2,084 research outputs found
The Formation of Fragments at Corotation in Isothermal Protoplanetary Disks
Numerical hydrodynamics simulations have established that disks which are
evolved under the condition of local isothermality will fragment into small
dense clumps due to gravitational instabilities when the Toomre stability
parameter is sufficiently low. Because fragmentation through disk
instability has been suggested as a gas giant planet formation mechanism, it is
important to understand the physics underlying this process as thoroughly as
possible. In this paper, we offer analytic arguments for why, at low ,
fragments are most likely to form first at the corotation radii of growing
spiral modes, and we support these arguments with results from 3D hydrodynamics
simulations.Comment: 21 pages, 1 figur
Turbulence-plankton interactions : a new cartoon
Author Posting. © John Wiley & Sons, 2009. This is the author's version of the work. It is posted here by permission of John Wiley & Sons for personal use, not for redistribution. The definitive version was published in Marine Ecology 30 (2009): 133-150, doi:10.1111/j.1439-0485.2009.00288.x.Climate change will alter turbulence intensity, motivating greater attention to mechanisms of
turbulence effects on organisms. Many analytic and analog models used to simulate and assess
effects of turbulence on plankton rely on a one-dimensional simplification of the dissipative
scales of turbulence, i.e., simple, steady, uniaxial shears, as produced in Couette vessels. There
shear rates are constant and spatially uniform, and hence so is vorticity. Studies in such Couette
flows have greatly informed, spotlighting stable orientations of nonspherical particles and
predictable, periodic, rotational motions of steadily sheared particles in Jeffery orbits that steepen
concentration gradients around nutrient-absorbing phytoplankton and other chemically (re)active
particles. Over the last decade, however, turbulence research within fluid dynamics has focused
on the structure of dissipative vortices in space and time and on spatially and temporally varying
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vorticity fields in particular. Because steadily and spatially uniformly sheared flows are
exceptional, so therefore are stable orientations for particles in turbulent flows. Vorticity
gradients, finite net diffusion of vorticity and small radii of curvature of streamlines are
ubiquitous features of turbulent vortices at dissipation scales that are explicitly excluded from
simple, steady Couette flows. All of these flow components contribute instabilities that cause
rotational motions of particles and so are important to simulate in future laboratory devices
designed to assess effects of turbulence on nutrient uptake, particle coagulation and predatorprey
encounter in the plankton. The Burgers vortex retains these signature features of turbulence
and provides a simplified “cartoon” of vortex structure and dynamics that nevertheless obeys the
Navier-Stokes equations. Moreover, this idealization closely resembles many dissipative
vortices observed in both the laboratory and the field as well as in direct numerical simulations
of turbulence. It is simple enough to allow both simulation in numerical models and fabrication
of analog devices that selectively reproduce its features. Exercise of such numerical and analog
models promises additional insights into mechanisms of turbulence effects on passive trajectories
and local accumulations of both living and nonliving particles, into solute exchange with living
and nonliving particles and into more subtle influences on sensory processes and swimming
trajectories of plankton, including demersal organisms and settling larvae in turbulent bottom
boundary layers. The literature on biological consequences of vortical turbulence has focused
primarily on the smallest, Kolmogorov-scale vortices of length scale η. Theoretical dissipation
spectra and direct numerical simulation, however, indicate that typical dissipative vortices with
radii of 7η to 8η, peak azimuthal speeds of order 1 cm s-1 and lifetimes of order 10 s as a
minimum (and much longer for moderate pelagic turbulence intensities) deserve new attention in
studies of biological effects of turbulence.This research was supported by collaborative U.S. National Science Foundation grant (OCE-
0724744) to Jumars and Karp-Boss
Validation of the particle size distribution obtained with the laser in-situ scattering and transmission (LISST) meter in flow-through mode
High spatial and temporal resolution estimates of the particle size distribution (PSD) in the surface ocean can enable improved understanding of biogeochemistry and ecosystem dynamics. Oceanic PSD measurements remain rare due to the time-consuming, manual sampling methods of common particle sizing instruments. Here, we evaluate the utility of measuring particle size data at high spatial resolution with a commercially-available submersible laser di raction particle sizer (LISST-100X, Sequoia Scientific), operating in an automated mode with continuously flowing seawater. The LISST PSD agreed reasonably well with discrete PSD measurements obtained with a Coulter Counter and data from the flow-through sampling Imaging Flow-Cytobot, validating our methodology. Total particulate area and Volume derived from the LISST PSD agreed well with beam-attenuation and particulate organic carbon respectively, further validating the LISST PSD. Furthermore, When compared to the measured spectral characteristics of particulate beam attenuation, we find a significant correlation. However, no significant relationship between the PSD and spectral particulate backscattering was found
Planetary Formation Scenarios Revistied: Core-Accretion Versus Disk Instability
The core-accretion and disk instability models have so far been used to
explain planetary formation. These models have different conditions, such as
planet mass, disk mass, and metallicity for formation of gas giants. The
core-accretion model has a metallicity condition ([Fe/H] > −1.17 in the
case of G-type stars), and the mass of planets formed is less than 6 times that
of the Jupiter mass MJ. On the other hand, the disk instability model does not
have the metallicity condition, but requires the disk to be 15 times more
massive compared to the minimum mass solar nebulae model. The mass of planets
formed is more than 2MJ. These results are compared to the 161 detected planets
for each spectral type of the central stars. The results show that 90% of the
detected planets are consistent with the core-accretion model regardless of the
spectral type. The remaining 10% are not in the region explained by the
core-accretion model, but are explained by the disk instability model. We
derived the metallicity dependence of the formation probability of gas giants
for the core-accretion model. Comparing the result with the observed fraction
having gas giants, they are found to be consistent. On the other hand, the
observation cannot be explained by the disk instability model, because the
condition for gas giant formation is independent of the metallicity.
Consequently, most of planets detected so far are thought to have been formed
by the core-accretion process, and the rest by the disk instability process.Comment: accepted for publication in The Astrophysical Journa
Prevalence of malnutrition among under five year olds attending the maternal child health clinic at Busia County Refferal Hospital
Objective: To determine the prevalence of malnutrition among under five-year-old children attending the maternal child health clinic (MCH) at Busia county referral hospital.Study Design: A retrospective studyStudy Setting: Busia County Referral Hospital in Busia County, western KenyaStudy Subjects/Participants: All children under five years attending the maternal child health clinic in BCRH from 1st June to 1st august 2018.Results: Majority of the children were of good nutritional status 65%, followed by those who have mild acute malnutrition 23% and finally marasmus and kwashiorkor at 9.35% and 1.1% respectively. The most affected gender was females at 53.4%. Age group 6-23months was the majority at 79% followed by 0-5months at 11.04% and finally 24-59months at 9.9%. 84% had a MUAC reading that was normal and indicated normal nutritional status while just 11% had a MUAC reading that indicated risk for acute malnutrition and 5% had MUAC reading that indicated poor nutrition.Conclusion: Majority of the children are of good nutritional status. The challenge is that those of poor nutritional status are significant as the prevalence is at 35%. The recommendation is that the county government should set aside enough funds to oversee countywide campaigns on nutrition, targeting specific cohorts within the community
Molecular Line Emission from Gravitationally Unstable Protoplanetary Disks
In the era of high resolution submillimeter interferometers, it will soon be
possible to observe the neutral circumstellar medium directly involved in gas
giant planet (GGP) formation at physical scales previously unattainable. In
order to explore possible signatures of gas giant planet formation via disk
instabilities, we have combined a 3D, non-local thermodynamic equilibrium (LTE)
radiative transfer code with a 3D, finite differences hydrodynamical code to
model molecular emission lines from the vicinity of a 1.4 M_J self-gravitating
proto-GGP. Here, we explore the properties of rotational transitions of the
commonly observed dense gas tracer, HCO+. Our main results are the following:
1. Very high lying HCO+ transitions (e.g. HCO+ J=7-6) can trace dense planet
forming clumps around circumstellar disks. Depending on the molecular
abundance, the proto-GGP may be directly imageable by the Atacama Large
Millimeter Array (ALMA). 2. HCO+ emission lines are heavily self-absorbed
through the proto-GGP's dense molecular core. This signature is nearly
ubiquitous, and only weakly dependent on assumed HCO+ abundances. The
self-absorption features are most pronounced at higher angular resolutions.
Dense clumps that are not self-gravitating only show minor self-absorption
features. 3. Line temperatures are highest through the proto-GGP at all assumed
abundances and inclination angles. Conversely, due to self-absorption in the
line, the velocity-integrated intensity may not be. High angular resolution
interferometers such as the Submillimeter Array (SMA) and ALMA may be able to
differentiate between competing theories of gas giant planet formation.Comment: 10 pages, 13 figures; Accepted by Ap
The Thermal Regulation of Gravitational Instabilities in Protoplanetary Disks II. Extended Simulations with Varied Cooling Rates
In order to investigate mass transport and planet formation by gravitational
instabilities (GIs), we have extended our 3-D hydrodynamic simulations of
protoplanetary disks from a previous paper. Our goal is to determine the
asymptotic behavior of GIs and how it is affected by different constant cooling
times. Initially, Rdisk = 40 AU, Mdisk = 0.07 Mo, M* = 0.5 Mo, and Qmin = 1.8.
Sustained cooling, with tcool = 2 orps (outer rotation periods, 1 orp ~ 250
yrs), drives the disk to instability in ~ 4 orps. This calculation is followed
for 23.5 orps. After 12 orps, the disk settles into a quasi-steady state with
sustained nonlinear instabilities, an average Q = 1.44 over the outer disk, a
well-defined power-law Sigma(r), and a roughly steady Mdot ~ 5(-7) Mo/yr. The
transport is driven by global low-order spiral modes. We restart the
calculation at 11.2 orps with tcool = 1 and 1/4 orp. The latter case is also
run at high azimuthal resolution. We find that shorter cooling times lead to
increased Mdots, denser and thinner spiral structures, and more violent dynamic
behavior. The asymptotic total internal energy and the azimuthally averaged
Q(r) are insensitive to tcool. Fragmentation occurs only in the high-resolution
tcool = 1/4 orp case; however, none of the fragments survive for even a quarter
of an orbit. Ring-like density enhancements appear and grow near the boundary
between GI active and inactive regions. We discuss the possible implications of
these rings for gas giant planet formation.Comment: Due to document size restrictions, the complete manuscript could not
be posted on astroph. Please go to http://westworld.astro.indiana.edu to
download the full document including figure
An m sin i = 24 Earth Mass Planetary Companion To The Nearby M Dwarf GJ 176
We report the detection of a planetary companion with a minimum mass of m sin
i = 0.0771 M_Jup = 24.5 M_Earth to the nearby (d = 9.4 pc) M2.5V star GJ 176.
The star was observed as part of our M dwarf planet search at the Hobby-Eberly
Telescope (HET). The detection is based on 5 years of high-precision
differential radial velocity (RV) measurements using the
High-Resolution-Spectrograph (HRS). The orbital period of the planet is 10.24
d. GJ 176 thus joins the small (but increasing) sample of M dwarfs hosting
short-periodic planets with minimum masses in the Neptune-mass range. Low mass
planets could be relatively common around M dwarfs and the current detections
might represent the tip of a rocky planet population.Comment: 13 pages preprint, 3 figures, submitted to Ap
Fragmentation Instability of Molecular Clouds: Numerical Simulations
We simulate fragmentation and gravitational collapse of cold, magnetized
molecular clouds. We explore the nonlinear development of an instability
mediated by ambipolar diffusion, in which the collapse rate is intermediate to
fast gravitational collapse and slow quasistatic collapse. Initially uniform
stable clouds fragment into elongated clumps with masses largely determined by
the cloud temperature, but substantially larger than the thermal Jeans mass.
The clumps are asymmetric, with significant rotation and vorticity, and lose
magnetic flux as they collapse. The clump shapes, intermediate collapse rates,
and infall profiles may help explain observations not easily fit by
contemporary slow or rapid collapse models.Comment: 25pp, 20 small eps figures, in press ApJ, April 1, 200
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