1,361 research outputs found
Use of acrylic sheet molds for elastomeric products
Molds constructed of acrylic sheet are more easily machined than metal, are transparent to ensure complete filling during injection, and have smooth surfaces free of contamination. Technique eliminates flashing on molded parts and mold release agents
Imaging the Haro 6-10 Infrared Companion
We present an infrared imaging study of the low-mass pre-main-sequence binary system Haro 6-10. This system is one of a handful in which the optically visible primary has the characteristics of a normal T Tauri star, while the secondary is a so-called "infrared companion" (IRC), a strongly extincted object that emits most of its luminosity in the infrared. A speckle holographic technique was used to produce nearly diffraction-limited images on three nights over a 1 yr period starting in late 1997. The images show that the IRC is obscured and surrounded by a compact, irregular, and variable nebula. This structure is in striking contrast to the well-ordered edge-on disk associated with HK Tauri B, the extincted companion to another T Tauri star of similar age. A new, resolved intensity peak was found 0".4 southwest of the IRC. We suggest that it may represent light scattered by a clump of dusty material illuminated by starlight escaping along an outflow-carved cavity in the IRC envelope. The primary star became fainter and the companion became more extended during the observing period
Mid-infrared Imaging of a Circumstellar Disk Around HR 4796: Mapping the Debris of Planetary Formation
We report the discovery of a circumstellar disk around the young A0 star, HR
4796, in thermal infrared imaging carried out at the W.M. Keck Observatory. By
fitting a model of the emission from a flat dusty disk to an image at
lambda=20.8 microns, we derive a disk inclination, i = 72 +6/-9 deg from face
on, with the long axis of emission at PA 28 +/-6 deg. The intensity of emission
does not decrease with radius as expected for circumstellar disks but increases
outward from the star, peaking near both ends of the elongated structure. We
simulate this appearance by varying the inner radius in our model and find an
inner hole in the disk with radius R_in = 55+/-15 AU. This value corresponds to
the radial distance of our own Kuiper belt and may suggest a source of dust in
the collision of cometesimals. By contrast with the appearance at 20.8 microns,
excess emission at lambda = 12.5 microns is faint and concentrated at the
stellar position. Similar emission is also detected at 20.8 microns in residual
subtraction of the best-fit model from the image. The intensity and ratio of
flux densities at the two wavelengths could be accounted for by a tenuous dust
component that is confined within a few AU of the star with mean temperature of
a few hundred degrees K, similar to that of zodiacal dust in our own solar
system. The morphology of dust emission from HR 4796 (age 10 Myr) suggests that
its disk is in a transitional planet-forming stage, between that of massive
gaseous proto-stellar disks and more tenuous debris disks such as the one
detected around Vega.Comment: 9 pages, 4 figures as LaTex manuscript and postscript files in
gzipped tar file. Accepted for publication in Astrophysical Journal Letters.
http://upenn5.hep.upenn.edu/~davidk/hr4796.htm
Mechanism of the fluxional behaviour in (1–5-η-cycloheptadienyl)-(1–5-η-cycloheptatrienyl)iron
Evidence for a 1.2-shift mechanism of the 1-5-η-cycloheptatrienyl moiety with respect to the central iron atom of the title compound is presented together with absolute assignments of the ^(13)C n.m.r. chemical shifts of the C_(7)H_(7) ring. A low-temperature rocking motion of both rings can be frozen out at -70 °C
Analyzing lepton flavor universality in the decays
Lepton flavor universality can be tested in the semileptonic decays
where denotes either the
ground state (with ) or its orbital excitations
(with ) and (with ).
We calculate the differential decay rates as well as the branching fractions of
these decays for both tauonic and muonic modes with form factors obtained from
a covariant confined quark model previously developed by us. We present results
for the rate ratios of the tauonic and muonic modes which provide important
tests of lepton flavor universality in forthcoming experiments.Comment: some references adde
A search for L dwarf binary systems
We present analysis of HST Planetary Camera images of twenty L dwarfs
identified in the course of the Two Micron All-Sky Survey. Four of the targets
have faint, red companions at separations between 0.07 and 0.29 arcseconds (1.6
to 7.6 AU). In three cases, the bolometric magnitudes of the components differ
by less than 0.3 magnitudes. Since the cooling rate for brown dwarfs is a
strong function of mass, similarity in luminosities implies comparable masses.
The faint component in the 2M0850 system, however, is over 1.3 magnitudes
fainter than the primary in the I-band, and ~0.8 magnitudes fainter in M(bol).
Indeed, 2M0850B is ~0.8 magnitudes fainter in I than the lowest luminosity L
dwarf currently known, while the absolute magnitude we deduce at J is almost
identical with M_J for Gl 229B. Theoretical models indicate a mass ratio of
\~0.75. The mean separation of the L dwarf binaries in the current sample is
smaller by a factor of two than amongst M dwarfs. We discuss the implications
of these results for the temperature scale in the L/T transition region and for
the binary frequency amongst L dwarfs.Comment: 38 pages, 11 figures; accepted for A
A Pulsed Eddy Current Method for Examining Thin-Walled Stainless Steel Tubing
A bellows is fabricated from a 12-in. section of type 321 or type 216 stainless steel tubing. In order to ensure that the bellows will survive the rigors of the production environment, it is essential that the tubing be free of all “scratch like” defects. A feasibility study was conducted to determine if an eddy current method could be developed to nondestructively examine this tubing
NICMOS Imaging of the HR 4796A Circumstellar Disk
We report the first near infrared (NIR) imaging of a circumstellar annular
disk around the young (~8 Myr), Vega-like star, HR 4796A. NICMOS coronagraph
observations at 1.1 and 1.6 microns reveal a ring-like symmetrical structure
peaking in reflected intensity 1.05 arcsec +/- 0.02 arcsec (~ 70 AU) from the
central A0V star. The ring geometry, with an inclination of 73.1 deg +/- 1.2
deg and a major axis PA of 26.8 deg +/- 0.6 deg, is in good agreement with
recent 12.5 and 20.8 micron observations of a truncated disk (Koerner, et al.
1998). The ring is resolved with a characteristic width of less than 0.26
arcsec (17 AU) and appears abruptly truncated at both the inner and outer
edges. The region of the disk-plane inward of ~60 AU appears to be relatively
free of scattering material. The integrated flux density of the part of the
disk that is visible (greater than 0.65 arcsec from the star) is found to be
7.5 +/- 0.5 mJy and 7.4 +/- 1.2 mJy at 1.1 and 1.6 microns, respectively.
Correcting for the unseen area of the ring yields total flux densities of 12.8
+/- 1.0 mJy and 12.5 +/- 2.0 mJy, respectively (Vega magnitudes = 12.92 /+-
0.08 and 12.35 +/-0.18). The NIR luminosity ratio is evaluated from these
results and ground-based photometry of the star. At these wavelengths
Ldisk(lambda)/L*(lambda) = 1.4 +/- 0.2E-3 and 2.4 +/- 0.5E-3, giving reasonable
agreement between the stellar flux scattered in the NIR and that which is
absorbed in the visible and re-radiated in the thermal infrared. The somewhat
red reflectance of the disk at these wavelengths implies mean particle sizes in
excess of several microns, larger than typical interstellar grains. The
confinement of material to a relatively narrow annular zone implies dynamical
constraints on the disk particles by one or more as yet unseen bodies.Comment: 14 pages, 1 figure for associated gif file see:
http://nicmosis.as.arizona.edu:8000/AAS99/FIGURE1_HR4796A_ApJL.gif . Accepted
13 January 1999, Astrophyical Journal Letter
Continuum and CO/HCO+ Emission from the Disk Around the T Tauri Star LkCa 15
We present OVRO Millimeter Array lambda = 3.4 - 1.2 mm dust continuum and
spectral line observations of the accretion disk encircling the T Tauri star
LkCa 15. The 1.2 mm dust continuum emission is resolved, and gives a minimum
diameter of 190 AU and an inclination angle of 57+/-5 degrees. There is a
noticeable, but at present poorly constrained, decrease in the continuum
spectral slope with frequency that may result from the coupled processes of
grain growth and dust settling. Imaging of the fairly intense emission from the
lowest rotational transitions of CO, 13CO and HCO+ reveals a rotating disk and
emission extends to 750 AU and the characteristic radius of the disk is
determined to be around 425 AU (HWHM) based on model fits to the CO velocity
field. The disk mass derived from the CO isotopologues with ``typical'' dense
cloud abundances is still nearly two orders of magnitude less than that
inferred from the dust emission, which is probably due to extensive molecular
depletion in the cold, dense disk midplane. N2H+ 1-0 emission has also been
detected which, along with HCO+, sets a lower limit to the fractional
ionization of 10^{-8} in the near-surface regions of protoplanetary disks. This
first detection of N2H+ in circumstellar disks has also made possible a
determination of the N2/CO ratio (~2) that is at least an order of magnitude
larger than those in the envelopes of young stellar objects and dense clouds.
The large N2/CO ratio indicates that our observations probe disk layers in
which CO is depleted but some N2 remains in the gas phase. Such differential
depletion can lead to large variations in the fractional ionization with height
in the outer reaches of circumstellar disks, and may help to explain the
relative nitrogen deficiency observed in comets.Comment: Submitted to ApJ, 28 pages, 7 figure
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