324 research outputs found
X-ray Emission from Wind Blown Bubbles. III. ASCA SIS Observations of NGC6888
We present ASCA SIS observations of the wind-blown bubble NGC6888. Owing to
the higher sensitivity of the SIS for higher energy photons compared to the
ROSAT PSPC, we are able to detect a T ~ 8x10^6 K plasma component in addition
to the T ~ 1.3x10^6 K component previously detected in PSPC observations. No
significant temperature variations are detected within NGC6888. Garcia-Segura &
Mac Low's (1995) analytical models of WR bubbles constrained by the observed
size, expansion velocity, and mass of the nebular shell under-predict the
stellar wind luminosity, and cannot reproduce simultaneously the observed X-ray
luminosity, spectrum, surface brightness profile, and SIS count rate of
NGC6888's bubble interior. The agreement between observations and expectations
from models can be improved if one or more of the following ad hoc assumptions
are made: (1) the stellar wind luminosity was weaker in the past, (2) the
bubble is at a special evolutionary stage and the nebular shell has recently
been decelerated to 1/2 of its previous expansion velocity, and (3) the heat
conduction between the hot interior and the cool nebular shell is suppressed.
Chandra and XMM-Newton observations with high spatial resolution and high
sensitivity are needed to determine accurately the physical conditions
NGC6888's interior hot gas for critical comparisons with bubble models.Comment: 24 pages, 6 figures; accepted for Astrophysical Journal, Nov 1, 2005
issu
Massive stars and the energy balance of the interstellar medium. II. The 35 solar mass star and a solution to the "missing wind problem"
We continue our numerical analysis of the morphological and energetic
influence of massive stars on their ambient interstellar medium for a 35 solar
mass star that evolves from the main sequence through red supergiant and
Wolf-Rayet phases, until it ultimately explodes as a supernova. We find that
structure formation in the circumstellar gas during the early main-sequence
evolution occurs as in the 60 solar mass case but is much less pronounced
because of the lower mechanical wind luminosity of the star. Since on the other
hand the shell-like structure of the HII region is largely preserved, effects
that rely on this symmetry become more important. At the end of the stellar
lifetime 1% of the energy released as Lyman continuum radiation and stellar
wind has been transferred to the circumstellar gas. From this fraction 10% is
kinetic energy of bulk motion, 36% is thermal energy, and the remaining 54% is
ionization energy of hydrogen. The sweeping up of the slow red supergiant wind
by the fast Wolf-Rayet wind produces remarkable morphological structures and
emission signatures, which are compared with existing observations of the
Wolf-Rayet bubble S308. Our model reproduces the correct order of magnitude of
observed X-ray luminosity, the temperature of the emitting plasma as well as
the limb brightening of the intensity profile. This is remarkable, because
current analytical and numerical models of Wolf-Rayet bubbles fail to
consistently explain these features. A key result is that almost the entire
X-ray emission in this stage comes from the shell of red supergiant wind swept
up by the shocked Wolf-Rayet wind rather than from the shocked Wolf-Rayet wind
itself as hitherto assumed and modeled. This offers a possible solution to what
is called the ``missing wind problem'' of Wolf-Rayet bubbles.Comment: 52 pages, 20 figures, 2 tables, accepted for publication in the
Astrophysical Journa
Strong extinction of a laser beam by a single molecule
We present an experiment where a single molecule strongly affects the
amplitude and phase of a laser field emerging from a subwavelength aperture. We
achieve a visibility of -6% in direct and +10% in cross-polarized detection
schemes. Our analysis shows that a close to full extinction should be possible
using near-field excitation.Comment: 5 pages, 4 figures, submitted to PR
Ultraviolet photoelectron spectroscopy of Nb â 4 to Nb â 200
Abstract. We present UV (hÎœ = 6.42 eV) photoelectron spectra of niobium cluster anions Nb â n in the size range from n = 4 to n = 200. The spectra exhibit a variety of patterns, which can be related to the geometrical structures of the clusters. The charging energies of the larger clusters are in very good agreement with the metallic sphere model. Nevertheless a strong size dependence of the work function is observed, which underlines the special properties of this transition metal. PACS. 33.60.Cv Ultraviolet and vacuum ultraviolet photoelectron spectra -36.40.Cg Electronic and magnetic properties of clusters -73.22.-f Electronic structure of nanoscale materials
Metal nanoparticles in strongly confined beams: transmission, reflection and absorption
We investigate the interaction of tightly focused light with the
surface-plasmon-polariton resonances of metal nanospheres. In particular, we
compute the scattering and absorption ratios as well as transmission and
reflection coefficients. Inspired by our previous work in Ref. [1], we discuss
how well a metal nanoparticle approximates a point-like dipolar radiator. We
find that a 100 nm silver nanosphere is very close to such an ideal oscillator.
Our results have immediate implications for single nanoparticle spectroscopy
and microscopy as well as plasmonics.Comment: 6 pages, 4 figure
An Interstellar Conduction Front Within a Wolf-Rayet Ring Nebula Observed with the GHRS
With the High Resolution Spectrograph aboard the Hubble Space Telescope we
obtained high signal-to-noise (S/N > 200 - 600 per 17 km/s resolution element)
spectra of narrow absorption lines toward the Wolf-Rayet star HD 50896. The
ring nebula S308 that surrounds this star is thought to be caused by a
pressure-driven bubble bounded by circumstellar gas (most likely from a red
supergiant or luminous blue variable progenitor) pushed aside by a strong
stellar wind. Our observation has shown for the first time that blueshifted
(approximately 70 km/s relative to the star) absorption components of C IV and
N V arise in a conduction front between the hot interior of the bubble and the
cold shell of swept-up material. These lines set limits on models of the
conduction front. Nitrogen in the shell appears to be overabundant by a factor
~10. The P Cygni profiles of N V and C IV are variable, possibly due to a
suspected binary companion to HD 50896.Comment: 32 pages, Latex, to appear in the Astrophysical Journal, April, 199
Wind-Blown Bubbles around Evolved Stars
Most stars will experience episodes of substantial mass loss at some point in
their lives. For very massive stars, mass loss dominates their evolution,
although the mass loss rates are not known exactly, particularly once the star
has left the main sequence. Direct observations of the stellar winds of massive
stars can give information on the current mass-loss rates, while studies of the
ring nebulae and HI shells that surround many Wolf-Rayet (WR) and luminous blue
variable (LBV) stars provide information on the previous mass-loss history. The
evolution of the most massive stars, (M > 25 solar masses), essentially follows
the sequence O star to LBV or red supergiant (RSG) to WR star to supernova. For
stars of mass less than 25 solar masses there is no final WR stage. During the
main sequence and WR stages, the mass loss takes the form of highly supersonic
stellar winds, which blow bubbles in the interstellar and circumstellar medium.
In this way, the mechanical luminosity of the stellar wind is converted into
kinetic energy of the swept-up ambient material, which is important for the
dynamics of the interstellar medium. In this review article, analytic and
numerical models are used to describe the hydrodynamics and energetics of
wind-blown bubbles. A brief review of observations of bubbles is given, and the
degree to which theory is supported by observations is discussed.Comment: To be published as a chapter in 'Diffuse Matter from Star Forming
Regions to Active Galaxies' - A volume Honouring John Dyson. Eds. T. W.
Harquist, J. M. Pittard and S. A. E. G. Falle. 22 pages, 12 figure
A Failed Gamma-Ray Burst with Dirty Energetic Jets Spirited Away? New Implications for the GRB-SN Connection from Supernova 2002ap
(Abridged) SN 2002ap is an interesting event with broad spectral features
like the famous SN 1998bw / GRB 980425. Here we examine the recently proposed
jet hypothesis from SN 2002ap by a spectropolarimetric observation. We show
that jets should be moving at about 0.23c with a jet kinetic energy of ~5 x
10^{50} erg, a similar energy scale to the GRB jets. The weak radio emission
from SN 2002ap has been used to argue against the jet hypothesis, but we show
that this problem can be avoided. However, the jet cannot be kept ionized
because of adiabatic cooling without external photoionization or heating
source. We found that only the radioactivity of 56Ni is a possible source,
indicating that the jet is formed and ejected from central region of the core
collapse. Then we point out that the jet will eventually sweep up enough
interstellar medium and generate shocks in a few to 10 years, producing strong
radio emission that can be spatially resolved, giving us a clear test for the
jet hypothesis. Discussions are given on possible implications for the GRB-SN
connection in the case that the jet is real. We suggest existence of two
distinct classes of GRBs from similar core-collapse events but by completely
different mechanisms. Cosmologically distant GRBs (~10^{50} erg) are collimated
jets generated by central activity of core collapses. SN 2002ap could be a
failed GRB of this type with a large baryon load. On the other hand, much less
energetic ones like GRB 980425 are rather isotropic, which may be produced by
hydrodynamical shock acceleration at the outer envelope. We propose that the
radioactive ionization for the SN 2002ap jet may give a new explanation also
for the X-ray line features often observed in GRB afterglows.Comment: 14 pages, 5 figures. Version accepted to Ap
Diffuse X-Ray Emission from the Quiescent Superbubble M17, the Omega Nebula
The emission nebula M17 contains a young ~1 Myr-old open cluster; the winds
from the OB stars of this cluster have blown a superbubble around the cluster.
ROSAT observations of M17 detected diffuse X-ray emission peaking at the
cluster and filling the superbubble interior. The young age of the cluster
suggests that no supernovae have yet occurred in M17; therefore, it provides a
rare opportunity to study hot gas energized solely by shocked stellar winds in
a quiescent superbubble. We have analyzed the diffuse X-ray emission from M17,
and compared the observed X-ray luminosity of ~2.5*10^33 ergs/s and the hot gas
temperature of ~8.5*10^6 K and mass of ~1 M_Sun to model predictions. We find
that bubble models with heat conduction overpredict the X-ray luminosity by two
orders of magnitude; the strong magnetic fields in M17, as measured from HI
Zeeman observations, have most likely inhibited heat conduction and associated
mass evaporation. Bubble models without heat conduction can explain the X-ray
properties of M17, but only if cold nebular gas can be dynamically mixed into
the hot bubble interior and the stellar winds are clumpy with mass-loss rates
reduced by a factor of >=3. Future models of the M17 superbubble must take into
account the large-scale density gradient, small-scale clumpiness, and strong
magnetic field in the ambient interstellar medium.Comment: 21 pages, 4 figures, to be published in the Astrophysical Journal,
June 200
Heralded single photon absorption by a single atom
The emission and absorption of single photons by single atomic particles is a
fundamental limit of matter-light interaction, manifesting its quantum
mechanical nature. At the same time, as a controlled process it is a key
enabling tool for quantum technologies, such as quantum optical information
technology [1, 2] and quantum metrology [3, 4, 5, 6]. Controlling both emission
and absorption will allow implementing quantum networking scenarios [1, 7, 8,
9], where photonic communication of quantum information is interfaced with its
local processing in atoms. In studies of single-photon emission, recent
progress includes control of the shape, bandwidth, frequency, and polarization
of single-photon sources [10, 11, 12, 13, 14, 15, 16, 17], and the
demonstration of atom-photon entanglement [18, 19, 20]. Controlled absorption
of a single photon by a single atom is much less investigated; proposals exist
but only very preliminary steps have been taken experimentally such as
detecting the attenuation and phase shift of a weak laser beam by a single atom
[21, 22], and designing an optical system that covers a large fraction of the
full solid angle [23, 24, 25]. Here we report the interaction of single
heralded photons with a single trapped atom. We find strong correlations of the
detection of a heralding photon with a change in the quantum state of the atom
marking absorption of the quantum-correlated heralded photon. In coupling a
single absorber with a quantum light source, our experiment demonstrates
previously unexplored matter-light interaction, while opening up new avenues
towards photon-atom entanglement conversion in quantum technology.Comment: 10 pages, 4 figure
- âŠ