7,105 research outputs found
Relationship, through geologic time, of days per lunar month to growth increments in fossil and recent molluscan shells Semiannual status report, 14 Sep. 1967 - 14 Mar. 1968
Relationship of geologic time and days per lunar month to growth patterns in fossil and recent molluscan shell
The Abundance of Molecular Hydrogen and its Correlation with Midplane Pressure in Galaxies: Non-Equilibrium, Turbulent, Chemical Models
Observations of spiral galaxies show a strong linear correlation between the
ratio of molecular to atomic hydrogen surface density R_mol and midplane
pressure. To explain this, we simulate three-dimensional, magnetized
turbulence, including simplified treatments of non-equilibrium chemistry and
the propagation of dissociating radiation, to follow the formation of H_2 from
cold atomic gas. The formation time scale for H_2 is sufficiently long that
equilibrium is not reached within the 20-30 Myr lifetimes of molecular clouds.
The equilibrium balance between radiative dissociation and H_2 formation on
dust grains fails to predict the time-dependent molecular fractions we find. A
simple, time-dependent model of H_2 formation can reproduce the gross behavior,
although turbulent density perturbations increase molecular fractions by a
factor of few above it. In contradiction to equilibrium models, radiative
dissociation of molecules plays little role in our model for diffuse radiation
fields with strengths less than ten times that of the solar neighborhood,
because of the effective self-shielding of H_2. The observed correlation of
R_mol with pressure corresponds to a correlation with local gas density if the
effective temperature in the cold neutral medium of galactic disks is roughly
constant. We indeed find such a correlation of R_mol with density. If we
examine the value of R_mol in our local models after a free-fall time at their
average density, as expected for models of molecular cloud formation by
large-scale gravitational instability, our models reproduce the observed
correlation over more than an order of magnitude range in density.Comment: 24 pages, 4 figures, accepted for publication in Astrophys. J,
changes include addition of models with higher radiation fields and
substantial clarification of the narrativ
Applications of inertial navigation and modern control theory to the all weather landing problem
Inertial navigation and automatic landing control theory applied to instrument landing proble
Ultrasensitivity in phosphorylation-dephosphorylation cycles with little substrate
Cellular decision-making is driven by dynamic behaviours, such as the preparations for sunrise enabled by circadian rhythms and the choice of cell fates enabled by positive feedback. Such behaviours are often built upon ultrasensitive responses where a linear change in input generates a sigmoidal change in output. Phosphorylation-dephosphorylation cycles are one means to generate ultrasensitivity. Using bioinformatics, we show that in vivo levels of kinases and phosphatases frequently exceed the levels of their corresponding substrates in budding yeast. This result is in contrast to the conditions often required by zero-order ultrasensitivity, perhaps the most well known means for how such cycles become ultrasensitive. We therefore introduce a mechanism to generate ultrasensitivity when numbers of enzymes are higher than numbers of substrates. Our model combines distributive and non-distributive actions of the enzymes with two-stage binding and concerted allosteric transitions of the substrate. We use analytical and numerical methods to calculate the Hill number of the response. For a substrate with [Formula: see text] phosphosites, we find an upper bound of the Hill number of [Formula: see text], and so even systems with a single phosphosite can be ultrasensitive. Two-stage binding, where an enzyme must first bind to a binding site on the substrate before it can access the substrate's phosphosites, allows the enzymes to sequester the substrate. Such sequestration combined with competition for each phosphosite provides an intuitive explanation for the sigmoidal shifts in levels of phosphorylated substrate. Additionally, we find cases for which the response is not monotonic, but shows instead a peak at intermediate levels of input. Given its generality, we expect the mechanism described by our model to often underlay decision-making circuits in eukaryotic cells
The geometry of extended null supersymmetry in M-theory
For supersymmetric spacetimes in eleven dimensions admitting a null Killing
spinor, a set of explicit necessary and sufficient conditions for the existence
of any number of arbitrary additional Killing spinors is derived. The necessary
and sufficient conditions are comprised of algebraic relationships, linear in
the spinorial components, between the spinorial components and their first
derivatives, and the components of the spin connection and four-form. The
integrability conditions for the Killing spinor equation are also analysed in
detail, to determine which components of the field equations are implied by
arbitrary additional supersymmetries and the four-form Bianchi identity. This
provides a complete formalism for the systematic and exhaustive investigation
of all spacetimes with extended null supersymmetry in eleven dimensions. The
formalism is employed to show that the general bosonic solution of eleven
dimensional supergravity admitting a structure defined by four Killing
spinors is either locally the direct product of with a
seven-manifold of holonomy, or locally the Freund-Rubin direct product of
with a seven-manifold of weak holonomy. In addition, all
supersymmetric spacetimes admitting a
structure are classified.Comment: 36 pages, latex; v2, section classifying all spacetimes admitting a
structure included; v3, typos
corrected. Final version to appear in Phys.Rev.
Fast Molecular Cloud Destruction Requires Fast Cloud Formation
A large fraction of the gas in the Galaxy is cold, dense, and molecular. If
all this gas collapsed under the influence of gravity and formed stars in a
local free-fall time, the star formation rate in the Galaxy would exceed that
observed by more than an order of magnitude. Other star-forming galaxies behave
similarly. Yet observations and simulations both suggest that the molecular gas
is indeed gravitationally collapsing, albeit hierarchically. Prompt stellar
feedback offers a potential solution to the low observed star formation rate if
it quickly disrupts star-forming clouds during gravitational collapse. However,
this requires that molecular clouds must be short-lived objects, raising the
question of how so much gas can be observed in the molecular phase. This can
occur only if molecular clouds form as quickly as they are destroyed,
maintaining a global equilibrium fraction of dense gas. We therefore examine
cloud formation timescales. We first demonstrate that supernova and superbubble
sweeping cannot produce dense gas at the rate required to match the cloud
destruction rate. On the other hand, Toomre gravitational instability can reach
the required production rate. We thus argue that, although dense, star-forming
gas may last only around a single global free-fall time, the dense gas in
star-forming galaxies can globally exist in a state of dynamic equilibrium
between formation by gravitational instability, and disruption by stellar
feedback. At redshift z >~ 2, the Toomre instability timescale decreases,
resulting in a prediction of higher molecular gas fractions at early times, in
agreement with observations.Comment: 7 pages, no figures, ApJL accepted; v3: corrected several errors,
added discussion, no change in conclusion
Temperature Fluctuations driven by Magnetorotational Instability in Protoplanetary Disks
The magnetorotational instability (MRI) drives magnetized turbulence in
sufficiently ionized regions of protoplanetary disks, leading to mass
accretion. The dissipation of the potential energy associated with this
accretion determines the thermal structure of accreting regions. Until
recently, the heating from the turbulence has only been treated in an
azimuthally averaged sense, neglecting local fluctuations. However, magnetized
turbulence dissipates its energy intermittently in current sheet structures. We
study this intermittent energy dissipation using high resolution numerical
models including a treatment of radiative thermal diffusion in an optically
thick regime. Our models predict that these turbulent current sheets drive
order unity temperature variations even where the MRI is damped strongly by
Ohmic resistivity. This implies that the current sheet structures where energy
dissipation occurs must be well resolved to correctly capture the flow
structure in numerical models. Higher resolutions are required to resolve
energy dissipation than to resolve the magnetic field strength or accretion
stresses. The temperature variations are large enough to have major
consequences for mineral formation in disks, including melting chondrules,
remelting calcium-aluminum rich inclusions, and annealing silicates; and may
drive hysteresis: current sheets in MRI active regions could be significantly
more conductive than the remainder of the disk.Comment: 16 pages, 13 figures, ApJ In Press, updated to match proof
Determination of Shapes of Boattail Bodies of Revolution for Minimum Wave Drag
By use of an approximate equation for the wave drag of slender bodies of revolution in a supersonic flow field, the optimum shapes of certain boattail bodies are determined for minimum wave drag. The properties of three specific families of bodies are determined, the first family consisting of bodies having a given length and base area and a contour passing through a prescribed point between the nose and base, the second family having fixed length, base area, and maximum area, and the third family having given length, volume, and base area. The method presented is easily generalized to determine minimum-wave-drag profile shapes which have contours that must pass through any prescribed number of points. According to linearized theory, the optimum profiles are found to have infinite slope at the nose but zero radius of curvature so that the bodies appear to have pointed noses, a zero slope at the body base, and no variation of wave drag with Mach number. For those bodies having a specified intermediate.diameter (that is, location and magnitude given), the maximum body diameter is shown to be larger, in general, than the specified diameter. It is also shown that, for bodies having a specified maximum diameter, the location of the maximum diameter is not arbitrary but is determined from the ratio of base diameter to maximum diameter
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