34,114 research outputs found
Variations in mid-ocean ridge CO2 emissions driven by glacial cycles
The geological record shows links between glacial cycles and volcanic
productivity, both subaerially and at mid-ocean ridges. Sea-level-driven
pressure changes could also affect chemical properties of mid-ocean ridge
volcanism. We consider how changing sea-level could alter the CO2 emissions
rate from mid-ocean ridges, on both the segment and global scale. We develop a
simplified transport model for a highly incompatible element through a
homogenous mantle; variations in the melt concentration the emission rate of
the element are created by changes in the depth of first silicate melting. The
model predicts an average global mid-ocean ridge CO2 emissions-rate of 53
Mt/yr, in line with other estimates. We show that falling sea level would cause
an increase in ridge CO2 emissions with a lag of about 100 kyrs after the
causative sea level change. The lag and amplitude of the response are sensitive
to mantle permeability and plate spreading rate. For a reconstructed sea-level
time series of the past million years, we predict variations of up to 12% (7
Mt/yr) in global mid-ocean ridge CO2 emissions. The magnitude and timing of the
predicted variations in CO2 emissions suggests a potential role for ridge
carbon emissions in glacial cycles
Analysis of integration error in the ODP-L program and the effect of selenopotential parameters on the solution vector
Analysis of integration error in lunar orbit trajectory program and effect of selenopotential parameters on solution vecto
The age structure of stellar populations in the solar vicinity. Clues of a two-phase formation history of the Milky Way disk
We analyze high quality abundances data of solar neighborhood stars and show
that there are two distinct regimes of [alpha/Fe] versus age which we identify
as the epochs of the thick and thin disk formation. A tight correlation between
metallicity and [alpha/Fe] versus age is clearly identifiable on thick disk
stars, implying that this population formed from a well mixed ISM, over a time
scale of 4-5 Gyr. Thick disk stars vertical velocity dispersion correlate with
age, with the youngest objects having as small scale heights as those of thin
disk stars. A natural consequence of these two results is that a vertical
metallicity gradient is expected in this population. We suggest that the thick
disk set the initial conditions for the formation of the inner thin disk. This
provides also an explanation of the apparent coincidence between the step in
metallicity at 7-10 kpc in the thin disk and the confinment of the thick disk
at about R<10 kpc. We suggest that the outer thin disk developped outside the
influence of the thick disk, but also that the high alpha-enrichment of the
outer regions may originate from a primordial pollution by the gas expelled
from the thick disk. Local metal-poor thin disk stars, whose properties are
best explained by an origin in the outer disk, are shown to be as old as the
youngest thick disk (9-10 Gyr), implying that the outer thin disk started to
form while the thick disk formation was still on-going in the inner Galaxy. We
point out that, given the tight age-abundance relations in the thick disk, an
inside-out process would give rise to a radial gradient in abundances in this
population which is not observed. Finally, we argue that the data discussed
here leave little room for radial migration, either to have contaminated the
solar vicinity, or, to have redistributed stars in significant proportion
across the solar annulus.Comment: Accepted in A&A, Revised version with new figures and extended
discussio
On the Evolutionary History of Stars and their Fossil Mass and Light
The total extragalactic background radiation can be an important test of the
global star formation history (SFH). Using direct observational estimates of
the SFH, along with standard assumptions about the initial mass function (IMF),
we calculate the total extragalactic background radiation and the observed
stellar density today. We show that plausible SFHs allow a significant range in
each quantity, but that their ratio is very tightly constrained. Current
estimates of the stellar mass and extragalactic background are difficult to
reconcile, as long as the IMF is fixed to the Salpeter slope above 1 Msun. The
joint confidence interval of these two quantities only agrees with that
determined from the allowed range of SFH fits at the 3-sigma level, and for our
best-fit values the discrepancy is about a factor of two. Alternative energy
sources that contribute to the background, such as active galactic nuclei
(AGN), Population III stars, or decaying particles, appear unlikely to resolve
the discrepancy. However, changes to the IMF allow plausible solutions to the
background problem. The simplest is an average IMF with an increased
contribution from stars around 1.5--4 Msun. A ``paunchy'' IMF of this sort
could emerge as a global average if low mass star formation is suppressed in
galaxies experiencing rapid starbursts. Such an IMF is consistent with
observations of star-forming regions, and would help to reconcile the fossil
record of star formation with the directly observed SFH.Comment: 21 pages, 7 figures, 3 tables; submitted to Monthly Notice
Private Database Queries Using Quantum States with Limited Coherence Times
We describe a method for private database queries using exchange of quantum
states with bits encoded in mutually incompatible bases. For technology with
limited coherence time, the database vendor can announce the encoding after a
suitable delay to allow the user to privately learn one of two items in the
database without the ability to also definitely infer the second item. This
quantum approach also allows the user to choose to learn other functions of the
items, such as the exclusive-or of their bits, but not to gain more information
than equivalent to learning one item, on average. This method is especially
useful for items consisting of a few bits by avoiding the substantial overhead
of conventional cryptographic approaches.Comment: extended to generalized (POVM) measurement
Phase transitions, double-scaling limit, and topological strings
Topological strings on Calabi--Yau manifolds are known to undergo phase
transitions at small distances. We study this issue in the case of perturbative
topological strings on local Calabi--Yau threefolds given by a bundle over a
two-sphere. This theory can be regarded as a q--deformation of Hurwitz theory,
and it has a conjectural nonperturbative description in terms of q--deformed 2d
Yang--Mills theory. We solve the planar model and find a phase transition at
small radius in the universality class of 2d gravity. We give strong evidence
that there is a double--scaled theory at the critical point whose all genus
free energy is governed by the Painlev\'e I equation. We compare the critical
behavior of the perturbative theory to the critical behavior of its
nonperturbative description, which belongs to the universality class of 2d
supergravity. We also give evidence for a new open/closed duality relating
these Calabi--Yau backgrounds to open strings with framing.Comment: 49 pages, 3 eps figures; section added on non-perturbative proposal
and 2d gravity; minor typos correcte
Investigation of light scattering in highly reflecting pigmented coatings. Volume 2 - Classical investigations, theoretical and experimental Final report, 1 May 30 Sep. 1966
Light scattering theory applied to particle arrays to explain scattering in highly reflecting paint coating
When the Milky Way turned off the lights: APOGEE provides evidence of star formation quenching in our Galaxy
Quenching, the cessation of star formation, is one of the most significant
events in the life cycle of galaxies. We show here the first evidence that the
Milky Way experienced a generalised quenching of its star formation at the end
of its thick disk formation 9 Gyr ago. Elemental abundances of stars
studied as part of the APOGEE survey reveal indeed that in less than 2
Gyr the star formation rate in our Galaxy dropped by an order-of-magnitude.
Because of the tight correlation between age and alpha abundance, this event
reflects in the dearth of stars along the inner disk sequence in the
[Fe/H]-[/Fe] plane. Before this phase, which lasted about 1.5 Gyr, the
Milky Way was actively forming stars. Afterwards, the star formation resumed at
a much lower level to form the thin disk. These events are very well matched by
the latest observation of MW-type progenitors at high redshifts. In late type
galaxies, quenching is believed to be related to a long and secular exhaustion
of gas. In our Galaxy, it occurred on a much shorter time scale, while the
chemical continuity before and after the quenching indicates that it was not
due to the exhaustion of the gas. While quenching is generally associated with
spheroids, our results show that it also occurs in galaxies like the Milky Way,
possibly when they are undergoing a morphological transition from thick to thin
disks. Given the demographics of late type galaxies in the local universe, in
which classical bulges are rare, we suggest further that this may hold true
generally in galaxies with mass lower than or approximately , where
quenching could be directly a consequence of thick disk formation. We emphasize
that the quenching phase in the Milky Way could be contemporaneous with, and
related to, the formation of the bar. We sketch a scenario on how a strong bar
may inhibit star formation.Comment: 17 pages, 8 figures. Published versio
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