874 research outputs found
The non-zero baryon number formulation of QCD
We discuss the non-zero baryon number formulation of QCD in the quenched
limit at finite temperature. This describes the thermodynamics of gluons in the
background of static quark sources. Although a sign problem remains in this
theory, our simulation results show that it can be handled quite well
numerically. The transition region gets shifted to smaller temperatures and the
transition region broadens with increasing baryon number. Although the action
is in our formulation explicitly Z(3) symmetric the Polyakov loop expectation
value becomes non-zero already in the low temperature phase and the heavy quark
potential gets screened at non-vanishing number density already this phase.Comment: LATTICE99(Finite Temperature and Density), Latex2e using espcrc2.sty,
3 pages, 7 figure
Can gas in young debris disks be constrained by their radial brightness profiles?
Disks around young stars are known to evolve from optically thick,
gas-dominated protoplanetary disks to optically thin, almost gas-free debris
disks. It is thought that the primordial gas is largely removed at ages of ~10
Myr, but it is difficult to discern the true gas densities from gas
observations. This suggests using observations of dust: it has been argued that
gas, if present with higher densities, would lead to flatter radial profiles of
the dust density and surface brightness than those actually observed. However,
here we show that these profiles are surprisingly insensitive to variation of
the parameters of a central star, location of the dust-producing planetesimal
belt, dustiness of the disk and - most importantly - the parameters of the
ambient gas. This result holds for a wide range of gas densities (three orders
of magnitude), for different radial distributions of the gas temperature, and
different gas compositions. The brightness profile slopes of -3...-4 we find
are the same that were theoretically found for gas-free debris disks, and they
are the same as actually retrieved from observations of many debris disks. Our
specific results for three young (10-30 Myr old), spatially resolved, edge-on
debris disks (beta Pic, HD 32297, and AU Mic) show that the observed radial
profiles of the surface brightness do not pose any stringent constraints on the
gas component of the disk. We cannot exclude that outer parts of the systems
may have retained substantial amounts of primordial gas which is not evident in
the gas observations (e.g. as much as 50 Earth masses for beta Pic). However,
the possibility that gas, most likely secondary, is only present in little to
moderate amounts, as deduced from gas detections (e.g. ~0.05 Earth masses in
the beta Pic disk), remains open, too.Comment: Accepted for publication in Astronomy and Astrophysic
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
Herschel PACS Observations and Modeling of Debris Disks in the Tucana-Horologium Association
We present Herschel PACS photometry of seventeen B- to M-type stars in the 30
Myr-old Tucana-Horologium Association. This work is part of the Herschel Open
Time Key Programme "Gas in Protoplanetary Systems" (GASPS). Six of the
seventeen targets were found to have infrared excesses significantly greater
than the expected stellar IR fluxes, including a previously unknown disk around
HD30051. These six debris disks were fitted with single-temperature blackbody
models to estimate the temperatures and abundances of the dust in the systems.
For the five stars that show excess emission in the Herschel PACS photometry
and also have Spitzer IRS spectra, we fit the data with models of optically
thin debris disks with realistic grain properties in order to better estimate
the disk parameters. The model is determined by a set of six parameters:
surface density index, grain size distribution index, minimum and maximum grain
sizes, and the inner and outer radii of the disk. The best fitting parameters
give us constraints on the geometry of the dust in these systems, as well as
lower limits to the total dust masses. The HD105 disk was further constrained
by fitting marginally resolved PACS 70 micron imaging.Comment: 15 pages, 7 figures, Accepted to Ap
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