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

Bio-Inspired Band-Gap Tunable Elastic Optical Multilayer Fibers

Engineering and Applied Science

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

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

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

Chromospheric activity, lithium and radial velocities of single late-type stars possible members of young moving groups

We present here high resolution echelle spectra taken during three observing runs of 14 single late-type stars identified in our previous studies (Montes et al. 2001b, hereafter Paper I) as possible members of different young stellar kinematic groups (Local Association (20 - 150 Myr), Ursa Major group (300 Myr), Hyades supercluster (600 Myr), and IC 2391 supercluster (35 Myr)). Radial velocities have been determined by cross correlation with radial velocity standard stars and used together with precise measurements of proper motions and parallaxes taken from Hipparcos and Tycho-2 Catalogues, to calculate Galactic space motions (U, V, W) and to apply Eggen's kinematic criteria. The chromospheric activity level of these stars have been analysed using the information provided for several optical spectroscopic features (from the Ca II H & K to Ca II IRT lines) that are formed at different heights in the chromosphere. The Li I 6707.8 AA line equivalent width (EW) has been determined and compared in the EW(Li I) versus spectral type diagram with the EW(Li I) of stars members of well known young open clusters of different ages, in order to obtain an age estimation. All these data allow us to analyse in more detail the membership of these stars in the different young stellar kinematic groups. Using both kinematic and spectroscopic criteria we have confirmed PW And, V368 Cep, V383 Lac, EP Eri, DX Leo, HD 77407, and EK Dra as members of the Local Association and V834 Tau, pi^{1} UMa, and GJ 503.2 as members of the Ursa Major group. A clear rotation-activity dependence has been found in these stars.Comment: Latex file with 19 pages, 7 figures tar'ed gzip'ed. Full postscript (text, figures and tables) available at http://www.ucm.es/info/Astrof/p_skg_stars_I_fv.ps.gz Accepted for publication in: Astronomy & Astrophysics (A&A