37,668 research outputs found
The creeping motion of liquid drops through a circular tube of comparable diameter
The creeping motion through a circular tube of neutrally buoyant Newtonian drops which have an undeformed radius comparable to that of the tube was studied experimentally. Both a Newtonian and a viscoelastic suspending fluid were used in order to determine the influence of viscoelasticity. The extra pressure drop owing to the presence of the suspended drops, the shape and velocity of the drops, and the streamlines of the flow are reported for various viscosity ratios, total flow rates and drop sizes
Migration of rigid spheres in a two-dimensional unidirectional shear flow of a second-order fluid
The lateral migration of a neutrally buoyant rigid sphere suspended in a second-order fluid is studied theoretically for unidirectional two-dimensional flows. The results demonstrate the existence of migration induced by normal stresses whenever there is a lateral variation of the shear rate in the undisturbed flow. The migration occurs in the direction of decreasing absolute shear rate, which is towards the centre-line for a plane Poiseuille flow and towards the outer cylinder wall for Couette flow. The direction of migration agrees with existing experimental data for a viscoelastic suspending fluid, and qualitative agreement is found between the theoretically predicted and experimentally measured sphere trajectories
Circumbinary Molecular Rings Around Young Stars in Orion
We present high angular resolution 1.3 mm continuum, methyl cyanide molecular
line, and 7 mm continuum observations made with the Submillimeter Array and the
Very Large Array, toward the most highly obscured and southern part of the
massive star forming region OMC1S located behind the Orion Nebula. We find two
flattened and rotating molecular structures with sizes of a few hundred
astronomical units suggestive of circumbinary molecular rings produced by the
presence of two stars with very compact circumstellar disks with sizes and
separations of about 50 AU, associated with the young stellar objects 139-409
and 134-411. Furthermore, these two circumbinary rotating rings are related to
two compact and bright {\it hot molecular cores}. The dynamic mass of the
binary systems obtained from our data are 4 M for 139-409 and
0.5 M for 134-411. This result supports the idea that
intermediate-mass stars will form through {\it circumstellar disks} and
jets/outflows, as the low mass stars do. Furthermore, when intermediate-mass
stars are in multiple systems they seem to form a circumbinary ring similar to
those seen in young, multiple low-mass systems (e.g., GG Tau and UY Aur).Comment: Accepted by Astronomy and Astrophysic
A Tidal Flare Candidate in Abell 1795
As part of our ongoing archival X-ray survey of galaxy clusters for tidal
flares, we present evidence of an X-ray transient source within 1 arcmin of the
core of Abell 1795. The extreme variability (a factor of nearly 50), luminosity
(> 2 x 10^42 erg s^{-1}), long duration (> 5 years) and supersoft X-ray
spectrum (< 0.1 keV) are characteristic signatures of a stellar tidal
disruption event according to theoretical predictions and to existing X-ray
observations, implying a massive >~10^5 M_sun black hole at the centre of that
galaxy. The large number of X-ray source counts (~700) and long temporal
baseline (~12 years with Chandra and XMM-Newton) make this one of the
best-sampled examples of any tidal flare candidate to date. The transient may
be the same EUV source originally found contaminating the diffuse ICM
observations of Bowyer et al. (1999), which would make it the only tidal flare
candidate with reported EUV observations and implies an early source luminosity
1-2 orders of magnitude greater. If the host galaxy is a cluster member then it
must be a dwarf galaxy, an order of magnitude less massive than the quiescent
galaxy Henize 2-10 which hosts a massive black hole that is difficult to
reconcile with its low mass. The unusual faintness of the host galaxy may be
explained by tidal stripping in the cluster core.Comment: Accepted by MNRAS 2013 July 23. 27 pages, 10 figure
Self-shadowing Effects of Slim Accretion Disks in Active Galactic Nuclei: Diverse Appearance of the Broad-line Region
Supermassive black holes in active galactic nuclei (AGNs) undergo a wide
range of accretion rates, which lead to diversity of appearance. We consider
the effects of anisotropic radiation from accretion disks on the broad-line
region (BLR), from the Shakura-Sunyaev regime to slim disks with
super-Eddington accretion rates. The geometrically thick funnel of the inner
region of slim disks produces strong self-shadowing effects that lead to very
strong anisotropy of the radiation field. We demonstrate that the degree of
anisotropy of the radiation fields grows with increasing accretion rate. As a
result of this anisotropy, BLR clouds receive different spectral energy
distributions depending on their location relative to the disk, resulting in
diverse observational appearance of the BLR. We show that the self-shadowing of
the inner parts of the disk naturally produces two dynamically distinct regions
of the BLR, depending on accretion rate. These two regions manifest themselves
as kinematically distinct components of the broad H line profile with
different line widths and fluxes, which jointly account for the Lorentzian
profile generally observed in narrow-line Seyfert 1 galaxies. In the time
domain, these two components are expected reverberate with different time lags
with respect to the varying ionizing continuum, depending on the accretion rate
and the viewing angle of the observer. The diverse appearance of the BLR due to
the anisotropic ionizing energy source can be tested by reverberation mapping
of H and other broad emission lines (e.g., \feii), providing a new tool
to diagnose the structure and dynamics of the BLR. Other observational
consequences of our model are also explored.Comment: emulatapj style, 15 pages, 6 figures, in pres
VLA Imaging of the Disk Surrounding the Nearby Young Star TW Hya
The TW Hya system is perhaps the closest analog to the early solar nebula. We
have used the Very Large Array to image TW Hya at wavelengths of 7mm and 3.6 cm
with resolutions 0.1 arcseconds (about 5 AU) and 1.0 arcseconds (about 50 AU),
respectively. The 7mm emission is extended and appears dominated by a dusty
disk of radius larger than 50 AU surrounding the star. The 3.6 cm emission is
unresolved and likely arises from an ionized wind or gyrosynchrotron activity.
The dust spectrum and spatially resolved 7mm images of the TW Hya disk are
fitted by a simple model with temperature and surface density described by
radial power laws, and . These
properties are consistent with an irradiated gaseous accretion disk of mass
with an accretion rate and viscosity parameter . The estimates of
mass and mass accretion rates are uncertain as the gas-to-dust ratio in the TW
Hya disk may have evolved from the standard interstellar value.Comment: 13 pages, 3 figures, accepted by ApJ Letter
A halo and a blow-out in NGC 253
We present the discovery of a 'synchrotron halo' in NGC 253. NGC 253 is an inclined, dusty, barred Sc galaxy in the Sculptor Group. It is also one of the prototype nuclear star burst galaxies. Very Large Array observations at 327 MHz and 1.4 GHz have revealed a synchrotron emitting 'halo' extending 9 kpc above and below the plane of NGC 253 (assuming a distance of 4 Mpc). The spectral index for the radio emission steepens from -0.7 in the disk to -1.0 in the halo. The fractional polarization at 1.4 GHz increases from less than or equal to 2 percent in the disk up to 20 percent in the halo, and the magnetic field vectors project parallel to the plane over most of the halo
Efficient out-coupling and beaming of Tamm optical states via surface plasmon polariton excitation
We present evidence of optical Tamm states to surface plasmon polariton (SPP) coupling. We experimentally demonstrate that for a Bragg stack with a thin metal layer on the surface, hybrid Tamm-SPP modes may be excited when a grating on the air-metal interface is introduced. Out-coupling via the grating to free space propagation is shown to enhance the transmission as well as the directionality and polarization selection for the transmitted beam. We suggest that this system will be useful on those devices, where a metallic electrical contact as well as beaming and polarization control is needed
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