4,888 research outputs found
A Lucky Imaging search for stellar sources near 74 transit hosts
Many transiting planet host stars lack high resolution imaging and thus close
stellar sources can be missed. Those unknown stars potentially bias the
derivation of the planetary and stellar parameters from the transit light
curve, no matter if they are bound or not. In addition, bound stellar
companions interact gravitationally with the exoplanet host star, the disk and
the planets and can thus influence the formation and evolution of the planetary
system strongly. We extended our high-resolution Lucky Imaging survey for close
stellar sources by 74 transiting planet host stars. 39 of these stars lack
previous high-resolution imaging, 23 are follow up observations of companions
or companion candidates, and the remaining stars have been observed by others
with AO imaging though in different bands. We determine the separation of all
new and known companion candidates and estimate the flux ratio in the observed
bands. All observations were carried out with the Lucky Imaging camera AstraLux
Norte at the Calar Alto 2.2 m telescope in i' and z' passbands. We find new
stellar sources within 1 arcsec to HAT-P-27, HAT-P-28, HAT-P-35, WASP-76, and
WASP-103, and between 1 and 4 arcsec to HAT-P-29 and WASP-56.Comment: 6 pages, 1 figure, accepted for publication in Astronomy &
Astrophysic
Multi-terminal Thermoelectric Transport in a Magnetic Field: Bounds on Onsager Coefficients and Efficiency
Thermoelectric transport involving an arbitrary number of terminals is
discussed in the presence of a magnetic field breaking time-reversal symmetry
within the linear response regime using the Landauer-B\"uttiker formalism. We
derive a universal bound on the Onsager coefficients that depends only on the
number of terminals. This bound implies bounds on the efficiency and on
efficiency at maximum power for heat engines and refrigerators. For isothermal
engines pumping particles and for absorption refrigerators these bounds become
independent even of the number of terminals. On a technical level, these
results follow from an original algebraic analysis of the asymmetry index of
doubly substochastic matrices and their Schur complements.Comment: 31 pages, 9 figures, New J. Phys., in pres
Star formation environments and the distribution of binary separations
We have carried out K-band speckle observations of a sample of 114 X-ray
selected weak-line T Tauri stars in the nearby Scorpius-Centaurus OB
association. We find that for binary T Tauri stars closely associated to the
early type stars in Upper Scorpius, the youngest subgroup of the OB
association, the peak in the distribution of binary separations is at 90 A.U.
For binary T Tauri stars located in the direction of an older subgroup, but not
closely associated to early type stars, the peak in the distribution is at 215
A.U. A Kolmogorov-Smirnov test indicates that the two binary populations do not
result from the same distibution at a significance level of 98%. Apparently,
the same physical conditions which facilitate the formation of massive stars
also facilitate the formation of closer binaries among low-mass stars, whereas
physical conditions unfavorable for the formation of massive stars lead to the
formation of wider binaries among low-mass stars. The outcome of the binary
formation process might be related to the internal turbulence and the angular
momentum of molecular cloud cores, magnetic field, the initial temperature
within a cloud, or - most likely - a combination of all of these. We conclude
that the distribution of binary separations is not a universal quantity, and
that the broad distribution of binary separations observed among main-sequence
stars can be explained by a superposition of more peaked binary distributions
resulting from various star forming environments. The overall binary frequency
among pre-main-sequence stars in individual star forming regions is not
necessarily higher than among main-sequence stars.Comment: 7 pages, Latex, 4 Postscript figures; also available at
http://spider.ipac.caltech.edu/staff/brandner/pubs/pubs.html ; accepted for
publication in ApJ Letter
VLT spectra of the companion candidate Cha Ha 5/cc 1
We obtained optical and infrared spectra of Cha Ha 5/cc 1, a faint possibly
sub-stellar companion candidate next to the M6-type brown dwarf candidate Cha
Ha 5 in Cha I, using FORS1 and ISAAC at the VLT. The VRIJHK colors of Cha Ha
5/cc 1 are consistent with either an L-type companion or a K-type background
giant. Our spectra show that the companion candidate actually is a background
star.Comment: IAU 211 Symp. "Brown dwarfs" poster proceedings (in press
Universal Coherence-Induced Power Losses of Quantum Heat Engines in Linear Response
We introduce a universal scheme to divide the power output of a periodically
driven quantum heat engine into a classical contribution and one stemming
solely from quantum coherence. Specializing to Lindblad-dynamics and small
driving amplitudes, we derive general upper bounds on both, the coherent and
the total power. These constraints imply that, in the linear-response regime,
coherence inevitably leads to power losses. To illustrate our general analysis,
we explicitly work out the experimentally relevant example of a single-qubit
engine.Comment: 7+4 pages, 2 figure
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