (the green line),
6300 A and 6364 A (the two red lines) - in the coma of comets and to determine
the parent species of the oxygen atoms using the green to red-doublet emission
intensity ratio (G/R ratio) and the lines velocity widths. We acquired at the
ESO VLT high-resolution spectroscopic observations of comets C/2002 T7
(LINEAR), 73P-C/Schwassmann-Wachmann 3, 8P/Tuttle, and, 103P/Hartley 2 when
they were close to the Earth (< 0.6 au). Using the observed spectra, we
determined the intensities and the widths of the three [OI] lines. We have
spatially extracted the spectra in order to achieve the best possible
resolution of about 1-2", i.e., nucleocentric projected distances of 100 to 400
km depending on the geocentric distance of the comet. We have decontaminated
the [OI] green line from C2 lines blends. It is found that the observed G/R
ratio on all four comets varies as a function of nucleocentric projected
distance. This is mainly due to the collisional quenching of O(1S) and O(1D) by
water molecules in the inner coma. The observed green emission line width is
about 2.5 km/s and decreases as the distance from the nucleus increases which
can be explained by the varying contribution of CO2 to the O(1S) production in
the innermost coma. The photodissociation of CO2 molecules seems to produce
O(1S) closer to the nucleus while the water molecule forms all the O(1S) and
O(1D) atoms beyond 1000 km. Thus we conclude that the main parent species
producing O(1S) and O(1D) in the inner coma is not always the same. The
observations have been interpreted in the framework of the
coupled-chemistry-emission model of Bhardwaj & Raghuram (2012) and the upper
limits of CO2 relative abundances are derived from the observed G/R ratios.
Measuring the [OI] lines could indeed provide a new way to determine the CO2
relative abundance in comets.Comment: accepted for publication in A&A, the abstract is shortene
Chandra Observation of an X-ray Flare at Saturn: Evidence for Direct Solar Control on Saturn's Disk X-ray Emissions
Saturn was observed by Chandra ACIS-S on 20 and 26-27 January 2004 for one
full Saturn rotation (10.7 hr) at each epoch. We report here the first
observation of an X-ray flare from Saturn's non-auroral (low-latitude) disk,
which is seen in direct response to an M6-class flare emanating from a sunspot
that was clearly visible from both Saturn and Earth. Saturn's disk X-ray
emissions are found to be variable on time scales of hours to weeks to months,
and correlated with solar F10.7 cm flux. Unlike Jupiter, X-rays from Saturn's
polar (auroral) region have characteristics similar to those from its disk.
This report, combined with earlier studies, establishes that disk X-ray
emissions of the giant planets Saturn and Jupiter are directly regulated by
processes happening on the Sun. We suggest that these emissions could be
monitored to study X-ray flaring from solar active regions when they are on the
far side and not visible to Near-Earth space weather satellites.Comment: Total 12 pages including 4 figure
I0-Jupiter system: A unique case of Moon-Planet interaction
Io and Jupiter constitute a moon-planet system that is unique in our solar
system. Io is the most volcanically active planetary body, while Jupiter is the
first among the planets in terms of size, mass, magnetic field strength, spin
rate, and volume of the magnetosphere. That Io is electrodynamically linked to
Jupiter is known for nearly four decades from the radio emissions. Io
influences Jupiter by supplying heavy ions to its magnetosphere, which
dominates its energetic and dynamics. Jupiter influences Io by tidally heating
its interior, which in turn drives the volcanic activity on Io. The role of Io
and Jupiter in their mutual interaction and the nature of their coupling were
first elaborated in greater detail by the two Voyagers flybys in 1979.
Subsequent exploration of this system by ground-based and Earth-satellite-borne
observatories and by the Galileo orbiter mission has improved our understanding
of the highly complex electrodynamical interaction between Io and Jupiter many
fold. A distinct feature of this interaction has been discovered in Jupiter's
atmosphere as a auroral-like bright emission spot along with a comet-like tail
in infrared (IR), ultraviolet (UV), and visible wavelengths at the foot of Io
flux tube (IFT). The HST and Galileo and Cassini imagining experiments have
observed emissions from the Io's atmosphere at UV and visible wavelengths,
which could be produced by energetic electrons in IFT. In this paper an
overview on these aspects of the Io-Jupiter system is presented, which by
virtue of its electrodynamical coupling, has implications for the extra-solar
planetary system and binary stars.Comment: 7 pages, 4 figures, Proc.36th ESLAB Symposium "Earth-Like Planets and
Moons", June 3-7, 2002, ESTEC, The Netherlands, ESA SP-514, Ed. Bruce
Battric
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