3,315 research outputs found
A Side of Mercury Not Seen By Mariner 10
More than 60,000 images of Mercury were taken at ~29 deg elevation during two
sunrises, at 820 nm, and through a 1.35 m diameter off-axis aperture on the
SOAR telescope. The sharpest resolve 0.2" (140 km) and cover 190-300 deg
longitude -- a swath unseen by the Mariner 10 spacecraft -- at complementary
phase angles to previous ground-based optical imagery. Our view is comparable
to that of the Moon through weak binoculars. Evident are the large crater
Mozart shadowed on the terminator, fresh rayed craters, and other albedo
features keyed to topography and radar reflectivity, including the putative
huge ``Basin S'' on the limb. Classical bright feature Liguria resolves across
the northwest boundary of the Caloris basin into a bright splotch centered on a
sharp, 20 km diameter radar crater, and is the brightest feature within a
prominent darker ``cap'' (Hermean feature Solitudo Phoenicis) that covers the
northern hemisphere between longitudes 140-250 deg. The cap may result from
space weathering that darkens via a magnetically enhanced flux of the solar
wind, or that reddens low latitudes via high solar insolation.Comment: 7 pages, 4 PDF figures, pdfLaTeX, typos corrected, Fig. 2 modified
slightly to add crater diameters not given in published versio
Two-body effects in the decay rate of atomic levels
Recoil corrections to the atomic decay rate are considered in the order of
Zm/M . The expressions are treated exactly without any expansion over Z alpha.
The expressions obtained are valid both for muonic atoms (for which they
contribute on the level of a few percent in high Z ions) and for electronic
atoms. Explicit results for Lyman-alpha transitions for low-Z of the order
(Zm/M)(Z alpha)^2 are also presented.Comment: 5 pages, 1 table, email: [email protected]
Stellar Dynamics and the implications on the merger evolution in NGC6240
We report near-infrared integral field spectroscopy of the luminous merging
galaxy NGC 6240. Stellar velocities show that the two K-band peaks separated by
1.6arcsec are the central parts of inclined, rotating disk galaxies with equal
mass bulges. The dynamical masses of the nuclei are much larger than the
stellar mass derived from the K-band light, implying that the progenitor
galaxies were galaxies with massive bulges. The K-band light is dominated by
red supergiants formed in the two nuclei in starbursts, triggered ~2x10^7 years
ago, possibly by the most recent perigalactic approach. Strong feedback effects
of a superwind and supernovae are responsible for a short duration burst
(~5x10^6 years) which is already decaying. The two galaxies form a
prograde-retrograde rotating system and from the stellar velocity field it
seems that one of the two interacting galaxies is subject to a prograde
encounter. Between the stellar nuclei is a prominent peak of molecular gas
(H_2, CO). The stellar velocity dispersion peaks there indicating that the gas
has formed a local, self-gravitating concentration decoupled from the stellar
gravitational potential. NGC 6240 has previously been reported to fit the
paradigm of an elliptical galaxy formed through the merger of two galaxies.
This was based on the near-infrared light distribution which follows a
r^1/4-law. Our data cast strong doubt on this conclusion: the system is by far
not relaxed, rotation plays an important role, as does self-gravitating gas,
and the near-infrared light is dominated by young stars.Comment: 34 pages, 11 figures, using AASTEX 5.0rc3.1, paper submitted to the
Astrophysical Journal, revised versio
Dressed test particles, oscillation centres and pseudo-orbits
A general semi-analytical method for accurate and efficient numerical
calculation of the dielectrically screened ("dressed") potential around a
non-relativistic test particle moving in an isotropic, collisionless,
unmagnetised plasma is presented. The method requires no approximations and is
illustrated using results calculated for two cases taken from the MSc thesis of
the first author: test particles with velocities above and below the ion sound
speed in plasmas with Maxwellian ions and warm electrons. The idea that the
fluctuation spectrum of a plasma can be described as a superposition of the
fields around \emph{non-interacting} dressed test particles is an expression of
the quasiparticle concept, which has also been expressed in the development of
the oscillation-centre and pseudo-orbit formalisms.Comment: 14 pages to Plasma Physics and Controlled Fusion for publication with
a cluster of papers associated with workshop Stability and Nonlinear Dynamics
of Plasmas, October 31, 2009 Atlanta, GA on occasion of the 65th birthday of
R.L. Dewar. Version 2: Reference [27] added in Sec. 5. Version 3: Revised in
response to referee
The optically-powerful quasar E1821+643 is associated with a 300-kpc scale FRI radio structure
We present a deep image of the optically-powerful quasar E1821+643 at 18cm
made with the Very Large Array (VLA). This image reveals radio emission, over
280 kpc in extent, elongated way beyond the quasar's host galaxy. Its radio
structure has decreasing surface brightness with increasing distance from the
bright core, characteristic of FRI sources (Fanaroff & Riley 1974). Its radio
luminosity at 5GHz falls in the classification for `radio-quiet' quasars (it is
only 10^23.9 W/Hz/sr; see e.g. Kellermann et al 1994). Its radio luminosity at
151MHz (which is 10^25.3 W/Hz/sr) is at the transition luminosity observed to
separate FRIs and FRIIs. Hitherto, no optically-powerful quasar had been found
to have a conventional FRI radio structure. For searches at low-frequency this
is unsurprising given current sensitivity and plausible radio spectral indices
for radio-quiet quasars. We demonstrate the inevitability of the extent of any
FRqI radio structures being seriously under-estimated by existing targetted
follow-up observations of other optically-selected quasars, which are typically
short exposures of z > 0.3 objects, and discuss the implications for the
purported radio bimodality in quasars.
The nature of the inner arcsec-scale jet in E1821+643, together with its
large-scale radio structure, suggest that the jet-axis in this quasar is
precessing (cf. Galactic jet sources such as SS433). A possible explanation for
this is that its central engine is a binary whose black holes have yet to
coalesce. The ubiquity of precession in `radio-quiet' quasars, perhaps as a
means of reducing the observable radio luminosity expected in highly-accreting
systems, remains to be established.Comment: Accepted by ApJ Letters; higher quality versions of figures available
at http://www-astro.physics.ox.ac.uk/~km
Obliquely propagating electromagnetic waves in magnetized kappa plasmas
Velocity distribution functions (VDFs) that exhibit a power-law dependence on
the high-energy tail have been the subject of intense research by the plasma
physics community. Such functions, known as kappa or superthermal
distributions, have been found to provide a better fitting to the VDFs measured
by spacecraft in the solar wind. One of the problems that is being addressed on
this new light is the temperature anisotropy of solar wind protons and
electrons. In the literature, the general treatment for waves excited by
(bi-)Maxwellian plasmas is well-established. However, for kappa distributions,
the wave characteristics have been studied mostly for the limiting cases of
purely parallel or perpendicular propagation, relative to the ambient magnetic
field. Contributions to the general case of obliquely-propagating
electromagnetic waves have been scarcely reported so far. The absence of a
general treatment prevents a complete analysis of the wave-particle interaction
in kappa plasmas, since some instabilities can operate simultaneously both in
the parallel and oblique directions. In a recent work, Gaelzer and Ziebell [J.
Geophys. Res. 119, 9334 (2014)] obtained expressions for the dielectric tensor
and dispersion relations for the low-frequency, quasi-perpendicular dispersive
Alfv\'en waves resulting from a kappa VDF. In the present work, the formalism
introduced by Ref. 1 is generalized for the general case of electrostatic
and/or electromagnetic waves propagating in a kappa plasma in any frequency
range and for arbitrary angles. An isotropic distribution is considered, but
the methods used here can be easily applied to more general anisotropic
distributions, such as the bi-kappa or product-bi-kappa.Comment: Accepted for publication in Physics of Plasmas; added references for
section
A Puzzling X-Ray Source Found in the chandra Deep Field South
In this letter we report the detection of an extremely strong X-ray emission
line in the 940ks chandra ACIS-I spectrum of CXO CDFS J033225.3-274219. The
source was identified as a Type1 AGN at redshift of z = 1.617, with 2.0 -- 10.0
keV rest frame X-ray luminosity of ~ 10^44 ergs s^-1. The emission line was
detected at 6.2^{+0.2}_{-0.1} keV, with an equivalent width (EW) of
4.4^{+3.2}_{-1.4} keV, both quantities referring to the observed frame. In the
rest frame, the line is at 16.2^{+0.4}_{-0.3} keV with an EW of
11.5^{+8.3}_{-3.7} keV. An X-ray emission line at similar energy (~ 17 keV,
rest frame) in QSO PKS 2149-306 was discovered before using ASCA data. We
reject the possibility that the line is due to a statistical or instrumental
artifact. The line is most likely due to blueshifted Fe-K emission from an
relativistic outflow, probably an inner X-ray jet, with velocities of the order
of ~ 0.6-0.7c. Other possible explanations are also discussed
Hemophilic Siblings With Chronic Hepatitis C: Familial Aggregation of Spontaneous and Treatment-Related Viral Clearance
Hemophilic siblings provide a unique population to explore the natural history of chronic hepatitis C
Gas Dynamics in the Luminous Merger NGC 6240
We report 0.5"x0.9" resolution, interferometric observations of the 1.3 mm CO
J=2-1 line in the infrared luminous galactic merger NGC 6240. About half of the
CO flux is concentrated in a rotating but highly turbulent, thick disk
structure centered between the two radio and near-infrared nuclei. A number of
gas features connect this ~500 pc diameter central disk to larger scales.
Throughout this region the molecular gas has local velocity widths which exceed
300 km/s FWHM and even reach FWZP line widths of 1000 km/s in a number of
directions. The mass of the central gas concentration constitutes a significant
fraction of the dynamical mass, M_gas(R<470 pc) ~ 2-4x10^9 M_o ~ 0.3-0.7 M_dyn.
We conclude that NGC 6240 is in an earlier merging stage than the prototypical
ultraluminous galaxy, Arp 220. The interstellar gas in NGC 6240 is in the
process of settling between the two progenitor stellar nuclei, is dissipating
rapidly and will likely form a central thin disk. In the next merger stage, NGC
6240 may well experience a major starburst like that observed in Arp 220.Comment: To be published in Ap.J.; 7 figure
On the segmentation of astronomical images via level-set methods
Astronomical images are of crucial importance for astronomers since they
contain a lot of information about celestial bodies that can not be directly
accessible. Most of the information available for the analysis of these objects
starts with sky explorations via telescopes and satellites. Unfortunately, the
quality of astronomical images is usually very low with respect to other real
images and this is due to technical and physical features related to their
acquisition process. This increases the percentage of noise and makes more
difficult to use directly standard segmentation methods on the original image.
In this work we will describe how to process astronomical images in two steps:
in the first step we improve the image quality by a rescaling of light
intensity whereas in the second step we apply level-set methods to identify the
objects. Several experiments will show the effectiveness of this procedure and
the results obtained via various discretization techniques for level-set
equations.Comment: 24 pages, 59 figures, paper submitte
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