95 research outputs found
Bio-inspired band-gap tunable elastic optical multilayer fibers.
The concentrically-layered photonic structure found in the tropical fruit Margaritaria nobilis serves as inspiration for photonic fibers with mechanically tunable band-gap. The fibers show the spectral filtering capabilities of a planar Bragg stack while the microscopic curvature decreases the strong directional chromaticity associated with flat multilayers. Elongation of the elastic fibers results in a shift of the reflection of over 200 nm.Financial support from
the US Air Force Offi ce of Scientifi c Research Multidisciplinary University
Research Initiative under award numbers FA9550-09-1-0669-DOD35CAP,
FA9550-10-1-0020 and the UK Engineering and Physical Sciences
Research Council EP/G060649/1 is gratefully acknowledged. M.Ko.
acknowledges the fi nancial support from the Alexander von Humboldt
Foundation in form of a Feodor Lynen postdoctoral research fellowship.
This work was performed in part at the Center for Nanoscale Systems
(CNS), a member of the National Nanotechnology Infrastructure Network
(NNIN), which is supported by the National Science Foundation under
NSF award no. ECS-0335765. CNS is part of Harvard University
Structures of the eukaryotic ribosome and its translational states in situ
Ribosomes translate genetic information into primary structure. During translation, various cofactors transiently bind to the ribosome that undergoes prominent conformational and structural changes. Different translational states of ribosomes have been well characterized in vitro. However, to which extent the known translational states are representative of the native situation inside cells has thus far only been addressed in prokaryotes. Here, we apply cryo-electron tomography to cryo-FIB milled Dictyostelium discoideum cells combined with subtomogram averaging and classification. We obtain an in situ structure that is locally resolved up to 3 Angstrom, the distribution of eukaryotic ribosome translational states, and unique arrangement of rRNA expansion segments. Our work demonstrates the use of in situ structural biology techniques for identifying distinct ribosome states within the cellular environment
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
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
51 Eri and GJ 3305: A 10-15 Myr old binary star system at 30 parsecs
Following the suggestion of Zuckerman et al. (2001, ApJ, 562, L87), we
consider the evidence that 51 Eri (spectral type F0) and GJ 3305 (M0),
historically classified as unrelated main sequence stars in the solar
neighborhood, are instead a wide physical binary system and members of the
young beta Pic moving group (BPMG). The BPMG is the nearest (d < 50 pc) of
several groups of young stars with ages around 10 Myr that are kinematically
convergent with the Oph-Sco-Cen Association (OSCA), the nearest OB star
association. Combining SAAO optical photometry, Hobby-Eberly Telescope
high-resolution spectroscopy, Chandra X-ray data, and UCAC2 catalog kinematics,
we confirm with high confidence that the system is indeed extremely young. GJ
3305 itself exhibits very strong magnetic activity but has rapidly depleted
most of its lithium. The 51 Eri/GJ 3305 system is the westernmost known member
of the OSCA, lying 110 pc from the main subgroups. The system is similar to the
BPMG wide binary HD 172555/CD -64d1208 and the HD 104237 quintet, suggesting
that dynamically fragile multiple systems can survive the turbulent
environments of their natal giant molecular cloud complexes, while still being
imparted high dispersion velocities. Nearby young systems such as these are
excellent targets for evolved circumstellar disk and planetary studies, having
stellar ages comparable to that of the late phases of planet formation.Comment: 27 pages, 7 figures. Accepted for publication in the Astronomical
Journal. For a version with high resolution figures, see
http://www.astro.psu.edu/users/edf/51Eri.pd
The Angular Expansion and Distance of the Planetary Nebula BD+30 3639
The WFPC2 camera aboard the HST was used to obtain images of the planetary
nebula BD+30 3639 at two epochs separated by 5.663 years. The expansion of the
nebula in the H-alpha and [N II] bands has been measured using several methods.
Detailed expansion maps for both emission lines were constructed from nearly
200 almost independent features. There is good agreement between the
(independent) H-alpha and [N II] proper motions. The central velocity split is
measured from the STIS echelle spectra of the C II] 2326A multiplet to be
+/-36.3 km/s at a position angle of 99 degrees. The angular displacement along
this slit position was measured to be 4.25 mas/yr at 2".47 from the center. We
constructed a tilted ellipsoidal shell model by fitting the radio brightness
variation of the 5 and 15 GHz VLA observations, and making use of the
ground-based echelle spectra from Bryce & Mellema (1999), to estimate distance.
Our model has an axial ratio of 1.56, is inclined to the line of sight by 9.7
degrees, and exhibits an expansion in the plane of the sky which is 2/3 that in
the radial direction, leading to a distance of 1.2 kpc. Not all the kinematic
data fits this simple model, so the distance must still be regarded as
uncertain. Based on the recent model atmosphere of Crowther et al. (2002), a
distance of 1.2 kpc implies a stellar luminosity of 4250 L_sun. The kinematic
age of the nebula varies somewhat from region to region; a good average value
is 800 years, while the expansion along the position of the echelle slit gives
about 600 years.Comment: 21 pages, 9 figures, accepted by AJ, May 200
X-Ray Emission from Central Binary Systems of Planetary Nebulae
We study the conditions under which a main sequence binary companion to the
central ionizing star of a planetary nebula (PN) might become magnetically
active and thereby display strong X-ray luminosity. Since most PNe are older
than few billion years, any main sequence companion will rotate too slowly to
have magnetic activity and hence bright X-ray emission, unless it is spun-up.
We demonstrate that if the orbital separation during the AGB phase of the PN
progenitor is less than 30-60 AU, main sequence companions in the spectral type
range F7 to M4 will accrete enough angular momentum from the AGB wind to rotate
rapidly, become magnetically active, and exhibit strong X-ray luminosities.
Lower mass M stars and brown dwarfs can also become magnetically active, but
they should have small orbital separations and hence are less likely to survive
the AGB phase of the progenitor. We estimate that 20-30 per cent of elliptical
PNe and 30-50 per cent of bipolar PN are likely to have magnetically active
companions which will reveal themselves in X-ray observations. Re-analysis of
Chandra X-ray Observatory spectroscopy of the compact central source of NGC
7293 indicates that the emitting region of this object possesses abundance
anomalies similar to those of coronally active main-sequence stars.Comment: 15 pages, Submitted to Ap
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
Chandra X-ray Observatory Detection of Extended X-ray Emission from the Planetary Nebula BD+303639
We report the detection of well resolved, extended X-ray emission from the
young planetary nebula BD+303639 using the Advanced CCD Imaging Spectrometer
(ACIS) aboard the Chandra X-ray Observatory. The X-ray emission from BD+303639
appears to lie within, but is concentrated to one side of, the interior of the
shell of ionized gas seen in high-resolution optical and IR images. The
relatively low X-ray temperature (Tx ~ 3x10^6 K) and asymmetric morphology of
the X-ray emission suggests that conduction fronts are present and/or mixing of
shock-heated and photoionized gas has occurred and, furthermore, hints at the
presence of magnetic fields. The ACIS spectrum suggests that the X-ray emitting
region is enriched in the products of helium burning. Our detection of extended
X-ray emission from BD+303639 demonstrates the power and utility of Chandra
imaging as applied to the study of planetary nebulae.Comment: 9 pages, 3 figures; to be published in the Astrophysical Journal
(Letters
Rotational modulation of the photospheric and chromospheric activity in the young, single K2-dwarf PW And
High resolution echelle spectra of PW And (HD~1405) have been taken during
eight observing runs from 1999 to 2002. The detailed analysis of the spectra
allow us to determine its spectral type (K2V), mean heliocentric radial
velocity (V_hel = -11.15 km/s) rotational velocity (vsin{i} = 22.6 km/s), and
equivalent width of the lithium line 6707.8 AA (EW(LiI) = 273 mAA). The
kinematic (Galactic Velocity (U, V, W)) confirms its membership of the Local
Association moving group, in agreement with the age (30 to 80 Myrs) inferred
from the color magnitude diagram and the lithium equivalent width. Photospheric
activity (presence of cool spots that disturb the profiles of the photospheric
lines) has been detected as changes in the the bisectors of the cross
correlation function (CCF) resulting of cross-correlate the spectra of PW And
with the spectrum of a non active star of similar spectral type. These
variations of the CCF bisectors are related to the variations in the measured
radial velocities and are modulated with a period similar to the photometric
period of the star. At the same time, chromospheric activity has been analyzed,
using the spectral subtraction technique and simultaneous spectroscopic
observations of the H_alpha, H_beta, NaI D_1 and D_2$, HeI D_3, MgI b triplet,
CaII H&K, and CaII infrared triplet lines. A flare was observed during the last
observing run of 2001, showing an enhancement in the observed chromospheric
lines. A less powerful flare was observed on 2002 August 23. The variations of
the chromospheric activity indicators seem to be related to the photospheric
activity. A correlation between radial velocity, changes in the CCF bisectors
and equivalent width of different chromospheric lines is observed with a
different behaviour between epochs 1999, 2001 and 2002.Comment: Latex file with 20 pages, 21 figures tar'ed gzip'ed. Full postscript
(text, figures and tables) available at
http://www.ucm.es/info/Astrof/users/dmg/pub_dmg.html Accepted for publication
in: Astronomy & Astrophysics (A&A
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