39,298 research outputs found
Dynamics of pebbles in the vicinity of a growing planetary embryo: hydro-dynamical simulations
Understanding the growth of the cores of giant planets is a difficult
problem. Recently, Lambrechts and Johansen (2012; LJ12) proposed a new model in
which the cores grow by the accretion of pebble-size objects, as the latter
drift towards the star due to gas drag. Here, we investigate the dynamics of
pebble-size objects in the vicinity of planetary embryos of 1 and 5 Earth
masses and the resulting accretion rates. We use hydrodynamical simulations, in
which the embryo influences the dynamics of the gas and the pebbles suffer gas
drag according to the local gas density and velocities. The pebble dynamics in
the vicinity of the planetary embryo is non-trivial, and it changes
significantly with the pebble size. Nevertheless, the accretion rate of the
embryo that we measure is within an order of magnitude of the rate estimated in
LJ12 and tends to their value with increasing pebble-size. We conclude that the
model by LJ12 has the potential to explain the rapid growth of giant planet
cores. The actual accretion rates however, depend on the surface density of
pebble size objects in the disk, which is unknown to date.Comment: In press in Astronomy and Astrophysic
On the Destruction of Musical Instruments
In this article, I aim to provide an account of the peculiar reasons that motivate our negative reaction whenever we see musical instruments being mistreated and destroyed. Stephen Davies has suggested that this happens because we seem to treat musical instruments as we treat human beings, at least in some relevant respects. I argue in favour of a different explanation, one that is based on the nature of music as an art form. The main idea behind my account is that musical instruments are not mere tools for the production of art; rather, they are involved in an essential way in artistic appreciation of music. This fact not only grounds our negative reaction to their mistreatment and destruction but also has a normative force that is lacked by the account proposed by Davies
Collisional Cascades in Planetesimal Disks I. Stellar Flybys
We use a new multiannulus planetesimal accretion code to investigate the
evolution of a planetesimal disk following a moderately close encounter with a
passing star. The calculations include fragmentation, gas and
Poynting-Robertson drag, and velocity evolution from dynamical friction and
viscous stirring. We assume that the stellar encounter increases planetesimal
velocities to the shattering velocity, initiating a collisional cascade in the
disk. During the early stages of our calculations, erosive collisions damp
particle velocities and produce substantial amounts of dust. For a wide range
of initial conditions and input parameters, the time evolution of the dust
luminosity follows a simple relation, L_d/L_{\star} = L_0 / [alpha +
(t/t_d)^{beta}]. The maximum dust luminosity L_0 and the damping time t_d
depend on the disk mass, with L_0 proportional to M_d and t_d proportional to
M_d^{-1}. For disks with dust masses of 1% to 100% of the `minimum mass solar
nebula' (1--100 earth masses at 30--150 AU), our calculations yield t_d approx
1--10 Myr, alpha approx 1--2, beta = 1, and dust luminosities similar to the
range observed in known `debris disk' systems, L_0 approx 10^{-3} to 10^{-5}.
Less massive disks produce smaller dust luminosities and damp on longer
timescales. Because encounters with field stars are rare, these results imply
that moderately close stellar flybys cannot explain collisional cascades in
debris disk systems with stellar ages of 100 Myr or longer.Comment: 33 pages of text, 12 figures, and an animation. The paper will appear
in the March 2002 issue of the Astronmomical Journal. The animation and a
copy of the paper with full resolution figures are at S. Kenyon's planet
formation website: http://cfa-www.harvard.edu/~kenyon/p
Spartan Daily, October 27, 1964
Volume 52, Issue 25https://scholarworks.sjsu.edu/spartandaily/4635/thumbnail.jp
Dust in stationary and flowing plasmas
This thesis contains work of a computational and theoretical nature. The floating
potential of dust grains immersed in plasma is investigated via particle-in-cell
simulation for a range of parameters. In particular, work is focused on the charging
of grains large with respect to the electron Debye length. Numerical fits are given for
the floating potential of large grains in stationary and flowing plasma. A modified
version of the well known orbit-motion-limited (OML) theory is developed for large
dust grains. The modified OML theory is shown to be in good agreement with
simulation. This modified theory is then adapted for use with flowing plasmas. In
the case of flowing plasma, for low ion temperatures and flow speeds upwards of
Mach 1, interesting and unexpected effects are seen in the potential and density
distribution around dust grains, these are investigated and discussed. Finally, the
application of this work is outlined with particular focus on dust grains in a
tokamak plasma environment
The fluid mechanics of bubbly drinks
Bubbly drinks are surprisingly attractive. There is something about the
nature of the these beverages that make them preferable among other choices. In
this article we explore the physics involved in this particular kind of
two-phase, mass-transfer-driven flows.Comment: Extended version of Zenit R and Rodr\'iguez-Rodr\'iguez J. The fluid
mechanics of bubbly drinks. Physics Today, Vol. 71(11) pp. 44-50, November
201
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