7,601 research outputs found
Wind work on the geostrophic ocean circulation: An observational study of the effect of small scales in the wind stress
We use QuikSCAT scatterometer data, together with geostrophic surface currents calculated from a combination of satellite altimetry, gravity and drifter data, to investigate the rate of work done on the geostrophic circulation by wind stress. In particular, we test the suggestion that accounting for ocean currents in the calculation of stress from 10 m winds can result in a reduction of 20–35% in the wind work, compared with an
approximate calculation in which currents are not accounted for. We calculate the predicted effect of accounting for ocean currents to be a reduction in power of about 0.19 TW, and find a total power input from observations which include this effect to be 0.76 TW, smaller than earlier estimates by about the right amount. By recalculating the power input using smoothed wind stresses or currents, we demonstrate that the effect of
ocean currents is visible in the midlatitude data, and close to the predicted value. Proof that the data are adequate to resolve the effect in the tropics, however, is lacking, suggesting that additional processes may also be important in this region
The central density of a neutron star is unaffected by a binary companion at linear order in
Recent numerical work by Wilson, Mathews, and Marronetti [J. R. Wilson, G. J.
Mathews and P. Marronetti, Phys. Rev. D 54, 1317 (1996)] on the coalescence of
massive binary neutron stars shows a striking instability as the stars come
close together: Each star's central density increases by an amount proportional
to 1/(orbital radius). This overwhelms any stabilizing effects of tidal
coupling [which are proportional to 1/(orbital radius)^6] and causes the stars
to collapse before they merge. Since the claimed increase of density scales
with the stars' mass, it should also show up in a perturbation limit where a
point particle of mass orbits a neutron star. We prove analytically that
this does not happen; the neutron star's central density is unaffected by the
companion's presence to linear order in . We show, further, that the
density increase observed by Wilson et. al. could arise as a consequence of not
faithfully maintaining boundary conditions.Comment: 3 pages, REVTeX, no figures, submitted to Phys Rev D as a Rapid
Communicatio
Imaging the Thermal and Kinematic Sunyaev-Zel'dovich Effect Signals in a Sample of Ten Massive Galaxy Clusters: Constraints on Internal Velocity Structures and Bulk Velocities
We have imaged the Sunyaev-Zel'dovich (SZ) effect signals at 140 and 270 GHz
towards ten galaxy clusters with Bolocam and AzTEC/ASTE. We also used Planck
data to constrain the signal at large angular scales, Herschel-SPIRE images to
subtract the brightest galaxies that comprise the cosmic infrared background
(CIB), Chandra imaging to map the electron temperature of the
intra-cluster medium (ICM), and HST imaging to derive models of each galaxy
cluster's mass density. The galaxy clusters gravitationally lens the background
CIB, which produced an on-average reduction in brightness towards the galaxy
clusters' centers after the brightest galaxies were subtracted. We corrected
for this deficit, which was between 5-25% of the 270 GHz SZ effect signal
within . Using the SZ effect measurements, along with the X-ray
constraint on , we measured each galaxy cluster's average line of sight
(LOS) velocity within , with a median per-cluster uncertainty
of +-700 km/s. We found an ensemble-mean of 430+-210 km/s, and an
intrinsic cluster-to-cluster scatter of 470+-340 km/s. We also
obtained maps of over each galaxy cluster's face with an angular
resolution of 70". All four galaxy clusters previously identified as having a
merger oriented along the LOS showed an excess variance in these maps at a
significance of 2-4, indicating an internal rms of 1000
km/s. None of the six galaxy clusters previously identified as relaxed or plane
of sky mergers showed any such excess variance.Comment: Accepted for publication in Ap
Submm/mm Galaxy Counterpart Identification Using a Characteristic Density Distribution
We present a new submm/mm galaxy counterpart identification technique which
builds on the use of Spitzer IRAC colors as discriminators between likely
counterparts and the general IRAC galaxy population. Using 102 radio- and
SMA-confirmed counterparts to AzTEC sources across three fields (GOODS-N,
GOODS-S, and COSMOS), we develop a non-parametric IRAC color-color
characteristic density distribution (CDD), which, when combined with positional
uncertainty information via likelihood ratios, allows us to rank all potential
IRAC counterparts around SMGs and calculate the significance of each ranking
via the reliability factor. We report all robust and tentative radio
counterparts to SMGs, the first such list available for AzTEC/COSMOS, as well
as the highest ranked IRAC counterparts for all AzTEC SMGs in these fields as
determined by our technique. We demonstrate that the technique is free of radio
bias and thus applicable regardless of radio detections. For observations made
with a moderate beamsize (~18"), this technique identifies ~85 per cent of SMG
counterparts. For much larger beamsizes (>30"), we report identification rates
of 33-49 per cent. Using simulations, we demonstrate that this technique is an
improvement over using positional information alone for observations with
facilities such as AzTEC on the LMT and SCUBA-2 on JCMT.Comment: 30 pages, 9 figures, 5 tables. Accepted for publication in MNRA
Evolving Einstein's Field Equations with Matter: The ``Hydro without Hydro'' Test
We include matter sources in Einstein's field equations and show that our
recently proposed 3+1 evolution scheme can stably evolve strong-field
solutions. We insert in our code known matter solutions, namely the
Oppenheimer-Volkoff solution for a static star and the Oppenheimer-Snyder
solution for homogeneous dust sphere collapse to a black hole, and evolve the
gravitational field equations. We find that we can evolve stably static,
strong-field stars for arbitrarily long times and can follow dust sphere
collapse accurately well past black hole formation. These tests are useful
diagnostics for fully self-consistent, stable hydrodynamical simulations in 3+1
general relativity. Moreover, they suggest a successive approximation scheme
for determining gravitational waveforms from strong-field sources dominated by
longitudinal fields, like binary neutron stars: approximate quasi-equilibrium
models can serve as sources for the transverse field equations, which can be
evolved without having to re-solve the hydrodynamical equations (``hydro
without hydro'').Comment: 4 postscript figures. Submitted to Phys. Rev. D15 as a Brief Repor
Radiation thermo-chemical models of protoplanetary disks II. Line diagnostics
Methods. We use the recently developed disk code ProDiMo to calculate the
physico-chemical structure of protoplanetary disks and apply the Monte-Carlo
line radiative transfer code RATRAN to predict observable line profiles and
fluxes. We consider a series of Herbig Ae type disk models ranging from 10^-6
M_Sun to 2.2 10^-2 M_Sun (between 0.5 and 700 AU) to discuss the dependency of
the line fluxes and ratios on disk mass for otherwise fixed disk parameters.
Results. We find the [CII] 157.7 mum line to originate in LTE from the surface
layers of the disk, where Tg > Td . The total emission is dominated by surface
area and hence depends strongly on disk outer radius. The [OI] lines can be
very bright (> 10^-16 W/m^2) and form in slightly deeper and closer regions
under non-LTE conditions. The high-excitation [OI] 145.5 mum line, which has a
larger critical density, decreases more rapidly with disk mass than the 63.2
mum line. Therefore, the [OI] 63.2 mum/145.5 mum ratio is a promising disk mass
indicator, especially as it is independent of disk outer radius for Rout > 200
AU. CO is abundant only in deeper layers A_V >~ 0.05. For too low disk masses
(M_disk <~10^-4 M_Sun) the dust starts to become transparent, and CO is almost
completely photo-dissociated. For masses larger than that the lines are an
excellent independent tracer of disk outer radius and can break the outer
radius degeneracy in the [OI] 63.2 mum/[CII]157.7 mum line ratio. Conclusions.
The far-IR fine-structure lines of [CII] and [OI] observable with Herschel
provide a promising tool to measure the disk gas mass, although they are mainly
generated in the atomic surface layers. In spatially unresolved observations,
none of these lines carry much information about the inner, possibly hot
regions < 30 AU.Comment: accepted for publication in A&
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