496 research outputs found
Tidal torques. A critical review of some techniques
We point out that the MacDonald formula for body-tide torques is valid only
in the zeroth order of e/Q, while its time-average is valid in the first order.
So the formula cannot be used for analysis in higher orders of e/Q. This
necessitates corrections in the theory of tidal despinning and libration
damping.
We prove that when the inclination is low and phase lags are linear in
frequency, the Kaula series is equivalent to a corrected version of the
MacDonald method. The correction to MacDonald's approach would be to set the
phase lag of the integral bulge proportional to the instantaneous frequency.
The equivalence of descriptions gets violated by a nonlinear
frequency-dependence of the lag.
We explain that both the MacDonald- and Darwin-torque-based derivations of
the popular formula for the tidal despinning rate are limited to low
inclinations and to the phase lags being linear in frequency. The
Darwin-torque-based derivation, though, is general enough to accommodate both a
finite inclination and the actual rheology.
Although rheologies with Q scaling as the frequency to a positive power make
the torque diverge at a zero frequency, this reveals not the impossible nature
of the rheology, but a flaw in mathematics, i.e., a common misassumption that
damping merely provides lags to the terms of the Fourier series for the tidal
potential. A hydrodynamical treatment (Darwin 1879) had demonstrated that the
magnitudes of the terms, too, get changed. Reinstating of this detail tames the
infinities and rehabilitates the "impossible" scaling law (which happens to be
the actual law the terrestrial planets obey at low frequencies).Comment: arXiv admin note: sections 4 and 9 of this paper contain substantial
text overlap with arXiv:0712.105
Eccentricities of Planets in Binary Systems
The most puzzling property of the extrasolar planets discovered by recent
radial velocity surveys is their high orbital eccentricities, which are very
difficult to explain within our current theoretical paradigm for planet
formation. Current data reveal that at least 25% of these planets, including
some with particularly high eccentricities, are orbiting a component of a
binary star system. The presence of a distant companion can cause significant
secular perturbations in the orbit of a planet. At high relative inclinations,
large-amplitude, periodic eccentricity perturbations can occur. These are known
as "Kozai cycles" and their amplitude is purely dependent on the relative
orbital inclination. Assuming that every planet host star also has a (possibly
unseen, e.g., substellar) distant companion, with reasonable distributions of
orbital parameters and masses, we determine the resulting eccentricity
distribution of planets and compare it to observations? We find that
perturbations from a binary companion always appear to produce an excess of
planets with both very high (e>0.6) and very low (e<0.1) eccentricities. The
paucity of near-circular orbits in the observed sample implies that at least
one additional mechanism must be increasing eccentricities. On the other hand,
the overproduction of very high eccentricities observed in our models could be
combined with plausible circularization mechanisms (e.g., friction from
residual gas) to create more planets with intermediate eccentricities
(e=0.1-0.6).Comment: 8 pages, to appear in "Close Binaries in the 21st Century: New
Opportunities and Challenges", ed. A. Gimenez et al. (Springer
Cosmic ray short burst observed with the Global Muon Detector Network (GMDN) on June 22, 2015
We analyze the short cosmic ray intensity increase ("cosmic ray burst": CRB)
on June 22, 2015 utilizing a global network of muon detectors and derive the
global anisotropy of cosmic ray intensity and the density (i.e. the
omnidirectional intensity) with 10-minute time resolution. We find that the CRB
was caused by a local density maximum and an enhanced anisotropy of cosmic rays
both of which appeared in association with Earth's crossing of the heliospheric
current sheet (HCS). This enhanced anisotropy was normal to the HCS and
consistent with a diamagnetic drift arising from the spatial gradient of cosmic
ray density, which indicates that cosmic rays were drifting along the HCS from
the north of Earth. We also find a significant anisotropy along the HCS,
lasting a few hours after the HCS crossing, indicating that cosmic rays
penetrated into the inner heliosphere along the HCS. Based on the latest
geomagnetic field model, we quantitatively evaluate the reduction of the
geomagnetic cut-off rigidity and the variation of the asymptotic viewing
direction of cosmic rays due to a major geomagnetic storm which occurred during
the CRB and conclude that the CRB is not caused by the geomagnetic storm, but
by a rapid change in the cosmic ray anisotropy and density outside the
magnetosphere.Comment: accepted for the publication in the Astrophysical Journa
An Oort cloud origin for the high-inclination, high-perihelion Centaurs
We analyse the origin of three Centaurs with perihelia in the range 15 AU to
30 AU, inclinations above 70 deg and semi-major axes shorter than 100 AU. Based
on long-term numerical simulations we conclude that these objects most likely
originate from the Oort cloud rather than the Kuiper Belt or Scattered Disc. We
estimate that there are currently between 1 and 200 of these high-inclination,
high-perihelion Centaurs with absolute magnitude H<8.Comment: Accepted for publication in MNRA
The GAPS Experiment to Search for Dark Matter using Low-energy Antimatter
The GAPS experiment is designed to carry out a sensitive dark matter search
by measuring low-energy cosmic ray antideuterons and antiprotons. GAPS will
provide a new avenue to access a wide range of dark matter models and masses
that is complementary to direct detection techniques, collider experiments and
other indirect detection techniques. Well-motivated theories beyond the
Standard Model contain viable dark matter candidates which could lead to a
detectable signal of antideuterons resulting from the annihilation or decay of
dark matter particles. The dark matter contribution to the antideuteron flux is
believed to be especially large at low energies (E < 1 GeV), where the
predicted flux from conventional astrophysical sources (i.e. from secondary
interactions of cosmic rays) is very low. The GAPS low-energy antiproton search
will provide stringent constraints on less than 10 GeV dark matter, will
provide the best limits on primordial black hole evaporation on Galactic length
scales, and will explore new discovery space in cosmic ray physics.
Unlike other antimatter search experiments such as BESS and AMS that use
magnetic spectrometers, GAPS detects antideuterons and antiprotons using an
exotic atom technique. This technique, and its unique event topology, will give
GAPS a nearly background-free detection capability that is critical in a
rare-event search. GAPS is designed to carry out its science program using
long-duration balloon flights in Antarctica. A prototype instrument was
successfully flown from Taiki, Japan in 2012. GAPS has now been approved by
NASA to proceed towards the full science instrument, with the possibility of a
first long-duration balloon flight in late 2020. Here we motivate low-energy
cosmic ray antimatter searches and discuss the current status of the GAPS
experiment and the design of the payload.Comment: 8 pags, 3 figures, Proc. 35th International Cosmic Ray Conference
(ICRC 2017), Busan, Kore
The formation of the eccentric-orbit millisecond pulsar J1903+0327 and the origin of single millisecond pulsars
The millisecond pulsar J1903+0327 is accompanied by an ordinary G-dwarf star
in an unusually wide (\,days) and eccentric () orbit. The standard model for producing MSPs fails to explain the
orbital characteristics of this extraordinary binary, and alternative binary
models are unable to explain the observables. We present a triple-star model
for producing MSPs in relatively wide eccentric binaries with a normal
(main-sequence) stellar companion. We start from a stable triple system
consisting of a Low-Mass X-ray Binary (LMXB) with an orbital period of at least
1 day, accompanied by a G-dwarf in a wide and possibly eccentric orbit.
Variations in the initial conditions naturally provide a satisfactory
explanation for the unexplained triple component in the eclipsing soft X-ray
transient 4U~2129+47 or the cataclysmic variable EC 19314-5915. The best
explanation for J1903, however, results from the expansion of the orbit of the
LMXB, driven by the mass transfer from the evolving donor star to its neutron
star companion, which causes the triple eventually to becomes dynamically
unstable. Using numerical computations we show that, depending on the precise
system configuration at the moment the triple becomes dynamically unstable, the
ejection of each of the three components is possible. If the donor star of the
LMXB is ejected, a system resembling J1903, will result. If the neutron star is
ejected, a single MSP results. This model therefore also provides a
straightforward mechanism for forming single MSP in the Galactic disk. We
conclude that the Galaxy contains some 30--300 binaries with characteristics
similar to J1903, and about an order of magnitude fewer single millisecond
pulsars produced with the proposed triple scenario.Comment: ApJ accepted for publicatio
The CORALIE survey for southern extra-solar planets XV. Discovery of two eccentric planets orbiting HD4113 and HD156846
We report the detection of two very eccentric planets orbiting HD4113 and
HD156846 with the CORALIE Echelle spectrograph mounted on the 1.2-m Euler Swiss
telescope at La Silla. The first planet, HD4113b, has minimum mass of
, a period of days and an
eccentricity of . It orbits a metal rich G5V star at
AU which displays an additional radial velocity drift of 28 m s/yr
observed during 8 years. The combination of the radial-velocity data and the
non-detection of any main sequence stellar companion in our high contrast
images taken at the VLT with NACO/SDI, characterizes the companion as a
probable brown dwarf or as a faint white dwarf. The second planet, \object{HD
156846 b}, has minimum mass of M, a period
of days, an eccentricity of and is located
at AU from its parent star. HD156846 is a metal rich G0 dwarf and is
also the primary of a wide binary system ( AU, years). Its
stellar companion, \object{IDS 17147-1914 B}, is a M4 dwarf. The very high
eccentricities of both planets can be explained by Kozai oscillations induced
by the presence of a third object.Comment: 4 pages, 5 figures, A&A Letter accepte
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