4,227 research outputs found
The Becklin-Neugebauer Object as a Runaway B Star, Ejected 4000 years ago from the theta^1C system
We attempt to explain the properties of the Becklin-Neugebauer (BN) object as
a runaway B star, as originally proposed by Plambeck et al. (1995). This is one
of the best-studied bright infrared sources, located in the Orion Nebula
Cluster -- an important testing ground for massive star formation theories.
From radio observations of BN's proper motion, we trace its trajectory back
to Trapezium star theta^1C, the most massive (45 Msun) in the cluster and a
relatively tight (17 AU) visual binary with a B star secondary. This origin
would be the most recent known runaway B star ejection event, occurring only
\~4000 yr ago and providing a unique test of models of ejection from multiple
systems of massive stars. Although highly obscured, we can constrain BN's mass
(~7 Msun) from both its bolometric luminosity and the recoil of theta^1C.
Interaction of a runaway B star with dense ambient gas should produce a compact
wind bow shock. We suggest that X-ray emission from this shocked gas may have
been seen by Chandra: the offset from the radio position is ~300 AU in the
direction of BN's motion. Given this model, we constrain the ambient density,
wind mass-loss rate and wind velocity. BN made closest approach to the massive
protostar, source ``I'', 500 yr ago. This may have triggered enhanced accretion
and thus outflow, consistent with previous interpretations of the outflow being
a recent (~10^3 yr) "explosive" event.Comment: 6 pages, accepted to ApJ Letter
VLTI/MIDI 10 micron interferometry of the forming massive star W33A
We report on resolved interferometric observations with VLTI/MIDI of the
massive young stellar object (MYSO) W33A. The MIDI observations deliver
spectrally dispersed visibilities with values between 0.03 and 0.06, for a
baseline of 45m over the wavelength range 8-13 micron. The visibilities
indicate that W33A has a FWHM size of approximately 120AU (0.030'') at 8 micron
which increases to 240AU at 13 micron, scales previously unexplored among
MYSOs. This observed trend is consistent with the temperature falling off with
distance. 1D dust radiative transfer models are simultaneously fit to the
visibility spectrum, the strong silicate feature and the shape of the mid
infrared spectral energy distribution (SED). For any powerlaw density
distribution, we find that the sizes (as implied by the visibilities) and the
stellar luminosity are incompatible. A reduction to a third of W33A's
previously adopted luminosity is required to match the visibilities; such a
reduction is consistent with new high resolution 70 micron data from Spitzer's
MIPSGAL survey. We obtain best fits for models with shallow dust density
distributions of r^(-0.5) and r^(-1.0) and for increased optical depth in the
silicate feature produced by decreasing the ISM ratio of graphite to silicates
and using optical grain properties by Ossenkopf et al. (1992).Comment: 4 pages, 4 figures. Accepted for ApJ letter
In Situ Label-Free Study of Protein Adsorption on Nanoparticles
[Image: see text] Improving the design of nanoparticles for use as drug carriers or biosensors requires a better understanding of the proteinânanoparticle interaction. Here, we present a new tool to investigate this interaction in situ and without additional labeling of the proteins and/or nanoparticles. By combining nonresonant second-harmonic light scattering with a modified Langmuir model, we show that it is possible to gain insight into the adsorption behavior of blood proteins, namely fibrinogen, human serum albumin, and transferrin, onto negatively charged polystyrene nanoparticles. The modified Langmuir model gives us access to the maximum amount of adsorbed protein, the apparent binding constant, and Gibbs free energy. Furthermore, we employ the method to investigate the influence of the nanoparticle size on the adsorption of human serum albumin and find that the amount of adsorbed protein increases more than the surface area per nanoparticle for larger diameters
How Anisotropic is our Universe?
Large-scale cosmic microwave background anisotropies in homogeneous, globally
anisotropic cosmologies are investigated. We perform a statistical analysis in
which the four-year data from the Cosmic Background Explorer satellite is
searched for the specific anisotropy patterns predicted by these models and
thereby set definitive upper limits on the amount of shear, and
vorticity, , which are orders of magnitude stronger than previous
constraints. We comment on how these results might impact our understanding of
primordial global anisotropy.Comment: 12 pages (1 figure), uses RevTex and psfig, submitted to PR
The Sagnac Effect in curved space-times from an analogy with the Aharonov-Bohm Effect
In the context of the natural splitting, the standard relative dynamics can
be expressed in terms of gravito-electromagnetic fields, which allow to
formally introduce a gravito-magnetic Aharonov-Bohm effect. We showed elsewhere
that this formal analogy can be used to derive the Sagnac effect in flat
space-time as a gravito-magnetic Aharonov-Bohm effect. Here, we generalize
those results to study the General Relativistic corrections to the Sagnac
effect in some stationary and axially symmetric geometries, such as the
space-time around a weakly gravitating and rotating source, Kerr space-time,
G\"{odel} universe and Schwarzschild space-time.Comment: 14 pages, 1 EPS figure, LaTeX, accepted for publication in General
Relativity and Gravitatio
An equatorial wind from the massive young stellar object S140 IRS 1
The discovery of the second equatorial ionized stellar wind from a massive
young stellar object is reported. High resolution radio continuum maps of S140
IRS 1 reveal a highly elongated source that is perpendicular to the larger
scale bipolar molecular outflow. This picture is confirmed by location of a
small scale monopolar near-IR reflection nebula at the base of the blueshifted
lobe. A second epoch of observations over a five year baseline show little
ordered outward proper motion of clumps as would have been expected for a jet.
A third epoch, taken only 50 days after the second, did show significant
changes in the radio morphology. These radio properties can all be understood
in the context of an equatorial wind driven by radiation pressure from the
central star and inner disc acting on the gas in the surface layers of the disc
as proposed by Drew et al. (1998). This equatorial wind system is briefly
compared with the one in S106IR, and contrasted with other massive young
stellar objects that drive ionized jets.Comment: 19 pages, 5 figures, accepted by ApJ, minor changes in light of
referees repor
Chemoprevention of lung cancerâfrom biology to clinical reality
Lung cancer is the commonest cause of cancer death in developed countries and throughout the world. Cigarette smoking is the main risk factor for lung cancer and ex-smokers today comprise âŒ50% of all new lung cancer cases. Chemoprevention builds on the concepts of field of cancerization and multistep carcinogenesis and can be defined as the use of natural or chemical compounds to prevent, inhibit or reverse the process of carcinogenesis. So far, chemoprevention studies in lung cancer have failed to reduce lung cancer mortality. New developments in biotechnology have made it possible to define more accurately high-risk populations, make earlier diagnosis possible, and allow more specific targeted therapies to be developed. Both the development and validation of biomarkers, for the selection of high-risk study populations and for response evaluation in chemoprevention studies, are important for the faster turnover of studies evaluating new agents. This article reviews the current status and describes the perspectives for new approaches in the chemoprevention of lung cance
Constraints on the Electrical Charge Asymmetry of the Universe
We use the isotropy of the Cosmic Microwave Background to place stringent
constraints on a possible electrical charge asymmetry of the universe. We find
the excess charge per baryon to be in the case of a uniform
distribution of charge, where is the charge of the electron. If the charge
asymmetry is inhomogeneous, the constraints will depend on the spectral index,
, of the induced magnetic field and range from
() to (). If one could further
assume that the charge asymmetries of individual particle species are not
anti-correlated so as to cancel, this would imply, for photons, ; for neutrinos, ; and for heavy (light) dark
matter particles ().Comment: New version to appear in JCA
Three flavor neutrino oscillation analysis of the Superkamiokande atmospheric neutrino data
Superkamiokande atmospheric neutrino data for 535 days are analyzed in the
framework of three flavor oscillations with mass hierarchy. It is shown that
the best fit point is very close to the pure maximal
case and
eV. The allowed region at 90 %CL is given and the implications to the long
baseline experiments are briefly discussed.Comment: 8 pages + 1 Table + 28 PostScript figures updated using 535 day dat
Signal-to-Noise Eigenmode Analysis of the Two-Year COBE Maps
To test a theory of cosmic microwave background fluctuations, it is natural
to expand an anisotropy map in an uncorrelated basis of linear combinations of
pixel amplitudes --- statistically-independent for both the noise and the
signal. These -eigenmodes are indispensible for rapid Bayesian analyses of
anisotropy experiments, applied here to the recently-released two-year COBE
{\it dmr} maps and the {\it firs} map. A 2-parameter model with an overall
band-power and a spectral tilt describes well inflation-based
theories. The band-powers for {\it all} the {\it dmr} + GHz
and {\it firs} 170 GHz maps agree, , and
are largely independent of tilt and degree of (sharp) -filtering. Further,
after optimal -filtering, the {\it dmr} maps reveal the same
tilt-independent large scale features and correlation function. The unfiltered
{\it dmr} + index is ; increasing the
-filtering gives a broad region at (1.0--1.2)0.5, a jump to
(1.4--1.6)0.5, then a drop to 0.8, the higher values clearly seen to be
driven by -power spectrum data points that do not fit single-tilt models.
These indices are nicely compatible with inflation values (0.8--1.2), but
not overwhelmingly so.Comment: submitted to Phys.Rev.Letters, 4 pages, uuencoded compressed
PostScript; also bdmr2.ps.Z, via anonymous ftp to ftp.cita.utoronto.ca, cd to
/pub/dick/yukawa; CITA-94-2
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