2,094 research outputs found
To what distances do we know the confining potential?
We argue that asymptotically linear static potential is built in into the
common procedure of extracting it from lattice Wilson loop measurements. To
illustrate the point, we extract the potential by the standard lattice method
in a model vacuum made of instantons. A beautiful infinitely rising linear
potential is obtained in the case where the true potential is actually
flattening. We argue that the flux tube formation might be also an artifact of
the lattice procedure and not necessarily a measured physical effect.
We conclude that at present the rising potential is known for sure up to no
more than about 0.7 fm. It may explain why no screening has been clearly
observed so far for adjoint sources and for fundamental sources but with
dynamical fermions.
Finally, we speculate on how confinement could be achieved even if the static
potential in the pure glue theory is not infinitely rising.Comment: 16 pages, 5 figures. Additional arguments presented, a new figure and
references adde
Can a Logarithmically Running Coupling Mimic a String Tension?
It is shown that a Coulomb potential using a running coupling slightly
modified from the perturbative form can produce an interquark potential that
appears nearly linear over a large distance range. Recent high-statistics SU(2)
lattice gauge theory data fit well to this potential without the need for a
linear string-tension term. This calls into question the accuracy of string
tension measurements which are based on the assumption of a constant
coefficient for the Coulomb term. It also opens up the possibility of obtaining
an effectively confining potential from gluon exchange alone.Comment: 13 pages, LaTeX, two figures not included, available from author.
revision - Line lengths fixed so it will tex properl
Generalized Farey trees, transfer Operators and phase transitions
We consider a family of Markov maps on the unit interval, interpolating
between the tent map and the Farey map. The latter map is not uniformly
expanding. Each map being composed of two fractional linear transformations,
the family generalizes many particular properties which for the case of the
Farey map have been successfully exploited in number theory. We analyze the
dynamics through the spectral analysis of generalized transfer operators.
Application of the thermodynamic formalism to the family reveals first and
second order phase transitions and unusual properties like positivity of the
interaction function.Comment: 39 pages, 10 figure
High-Contrast NIR Polarization Imaging of MWC480
One of the key predictions of modeling from the IR excess of Herbig Ae stars
is that for protoplanetary disks, where significant grain growth and settling
has occurred, the dust disk has flattened to the point that it can be partially
or largely shadowed by the innermost material at or near the dust sublimation
radius. When the self-shadowing has already started, the outer disk is expected
to be detected in scattered light only in the exceptional cases that the scale
height of the dust disk at the sublimation radius is smaller than usual.
High-contrast imaging combined with the IR spectral energy distribution allow
us to measure the degree of flattening of the disk, as well as to determine the
properties of the outer disk. We present polarimetric differential imaging in
band obtained with Subaru/HiCIAO of one such system, MWC 480. The HiCIAO
data were obtained at a historic minimum of the NIR excess. The disk is
detected in scattered light from 0\farcs2-1\farcs0 (27.4-137AU). Together with
the marginal detection of the disk from 1998 February 24 by HST/NICMOS, our
data constrain the opening half angle for the disk to lie between
1.3. When compared with similar measures in CO for
the gas disk from the literature, the dust disk subtends only 30% of the
gas disk scale height (H/R0.03). Such a dust disk is a factor of 5-7
flatter than transitional disks, which have structural signatures that giant
planets have formed.Comment: 21 pages, 6 figures, 1 table, ApJ accepted 2012-05-0
Cometary Dust in the Debris Disks of HD 31648 and HD 163296: Two ``Baby'' beta Pics
The debris disks surrounding the pre-main sequence stars HD 31648 and HD
163296 were observed spectroscopically between 3 and 14 microns. Both possess a
silicate emission feature at 10 microns which resembles that of the star beta
Pictoris and those observed in solar system comets. The structure of the band
is consistent with a mixture of olivine and pyroxene material, plus an
underlying continuum of unspecified origin. The similarity in both size and
structure of the silicate band suggests that the material in these systems had
a processing history similar to that in our own solar system prior to the time
that the grains were incorporated into comets.Comment: 17 pages, AASTeX, 5 eps figures, accepted for publication in Ap.
Observations of 51 Ophiuchi with MIDI at the VLTI
We present interferometric observations of the Be star 51 Ophiuchi. These
observations were obtained during the science demonstration phase of the MIDI
instrument at the Very Large Telescope Interferometer (VLTI). Using MIDI, a
Michelson 2 beam combiner that operates at the N band (8 to 13 microns), we
obtained for the first time observations of 51 Oph in the mid-infrared at
high-angular resolution. It is currently known that this object presents a
circumstellar dust and gas disk that shows a very different composition from
other Herbig Ae disks. The nature of the 51 Oph system is still a mystery to be
solved. Does it have a companion? Is it a protoplanetary system? We still don't
know. Observations with MIDI at the VLTI allowed us to reach high-angular
resolution (20 mas).We have several uv points that allowed us to constrain the
disk model. We have modeled 51 Oph visibilities and were able to constrain the
size and geometry of the 51 Oph circumstellar disk.Comment: 5 pages, 3 figures, 2 tables, to be published in the proceedings of
"The Power of Optical / IR Interferometry: Recent Scientific Results and 2nd
Generation VLTI Instrumentation", Garching, April 4-8, 200
Keep up production and save grain
The Oklahoma Cooperative Extension Service periodically issues revisions to its publications. The most current edition is made available. For access to an earlier edition, if available for this title, please contact the Oklahoma State University Library Archives by email at [email protected] or by phone at 405-744-6311
GASPS observations of Herbig Ae/Be stars with PACS/Herschel. The atomic and molecular content of their protoplanetary discs
We observed a sample of 20 representative Herbig Ae/Be stars and five A-type
debris discs with PACS onboard of Herschel. The observations were done in
spectroscopic mode, and cover far-IR lines of [OI], [CII], CO, CH+, H2O and OH.
We have a [OI]63 micron detection rate of 100% for the Herbig Ae/Be and 0% for
the debris discs. [OI]145 micron is only detected in 25%, CO J=18-17 in 45%
(and less for higher J transitions) of the Herbig Ae/Be stars and for [CII] 157
micron, we often found spatially variable background contamination. We show the
first detection of water in a Herbig Ae disc, HD 163296, which has a settled
disc. Hydroxyl is detected as well in this disc. CH+, first seen in HD 100546,
is now detected for the second time in a Herbig Ae star, HD 97048. We report
fluxes for each line and use the observations as line diagnostics of the gas
properties. Furthermore, we look for correlations between the strength of the
emission lines and stellar or disc parameters, such as stellar luminosity, UV
and X-ray flux, accretion rate, PAH band strength, and flaring. We find that
the stellar UV flux is the dominant excitation mechanism of [OI]63 micron, with
the highest line fluxes found in those objects with a large amount of flaring
and greatest PAH strength. Neither the amount of accretion nor the X-ray
luminosity has an influence on the line strength. We find correlations between
the line flux of [OI]63 micron and [OI]145 micron, CO J = 18-17 and [OI]6300
\AA, and between the continuum flux at 63 micron and at 1.3 mm, while we find
weak correlations between the line flux of [OI]63 micron and the PAH
luminosity, the line flux of CO J = 3-2, the continuum flux at 63 micron, the
stellar effective temperature and the Brgamma luminosity. (Abbreviated version)Comment: 20 pages, 29 figures, accepted by Astronomy and Astrophysic
Discovery of an 86 AU Radius Debris Ring Around HD 181327
HST/NICMOS PSF-subtracted coronagraphic observations of HD 181327 have
revealed the presence of a ring-like disk of circumstellar debris seen in 1.1
micron light scattered by the disk grains, surrounded by a di use outer region
of lower surface brightness. The annular disk appears to be inclined by 31.7
+/- 1.6 deg from face on with the disk major axis PA at 107 +/-2 deg . The
total 1.1 micron flux density of the light scattered by the disk (at 1.2" < r <
5.0") of 9.6 mJy +/- 0.8 mJy is 0.17% +/- 0.015% of the starlight. Seventy
percent of the light from the scattering grains appears to be confined in a 36
AU wide annulus centered on the peak of the radial surface brightness (SB)
profile 86.3 +/- 3.9 AU from the star, well beyond the characteristic radius of
thermal emission estimated from IRAS and Spitzer flux densities assuming
blackbody grains (~ 22 AU). The light scattered by the ring appears bilaterally
symmetric, exhibits directionally preferential scattering well represented by a
Henyey-Greenstein scattering phase function with g = 0.30 +/- 0.03, and has an
azimuthally medianed SB at the 86.3 AU radius of peak SB of 1.00 +/- 0.07 mJy
arcsec^-2. No photocentric offset is seen in the ring relative to the position
of the central star. A low surface brightness diffuse halo is seen in the
NICMOS image to a distance of ~ 4" Deeper 0.6 micron HST/ACS PSF-subtracted
coronagraphic observations reveal a faint outer nebulosity, asymmetrically
brighter to the North of the star. We discuss models of the disk and properties
of its grains, from which we infer a maximum vertical scale height of 4 - 8 AU
at the 87.6 AU radius of maximum surface density, and a total maximum dust mass
of collisionally replenished grains with minimum grain sizes of ~ 1 micron of ~
4 M(moon).Comment: 45 pages, 15 figures, accepted for publication in Ap
- …