805 research outputs found
Prevalence and diagnostic characteristics of non-clinical mitral regurgitation murmurs in North American Whippets
The radio lighthouse CU Virginis: the spindown of a single main sequence star
The fast rotating star CU Virginis is a magnetic chemically peculiar star
with an oblique dipolar magnetic field. The continuum radio emission has been
interpreted as gyrosyncrotron emission arising from a thin magnetospheric
layer. Previous radio observations at 1.4 GHz showed that a 100% circular
polarized and highly directive emission component overlaps to the continuum
emission two times per rotation, when the magnetic axis lies in the plane of
the sky. This sort of radio lighthouse has been proposed to be due to cyclotron
maser emission generated above the magnetic pole and propagating
perpendicularly to the magnetic axis. Observations carried out with the
Australia Telescope Compact Array at 1.4 and 2.5 GHz one year after this
discovery show that this radio emission is still present, meaning that the
phenomenon responsible for this process is steady on a timescale of years. The
emitted radiation spans at least 1 GHz, being observed from 1.4 to 2.5 GHz. On
the light of recent results on the physics of the magnetosphere of this star,
the possibility of plasma radiation is ruled out. The characteristics of this
radio lighthouse provides us a good marker of the rotation period, since the
peaks are visible at particular rotational phases. After one year, they show a
delay of about 15 minutes. This is interpreted as a new abrupt spinning down of
the star. Among several possibilities, a quick emptying of the equatorial
magnetic belt after reaching the maximum density can account for the magnitude
of the breaking. The study of the coherent emission in stars like CU Vir, as
well as in pre main sequence stars, can give important insight into the angular
momentum evolution in young stars. This is a promising field of investigation
that high sensitivity radio interferometers such as SKA can exploit.Comment: Accepted to MNRAS, 8 pages, 7 figures, updated versio
Developing a gas rocket performance prediction technique
A simple, semi-empirical performance correlation/prediction technique applicable to gaseous and liquid propellant rocket engines is presented. Excellent correlations were attained for over 100 test firings by adjusting the computation of the gaseous mixing of an unreactive, coaxial jet using a correlation factor, F, which resulted in prediction of the experimental combustion efficiency for each firing. Static pressure, mean velocity and turbulence intensity in the developing region of non-reactive coaxial jets, typical of those of coaxial injector elements were determined. Detailed profiles were obtained at twelve axial locations (extending from the nozzle exit for a distance of five diameters) downstream from a single element of the Bell Aerospace H2/O2 19-element coaxial injector. These data are compared with analytical predictions made using both eddy viscosity and turbulence kinetic energy mixing models and available computer codes. Comparisons were disappointing, demonstrating the necessity of developing improved turbulence models and computational techniques before detailed predictions of practical coaxial free jet flows are attempted
Shrinking binary and planetary orbits by Kozai cycles with tidal friction
At least two arguments suggest that the orbits of a large fraction of binary
stars and extrasolar planets shrank by 1-2 orders of magnitude after formation:
(i) the physical radius of a star shrinks by a large factor from birth to the
main sequence, yet many main-sequence stars have companions orbiting only a few
stellar radii away, and (ii) in current theories of planet formation, the
region within ~0.1 AU of a protostar is too hot and rarefied for a Jupiter-mass
planet to form, yet many "hot Jupiters" are observed at such distances. We
investigate orbital shrinkage by the combined effects of secular perturbations
from a distant companion star (Kozai oscillations) and tidal friction. We
integrate the relevant equations of motion to predict the distribution of
orbital elements produced by this process. Binary stars with orbital periods of
0.1 to 10 days, with a median of ~2 d, are produced from binaries with much
longer periods (10 d to 10^5 d), consistent with observations indicating that
most or all short-period binaries have distant companions (tertiaries). We also
make two new testable predictions: (1) For periods between 3 and 10 d, the
distribution of the mutual inclination between the inner binary and the
tertiary orbit should peak strongly near 40 deg and 140 deg. (2) Extrasolar
planets whose host stars have a distant binary companion may also undergo this
process, in which case the orbit of the resulting hot Jupiter will typically be
misaligned with the equator of its host star.Comment: Submitted to ApJ; 18 pages, 10 figure
Mergers of close primordial binaries
We study the production of main sequence mergers of tidally-synchronized
primordial short-period binaries. The principal ingredients of our calculation
are the angular momentum loss rates inferred from the spindown of open cluster
stars and the distribution of binary properties in young open clusters. We
compare our results with the expected number of systems that experience mass
transfer in post-main sequence phases of evolution and compute the
uncertainties in the theoretical predictions. We estimate that main-sequence
mergers can account for the observed number of single blue stragglers in M67.
Applied to the blue straggler population, this implies that such mergers are
responsible for about one quarter of the population of halo blue metal poor
stars, and at least one third of the blue stragglers in open clusters for
systems older than 1 Gyr. The observed trends as a function of age are
consistent with a saturated angular momentum loss rate for rapidly rotating
tidally synchronized systems. The predicted number of blue stragglers from main
sequence mergers alone is comparable to the number observed in globular
clusters, indicating that the net effect of dynamical interactions in dense
stellar environments is to reduce rather than increase the blue straggler
population. A population of subturnoff mergers of order 3-4% of the upper main
sequence population is also predicted for stars older than 4 Gyr, which is
roughly comparable to the small population of highly Li-depleted halo dwarfs.
Other observational tests are discussed.Comment: number of pages depends on font, margins, columns etc (58 with given
format), 14 figures, submitted to the Astrophysical Journa
Four ultra-short period eclipsing M-dwarf binaries in the WFCAM Transit Survey
We report on the discovery of four ultra-short period (P<0.18 days) eclipsing
M-dwarf binaries in the WFCAM Transit Survey. Their orbital periods are
significantly shorter than of any other known main-sequence binary system, and
are all significantly below the sharp period cut-off at P~0.22 days as seen in
binaries of earlier type stars. The shortest-period binary consists of two M4
type stars in a P=0.112 day orbit. The binaries are discovered as part of an
extensive search for short-period eclipsing systems in over 260,000 stellar
lightcurves, including over 10,000 M-dwarfs down to J=18 mag, yielding 25
binaries with P<0.23 days. In a popular paradigm, the evolution of short period
binaries of cool main-sequence stars is driven by loss of angular momentum
through magnetised winds. In this scheme, the observed P~0.22 day period
cut-off is explained as being due to timescales that are too long for
lower-mass binaries to decay into tighter orbits. Our discovery of low-mass
binaries with significantly shorter orbits implies that either these timescales
have been overestimated for M-dwarfs, e.g. due to a higher effective magnetic
activity, or that the mechanism for forming these tight M-dwarf binaries is
different from that of earlier type main-sequence stars.Comment: 22 pages, 17 figures, 3 tables Accepted for publication in MNRA
Solar-like oscillations and magnetic activity of the slow rotator EK Eri
We aim to understand the interplay between non-radial oscillations and
stellar magnetic activity and test the feasibility of doing asteroseismology of
magnetically active stars. We analyze 30 years of photometric time-series data,
3 years of HARPS radial velocity monitoring, and 3 nights of high-cadence HARPS
asteroseismic data. We construct a high-S/N HARPS spectrum that we use to
determine atmospheric parameters and chemical composition. Spectra observed at
different rotation phases are analyzed to search for signs of temperature or
abundance variations. An upper limit on the projected rotational velocity is
derived from very high-resolution CES spectra. We detect oscillations in EK Eri
with a frequency of the maximum power of nu_max = 320+/-32 muHz, and we derive
a peak amplitude per radial mode of ~0.15 m/s, which is a factor of ~3 lower
than expected. We suggest that the magnetic field may act to suppress
low-degree modes. Individual frequencies can not be extracted from the
available data. We derive accurate atmospheric parameters, refining our
previous analysis. We confirm that the main light variation is due to cool
spots, but that other contributions may need to be taken into account. We
suggest that the rotation period is twice the photometric period, i.e., P_rot =
2 P_phot = 617.6 d. We conclude from our derived parameters that v sin i < 0.40
km/s. We also link the time series of direct magnetic field measurements
available in the literature to our newly derived photometric ephemeris.Comment: 11 pages, 10 figures. Accepted by A&
First spectra of the W UMa system V524 Monocerotis
We present the first high-resolution spectra of the W UMa contact binary V524
Mon. The spectra of the two components are very similar, resembling a G5 and a
K0. We find the radial velocities and rotational velocities consistent with
corotation. We estimate the radii and the masses and derive a mass ratio
M_2/M_1 = 2.1. We confirm that V524 Mon is a W-type contact system, likely
enclosed in a common convective envelope, as found by Samec & Loflin (2003). We
do not find evidence for the expected level of emission in the chromospheric
CaII H and K lines, neither H_alpha, indicating that the magnetic activity is
much weaker than expected or that other processes are hampering chromospheric
emission.Comment: 5 pages, 3 figures. Accepted for A&A (in press
Pion-Lambda-Sigma Coupling Extracted from Hyperonic Atoms
The latest measurements of the atomic level width in Sigma-hyperonic Pb atom
offer the most accurate datum in the region of low-energy Sigma-hyperon
physics. Atomic widths are due to the conversion of Sigma-nucleon into
Lambda-nucleon. In high angular momentum states this conversion is dominated by
the one-pion exchange. A joint analysis of the data of the scattering of
negative-Sigma on proton converting into a Lambda and a neutron and of the
atomic widths allows to extract a pseudovector pion-hyperon-Sigma coupling
constant of 0.048 with a statistical error of +-0.005 and a systematic one of
+-0.004. This corresponds to a pseudoscalar coupling constant of 13.3 with a
statistical uncertainty of 1.4 and a systematic one of 1.1.Comment: 12 pages, 1 figure, Use of Revtex.st
- …