105,552 research outputs found
Inflating and Deflating Hot Jupiters: Coupled Tidal and Thermal Evolution of Known Transiting Planets
We examine the radius evolution of close-in giant planets with a planet
evolution model that couples the orbital-tidal and thermal evolution. For 45
transiting systems, we compute a large grid of cooling/contraction paths
forward in time, starting from a large phase space of initial semi-major axes
and eccentricities. Given observational constraints at the current time for a
given planet (semi-major axis, eccentricity, and system age) we find possible
evolutionary paths that match these constraints, and compare the calculated
radii to observations. We find that tidal evolution has two effects. First,
planets start their evolution at larger semi-major axis, allowing them to
contract more efficiently at earlier times. Second, tidal heating can
significantly inflate the radius when the orbit is being circularized, but this
effect on the radius is short-lived thereafter. Often circularization of the
orbit is proceeded by a long period while the semi-major axis slowly decreases.
Some systems with previously unexplained large radii that we can reproduce with
our coupled model are HAT-P-7, HAT-P-9, WASP-10, and XO-4. This increases the
number of planets for which we can match the radius from 24 (of 45) to as many
as 35 for our standard case, but for some of these systems we are required to
be viewing them at a special time around the era of current radius inflation.
This is a concern for the viability of tidal inflation as a general mechanism
to explain most inflated radii. Also, large initial eccentricities would have
to be common. We also investigate the evolution of models that have a floor on
the eccentricity, as may be due to a perturber. In this scenario we match the
extremely large radius of WASP-12b. (Abridged)Comment: 18 pages, 14 figures, 2 tables, Accepted for publication in Ap
A microprocessor-based table lookup approach for magnetic bearing linearization
An approach for producing a linear transfer characteristic between force command and force output of a magnetic bearing actuator without flux biasing is presented. The approach is microprocessor based and uses a table lookup to generate drive signals for the magnetic bearing power driver. An experimental test setup used to demonstrate the feasibility of the approach is described, and test results are presented. The test setup contains bearing elements similar to those used in a laboratory model annular momentum control device
The quiescent counterpart of the peculiar X-ray burster SAX J2224.9+5421
SAX J2224.9+5421 is an extraordinary neutron star low-mass X-ray binary.
Albeit discovered when it exhibited a ~10-s long thermonuclear X-ray burst, it
had faded to a 0.5-10 keV luminosity of Lx<8E32 (D/7.1 kpc)^2 erg/s only ~8 hr
later. It is generally assumed that neutron stars are quiescent (i.e., not
accreting) at such an intensity, raising questions about the trigger conditions
of the X-ray burst and the origin of the faint persistent emission. We report
on a ~51 ks XMM-Newton observation aimed to find clues explaining the unusual
behavior of SAX J2224.9+5421. We identify a likely counterpart that is detected
at Lx~5E31 (D/7.1 kpc)^2 erg/s (0.5-10 keV) and has a soft X-ray spectrum that
can be described by a neutron star atmosphere model with a temperature of ~50
eV. This would suggest that SAX J2224.9+5421 is a transient source that was in
quiescence during our XMM-Newton observation and experienced a very faint
(ceasing) accretion outburst at the time of the X-ray burst detection. We
consider one other potential counterpart that is detected at Lx~5E32 (D/7.1
kpc)^2 erg/s and displays an X-ray spectrum that is best described by power law
with a photon index of ~1.7. Similarly hard X-ray spectra are seen for a few
quiescent neutron stars and may be indicative of a relatively strong magnetic
field or the occurrence of low-level accretion.Comment: 7 pages, 2 figures, 2 tables. Accepted to Ap
A direct measurement of the heat release in the outer crust of the transiently accreting neutron star XTE J1709-267
The heating and cooling of transiently accreting neutron stars provides a
powerful probe of the structure and composition of their crust. Observations of
superbursts and crust cooling of accretion-heated neutron stars require more
heat release than is accounted for in current models. Obtaining firm
constraints on the depth and magnitude of this extra heat is challenging and
therefore its origin remains uncertain. We report on Swift and XMM-Newton
observations of the transient neutron star low-mass X-ray binary XTE J1709-267,
which were made in 2012 September-October when it transitioned to quiescence
after a ~10-week long accretion outburst. The source is detected with
XMM-Newton at a 0.5-10 keV luminosity of Lx~2E34 (D/8.5 kpc)^2 erg/s. The X-ray
spectrum consists of a thermal component that fits to a neutron star atmosphere
model and a non-thermal emission tail, which each contribute ~50% to the total
emission. The neutron star temperature decreases from ~158 to ~152 eV during
the ~8-hour long observation. This can be interpreted as cooling of a crustal
layer located at a column density of y~5E12 g/cm^2 (~50 m inside the neutron
star), which is just below the ignition depth of superbursts. The required heat
generation in the layers on top would be ~0.06-0.13 MeV per accreted nucleon.
The magnitude and depth rule out electron captures and nuclear fusion reactions
as the heat source, but it may be accounted for by chemical separation of light
and heavy nuclei. Low-level accretion offers an alternative explanation for the
observed variability.Comment: 6 pages, 4 figures, 1 table, accepted to ApJ Letters. Minor changes
according to referee report, revised version includes a discussion on the
alternative interpretation of residual accretio
Assessment of Models of Galactic Thermal Dust Emission Using COBE/FIRAS and COBE/DIRBE Observations
Accurate modeling of the spectrum of thermal dust emission at millimeter
wavelengths is important for improving the accuracy of foreground subtraction
for CMB measurements, for improving the accuracy with which the contributions
of different foreground emission components can be determined, and for
improving our understanding of dust composition and dust physics. We fit four
models of dust emission to high Galactic latitude COBE/FIRAS and COBE/DIRBE
observations from 3 millimeters to 100 microns and compare the quality of the
fits. We consider the two-level systems model because it provides a physically
motivated explanation for the observed long wavelength flattening of the dust
spectrum and the anticorrelation between emissivity index and dust temperature.
We consider the model of Finkbeiner, Davis, and Schlegel because it has been
widely used for CMB studies, and the generalized version of this model recently
applied to Planck data by Meisner and Finkbeiner. For comparison we have also
fit a phenomenological model consisting of the sum of two graybody components.
We find that the two-graybody model gives the best fit and the FDS model gives
a significantly poorer fit than the other models. The Meisner and Finkbeiner
model and the two-level systems model remain viable for use in Galactic
foreground subtraction, but the FIRAS data do not have sufficient
signal-to-noise ratio to provide a strong test of the predicted spectrum at
millimeter wavelengths.Comment: 17 pages, 7 figures. Accepted for publication in Ap
New hyperthermal thermosetting heterocyclic polymers
Polyimidazopyrrolone polymers, formed by the condensation of aromatic dianhydrides with aromatic tetraamines in various solvents, form moldings that resist degradation in air and retain great strength at 400 to 700 degrees F. The resins have good insulating properties, are easy to mold, and make good protective coatings
- …