565 research outputs found
Evaluation of high Reynolds number flow in a 180 degree turn-around-duct
Mean and turbulent velocities were measured for the flow in a 180 degree turn-around-duct over a Reynolds number range from 600,000 to greater than 900,000. The measurements were made in water using a forward scattering laser velocimeter. A duct of 100 x 10 cm constant cross-section, with a mean radius of curvature (centerline) of 10 cm was employed for the study. The measurements are in agreement with previous studies in that the use of local bulk velocity to nondimensionalize the mean and turbulent velocities reduce the Reynolds number variations. The basic phenomenon of relaminarization along the inner surface at the exit of the turn are similar to the flow observed at low Reynolds numbers. The separation bubble region shows a systematic variation with Reynolds number, however the Reynolds number effect may be of second order in the calculation of the overall flow. Large tangential, radial, and lateral turbulent velocities are measured along the outer surface of the turn
Optimal strategies of radial velocity observations in planet search surveys
Applications of the theory of optimal design of experiments to radial
velocity planet search surveys are considered. Different optimality criteria
are discussed, basing on the Fisher, Shannon, and Kullback-Leibler
informations. Algorithms of optimal scheduling of RV observations for two
important practical problems are considered. The first problem is finding the
time for future observations to yield the maximum improvement of the precision
of exoplanetary orbital parameters and masses. The second problem is finding
the most favourable time for distinguishing alternative orbital fits (the
scheduling of discriminating observations).
These methods of optimal planning are demonstrated to be potentially
efficient for multi-planet extrasolar systems, in particular for resonant ones.
In these cases, the optimal dates of observations are often concentrated in
quite narrow time segments.Comment: 8 pages, 2 figures, no tables, Accepted to MNRA
Variability of surface flows on the Sun and the implications for exoplanet detection
The published Mount Wilson Doppler-shift measurements of the solar velocity
field taken in 1967--1982 are revisited with a more accurate model, which
includes two terms representing the meridional flow and three terms
corresponding to the convective limb shift. Integration of the recomputed data
over the visible hemisphere reveals significant variability of the net radial
velocity at characteristic time scales of 0.1--10 years, with a standard
deviation of 1.4 \ms. This result is supported by independent published
observations. The implications for exoplanet detection include reduced
sensitivity of the Doppler method to Earth-like planets in the habitable zone,
and an elevated probability of false detections at periods of a few to several
years.Comment: Accepted in this form for publication in Ap
Orbital structure of the GJ876 extrasolar planetary system, based on the latest Keck and HARPS radial velocity data
We use full available array of radial velocity data, including recently
published HARPS and Keck observatory sets, to characterize the orbital
configuration of the planetary system orbiting GJ876. First, we propose and
describe in detail a fast method to fit perturbed orbital configuration, based
on the integration of the sensitivity equations inferred by the equations of
the original -body problem. Further, we find that it is unsatisfactory to
treat the available radial velocity data for GJ876 in the traditional white
noise model, because the actual noise appears autocorrelated (and demonstrates
non-white frequency spectrum). The time scale of this correlation is about a
few days, and the contribution of the correlated noise is about 2 m/s (i.e.,
similar to the level of internal errors in the Keck data). We propose a
variation of the maximum-likelihood algorithm to estimate the orbital
configuration of the system, taking into account the red noise effects. We
show, in particular, that the non-zero orbital eccentricity of the innermost
planet \emph{d}, obtained in previous studies, is likely a result of
misinterpreted red noise in the data. In addition to offsets in some orbital
parameters, the red noise also makes the fit uncertainties systematically
underestimated (while they are treated in the traditional white noise model).
Also, we show that the orbital eccentricity of the outermost planet is actually
ill-determined, although bounded by . Finally, we investigate
possible orbital non-coplanarity of the system, and limit the mutual
inclination between the planets \emph{b} and \emph{c} orbits by
, depending on the angular position of the mutual orbital
nodes.Comment: 36 pages, 11 figures, 3 tables; Accepted to Celestial Mechanics and
Dynamical Astronom
Analysis of the Vertical-tail Loads Measured During a Flight Investigation at Transonic Speeds of the Douglas X-3 Research Airplane
Substellar companions and isolated planetary mass objects from protostellar disc fragmentation
Self-gravitating protostellar discs are unstable to fragmentation if the gas
can cool on a time scale that is short compared to the orbital period. We use a
combination of hydrodynamic simulations and N-body orbit integrations to study
the long term evolution of a fragmenting disc with an initial mass ratio to the
star of M_disc/M_star = 0.1. For a disc which is initially unstable across a
range of radii, a combination of collapse and subsequent accretion yields
substellar objects with a spectrum of masses extending (for a Solar mass star)
up to ~0.01 M_sun. Subsequent gravitational evolution ejects most of the lower
mass objects within a few million years, leaving a small number of very massive
planets or brown dwarfs in eccentric orbits at moderately small radii. Based on
these results, systems such as HD 168443 -- in which the companions are close
to or beyond the deuterium burning limit -- appear to be the best candidates to
have formed via gravitational instability. If massive substellar companions
originate from disc fragmentation, while lower-mass planetary companions
originate from core accretion, the metallicity distribution of stars which host
massive substellar companions at radii of ~1 au should differ from that of
stars with lower mass planetary companions.Comment: 5 pages, accepted for publication in MNRA
A 1.1 to 1.9 GHz SETI Survey of the Kepler Field: I. A Search for Narrow-band Emission from Select Targets
We present a targeted search for narrow-band (< 5 Hz) drifting sinusoidal
radio emission from 86 stars in the Kepler field hosting confirmed or candidate
exoplanets. Radio emission less than 5 Hz in spectral extent is currently known
to only arise from artificial sources. The stars searched were chosen based on
the properties of their putative exoplanets, including stars hosting candidates
with 380 K > T_eq > 230 K, stars with 5 or more detected candidates or stars
with a super-Earth (R_p 50 day orbit. Baseband voltage data
across the entire band between 1.1 and 1.9 GHz were recorded at the Robert C.
Byrd Green Bank Telescope between Feb--Apr 2011 and subsequently searched
offline. No signals of extraterrestrial origin were found. We estimate that
fewer than ~1% of transiting exoplanet systems host technological civilizations
that are radio loud in narrow-band emission between 1-2 GHz at an equivalent
isotropically radiated power (EIRP) of ~1.5 x 10^21 erg s^-1, approximately
eight times the peak EIRP of the Arecibo Planetary Radar, and we limit the the
number of 1-2 GHz narrow-band-radio-loud Kardashev type II civilizations in the
Milky Way to be < 10^-6 M_solar^-1. Here we describe our observations, data
reduction procedures and results.Comment: Accepted to the Astrophysical Journa
Giant Planet Migration through the Action of Disk Torques and Planet Scattering
This paper presents a parametric study of giant planet migration through the
combined action of disk torques and planet-planet scattering. The torques
exerted on planets during Type II migration in circumstellar disks readily
decrease the semi-major axes, whereas scattering between planets increases the
orbital eccentricities. This paper presents a parametric exploration of the
possible parameter space for this migration scenario using two (initial)
planetary mass distributions and a range of values for the time scale of
eccentricity damping (due to the disk). For each class of systems, many
realizations of the simulations are performed in order to determine the
distributions of the resulting orbital elements of the surviving planets; this
paper presents the results of 8500 numerical experiments. Our goal is to study
the physics of this particular migration mechanism and to test it against
observations of extrasolar planets. The action of disk torques and
planet-planet scattering results in a distribution of final orbital elements
that fills the a-e plane, in rough agreement with the orbital elements of
observed extrasolar planets. In addition to specifying the orbital elements, we
characterize this migration mechanism by finding the percentages of ejected and
accreted planets, the number of collisions, the dependence of outcomes on
planetary masses, the time spent in 2:1 and 3:1 resonances, and the effects of
the planetary IMF. We also determine the distribution of inclination angles of
surviving planets and the distribution of ejection speeds for exiled planets.Comment: 46 pages including 15 figures; accepted to ICARU
Time-Varying Potassium in High-Resolution Spectra of the Type Ia Supernova 2014J
We present a time series of the highest resolution spectra yet published for
the nearby Type Ia supernova (SN) 2014J in M82. They were obtained at 11 epochs
over 33 days around peak brightness with the Levy Spectrograph (resolution
R~110,000) on the 2.4m Automated Planet Finder telescope at Lick Observatory.
We identify multiple Na I D and K I absorption features, as well as absorption
by Ca I H & K and several of the more common diffuse interstellar bands (DIBs).
We see no evolution in any component of Na I D, Ca I, or in the DIBs, but do
establish the dissipation/weakening of the two most blueshifted components of K
I. We present several potential physical explanations, finding the most
plausible to be photoionization of circumstellar material, and discuss the
implications of our results with respect to the progenitor scenario of SN
2014J.Comment: 11 pages, 8 figures, 3 tables, submitted to Ap
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