59 research outputs found
Suzaku Reveals Helium-burning Products in the X-ray Emitting Planetary Nebula BD+303639
BD+303639, the brightest planetary nebula at X-ray energies, was observed
with Suzaku, an X-ray observatory launched on 2005 July 10. Using the X-ray
Imaging Spectrometer, the K-lines from C VI, O VII, and O VIII were resolved
for the first time, and C/O, N/O, and Ne/O abundance ratios determined. The C/O
and Ne/O abundance ratios exceed the solar value by a factor of at least 30 and
5, respectively. These results indicate that the X-rays are emitted mainly by
helium shell-burning products.Comment: 12 pages, 4 figures, accepted for publication in The Astrophysical
Journal Letter
Diffraction microtomography with sample rotation: influence of a missing apple core in the recorded frequency space
Diffraction microtomography in coherent light is foreseen as a promising
technique to image transparent living samples in three dimensions without
staining. Contrary to conventional microscopy with incoherent light, which
gives morphological information only, diffraction microtomography makes it
possible to obtain the complex optical refractive index of the observed sample
by mapping a three-dimensional support in the spatial frequency domain. The
technique can be implemented in two configurations, namely, by varying the
sample illumination with a fixed sample or by rotating the sample using a fixed
illumination. In the literature, only the former method was described in
detail. In this report, we precisely derive the three-dimensional frequency
support that can be mapped by the sample rotation configuration. We found that,
within the first-order Born approximation, the volume of the frequency domain
that can be mapped exhibits a missing part, the shape of which resembles that
of an apple core. The projection of the diffracted waves in the frequency space
onto the set of sphere caps covered by the sample rotation does not allow for a
complete mapping of the frequency along the axis of rotation due to the finite
radius of the sphere caps. We present simulations of the effects of this
missing information on the reconstruction of ideal objects.Comment: 7 pages, 11 figures, presented at Focus On Microscopy 200
X-ray Imaging of Planetary Nebulae with Wolf-Rayet-type Central Stars: Detection of the Hot Bubble in NGC 40
We present the results of Chandra X-ray Observatory (CXO) observations of the
planetary nebulae (PNs) NGC 40 and Hen 2-99. Both PNs feature late-type
Wolf-Rayet central stars that are presently driving fast ~1000 km/s, massive
winds into denser, slow-moving (~10 km/s) material ejected during recently
terminated asymptotic giant branch (AGB) evolutionary phases. Hence, these
observations provide key tests of models of wind-wind interactions in PNs. In
NGC 40, we detect faint, diffuse X-ray emission distributed within a partial
annulus that lies nested within a ~40'' diameter ring of nebulosity observed in
optical and near-infrared images. Hen 2-99 is undetected. The inferred X-ray
temperature (T_X ~10^6 K) and luminosity (L_X ~ 2 X 10^30 ergs/s) of NGC 40 are
the lowest measured thus far for any PN displaying diffuse X-ray emission.
These results, combined with the ring-like morphology of the X-ray emission
from NGC 40, suggest that its X-ray emission arises from a ``hot bubble'' that
is highly evolved and is generated by a shocked, quasi-spherical fast wind from
the central star, as opposed to AGB or post-AGB jet activity. In constrast, the
lack of detectable X-ray emission from Hen 2-99 suggests that this PN has yet
to enter a phase of strong wind-wind shocks.Comment: 15 pages, 5 figures to appear in The Astrophysical Journa
ROSAT observations of X-ray emission from planetary nebulae
We have searched the entire ROSAT archive for useful observations to study
X-ray emission from Galactic planetary nebulae (PNs). The search yields a
sample of 63 PNs, which we call the ROSAT PN sample. About 20-25% of this
sample show X-ray emission; these include 13 definite detections and three
possible detections (at a 2-sigma level). All X-ray sources in these PNs are
concentrated near the central stars. Only A 30, BD+30 3639, and NGC 6543 are
marginally resolved by the ROSAT instruments. Three types of X-ray spectra are
seen in PNs. Type 1 consists of only soft X-ray emission (<0.5 keV), peaks at
0.1-0.2 keV, and can be fitted by blackbody models at temperatures 1-2 10^5 K.
Type 2 consists of harder X-ray emission, peaks at >0.5 keV, and can be fitted
by thin plasma emission models at temperatures of a few 10^6 K. Type 3 is a
composite of a bright Type 1 component and a fainter Type 2 component.
Unresolved soft sources with Type 1 spectra or the soft component of Type 3
spectra are most likely photospheric emission from the hot central stars.
Absorption cross sections are large for these soft-energy photons; therefore,
only large, tenuous, evolved PNs with hot central stars and small absorption
column densities have been detected. The origin of hard X-ray emission from PNs
is uncertain. PNs with Type 2 spectra are small, dense, young nebulae with
relatively cool (<<10^5 K) central stars, while PNs with Type 3 X-ray spectra
are large, tenuous, evolved nebulae with hot central stars. The hard X-ray
luminosities are also different between these two types of PNs, indicating
perhaps different origins of their hard X-ray emission. Future Chandra and XMM
observations with high spatial and spectral resolution will help to understand
the origin of hard X-ray emission from PNs.Comment: To be published in The Astrophysical Journal Supplement Series. 21
pages, 7 figures, 5 table
Three-dimensional holographic optical manipulation through a high-numerical-aperture soft-glass multimode fibre
Holographic optical tweezers (HOT) hold great promise for many applications in biophotonics, allowing the creation and measurement of minuscule forces on biomolecules, molecular motors and cells. Geometries used in HOT currently rely on bulk optics, and their exploitation in vivo is compromised by the optically turbid nature of tissues. We present an alternative HOT approach in which multiple three-dimensional (3D) traps are introduced through a high-numerical-aperture multimode optical fibre, thus enabling an equally versatile means of manipulation through channels having cross-section comparable to the size of a single cell. Our work demonstrates real-time manipulation of 3D arrangements of micro-objects, as well as manipulation inside otherwise inaccessible cavities. We show that the traps can be formed over fibre lengths exceeding 100 mm and positioned with nanometric resolution. The results provide the basis for holographic manipulation and other high-numerical-aperture techniques, including advanced microscopy, through single-core-fibre endoscopes deep inside living tissues and other complex environments
High-Resolution Spectroscopy of Some Very Active Southern Stars
We have obtained high-resolution echelle spectra of 18 solar-type stars that
an earlier survey showed to have very high levels of Ca II H and K emission.
Most of these stars belong to close binary systems, but 5 remain as probable
single stars or well-separated binaries that are younger than the Pleiades on
the basis of their lithium abundances and H-alpha emission. Three of these
probable single stars also lie more than 1 magnitude above the main sequence in
a color-magnitude diagram, and appear to have ages of 10 to 15 Myr. Two of
them, HD 202917 and HD 222259, also appear to have a kinematical association
with the pre-main sequence multiple system HD 98800.Comment: 25 figures, 3 table
Physical insight into light scattering by photoreceptor cell nuclei
A recent study showed that the rod photoreceptor cell nuclei in the retina of nocturnal and diurnal mammals differ considerably in architecture: the location of euchromatin and heterochromatin in the nucleus is interchanged. This inversion has significant implications for the refractive index distribution and the light scattering properties of the nucleus. Here, we extend previous two-dimensional analysis to three dimensions (3D) by using both a numerical finite-difference time-domain and an analytic Mie theory approach. We find that the specific arrangement of the chromatin phases in the nuclear core-shell models employed have little impact on the far-field scattering cross section. However, scattering in the near field, which is the relevant regime inside the retina, shows a significant difference between the two architectures. The "inverted" photoreceptor cell nuclei of nocturnal mammals act as collection lenses, with the lensing effect being much more pronounced in 3D than in two dimensions. This lensing helps to deliver light efficiently to the light-sensing outer segments of the rod photoreceptor cells and thereby improve night vision. (C) 2010 Optical Society of Americ
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