721 research outputs found
The Li Overabundance of J37: Diffusion or Accretion?
In September 2002 the discovery of a super Li-rich F-dwarf (J37) in NGC 6633,
an iron poor analogue of the better studied Hyades and Praecepe open clusters,
was announced. This unique star was thought to be the smoking gun for the
action of diffusion, models of which predict a narrow "Li-peak" at
approximately the correct temperature. However, with more detailed studies into
J37s abundance pattern this star provides firm evidence for the accretion of
planetesimals or other material from the circumstellar environment of new born
stars.
Thanks to the specific predictions made about the behaviour of Be abundances,
(the most striking of which being no Be in super-Li-rich dwarfs subject to
diffusion) the opposing diffusion/accretion predictions can be tested.
Initial modelling of the Be line indicates that J37 is as Be rich as it is Li
rich; log N(Be) = 2.25 +/- 0.25, and so is broadly consistent with an
accretion-fuelled enhancement. However, that both Li and Be are enhanced by
much more than the iron-peak elements (as determined in previous studies)
suggests that diffusion also plays a role in increasing the abundances of Li
and Be specifically.
Furthermore, a new data set from the UVES/UT2 combination has allowed the
elemental abundance of Iron to be measured, and the set of preliminary stellar
parameters determined; Teff ~ 7340 K, log g ~ 4.1, microturbulence ~ 4.3 km/s,
[Fe/H] ~ 0.50. This again provides distinct evidence for the effects of
accretion in J37 and requires a new synthesis of the Be doublet.Comment: 5 pages, 2 figures. Poster presented at IAU Symposium 224 "The A Star
Puzzle", 7-13 July 2004, Poprad, Slovaki
Beryllium Enhancement as Evidence for Accretion in a Lithium-Rich F Dwarf
The early F dwarf star ``J37'' in the open cluster NGC6633 shows an unusual
pattern of photospheric abundances, including an order of magnitude enhancement
of lithium and iron-peak elements, but an under-abundance of carbon. As a
consequence of its thin convection zone these anomalies have been attributed to
either radiative diffusion or the accretion of hydrogen-depleted material. By
comparing high resolution VLT/UVES spectra of J37 (and other F stars in NGC
6633) with syntheses of the Be ii doublet region at 3131 Ang, we establish that
J37 also has a Be abundance (A(Be)=3.0+/-0.5) that is at least ten times the
cosmic value. This contradicts radiative diffusion models that produce a Li
over-abundance, as they also predict photospheric Be depletion. Instead, since
Be is a highly refractory element, it supports the notion that J37 is the first
clear example of a star that has accreted volatile-depleted material with a
composition similar to chondritic meteorites, although some diffusion may be
necessary to explain the low C and O abundances.Comment: Accepted for publication in MNRAS letters, 5 page
Low Power Superconducting Microwave Applications and Microwave Microscopy
We briefly review some non-accelerator high-frequency applications of
superconductors. These include the use of high-Tc superconductors in front-end
band-pass filters in cellular telephone base stations, the High Temperature
Superconductor Space Experiment, and high-speed digital electronics. We also
present an overview of our work on a novel form of near-field scanning
microscopy at microwave frequencies. This form of microscopy can be used to
investigate the microwave properties of metals and dielectrics on length scales
as small as 1 mm. With this microscope we have demonstrated quantitative
imaging of sheet resistance and topography at microwave frequencies. An
examination of the local microwave response of the surface of a heat-treated
bulk Nb sample is also presented.Comment: 11 pages, including 6 figures. Presented at the Eight Workshop on RF
Superconductivity. To appear in Particle Accelerator
Multiple Andreev Reflections in Weak Links of Superfluid 3He-B
We calculate the current-pressure characteristics of a ballistic pinhole
aperture between two volumes of B-phase superfluid 3He. The most important
mechanism contributing to dissipative currents in weak links of this type is
the process of multiple Andreev reflections. At low biases this process is
significantly affected by relaxation due to inelastic
quasiparticle-quasiparticle collisions. In the numerical calculations,
suppression of the superfluid order parameter at surfaces is taken into account
self-consistently. When this effect is neglected, the theory may be developed
analytically like in the case of s-wave superconductors. A comparison with
experimental results is presented.Comment: 12 pages, 9 figures, RevTeX
Superconducting Material Diagnostics using a Scanning Near-Field Microwave Microscope
We have developed scanning near-field microwave microscopes which can image
electrodynamic properties of superconducting materials on length scales down to
about 2 m. The microscopes are capable of quantitative imaging of sheet
resistance of thin films, and surface topography. We demonstrate the utility of
the microscopes through images of the sheet resistance of a YBa2Cu3O7-d thin
film wafer, images of bulk Nb surfaces, and spatially resolved measurements of
Tc of a YBa2Cu3O7-d thin film. We also discuss some of the limitations of the
microscope and conclude with a summary of its present capabilities.Comment: 6 pages with 9 figures, Proceedings of the Applied Superconductivity
Conference 199
Phonon heat capacity and self-heating normal domains in NbTiN nanostrips
Self-heating normal domains in thin superconducting NbTiN nanostrips were
characterized via steady-state hysteretic current-voltage characteristics
measured at different substrate temperatures. The temperature dependence and
the magnitude of the current, which sustains a domain in equilibrium at
different voltages, can only be explained with a phonon heat capacity
noticeably less than expected for 3-d Debye phonons. This reduced heat capacity
coincides with the value obtained earlier from magnetoconductance and
photoresponse studies of the same films. The rate of heat flow from electrons
at a temperature Te to phonons in the substrate at a temperature TB is
proportional to (T_e^p - T_B^p) with the exponent p~3, which differs from the
exponents for heat flows mediated by the electron-phonon interaction or by
escaping of 3-d Debye phonons via the film/substrate interface. We attribute
both findings to the effect of the mean grain size on the phonon spectrum of
thin granular NbTiN films. Our findings are significant for understanding the
thermal transport in superconducting devices exploiting thin granular films
Quantitative imaging of dielectric permittivity and tunability with a near-field scanning microwave microscope
We describe the use of a near-field scanning microwave microscope to image
the permittivity and tunability of bulk and thin film dielectric samples on a
length scale of about 1 micron. The microscope is sensitive to the linear
permittivity, as well as to nonlinear dielectric terms, which can be measured
as a function of an applied electric field. We introduce a versatile finite
element model for the system, which allows quantitative results to be obtained.
We demonstrate use of the microscope at 7.2 GHz with a 370 nm thick barium
strontium titanate thin film on a lanthanum aluminate substrate. This technique
is nondestructive and has broadband (0.1-50 GHz) capability. The sensitivity of
the microscope to changes in relative permittivity is 2 at permittivity = 500,
while the nonlinear dielectric tunability sensitivity is 10^-3 cm/kV.Comment: 12 pages, 10 figures, to be published in Rev. Sci. Instrum., July,
200
Exploiting kinematics and UBVIc photometry to establish high fidelity membership of the open cluster Blanco 1
We present the results of a wide-field, high-precision UBVIc CCD photometric survey of the Galactic open cluster Blanco 1. Standardized photometry was acquired using the Y4Kcam on the SMARTS 1m telescope at CTIO. We have also determined new high-precision proper motions (ÏÎŒ = 0.3 mas yrâ1) over an eight square degree area down to V = 16.5. Combined with 1D kinematic data, our survey yields a complete list of cluster members down to ~ 0.5 M and new high-fidelity color-magnitude diagrams are presented for Blanco 1. Having established a bona fide membership catalog, astrophysical characteristics of solar-type cluster members such as X-ray activity and lithium abundance have been studied to gain more insights in the process of internal mixing and convection. Our new results should also help to better understand its peculiar location in the Milky Way and to unravel its dynamical histor
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