1,693 research outputs found
Triple-Star Candidates Among the Kepler Binaries
We present the results of a search through the photometric database of
eclipsing Kepler binaries (Prsa et al. 2011; Slawson et al. 2011) looking for
evidence of hierarchical triple star systems. The presence of a third star
orbiting the binary can be inferred from eclipse timing variations. We apply a
simple algorithm in an automated determination of the eclipse times for all
2157 binaries. The "calculated" eclipse times, based on a constant period
model, are subtracted from those observed. The resulting O-C (observed minus
calculated times) curves are then visually inspected for periodicities in order
to find triple-star candidates. After eliminating false positives due to the
beat frequency between the ~1/2-hour Kepler cadence and the binary period, 39
candidate triple systems were identified. The periodic O-C curves for these
candidates were then fit for contributions from both the classical Roemer delay
and so-called "physical" delay, in an attempt to extract a number of the system
parameters of the triple. We discuss the limitations of the information that
can be inferred from these O-C curves without further supplemental input, e.g.,
ground-based spectroscopy. Based on the limited range of orbital periods for
the triple star systems to which this search is sensitive, we can extrapolate
to estimate that at least 20% of all close binaries have tertiary companions.Comment: 19 pages, 13 figures, 3 tables; ApJ, 2013, 768, 33; corrected Fig. 7,
updated references, minor fixes to tex
The Fermi edge singularity of spin polarized electrons
We study the absorption spectrum of a two-dimensional electron gas (2DEG) in
a magnetic field. We find that that at low temperatures, when the 2DEG is spin
polarized, the absorption spectra, which correspond to the creation of spin up
or spin down electron, differ in magnitude, linewidth and filling factor
dependence. We show that these differences can be explained as resulting from
creation of a Mahan exciton in one case, and of a power law Fermi edge
singularity in the other.Comment: 4 pages, 4 figures, published in Phys. Rev. Let
Aerosol number-to-volume-relationship and relative humidity in the eastern Atlantic
J. Geophys. Res ., 105, 1987-1995.Measurementsa cquiredf rom the Office of Naval Research( ONR) Pelican research
aircraftd uringt he secondA erosolC haracterizationE xperiment( ACE 2) are analyzedt o derive
valuesf or the dry (RH = 40%) aerosonl umber-to-volumrea tio in the submicrons izer ange. This
ratioi s foundto ber elativelyc onstanwt,i tha meanv alueo f 168_ +2 1 gm- 3,i n agreemenwti th
previouss tudiese lsewhere.T he impacto f ambientr elativeh umidity (RH) on the dry
number-to-volumies alsoq uantifieda nd a procedurefo r estimatingth e dry from the ambientr atio
established.F inally, the feasibilityo f a remoter etrievalo f the aerosoln umberc oncentrationin
the submicrons izer ange,e ssentiallyth e cloudc ondensation ucleusc oncentrationa ctive at a
nominal0 .2% supersaturationis, partially assessed
Optical absorption to probe the quantum Hall ferromagnet at filling factor
Optical absorption measurements are used to probe the spin polarization in
the integer and fractional quantum Hall effect regimes. The system is fully
spin polarized only at filling factor and at very low
temperatures( mK). A small change in filling factor
() leads to a significant depolarization. This
suggests that the itinerant quantum Hall ferromagnet at is surprisingly
fragile against increasing temperature, or against small changes in filling
factor.Comment: 4 pages, 2 figure
Evidence for the disintegration of KIC 12557548 b
Context. The Kepler object KIC 12557548 b is peculiar. It exhibits
transit-like features every 15.7 hours that vary in depth between 0.2% and
1.2%. Rappaport et al. (2012) explain the observations in terms of a
disintegrating, rocky planet that has a trailing cloud of dust created and
constantly replenished by thermal surface erosion. The variability of the
transit depth is then a consequence of changes in the cloud optical depth.
Aims. We aim to validate the disintegrating-planet scenario by modeling the
detailed shape of the observed light curve, and thereby constrain the cloud
particle properties to better understand the nature of this intriguing object.
Methods. We analysed the six publicly-available quarters of raw Kepler data,
phase-folded the light curve and fitted it to a model for the trailing dust
cloud. Constraints on the particle properties were investigated with a
light-scattering code. Results. The light curve exhibits clear signatures of
light scattering and absorption by dust, including a brightening in flux just
before ingress correlated with the transit depth and explained by forward
scattering, and an asymmetry in the transit light curve shape, which is easily
reproduced by an exponentially decaying distribution of optically thin dust,
with a typical grain size of 0.1 micron. Conclusions. Our quantitative analysis
supports the hypothesis that the transit signal of KIC 12557548 b is due to a
variable cloud of dust, most likely originating from a disintegrating object.Comment: 5 pages, 4 figures. Accepted for publication in Astronomy and
Astrophysic
Absorption in the fractional quantum Hall regime: trion dichroism and spin polarization
We present measurements of optical interband absorption in the fractional
quantum Hall regime in a GaAs quantum well in the range 0 < nu < 1. We
investigate the mechanism of singlet trion absorption, and show that its
circular dichroism can be used as a probe of the spin polarization of the
ground state of the two-dimensional electron system (2DES). We find that at nu
= 1/3 the 2DES is fully spin-polarized. Increasing the filling factor results
in a gradual depolarization, with a sharp minimum in the dichroism near nu =
2/3. We find that in the range 0.5 < nu < 0.85 the 2DES remains partially
polarized for the broad range of magnetic fields from 2.75 to 11 Tesla. This is
consistent with the presence of a mixture of polarized and depolarized regions.Comment: 4 pages, 4 figures (Fig 4 is in color
The mass of the neutron star in Vela X-1 and tidally induced non-radial oscillations in GP Vel
We report new radial velocity observations of GP Vel/HD77581, the optical
companion to the eclipsing X-ray pulsar Vela X-1. Using data spanning more than
two complete orbits of the system, we detect evidence for tidally induced
non-radial oscillations on the surface of GP Vel, apparent as peaks in the
power spectrum of the residuals to the radial velocity curve fit. By removing
the effect of these oscillations (to first order) and binning the radial
velocities, we have determined the semi-amplitude of the radial velocity curve
of GP Vel to be K_o=22.6+/-1.5 km/s. Given the accurately measured
semi-amplitude of the pulsar's orbit, the mass ratio of the system is
0.081+/-0.005. We are able to set upper and lower limits on the masses of the
component stars as follows. Assuming GP Vel fills its Roche lobe then the
inclination angle of the system, i=70.1+/-2.6 deg. In this case we obtain the
masses of the two stars as M_x=2.27 +/-0.17 M_sun for the neutron star and
M_o=27.9+/-1.3 M_sun for GP Vel. Conversely, assuming the inclination angle is
i=90 deg, the ratio of the radius of GP Vel to the radius of its Roche lobe is
beta=0.89+/-0.03 and the masses of the two stars are M_x=1.88+/-0.13 M_sun and
M_o=23.1+/-0.2 M_sun. A range of solutions between these two sets of limits is
also possible, corresponding to other combinations of i and beta. In addition,
we note that if the zero phase of the radial velocity curve is allowed as a
free parameter, rather than constrained by the X-ray ephemeris, a significantly
improved fit is obtained with an amplitude of 21.2+/-0.7 km/s and a phase shift
of 0.033+/-0.007 in true anomaly. The apparent shift in the zero phase of the
radial velocity curve may indicate the presence of an additional radial
velocity component at the orbital period.Comment: Accepted for publication in Astronomy & Astrophysic
Magnetically Torqued Thin Accretion Disks
We compute the properties of a geometrically thin, steady accretion disk
surrounding a central rotating, magnetized star. The magnetosphere is assumed
to entrain the disk over a wide range of radii. The model is simplified in that
we adopt two (alternate) ad hoc, but plausible, expressions for the azimuthal
component of the magnetic field as a function of radial distance. We find a
solution for the angular velocity profile tending to corotation close to the
central star, and smoothly matching a Keplerian curve at a radius where the
viscous stress vanishes. The value of this ''transition'' radius is nearly the
same for both of our adopted B-field models. We then solve analytically for the
torques on the central star and for the disk luminosity due to gravity and
magnetic torques. When expressed in a dimensionless form, the resulting
quantities depend on one parameter alone, the ratio of the transition radius to
the corotation radius. For rapid rotators, the accretion disk may be powered
mostly by spin-down of the central star. These results are independent of the
viscosity prescription in the disk. We also solve for the disk structure for
the special case of an optically thick alpha disk. Our results are applicable
to a range of astrophysical systems including accreting neutron stars,
intermediate polar cataclysmic variables, and T Tauri systems.Comment: 9 sharper figs, updated reference
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