4,307 research outputs found
Radiative lepton flavor violating decays in the Randall Sundrum background with localized leptons
We study the radiative lepton flavor violating l_i -> l_j\gamma decays in the
two Higgs doublet model, respecting the Randall Sundrum scenario and estimate
the contributions of the KK modes of left (right) handed charged lepton
doublets (singlets) on the branching ratios. We observe that the branching
ratios are sensitive to the contributions of the charged lepton KK modes.Comment: 23 pages, 10 figures, 2 table
Formation of shear bands in drying colloidal dispersions
In directionally dried colloidal dispersions regular bands can appear behind the drying front, inclined at ±45° to the drying line. Although these features have been noted to share visual similarities with shear bands in metal, no physical mechanism for their formation has ever been suggested, until very recently. Here, through microscopy of silica and polystyrene dispersions, dried in Hele-Shaw cells, we demonstrate that the bands are indeed associated with local shear strains. We further show how the bands form, that they scale with the thickness of the drying layer, and that they are eliminated by the addition of salt to the drying dispersions. Finally, we reveal the origins of these bands in the compressive forces associated with drying
Flickering of 1.3 cm Sources in Sgr B2: Towards a Solution to the Ultracompact HII Region Lifetime Problem
Accretion flows onto massive stars must transfer mass so quickly that they
are themselves gravitationally unstable, forming dense clumps and filaments.
These density perturbations interact with young massive stars, emitting
ionizing radiation, alternately exposing and confining their HII regions. As a
result, the HII regions are predicted to flicker in flux density over periods
of decades to centuries rather than increasing monotonically in size as
predicted by simple Spitzer solutions. We have recently observed the Sgr B2
region at 1.3 cm with the VLA in its three hybrid configurations (DnC, CnB and
BnA) at a resolution of 0.25''. These observations were made to compare in
detail with matched continuum observations from 1989. At 0.25'' resolution, Sgr
B2 contains 41 UC HII regions, 6 of which are hypercompact. The new
observations of Sgr B2 allow comparison of relative peak flux densites for the
HII regions in Sgr B2 over a 23 year time baseline (1989-2012) in one of the
most source-rich massive star forming regions in the Milky Way. The new 1.3 cm
continuum images indicate that four of the 41 UC HII regions exhibit
significant changes in their peak flux density, with one source (K3) dropping
in peak flux density, and the other 3 sources (F10.303, F1 and F3) increasing
in peak flux density. The results are consistent with statistical predictions
from simulations of high mass star formation, suggesting that they offer a
solution to the lifetime problem for ultracompact HII regions.Comment: 12 pages, 3 figures, Accepted for publication in the Astrophysical
Journal Letter
W49A: A starburst triggered by expanding shells
W49A is a giant molecular cloud which harbors some of the most luminous
embedded clusters in the Galaxy. However, the explanation for this
starburst-like phenomenon is still under debate. Methods. We investigated
large-scale Spitzer mid-infrared images together with a Galatic Ring Survey
13CO J = 1-0 image, complemented with higher resolution (~ 11 arcsec) 13CO J =
2-1 and C18O J = 2-1 images over a ~ 15 x 13 pc^2 field obtained with the IRAM
30m telescope. Two expanding shells have been identified in the mid-infrared
images, and confirmed in the position-velocity diagrams made from the 13CO J =
2-1 and C18O J = 2-1 data. The mass of the averaged expanding shell, which has
an inner radius of ~ 3.3 pc and a thickness of ~ 0.41 pc, is about 1.9 x 10^4
M*. The total kinetic energy of the expanding shells is estimated to be ~ 10^49
erg which is probably provided by a few massive stars, whose radiation pressure
and/or strong stellar winds drive the shells. The expanding shells are likely
to have a common origin close to the two ultracompact Hii regions (source O and
source N), and their expansion speed is estimated to be ~ 5 km/s, resulting in
an age of ~ 3-7 x 10^5 years. In addition, on larger (~ 35 x 50 pc^2) scales,
remnants of two gas ejections have been identified in the 13CO J = 1 - 0 data.
Both ejections seem to have the same center as the expanding shells with a
total energy of a few times 10^50 erg. The main driving mechanism for the gas
ejections is unclear, but likely related to the mechanism which triggers the
starburst in W49A
An 8.5 GHz Arecibo survey of Carbon Recombination Lines toward Ultra-compact \HII regions: Physical properties of dense molecular material
We report here on a survey of carbon recombination lines (RLs) near 8.5 GHz
toward 17 ultra-compact \HII regions (\UCHII s). Carbon RLs are detected in 11
directions, indicating the presence of dense photodissociation regions (PDRs)
associated with the \UCHII s. In this paper, we show that the carbon RLs
provide important, complementary information on the kinematics and physical
properties of the ambient medium near \UCHII s. Non-LTE models for the carbon
line forming region are developed, assuming that the PDRs surround the \UCHII
s, and we constrained the model parameters by multi-frequency RL data. Modeling
shows that carbon RL emission near 8.5 GHz is dominated by stimulated emission
and hence we preferentially observe the PDR material that is in front of the
\UCHII continuum. We find that the relative motion between ionized gas and the
associated PDR is about half that estimated earlier, and has an RMS velocity
difference of 3.3 \kms. Our models also give estimates for the PDR density and
pressure. We found that the neutral density of PDRs is typically 5
10 \cmthree and \UCHII s can be embedded in regions with high ambient
pressure. Our results are consistent with a pressure confined \HII region model
where the stars are moving relative to the cloud core. Other models cannot be
ruled out, however. Interestingly, in most cases, the PDR pressure is an order
of magnitude larger than the pressure of the ionized gas. Further investigation
is needed to understand this large pressure difference.Comment: 28 pages, 7 figures, 5 tables (accepted for publication in ApJ
Expansion of W 3(OH)
A direct measurement of the expansion of W 3(OH) is made by comparing Very
Large Array images taken about 10 yr apart. The expansion is anisotropic with a
typical speed of 3 to 5 km/s, indicating a dynamical age of only 2300 yr. These
observations are inconsistent with either the freely expanding shell model or a
simple bow shock model. The most favored model is a slowly expanding shell-like
HII region, with either a fast rarefied flow or another less massive diffuse
ionized region moving towards the observer. There is also a rapidly evolving
source near the projected center of emission, perhaps related to the central
star.Comment: LaTeX file, 28 pages, includes 8 figures. To appear in ApJ in
December 10 (1998) issue. Also available at
http://www.submm.caltech.edu/~kawamura/w3oh_pp.p
OT FE-Box Test Procedures
The OT FE readout requirements is the precise (~0.5 ns) and efficient drift time measurement at an occupancy of ~4% to ensure single hit resolution. The acquired achievement of such performance on an assembled FE-Box is verify through a final test performed using a special FE-Tester. In this note the test procedures are described
The hyperyoung HII region in G24.78+0.08 A1
Context. G24.78+0.08 A1 is a 20 Msun star surrounded by a hypercompact (HC)
HII region, driving a CO bipolar outflow, and located at the center of a
massive rotating toroid undergoing infall towards the HC region. Recent water
maser observations suggest that the HC region is expanding and accretion onto
the star is halted. Aims. This study aims to confirm the expansion scenario
proposed for the HC region on the basis of recent H2O maser observations.
Methods. We carried out continuum VLA observations at 1.3cm and 7mm with the A
array plus Pie Town configuration to map the HC region towards G24 A1. Results.
The emission of the HC region has been resolved and shows a ring shape
structure. The profiles of the emission obtained by taking slices at different
angles passing through the barycenter of the HC region confirm the shell
structure of the emission. The ratio between the inner and the outer radius of
the shell, Ri/Ro, derived fitting the normalized brightness temperature profile
passing through the peak of the 7mm emission, is 0.9, which indicates that the
shell is thin. The deconvolved outer radius estimated from the fit is 590 AU.
These results imply that the HC region in G24 A1 cannot be described in terms
of a classical, homogeneous HII region but is instead an ionized shell. This
gives support to the model of an expanding wind-driven, ionized shell suggested
by the kinematics and distribution of the H2O masers associated with the HC
region. According to this model, the HC region is expanding on very short times
scales, 21-66 yr.Comment: 4 pages, 5 figures. Accepted for publication by A&A Lette
Radio Continuum and Recombination Line Study of UC HII Regions with Extended Envelopes
We have carried out 21 cm radio continuum observations of 16 UC HII regions
using the VLA (D-array) in search of associated extended emission. We have also
observed H76 recombination line towards all the sources and
He76 line at the positions with strong H76 line emission. The
UC HII regions have simple morphologies and large (>10) ratios of single-dish
to VLA fluxes. Extended emission was detected towards all the sources. The
extended emission consists of one to several compact components and a diffuse
extended envelope. All the UC HII regions but two are located in the compact
components, where the UC HII regions always correspond to their peaks. The
compact components with UC HII regions are usually smaller and denser than
those without UC HII regions. Our recombination line observations indicate that
the ultracompact, compact, and extended components are physically associated.
The UC HII regions and their associated compact components are likely to be
ionized by the same sources on the basis of the morphological relations
mentioned above. This suggests that almost all of the observed UC HII regions
are not `real' UC HII regions and that their actual ages are much greater than
their dynamical age (<10000 yr). We find that most of simple UC HII regions
previously known have large ratios of single-dish to VLA fluxes, similar to our
sources. Therefore, the `age problem' of UC HII regions does not seem to be as
serious as earlier studies argued. We present a simple model that explains
extended emission around UC HII regions. Some individual sources are discussed.Comment: 29 pages, 28 postscript figures, Accepted for publication in Ap
- …