216 research outputs found
Dynamic Thermal Analysis of a Power Amplifier
This paper presents dynamic thermal analyses of a power amplifier. All the
investigations are based on the transient junction temperature measurements
performed during the circuit cooling process. The presented results include the
cooling curves, the structure functions, the thermal time constant distribution
and the Nyquist plot of the thermal impedance. The experiments carried out
demonstrated the influence of the contact resistance and the position of the
entire cooling assembly on the obtained results.Comment: Submitted on behalf of TIMA Editions
(http://irevues.inist.fr/tima-editions
Role of anisotropy in the F\"orster energy transfer from a semiconductor quantum well to an organic crystalline overlayer
We consider the non-radiative resonant energy transfer from a two-dimensional
Wannier exciton (donor) to a Frenkel exciton of a molecular crystal overlayer
(acceptor). We characterize the effect of the optical anisotropy of the organic
subsystem on this process. Using realistic values of material parameters, we
show that it is possible to change the transfer rate within typically a factor
of two depending on the orientation of the crystalline overlayer. The resonant
matching of donor and acceptor energies is also partly tunable via the organic
crystal orientation.Comment: 6 pages, 8 figure
The White Dwarfs within 20 Parsecs of the Sun: Kinematics and Statistics
We present the kinematical properties, distribution of spectroscopic
subtypes, stellar population subcomponents of the white dwarfs within 20 pc of
the sun. We find no convincing evidence of halo white dwarfs in the total 20 pc
sample of 129 white dwarfs nor is there convincing evidence of genuine thick
disk subcomponent members within 20 parsecs. Virtually the entire 20 pc sample
likely belongs to the thin disk. The total DA to non-DA ratio of the 20 pc
sample is 1.6, a manifestation of deepening envelope convection which
transforms DA stars with sufficiently thin H surface layers into non-DAs. The
addition of 5 new stars to the 20 pc sample yields a revised local space
density of white dwarfs of M_{\sun}/yr and a
corresponding mass density of M_{\sun}/pc.
We find that at least 15% of the white dwarfs within 20 parsecs of the sun (the
DAZ and DZ stars) have photospheric metals that possibly originate from
accretion of circumstellar material (debris disks) around them. If this
interpretation is correct, this suggests the possibility that the same
percentage have planets or asteroid-like bodies orbiting them.Comment: Accepted for publication in The Astronomical Journa
LP 400-22, A very low-mass and high-velocity white dwarf
We report the identification of LP 400-22 (WD 2234+222) as a very low-mass
and high-velocity white dwarf. The ultraviolet GALEX and optical photometric
colors and a spectral line analysis of LP 400-22 show this star to have an
effective temperature of 11080+/-140 K and a surface gravity of log g =
6.32+/-0.08. Therefore, this is a helium core white dwarf with a mass of 0.17
M_solar. The tangential velocity of this white dwarf is 414+/-43 km/s, making
it one of the fastest moving white dwarfs known. We discuss probable
evolutionary scenarios for this remarkable object.Comment: accepted for publication in ApJ Letters, made minor correction
LP133-373 A New Chromospherically Active Eclipsing dMe Binary with a Distant, Cool White Dwarf Companion
We report the discovery of the partially eclipsing binary LP 133-373. Nearly identical eclipses along with observed photometric colors and spectroscopy indicate that it is a pair of chromospherically active dM4 stars in a circular 1.6 day orbit. Light and velocity curve modeling to our differential photometry and velocity data show that each star has a mass and radius of 0.340 ± 0.014 M☉ and 0.33 ± 0.02 R☉. The binary is itself part of a common proper motion pair with LP 133-374, a cool DC or possible DA white dwarf with a mass of 0.49-0.82 M☉, which would make the system at least 3 Gyr old
Infrared Signatures of Disrupted Minor Planets at White Dwarfs
Spitzer Space Observatory IRAC and MIPS photometric observations are
presented for 20 white dwarfs with T < 20,000 K and metal-contaminated
photospheres. A warm circumstellar disk is detected at GD 16 and likely at PG
1457-086, while the remaining targets fail to reveal mid-infrared excess
typical of dust disks, including a number of heavily polluted stars. Extending
previous studies, over 50% of all single white dwarfs with implied metal
accretion rates dM/dt > 3e8 g/s display a warm infrared excess from orbiting
dust; the likely result of a tidally-destroyed minor planet. This benchmark
accretion rate lies between the dust production rates of 1e6 g/s in the solar
system zodiacal cloud and 1e10 g/s often inferred for debris disks at main
sequence A-type stars. It is estimated that between 1% and 3% of all single
white dwarfs with cooling ages less than around 0.5 Gyr possess circumstellar
dust, signifying an underlying population of minor planets.Comment: 47 pages, accepted to Ap
LP 133-373: A New Chromospherically Active Eclipsing dMe Binary with a Distant, Cool White Dwarf Companion
We report the discovery of the partially eclipsing binary LP 133-373. Nearly identical eclipses along with observed photometric colors and spectroscopy indicate that it is a pair of chromospherically active dM4 stars in a circular 1.6 day orbit. Light and velocity curve modeling to our differential photometry and velocity data show that each star has a mass and radius of 0.340+/-0.014 Msolar and 0.33+/-0.02 Rsolar. The binary is itself part of a common proper motion pair with LP 133-374, a cool DC or possible DA white dwarf with a mass of 0.49-0.82 Msolar, which would make the system at least 3 Gyr ol
Understanding the rotational variability of K2 targets. HgMn star KIC 250152017 and blue horizontal branch star KIC 249660366
Ultraprecise space photometry enables us to reveal light variability even in
stars that were previously deemed constant. A large group of such stars show
variations that may be rotationally modulated. This type of light variability
is of special interest because it provides precise estimates of rotational
rates. We aim to understand the origin of the light variability of K2 targets
that show signatures of rotational modulation. We used phase-resolved
medium-resolution XSHOOTER spectroscopy to understand the light variability of
the stars KIC~250152017 and KIC~249660366, which are possibly rotationally
modulated. We determined the atmospheric parameters at individual phases and
tested the presence of the rotational modulation in the spectra. KIC 250152017
is a HgMn star, whose light variability is caused by the inhomogeneous surface
distribution of manganese and iron. It is only the second HgMn star whose light
variability is well understood. KIC 249660366 is a He-weak, high-velocity
horizontal branch star with overabundances of silicon and argon. The light
variability of this star is likely caused by a reflection effect in this
post-common envelope binary.Comment: 8 pages, accepted for publication in Astronomy & Astrophysic
Casimir forces from a loop integral formulation
We reformulate the Casimir force in the presence of a non-trivial background.
The force may be written in terms of loop variables, the loop being a curve
around the scattering sites. A natural path ordering of exponentials take place
when a particular representation of the scattering centres is given. The basic
object to be evaluated is a reduced (or abbreviated) classical pseudo-action
that can be operator valued.Comment: references added, text clarified in place
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