7,613 research outputs found
Magnetic games between a planet and its host star: the key role of topology
Magnetic interactions between a star and a close-in planet are postulated to
be a source of enhanced emissions and to play a role in the secular evolution
of the orbital system. Close-in planets generally orbit in the sub-alfv\'enic
region of the stellar wind, which leads to efficient transfers of energy and
angular momentum between the star and the planet. We model the magnetic
interactions occurring in close-in star-planet systems with three-dimensional,
global, compressible magneto-hydrodynamic numerical simulations of a planet
orbiting in a self-consistent stellar wind. We focus on the cases of magnetized
planets and explore three representative magnetic configurations. The Poynting
flux originating from the magnetic interactions is an energy source for
enhanced emissions in star-planet systems. Our results suggest a simple
geometrical explanation for ubiquitous on/off enhanced emissions associated
with close-in planets, and confirm that the Poynting fluxes can reach powers of
the order of W. Close-in planets are also showed to migrate due to
magnetic torques for sufficiently strong stellar wind magnetic fields. The
topology of the interaction significantly modifies the shape of the magnetic
obstacle that leads to magnetic torques. As a consequence, the torques can vary
by at least an order of magnitude as the magnetic topology of the interaction
varies.Comment: 15 pages, 6 figures, accepted for publication in The Astrophysical
Journa
Galaxy Selection and Clustering and Lyman alpha Absorber Identification
The effects of galaxy selection on our ability to constrain the nature of
weak Ly\alpha absorbers at low redshift are explored. Current observations
indicate the existence of a population of gas-rich, low surface brightness
(LSB) galaxies, and these galaxies may have large cross sections for Ly\alpha
absorption. Absorption arising in LSB galaxies may be attributed to HSB
galaxies at larger impact parameters from quasar lines of sight, so that the
observed absorption cross sections of galaxies may seem unreasonably large.
Thus it is not possible to rule out scenarios where LSB galaxies make
substantial contributions to Ly\alpha absorption using direct observations.
Less direct tests, where observational selection effects are taken into account
using simulations, should make it possible to determine the nature of Ly\alpha
absorbers by observing a sample of ~100 galaxies around quasar lines of sight
with well-defined selection criteria. Such tests, which involve comparing
simulated and observed plots of the unidentified absorber fractions and
absorbing galaxy fractions versus impact parameter, can distinguish between
scenarios where absorbers arise in particular galaxies and those where
absorbers arise in gas tracing the large scale galaxy distribution. Care must
be taken to minimize selection effects even when using these tests. Results
from such tests are likely to depend upon the limiting neutral hydrogen column
density. While not enough data are currently available to make a strong
conclusion about the nature of moderately weak absorbers, evidence is seen that
such absorbers arise in gas that is around or between galaxies that are often
not detected in surveys.Comment: 15 pages, 10 figures, accepted to the Astrophysical Journa
Damped Lyman Alpha Systems at z<1.65: The Expanded SDSS HST Sample
We present results of our HST Cycle 11 Survey for low-redshift (z<1.65) DLAs
in the UV spectra of quasars selected from the SDSS Early Data Release. These
quasars have strong intervening MgII-FeII systems which are known signatures of
high column density neutral gas. In total, UV observations of Ly-alpha
absorption in 197 MgII systems with z<1.65 and rest equivalent width (REW)
W2796 \ge 0.3A have now been obtained. The main results are: (1) 36(+/- 6)% of
systems with W2796 \ge 0.5 A and FeII W2600 \ge 0.5 A are DLAs. This increases
to 42(+/- 7)% for systems with W2796/W2600 0.1 A. (2) The
mean N(HI) of MgII systems with 0.3 A \le W2796 < 0.6 A is a factor of ~36
lower than that of systems with W2796 \ge 0.6 A. (3) The DLA incidence per unit
redshift is consistent with no evolution for z <~ 2 (Omega_L=0.7, Omega_M =
0.3), but exhibits significant evolution for z >~ 2. (4) Omega_{DLA} is
constant for 0.5<z<5.0 to within the uncertainties. This is larger than
Omega_{gas}(z=0) by a factor of ~2. (5) The slope of the N(HI) distribution
does not change significantly with redshift. However, the low redshift
distribution is marginally flatter due to the higher fraction of high N(HI)
systems in our sample. (6) Finally, using the precision of MgII survey
statistics, we find that there may be evidence of a decreasing Omega_{DLA} from
z=0.5 to z=0. We reiterate the conclusion of Hopkins, Rao, & Turnshek that very
high columns of neutral gas might be missed by DLA surveys because of their
very small cross sections, and therefore, that Omega_{DLA} might not include
the bulk of the neutral gas mass in the Universe. (Abridged)Comment: Accepted for publication in ApJ. 22 pages, 22 figure
Constraints on the Lyman continuum radiation from galaxies: first results with FUSE on Mrk 54
We present Far Ultraviolet Spectroscopic Explorer observations of the
star-forming galaxy Mrk 54 at z = 0.0448. The Lyman continuum radiation is not
detected above the HI absorption edge in our Galaxy. An upper limit is
evaluated by comparison with the background measured in regions of the detector
adjacent to the observed spectrum. A spectral window of 16 A, reasonably free
of additional HI Lyman series line absorption is used. No correction is needed
for molecular hydrogen absorption in our Galaxy but a foreground extinction of
0.29 mag is accounted for. An upper limit of 6.15 10^{-16} erg/cm^2/s/A is
obtained for the flux at ~ 900 A in the rest frame of Mrk 54. By comparison
with the number of ionizing photons derived from the H-alpha flux, this limit
translates into an upper limit of f_esc < 0.062 for the fraction of Lyman
continuum photons that escape the galaxy without being absorbed by interstellar
material. This limit compares with the limits obtained in three other nearby
galaxies and is compatible with the escape fractions predicted by models.
The upper limits obtained in nearby galaxies contrasts with the detection of
Lyman continuum flux in the composite spectrum of Lyman-break galaxies at z ~
3.4. The difficulties and implications of a comparison are discussed.Comment: 9 pages, 3 figures, accepted for publication in A&A include aa.cls
v5.0
Proximity effect between two superconductors spatially resolved by scanning tunneling spectroscopy
We present a combined experimental and theoretical study of the proximity
effect in an atomic-scale controlled junction between two different
superconductors. Elaborated on a Si(111) surface, the junction comprises a Pb
nanocrystal with an energy gap of 1.2 meV, connected to a crystalline atomic
monolayer of lead with a gap of 0.23 meV. Using in situ scanning tunneling
spectroscopy we probe the local density of states of this hybrid system both in
space and in energy, at temperatures below and above the critical temperature
of the superconducting monolayer. Direct and inverse proximity effects are
revealed with high resolution. Our observations are precisely explained with
the help of a self-consistent solution of the Usadel equations. In particular,
our results demonstrate that in the vicinity of the Pb islands, the Pb
monolayer locally develops a finite proximity-induced superconducting order
parameter, well above its own bulk critical temperature. This leads to a giant
proximity effect where the superconducting correlations penetrate inside the
monolayer a distance much larger than in a non-superconducting metal.Comment: 13 pages, 14 figures, accepted for publication in Physical Review
Preliminary Results on gamma gamma -> Ks K pi from CLEO
We analyzed 13.8 fb^{-1} of the integrated e+e- luminosity collected at 10.6
GeV center-of-mass energy with the CLEO II and II.V detectors to study
exclusive two-photon production of single hadronic resonances. We searched for
hadrons decaying into Ks K pi when both leptons remain undetected. In this
analysis we studied the detection efficiency and evaluated systematic errors
using independent data samples. We estimated 90% CL upper limits on the
products of the two-photon partial widths of (pseudo)scalar hadrons with masses
below 1.7 GeV/c2 and their branching fractions into Ks K pi. Our preliminary
results are marginally consistent with the first observation of eta(1440) in
two-photon collisions by the L3 experiment.Comment: 4 pages, 1 figure, proceedings contribution for PANIC'0
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