442 research outputs found
Hydrodynamic interaction in quasi-two-dimensional suspensions
Confinement between two parallel surfaces is found, theoretically and
experimentally, to drastically affect the hydrodynamic interaction between
colloid particles, changing the sign of the coupling, its decay with distance
and its concentration dependence. In particular, we show that three-body
effects do not modify the coupling at large distances as would be expected from
hydrodynamic screening.Comment: 8 pages, 2 figure
Persistence with Partial Survival
We introduce a parameter , called partial survival, in the persistence of
stochastic processes and show that for smooth processes the persistence
exponent changes continuously with , being the usual
persistence exponent. We compute exactly for a one-dimensional
deterministic coarsening model, and approximately for the diffusion equation.
Finally we develop an exact, systematic series expansion for , in
powers of , for a general Gaussian process with finite density of
zero crossings.Comment: 5 pages, 2 figures, references added, to appear in Phys.Rev.Let
Spatial Persistence of Fluctuating Interfaces
We show that the probability, P_0(l), that the height of a fluctuating
(d+1)-dimensional interface in its steady state stays above its initial value
up to a distance l, along any linear cut in the d-dimensional space, decays as
P_0(l) \sim l^(-\theta). Here \theta is a `spatial' persistence exponent, and
takes different values, \theta_s or \theta_0, depending on how the point from
which l is measured is specified. While \theta_s is related to fractional
Brownian motion, and can be determined exactly, \theta_0 is non-trivial even
for Gaussian interfaces.Comment: 5 pages, new material adde
Rotational Velocities of Individual Components in Very Low Mass Binaries
We present rotational velocities for individual components of 11 very low mass (VLM) binaries with spectral types between M7 and L7.5. These results are based on observations taken with the near-infrared spectrograph, NIRSPEC, and the Keck II laser guide star adaptive optics system. We find that the observed sources tend to be rapid rotators (v sin i > 10 km s^(–1)), consistent with previous seeing-limited measurements of VLM objects. The two sources with the largest v sin i, LP 349–25B and HD 130948C, are rotating at ~30% of their break-up speed, and are among the most rapidly rotating VLM objects known. Furthermore, five binary systems, all with orbital semimajor axes ≾3.5 AU, have component v sin i values that differ by greater than 3σ. To bring the binary components with discrepant rotational velocities into agreement would require the rotational axes to be inclined with respect to each other, and that at least one component is inclined with respect to the orbital plane. Alternatively, each component could be rotating at a different rate, even though they have similar spectral types. Both differing rotational velocities and inclinations have implications for binary star formation and evolution. We also investigate possible dynamical evolution in the triple system HD 130948A–BC. The close binary brown dwarfs B and C have significantly different v sin i values. We demonstrate that components B and C could have been torqued into misalignment by the primary star, A, via orbital precession. Such a scenario can also be applied to another triple system in our sample, GJ 569A–Bab. Interactions such as these may play an important role in the dynamical evolution of VLM binaries. Finally, we note that two of the binaries with large differences in component v sin i, LP 349–25AB and 2MASS 0746+20AB, are also known radio sources
A Correlation Between Circumstellar Disks and Rotation in the Upper Scorpius OB Association
We present projected rotational velocities for 20 early-type (B8-A9) and 74
late-type (F2-M8) members of the ~5 Myr old Upper Scorpius OB Association
derived from high dispersion optical spectra obtained with the High Resolution
Echelle Spectrometer (HIRES) on Keck I and the Magellan Inamori Kyocera Echelle
(MIKE) on the Magellan Clay telescope. The spectroscopic sample is composed of
stars and brown dwarfs with infrared signatures of circumstellar disks, both
primordial and debris, and non-excess sources of comparable spectral type. We
merge projected rotational velocities, accretion diagnostics, and Spitzer Space
Telescope Infrared Array Camera (IRAC) and Multiband Imaging Photometer for
Spitzer (MIPS) 24 micron photometry to examine the relationship between
rotation and circumstellar disks. The rotational velocities are strongly
correlated with spectral type, a proxy for mass, such that the median vsini for
B8-A9 type stars is: 195(+/-)70 km/s, F2-K4: 37.8(+/-)7.4 km/s, K5-K9:
13.8(+21.3/-8.2) km/s, M0-M5: 16.52(+/-)5.3 km/s, and M5.5-M8: 17.72(+/-)8.1
km/s. We find with a probability of >0.99 that M-type stars and brown dwarfs
having infrared excess suggestive of circumstellar disks rotate more slowly
than their non-excess counterparts. A similar correlation is present among
F2-K9 type stars, but only at the ~97% confidence level. Among the early-type
(B8-A9) members, rotational velocities of the debris-disk and non-disk
populations are indistinguishable. Considering the late-type (F2-M8) stars and
brown dwarfs, we find a low fraction of slowly rotating, non-excess sources
relative to younger star forming regions, suggesting that most have spun up
following disk dissipation. The few late-type (F2-M5) debris disk sources,
which may be representative of stars that have recently dispersed their inner
disks, are evenly divided between slow and moderate rotators.Comment: 41 pages, 11 figures, accepted for publication in the Astrophysical
Journa
Masses, Radii, and Cloud Properties of the HR 8799 Planets
The near-infrared colors of the planets directly imaged around the A star HR
8799 are much redder than most field brown dwarfs of the same effective
temperature. Previous theoretical studies of these objects have concluded that
the atmospheres of planets b, c, and d are unusually cloudy or have unusual
cloud properties. Some studies have also found that the inferred radii of some
or all of the planets disagree with expectations of standard giant planet
evolution models. Here we compare the available data to the predictions of our
own set of atmospheric and evolution models that have been extensively tested
against observations of field L and T dwarfs, including the reddest L dwarfs.
Unlike some previous studies we require mutually consistent choices for
effective temperature, gravity, cloud properties, and planetary radius. This
procedure thus yields plausible values for the masses, effective temperatures,
and cloud properties of all three planets. We find that the cloud properties of
the HR 8799 planets are not unusual but rather follow previously recognized
trends, including a gravity dependence on the temperature of the L to T
spectral transition--some reasons for which we discuss. We find the inferred
mass of planet b is highly sensitive to whether or not we include the H and K
band spectrum in our analysis. Solutions for planets c and d are consistent
with the generally accepted constraints on the age of the primary star and
orbital dynamics. We also confirm that, like in L and T dwarfs and solar system
giant planets, non-equilibrium chemistry driven by atmospheric mixing is also
important for these objects. Given the preponderance of data suggesting that
the L to T spectral type transition is gravity dependent, we present an
exploratory evolution calculation that accounts for this effect. Finally we
recompute the the bolometric luminosity of all three planets.Comment: 52 pages, 12 figures, Astrophysical Journal, in press. v2 features
minor editorial updates and correction
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