26,785 research outputs found
On the 2:1 Orbital Resonance in the HD 82943 Planetary System
We present an analysis of the HD 82943 planetary system based on a radial
velocity data set that combines new measurements obtained with the Keck
telescope and the CORALIE measurements published in graphical form. We examine
simultaneously the goodness of fit and the dynamical properties of the best-fit
double-Keplerian model as a function of the poorly constrained eccentricity and
argument of periapse of the outer planet's orbit. The fit with the minimum
chi_{nu}^2 is dynamically unstable if the orbits are assumed to be coplanar.
However, the minimum is relatively shallow, and there is a wide range of fits
outside the minimum with reasonable chi_{nu}^2. For an assumed coplanar
inclination i = 30 deg. (sin i = 0.5), only good fits with both of the lowest
order, eccentricity-type mean-motion resonance variables at the 2:1
commensurability, theta_1 and theta_2, librating about 0 deg. are stable. For
sin i = 1, there are also some good fits with only theta_1 (involving the inner
planet's periapse longitude) librating that are stable for at least 10^8 years.
The libration semiamplitudes are about 6 deg. for theta_1 and 10 deg. for
theta_2 for the stable good fit with the smallest libration amplitudes of both
theta_1 and theta_2. We do not find any good fits that are non-resonant and
stable. Thus the two planets in the HD 82943 system are almost certainly in 2:1
mean-motion resonance, with at least theta_1 librating, and the observations
may even be consistent with small-amplitude librations of both theta_1 and
theta_2.Comment: 24 pages, including 10 figures; accepted for publication in Ap
Quantitative spectroscopy of extreme helium stars - Model atmospheres and a non-LTE abundance analysis of BD+102179?
Extreme helium stars (EHe stars) are hydrogen-deficient supergiants of
spectral type A and B. They are believed to result from mergers in double
degenerate systems. In this paper we present a detailed quantitative non-LTE
spectral analysis for BD+102179, a prototype of this rare class of
stars, using UVES and FEROS spectra covering the range from 3100 to 10
000 {\AA}. Atmosphere model computations were improved in two ways. First,
since the UV metal line blanketing has a strong impact on the
temperature-density stratification, we used the Atlas12 code. Additionally, We
tested Atlas12 against the benchmark code Sterne3, and found only small
differences in the temperature and density stratifications, and good agreement
with the spectral energy distributions. Second, 12 chemical species were
treated in non-LTE. Pronounced non-LTE effects occur in individual spectral
lines but, for the majority, the effects are moderate to small. The
spectroscopic parameters give = 17 300300 K and
= 2.800.10, and an evolutionary mass of 0.550.05 . The star
is thus slightly hotter, more compact and less massive than found in previous
studies. The kinematic properties imply a thick-disk membership, which is
consistent with the metallicity Fe/H and -enhancement.
The refined light-element abundances are consistent with the white dwarf merger
scenario. We further discuss the observed helium spectrum in an appendix,
detecting dipole-allowed transitions from about 150 multiplets plus the most
comprehensive set of known/predicted isolated forbidden components to date.
Moreover, a so far unreported series of pronounced forbidden He I components is
detected in the optical-UV.Comment: Accepted for publication in MNRAS, 26 pages, 19 Figure
The Mid-infrared Fine-structure Lines of Neon as an Indicator of Star For mation Rate in Galaxies
The fine-structure lines of singly ([Ne II] 12.8 micron) and doubly ([Ne III]
15.6 micron) ionized neon are among the most prominent features in the
mid-infrared spectra of star-forming regions, and have the potential to be a
powerful new indicator of the star formation rate in galaxies. Using a sample
of star-forming galaxies with measurements of the fine-structure lines
available from the literature, we show that the sum of the [Ne II] and [Ne III]
luminosities obeys a tight, linear correlation with the total infrared
luminosity, over 5 orders of magnitude in luminosity. We discuss the formation
of the lines and their relation with the Lyman continuum luminosity. A simple
calibration between star formation rate and the [Ne II]+[Ne III] luminosity is
presented.Comment: To appear in ApJ. 8 page
Atomic data for neutron-capture elements III. Charge transfer rate coefficients for low-charge ions of Ge, Se, Br, Kr, Rb, and Xe
We present total and final-state resolved charge transfer (CT) rate
coefficients for low-charge Ge, Se, Br, Kr, Rb, and Xe ions reacting with
neutral hydrogen over the temperature range 10^2--10^6 K. Each of these
elements has been detected in ionized astrophysical nebulae, particularly
planetary nebulae. CT rate coefficients are a key ingredient for the ionization
equilibrium solutions needed to determine total elemental abundances from those
of the observed ions. A multi-channel Landau Zener approach was used to compute
rate coefficients for projectile ions with charges q=2-5, and for
singly-charged ions the Demkov approximation was utilized. Our results for
five-times ionized species are lower limits, due to the incompleteness of level
energies in the NIST database. In addition, we computed rate coefficients for
charge transfer ionization reactions between the neutral species of the above
six elements and ionized hydrogen. The resulting total and state-resolved CT
rate coefficients are tabulated and available at the CDS. In tandem with our
concurrent investigations of other important atomic processes in photoionized
nebulae, this work will enable robust investigations of neutron-capture element
abundances and nucleosynthesis via nebular spectroscopy.Comment: 11 pages, 4 figures, accepted for publication in Astronomy &
Astrophysic
Labeling Schemes for Bounded Degree Graphs
We investigate adjacency labeling schemes for graphs of bounded degree
. In particular, we present an optimal (up to an additive
constant) adjacency labeling scheme for bounded degree trees.
The latter scheme is derived from a labeling scheme for bounded degree
outerplanar graphs. Our results complement a similar bound recently obtained
for bounded depth trees [Fraigniaud and Korman, SODA 10], and may provide new
insights for closing the long standing gap for adjacency in trees [Alstrup and
Rauhe, FOCS 02]. We also provide improved labeling schemes for bounded degree
planar graphs. Finally, we use combinatorial number systems and present an
improved adjacency labeling schemes for graphs of bounded degree with
Throwing shoes...
What is the structure of the social? If we accept organismic metaphors, the social is analogous to the body, usually the human body. The foot/shoe is the most basic foundation, the ground upon which the rest of the socialbody rests. By standing on two feet, the hands are freed to become tool making and using appendages, and the mouth is thereby freed from carrying to bear words instead. In structural terms, the foot/shoe functions as base to the face?s superstructure. But when a shoe is thrown at the face of power, a double inversion comes into play. On the one hand, the base rises up to strike directly at the superstructure and to challenge materially the basis of its legitimacy. On the other hand, the most basic sign of development and civilization ? the shoe ? is removed from the bare foot. As well as turning the foot?s prosthesis into a projectile weapon, this move symbolically reaffirms the body against a becoming-face or a becoming-technology. This moment of unshod insubordination asserts a basic, naked, human dignity in the face of dehumanization. These themes of insubordination and rehumanization, structure and ideology, run through the various contributions to this issue of ephemer
Diagnosing hydrological limitations of a land surface model: application of JULES to a deep-groundwater chalk basin
Land surface models (LSMs) are prospective starting points to develop a global hyper-resolution model of the terrestrial water, energy, and biogeochemical cycles. However, there are some fundamental limitations of LSMs related to how meaningfully hydrological fluxes and stores are represented. A diagnostic approach to model evaluation and improvement is taken here that exploits hydrological expert knowledge to detect LSM inadequacies through consideration of the major behavioural functions of a hydrological system: overall water balance, vertical water redistribution in the unsaturated zone, temporal water redistribution, and spatial water redistribution over the catchment's groundwater and surface-water systems. Three types of information are utilized to improve the model's hydrology: (a) observations, (b) information about expected response from regionalized data, and (c) information from an independent physics-based model. The study considers the JULES (Joint UK Land Environmental Simulator) LSM applied to a deep-groundwater chalk catchment in the UK. The diagnosed hydrological limitations and the proposed ways to address them are indicative of the challenges faced while transitioning to a global high resolution model of the water cycle
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