A search for strong, ordered magnetic fields in Herbig Ae/Be stars

The origin of magnetic fields in intermediate-mass and high-mass stars is fundamentally a mystery. Clues toward solving this basic astrophysical problem can likely be found at the pre-main sequence (PMS) evolutionary stage. With this work, we perform the largest and most sensitive search for magnetic fields in pre-main sequence Herbig Ae/Be (HAeBe) stars. Sixty-eight observations of 50 HAeBe stars have been obtained in circularly polarised light using the FORS1 spectropolarimeter at the ESO VLT. An analysis of both Balmer and metallic lines reveals the possible presence of weak longitudinal magnetic fields in photospheric lines of two HAeBe stars, HD 101412 and BF Ori. The intensity of the longitudinal fields detected in HD 101412 and BF Ori suggest that they correspond to globally-ordered magnetic fields with surface intensities of order 1 kG. Monte Carlo simulations of the longitudinal field measurements of the undetected stars allow us to place an upper limits of about 300 G on the general presence of aligned magnetic dipole magnetic fields, and of about 500 G on perpendicular dipole fields. We find that the observed bulk incidence of magnetic HAeBe stars in our sample is 8-12%, in good agreement with that of magnetic main sequence stars of similar masses. We also find that the rms longitudinal field intensity of magnetically-detected HAeBe stars is similar to that of Ap stars and consistent with magnetic flux conservation during stellar evolution. These results are all in agreement with the hypothesis that the magnetic fields of main sequence Ap/Bp stars are fossils, which already exist within the stars at the pre-main sequence stage. Finally, we explore the ability of our new magnetic data to constrain magnetospheric accretion in Herbig Ae/Be stars.Comment: Accepted by Monthly Notices of the Royal Astronomical Society, 2007 January 11. Received 2007 January 11; in original form 2006 August 18. The paper contains 18 pages, 11 figures and 2 table

Probing Carrier Behavior at the Nanoscale in Gallium Nitride using Low Voltage Cathodoluminescence

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Depletion layer imaging using a gaseous secondary electron detector in an environmental scanning electron microscope

Abstract : We present a method for imaging depletion layers using the gaseous secondary electron detector (GSED) employed in environmental scanning electron microscopes. GSED images of a p-np-n junction were obtained from a Si P+PNP+PN power diode. Behavior of the junction contrast as a function of imaging conditions is unrelated to reported GSED contrast formation mechanisms [ A. L. Fletcher, B. L. Thiel, and A. M. Donald, J. Phys. D 30, 2249 (1997)]. Optimum imaging conditions are presented, and the contrast behavior is interpreted in terms of a previously unreported induced current component in GSED images. The presented technique is unique as it will enable imaging of depletion layers in uncoated semiconductor/oxide devices in controlled gaseous environments at elevated specimen temperatures

Probing carrier behaviour at the nanoscale in gallium nitride using low voltage cathodoluminescence

The increasing application of GaN in blue and UV light emitting diodes and lasers has generated considerable interest in its optical and electrical properties. These optical devices exhibit extremely high emission efficiencies despite the presence of a very high concentration of threading dislocations (108 1010 cm-2) that act as non-radiative recombination channels. This perceived contradiction can be been explained by small (< 100 nm) carrier diffusion lengths which effectively negate the effect of the threading dislocations on the radiative recombination efficiency. These short exciton and minority carrier diffusion lengths in GaN can be explored by cathodoluminescence (CL) microscopy and spectroscopy using a SEM equipped with a Schottky field emission gun operating at 1 kV

Single-electron transistors with wide operating temperature range

Abstract : Single-electron transistors are fabricated with a planar self-aligned process using chemical mechanical polishing. The method is demonstrated with Ti∕TiOxTi∕TiOx junctions and resistless lithography. The device characterization showed Coulomb blockade up to 433K433K. High temperature data allowed one to calculate the impact of the process variations on the charging energy and thus on a realistic operating temperature. It is found that single electron devices can have an operating temperature range similar to conventional silicon transistors, opening the door to hybrid designs. These approaches are promising because advanced functionality is created by an optimal combination of both technology strengths

Investigation of the magnetic field characteristics of Herbig Ae/Be stars: Discovery of the pre-main sequence progenitors of the magnetic Ap/Bp stars

We are investigating the magnetic characteristics of pre-main sequence Herbig Ae/Be stars, with the aim of (1) understanding the origin and evolution of magnetism in intermediate-mass stars, and (2) exploring the influence of magnetic fields on accretion, rotation and mass-loss at the early stages of evolution of A, B and O stars. We have begun by conducting 2 large surveys of Herbig Ae/Be stars, searching for direct evidence of photospheric magnetic fields via the longitudinal Zeeman effect. From observations obtained using FORS1 at the ESO-VLT and ESPaDOnS at the Canada-France-Hawaii Telescope, we report the confirmed detection of magnetic fields in 4 pre-main sequence A- and B-type stars, and the apparent (but as yet unconfirmed) detection of fields in 2 other such stars. We do not confirm the detection of magnetic fields in several stars reported by other authors to be magnetic: HD 139614, HD 144432 or HD 31649. One of the most evolved stars in the detected sample, HD 72106A, shows clear evidence of strong photospheric chemical peculiarity, whereas many of the other (less evolved) stars do not. The magnetic fields that we detect appear to have surface intensities of order 1 kG, seem to be structured on global scales, and appear in about 10% of the stars studied. Based on these properties, these magnetic stars appear to be pre-main sequence progenitors of the magnetic Ap/Bp stars.Comment: v2: Include comment regarding publication source To appear in the proceedings of "Solar Polarisation 4", held in Boulder, USA, Sept. 200

Response to "Comment on 'Carrier recombination near threading dislocations in GaN epilayers by low voltage cathodoluminescence'" [Appl. Phys. Lett. 97, 166101 (2010)]

Abstract : The authors are aware that there are number of theoretical models available to simulate cathodoluminescence (CL) contrast as a function of the distance, r, from a nonradiative defect. In our letter1 a simple expression, C(r)=C0 exp(−r/L)C(r)=C0 exp(−r/L), was chosen to compare the diffusion length, L, from GaN samples with different n-doping levels over a range of low electron beam energies. This approach is widely used in other studies of CL dislocation contrast as pointed out in the comment on the letter and it was found to provide an acceptable fit to the experimental CL contrast data

Biomechanical interactions of endodontically treated tooth implant-supported prosthesis under fatigue test with acoustic emission monitoring

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