722 research outputs found
FUSE and HST STIS Observations of Hot and Cold Gas in the AB Aurigae System
We present the first observations of a Herbig Ae star with a circumstellar
disk by the Far Ultraviolet Spectroscopic Explorer (FUSE), as well as a
simultaneous observation of the star obtained with the Hubble Space Telescope
Space Telescope Imaging Spectrograph (STIS). The spectra of AB Aurigae show
emission and absorption features arising from gasses that have a wide range in
temperature, from hot OVI emission to cold molecular hydrogen and CO
absorption. Emissions from the highly ionized species OVI and CIII present in
the FUSE spectrum are redshifted, while absorption features arising from
low-ionization species like OI, NI, and SiII are blueshifted and show
characteristic stellar wind line-profiles. We find the total column density of
molecular hydrogen toward AB Aur from the FUSE apectrum, N(H_2) = (6.8 +/- 0.5)
x 10^19 cm^-2. The gas kinetic temperature of the molecular hydrogen derived
from the ratio N(J=1)/N(J=0) is 65 +/- 4 K. The column density of the CO
observed in the STIS spectrum is N(CO) = (7.1 +/- 0.5) x 10^13 cm^-2, giving a
CO/H_2 ratio of (1.04 +/- 0.11) x 10^-6. We also use the STIS spectrum to find
the column density of HI, permitting us to calculate the total column density
of hydrogen atoms, the fractional abundance of H_2, and the gas-to-dust ratio.Comment: 5 pages, including 6 figures. LaTex2e (emulateapj5.sty). Accepted for
publication in ApJ Letter
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
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
High Resolution HST-STIS Spectra of CI and CO in the Beta Pictoris Circumstellar Disk
High resolution FUV echelle spectra showing absorption features arising from
CI and CO gas in the Beta Pictoris circumstellar (CS) disk were obtained on
1997 December 6 and 19 using the Space Telescope Imaging Spectrograph (STIS).
An unsaturated spin-forbidden line of CI at 1613.376 A not previously seen in
spectra of Beta Pictoris was detected, allowing for an improved determination
of the column density of CI at zero velocity relative to the star (the stable
component), N = (2-4) x 10^{16} cm^{-2}. Variable components with multiple
velocities, which are the signatures of infalling bodies in the Beta Pictoris
CS disk, are observed in the CI 1561 A and 1657 A multiplets. Also seen for the
first time were two lines arising from the metastable singlet D level of
carbon, at 1931 A and 1463 A The results of analysis of the CO A-X (0-0),
(1-0), and (2-0) bands are presented, including the bands arising from {13}^CO,
with much better precision than has previously been possible, due to the very
high resolution provided by the STIS echelle gratings. Only stable CO gas is
observed, with a column density N(CO) = (6.3 +/- 0.3) x 10^{14} cm{-2}. An
unusual ratio of the column densities of {12}^CO to {13}^CO is found (R = 15
+/- 2). The large difference between the column densities of CI and CO
indicates that photodissociation of CO is not the primary source of CI gas in
the disk, contrary to previous suggestion.Comment: 13 pages, including 6 figures. LaTex2e (emulateapj5.sty). Accepted
for publication in Ap
Constraints on the gas content of the Fomalhaut debris belt. Can gas-dust interactions explain the belt's morphology?
Context: The 440 Myr old main-sequence A-star Fomalhaut is surrounded by an
eccentric debris belt with sharp edges. This sort of a morphology is usually
attributed to planetary perturbations, but the orbit of the only planetary
candidate detected so far, Fomalhaut b, is too eccentric to efficiently shape
the belt. Alternative models that could account for the morphology without
invoking a planet are stellar encounters and gas-dust interactions.
Aims: We aim to test the possibility of gas-dust interactions as the origin
of the observed morphology by putting upper limits on the total gas content of
the Fomalhaut belt.
Methods: We derive upper limits on the CII 158 m and OI 63 m
emission by using non-detections from the Photodetector Array Camera and
Spectrometer (PACS) onboard the Herschel Space Observatory. Line fluxes are
converted into total gas mass using the non-local thermodynamic equilibrium
(non-LTE) code RADEX. We consider two different cases for the elemental
abundances of the gas: solar abundances and abundances similar to those
observed for the gas in the Pictoris debris disc.
Results: The gas mass is shown to be below the millimetre dust mass by a
factor of at least 3 (for solar abundances) respectively 300 (for
Pic-like abundances).
Conclusions: The lack of gas co-spatial with the dust implies that gas-dust
interactions cannot efficiently shape the Fomalhaut debris belt. The morphology
is therefore more likely due to a yet unseen planet (Fomalhaut c) or stellar
encounters.Comment: 5 pages, 3 figures, published in A&A; versions 2 and 3: language
editin
Numerical Study of the Two Color Attoworld
We consider QCD at very low temperatures and non-zero quark chemical
potential from lattice Monte Carlo simulations of the two-color theory in a
very small spatial volume (the attoscale). In this regime the quark number
rises in discrete levels in qualitative agreement with what is found
analytically at one loop on S3xS1 with radius R_S3 << 1/{\Lambda}_QCD. The
detailed level degeneracy, however, cannot be accounted for using weak coupling
arguments. At each rise in the quark number there is a corresponding spike in
the Polyakov line, also in agreement with the perturbative results. In addition
the quark number susceptibility shows a similar behaviour to the Polyakov line
and appears to be a good indicator of a confinement-deconfinement type of
transition.Comment: 18 pages, 10 figure
The deconfinement transition of finite density QCD with heavy quarks from strong coupling series
Starting from Wilson's action, we calculate strong coupling series for the
Polyakov loop susceptibility in lattice gauge theories for various small N_\tau
in the thermodynamic limit. Analysing the series with Pad\'e approximants, we
estimate critical couplings and exponents for the deconfinement phase
transition. For SU(2) pure gauge theory our results agree with those from
Monte-Carlo simulations within errors, which for the coarser N_\tau=1,2
lattices are at the percent level. For QCD we include dynamical fermions via a
hopping parameter expansion. On a N_\tau=1 lattice with N_f=1,2,3, we locate
the second order critical point where the deconfinement transition turns into a
crossover. We furthermore determine the behaviour of the critical parameters
with finite chemical potential and find the first order region to shrink with
growing \mu. Our series moreover correctly reflects the known Z(N) transition
at imaginary chemical potential.Comment: 18 pages, 7 figures, typos corrected, version published in JHE
The Dynamics of Sustained Reentry in a Loop Model with Discrete Gap Junction Resistance
Dynamics of reentry are studied in a one dimensional loop of model cardiac
cells with discrete intercellular gap junction resistance (). Each cell is
represented by a continuous cable with ionic current given by a modified
Beeler-Reuter formulation. For below a limiting value, propagation is found
to change from period-1 to quasi-periodic () at a critical loop length
() that decreases with . Quasi-periodic reentry exists from
to a minimum length () that is also shortening with .
The decrease of is not a simple scaling, but the bifurcation can
still be predicted from the slope of the restitution curve giving the duration
of the action potential as a function of the diastolic interval. However, the
shape of the restitution curve changes with .Comment: 6 pages, 7 figure
Fermion Back-Reaction and the Sphaleron
Using a simple model, a new sphaleron solution which incorporates finite
fermionic density effects is obtained. The main result is that the height of
the potential barrier (sphaleron energy) decreases as the fermion density
increases. This suggests that the rate of sphaleron-induced transitions
increases when the fermionic density increases. However the rate increase is
not expected to change significantly the predictions from the standard
sphaleron-induced baryogenesis scenarios.Comment: 11 pages, Revtex (2 figures available upon request), to appear in
Phys. Rev. D (Rapid Communication
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