3,829 research outputs found
Detection of turbulent thermal diffusion of particles in numerical simulations
The phenomenon of turbulent thermal diffusion in temperature-stratified
turbulence causing a non-diffusive turbulent flux of inertial and non-inertial
particles in the direction of the turbulent heat flux is found using direct
numerical simulations (DNS). In simulations with and without gravity, this
phenomenon is found to cause a peak in the particle number density around the
minimum of the mean fluid temperature for Stokes numbers less than 1, where the
Stokes number is the ratio of particle Stokes time to turbulent Kolmogorov time
at the viscous scale. Turbulent thermal diffusion causes the formation of
large-scale inhomogeneities in the spatial distribution of inertial particles.
The strength of this effect is maximum for Stokes numbers around unity, and
decreases again for larger values. The dynamics of inertial particles is
studied using Lagrangian modelling in forced temperature-stratified turbulence,
whereas non-inertial particles and the fluid are described using DNS in an
Eulerian framework.Comment: 9 pages, 6 figures, REVTEX4-1, extended pape
Charges on Strange Quark Nuggets in Space
Since Witten's seminal 1984 paper on the subject, searches for evidence of
strange quark nuggets (SQNs) have proven unsuccessful. In the absence of
experimental evidence ruling out SQNs, the validity of theories introducing
mechanisms that increase their stability should continue to be tested. To
stimulate electromagnetic SQN searches, particularly space searches, we
estimate the net charge that would develop on an SQN in space exposed to
various radiation baths (and showers) capable of liberating their less strongly
bound electrons, taking into account recombination with ambient electrons. We
consider, in particular, the cosmic background radiation, radiation from the
sun, and diffuse galactic and extragalactic -ray backgrounds. A
possible dramatic signal of SQNs in explosive astrophysical events is noted.Comment: CitationS added, new subsection added, more discussion, same
numerical result
The Eastern Arm of M83 Revisited: High-Resolution Mapping of 12CO 1-0 Emission
We have used the Owens Valley Millimeter Array to map 12CO (J=1-0) along a
3.5 kpc segment of M83's eastern spiral arm at resolutions of 6.5"x3.5", 10",
and 16". The CO emission in most of this segment lies along the sharp dust lane
demarking the inner edge of the spiral arm, but beyond a certain point along
the arm the emission shifts downstream from the dust lane to become better
aligned with the young stars seen in blue and H-beta images. This morphology
resembles that of the western arm of M100. Three possibilities, none of which
is wholly satisfactory, are considered to explain the deviation of the CO arm
from the dust lane: heating of the CO by UV radiation from young stars, heating
by low-energy cosmic rays, and a molecular medium consisting of two (diffuse
and dense) components which react differently to the density wave. Regardless,
the question of what CO emission traces along this spiral arm is a complicated
one. Strong tangential streaming is observed where the arm crosses the
kinematic major axis of the galaxy, implying that the shear becomes locally
prograde in the arms. Inferred from the streaming is a very high gas surface
density of about 230 solar masses/pc**2 and an arm-interarm contrast greater
than 2.3 in the part of the arm near the major axis. Using two different
criteria, we find that the gas at this location is well above the threshold for
gravitational instability -- much more clearly so than in either M51 or M100.Comment: Accepted for publication in ApJ. 25 pages, 5 figures. Manuscript in
LaTeX, figures in pdf. Fig 3 in colo
Rotating Nuclear Rings and Extreme Starbursts in Ultraluminous Galaxies
New high resolution interferometer data of 10 IR ultraluminous galaxies shows
the molecular gas is in rotating nuclear rings or disks with radii 300 to 800
pc. Most of the CO flux comes from a moderate-density, warm, intercloud medium
rather than self-gravitating clouds. Gas masses of ~ 5 x 10^9 Msun, 5 times
lower than the standard method are derived from a model of the molecular disks.
The ratio of molecular gas to dynamical mass, is M_gas/M_dyn ~ 1/6 with a
maximum ratio of gas to total mass surface density of 1/3. For the galaxies
VIIZw31, Arp193, and IRAS 10565+24, there is good evidence for rotating
molecular rings with a central gap. In addition to the rotating rings a new
class of star formation region is identified which we call an Extreme
Starburst. They have a characteristic size of only 100 pc., about 10^9 Msun of
gas and an IR luminosity of ~3 x 10^11 Lsun. Four extreme starbursts are
identified in the 3 closest galaxies in the sample Arp220, Arp193 and Mrk273.
They are the most prodigious star formation events in the local universe, each
representing about 1000 times as many OB stars as 30 Doradus. In Arp220, the CO
and 1.3 mm continuum maps show the two ``nuclei'' embedded in a central ring or
disk and a fainter structure extending 3 kpc to the east, normal to the nuclear
disk. There is no evidence that these sources really are the pre-merger nuclei.
They are compact, extreme starburst regions containing 10^9 Msun of dense
molecular gas and new stars, but no old stars. Most of the dust emission and
HCN emission arises in the two extreme starbursts. The entire bolometric
luminosity of Arp~220 comes from starbursts, not an AGN. In Mrk231, the disk
geometry shows that the molecular disk cannot be heated by the AGN; the far IR
luminosity of Mrk~231 is powered by a starburst, not the AGN. (Abridged)Comment: 97 pages Latex with aasms.sty, including 29 encapsulated Postscript
figures. Figs 18 and 23 are GIFs. 31 figures total. Text and higher quality
versions of figures available at
http://sbastk.ess.sunysb.edu/www/RINGS_ESB_PREPRINT.html To be published in
Ap. J., 10 Nov. 199
Global axisymmetric stability analysis for a composite system of two gravitationally coupled scale-free discs
In a composite system of gravitationally coupled stellar and gaseous discs,
we perform linear stability analysis for axisymmetric coplanar perturbations
using the two-fluid formalism. The background stellar and gaseous discs are
taken to be scale-free with all physical variables varying as powers of
cylindrical radius with compatible exponents. The unstable modes set in as
neutral modes or stationary perturbation configurations with angular frequency
.Comment: 7 pages using AAS styl
Hysteroscopic metroplasty: current status of the problem
Hysteroscopic metroplasty is very important for the restoration of the normal anatomy of the uterus, creation of the conditions for favorable obstetric outcomes. This review of the current scientific papers on the topic summarizes the data on the indications, preoperative preparation, and techniques of hysteroscopic metroplasty. It also presents the information on the approaches to the evaluation of the effectiveness of this kind of surgery, peculiarities of the postoperative care, preventive treatment of the intra- and postoperative complications. The paper provides state-of-the art information on the importance of the hysteroscopic metroplasty for the reproductive health of a woman and peculiarities of the gestation course after the dissection of the intrauterine septum.ΠΠΈΡΡΠ΅ΡΠΎΡΠΊΠΎΠΏΠΈΡΠ΅ΡΠΊΠ°Ρ ΠΌΠ΅ΡΡΠΎΠΏΠ»Π°ΡΡΠΈΠΊΠ° ΠΈΠΌΠ΅Π΅Ρ Π²Π°ΠΆΠ½ΠΎΠ΅ Π·Π½Π°ΡΠ΅Π½ΠΈΠ΅ Π΄Π»Ρ Π²ΠΎΡΡΡΠ°Π½ΠΎΠ²Π»Π΅Π½ΠΈΡ Π½ΠΎΡΠΌΠ°Π»ΡΠ½ΠΎΠΉ Π°Π½Π°ΡΠΎΠΌΠΈΠΈ ΠΌΠ°ΡΠΊΠΈ, ΡΠΎΠ·Π΄Π°Π½ΠΈΡ ΡΡΠ»ΠΎΠ²ΠΈΠΉ Π΄Π»Ρ Π±Π»Π°Π³ΠΎΠΏΡΠΈΡΡΠ½ΡΡ
Π°ΠΊΡΡΠ΅ΡΡΠΊΠΈΡ
ΠΈΡΡ
ΠΎΠ΄ΠΎΠ². Π ΠΎΠ±Π·ΠΎΡΠ΅ Π΄Π°Π½Π½ΡΡ
ΡΠΎΠ²ΡΠ΅ΠΌΠ΅Π½Π½ΠΎΠΉ Π»ΠΈΡΠ΅ΡΠ°ΡΡΡΡ ΠΎΠ±ΠΎΠ±ΡΠ΅Π½Ρ ΡΠ²Π΅Π΄Π΅Π½ΠΈΡ ΠΎ ΠΏΠΎΠΊΠ°Π·Π°Π½ΠΈΡΡ
, ΠΏΡΠ΅Π΄ΠΎΠΏΠ΅ΡΠ°ΡΠΈΠΎΠ½Π½ΠΎΠΉ ΠΏΠΎΠ΄Π³ΠΎΡΠΎΠ²ΠΊΠ΅, ΠΌΠ΅ΡΠΎΠ΄Π°Ρ
Π³ΠΈΡΡΠ΅ΡΠΎΡΠΊΠΎΠΏΠΈΡΠ΅ΡΠΊΠΎΠΉ ΠΌΠ΅ΡΡΠΎΠΏΠ»Π°ΡΡΠΈΠΊΠΈ. ΠΡΠΈΠ²Π΅Π΄Π΅Π½Π° ΠΈΠ½ΡΠΎΡΠΌΠ°ΡΠΈΡ ΠΎ ΡΠΏΠΎΡΠΎΠ±Π°Ρ
ΠΎΡΠ΅Π½ΠΊΠΈ ΡΡΡΠ΅ΠΊΡΠΈΠ²Π½ΠΎΡΡΠΈ Π΄Π°Π½Π½ΠΎΠΉ ΠΎΠΏΠ΅ΡΠ°ΡΠΈΠΈ, ΠΎΡΠΎΠ±Π΅Π½Π½ΠΎΡΡΡΡ
Π²Π΅Π΄Π΅Π½ΠΈΡ ΠΏΠΎΡΠ»Π΅ΠΎΠΏΠ΅ΡΠ°ΡΠΈΠΎΠ½Π½ΠΎΠ³ΠΎ ΠΏΠ΅ΡΠΈΠΎΠ΄Π°, ΠΏΡΠΎΡΠΈΠ»Π°ΠΊΡΠΈΠΊΠΈ ΠΈΠ½ΡΡΠ°- ΠΈ ΠΏΠΎΡΠ»Π΅ΠΎΠΏΠ΅ΡΠ°ΡΠΈΠΎΠ½Π½ΡΡ
ΠΎΡΠ»ΠΎΠΆΠ½Π΅Π½ΠΈΠΉ, Π° ΡΠ°ΠΊΠΆΠ΅ ΠΏΡΠ΅Π΄ΡΡΠ°Π²Π»Π΅Π½Ρ ΡΠΎΠ²ΡΠ΅ΠΌΠ΅Π½Π½ΡΠ΅ ΡΠ²Π΅Π΄Π΅Π½ΠΈΡ ΠΎ Π·Π½Π°ΡΠ΅Π½ΠΈΠΈ Π³ΠΈΡΡΠ΅ΡΠΎΡΠΊΠΎΠΏΠΈΡΠ΅ΡΠΊΠΎΠΉ ΠΌΠ΅ΡΡΠΎΠΏΠ»Π°ΡΡΠΈΠΊΠΈ Π΄Π»Ρ ΡΠ΅ΠΏΡΠΎΠ΄ΡΠΊΡΠΈΠ²Π½ΠΎΠ³ΠΎ Π·Π΄ΠΎΡΠΎΠ²ΡΡ ΠΆΠ΅Π½ΡΠΈΠ½Ρ, ΠΎΡΠΎΠ±Π΅Π½Π½ΠΎΡΡΡΡ
ΡΠ΅ΡΠ΅Π½ΠΈΡ Π±Π΅ΡΠ΅ΠΌΠ΅Π½Π½ΠΎΡΡΠΈ ΠΏΠΎΡΠ»Π΅ ΡΠ°ΡΡΠ΅ΡΠ΅Π½ΠΈΡ Π²Π½ΡΡΡΠΈΠΌΠ°ΡΠΎΡΠ½ΠΎΠΉ ΠΏΠ΅ΡΠ΅Π³ΠΎΡΠΎΠ΄ΠΊΠΈ
Rapid planetesimal formation in turbulent circumstellar discs
The initial stages of planet formation in circumstellar gas discs proceed via
dust grains that collide and build up larger and larger bodies (Safronov 1969).
How this process continues from metre-sized boulders to kilometre-scale
planetesimals is a major unsolved problem (Dominik et al. 2007): boulders stick
together poorly (Benz 2000), and spiral into the protostar in a few hundred
orbits due to a head wind from the slower rotating gas (Weidenschilling 1977).
Gravitational collapse of the solid component has been suggested to overcome
this barrier (Safronov 1969, Goldreich & Ward 1973, Youdin & Shu 2002). Even
low levels of turbulence, however, inhibit sedimentation of solids to a
sufficiently dense midplane layer (Weidenschilling & Cuzzi 1993, Dominik et al.
2007), but turbulence must be present to explain observed gas accretion in
protostellar discs (Hartmann 1998). Here we report the discovery of efficient
gravitational collapse of boulders in locally overdense regions in the
midplane. The boulders concentrate initially in transient high pressures in the
turbulent gas (Johansen, Klahr, & Henning 2006), and these concentrations are
augmented a further order of magnitude by a streaming instability (Youdin &
Goodman 2005, Johansen, Henning, & Klahr 2006, Johansen & Youdin 2007) driven
by the relative flow of gas and solids. We find that gravitationally bound
clusters form with masses comparable to dwarf planets and containing a
distribution of boulder sizes. Gravitational collapse happens much faster than
radial drift, offering a possible path to planetesimal formation in accreting
circumstellar discs.Comment: To appear in Nature (30 August 2007 issue). 18 pages (in referee
mode), 3 figures. Supplementary Information can be found at 0708.389
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