1,545 research outputs found
Production Mechanism for Quark Gluon Plasma in Heavy Ion Collisions
A general scheme is proposed here to describe the production of semi soft and
soft quarks and gluons that form the bulk of the plasma in ultra relativistic
heavy ion collisions. We show how to obtain rates as a function of time in a
self consistent manner, without any ad-hoc assumption. All the required
features - the dynamical nature of QCD vacuum, the non-Markovian nature of the
production, and quasi particle nature of the partons, and the importance of
quantum interference effects are naturally incorporated. We illustrate the
results with a realistic albeit toy model and show how almost all the currently
employed source terms are unreliable in their predictions. We show the rates in
the momentum space and indicate at the end how to extract the full phase-space
dependence.Comment: 4 pages, 4 figures, two colum
The GAPS Programme with HARPS-N at TNG VIII: Observations of the Rossiter-McLaughlin effect and characterisation of the transiting planetary systems HAT-P-36 and WASP-11/HAT-P-10
We determine the true and the projected obliquity of HAT-P-36 and
WASP-11/HAT-P-10 systems, respectively, which are both composed of a relatively
cool star and a hot-Jupiter planet. Thanks to the high-resolution spectrograph
HARPS-N, we observed the Rossiter-McLaughlin effect for both the systems by
acquiring precise radial-velocity measurements during planetary transit events.
We also present photometric observations comprising six light curves covering
five transit events, obtained using three medium-class telescopes and the
telescope-defocussing technique. One transit of WASP-11/HAT-P-10 was followed
simultaneously from two observatories. The three transit light curves of
HAT-P-36b show anomalies that are attributable to starspot complexes on the
surface of the parent star, in agreement with the analysis of its spectra that
indicate a moderate activity. By analysing the complete HATNet data set of
HAT-P-36, we estimated the stellar rotation period by detecting a periodic
photometric modulation in the light curve caused by star spots, obtaining
Prot=15.3 days, which implies that the inclination of the stellar rotational
axis with respect to the line of sight is 65 degree. We used the new
spectroscopic and photometric data to revise the main physical parameters and
measure the sky-projected misalignment angle of the two systems. We found
\lambda=-14 degree for HAT-P-36 and \lambda=7 degree for WASP-11/HAT-P-10,
indicating in both cases a good spin-orbit alignment. In the case of HAT-P-36,
we also measured its real obliquity, which turned out to be 25 degrees.Comment: 18 pages, 14 figure
Out of Equilibrium Thermal Field Theories - Finite Time after Switching on the Interaction - Wigner Transforms of Projected Functions
We study out of equilibrium thermal field theories with switching on the
interaction occurring at finite time using the Wigner transforms (in relative
space-time) of two-point functions.
For two-point functions we define the concept of projected function: it is
zero if any of times refers to the time before switching on the interaction,
otherwise it depends only on the relative coordinates. This definition includes
bare propagators, one-loop self-energies, etc. For the infinite-average-time
limit of the Wigner transforms of projected functions we define the analyticity
assumptions: (1) The function of energy is analytic above (below) the real
axis. (2) The function goes to zero as the absolute value of energy approaches
infinity in the upper (lower) semiplane.
Without use of the gradient expansion, we obtain the convolution product of
projected functions. We sum the Schwinger-Dyson series in closed form. In the
calculation of the Keldysh component (both, resummed and single self-energy
insertion approximation) contributions appear which are not the Wigner
transforms of projected functions, signaling the limitations of the method.
In the Feynman diagrams there is no explicit energy conservation at vertices,
there is an overall energy-smearing factor taking care of the uncertainty
relations.
The relation between the theories with the Keldysh time path and with the
finite time path enables one to rederive the results, such as the cancellation
of pinching, collinear, and infrared singularities, hard thermal loop
resummation, etc.Comment: 23 pages + 1 figure, Latex, corrected version, improved presentation,
version accepted for publication in Phys. Rev.
Landau-Pomeranchuk-Migdal effect in thermal field theory
In recent studies, the production rate of photons or lepton pairs by a quark
gluon plasma has been found to be enhanced due to collinear singularities. This
enhancement pattern is very dependent on rather strict collinearity conditions
between the photon and the quark momenta. It was estimated by neglecting the
collisional width of quasi-particles. In this paper, we study the modifications
of this collinear enhancement when we take into account the possibility for the
quarks to have a finite mean free path. Assuming a mean free path of order
, we find that only low invariant mass photons are
affected. The region where collision effects are important can be interpreted
as the region where the Landau-Pomeranchuk-Migdal effect plays a role in
thermal photon production by bremsstrahlung. It is found that this effect
modifies the spectrum of very energetic photons as well. Based on these results
and on a previous work on infrared singularities, we end this paper by a
reasonable physical picture for photon production by a quark gluon plasma, that
should be useful to set directions for future technical developments.Comment: 28 pages Latex document, 9 postscript figures, typos corrected,
semantics cleanup, final version to appear in Phys. Rev.
Millimeter observations and modeling of the AB Aurigae system
(Abriged) We present the results of millimeter observations and a suitable
chemical and radiative transfer model of the AB Aur (HD 31293) circumstellar
disk and surrounding envelope. The integral molecular content of this system is
studied by observing CO, CO, CS, HCO, DCO, HCO, HCN, HNC,
and SiO rotational lines with the IRAM 30-m antenna, while the disk is mapped
in the HCO(1-0) transition with the Plateau de Bure interferometer. Using a
flared disk model with a vertical temperature gradient and an isothermal
spherical envelope model with a shadowed midplane and two unshielded cones
together with a gas-grain chemical network, time-dependent abundances of
observationally important molecules are calculated. Then a 2D non-LTE line
radiative transfer code is applied to compute excitation temperatures of
several rotational transitions of HCO, CO, CO, and CS molecules. We
synthesize the HCO(1-0) interferometric map along with single-dish CO(2-1),
CO(2-1), HCO(1-0), HCO(3-2), CS(2-1), and CS(5-4) spectra and
compared them with the observations. Our disk model successfully reproduces
observed interferometric HCO(1-0) data, thereby constraining the following
disk properties: (1) the inclination angle \iota=17^{+6}_{-3}\degr, (2) the
position angle \phi=80\pm30\degr, (3) the size AU,
(4) the mass M_\mathrm{disk}=1.3\cdot10^{-2} M_{\sun} (with a factor of
uncertainty), and (5) that the disk is in Keplerian rotation.
Furthermore, indirect evidence for a local inhomogeneity of the envelope at
\ga600 AU is found...Comment: 62 pages, 13 figures, 7 tables, accepted for publication in Ap
Cold Disks: Spitzer Spectroscopy of Disks around Young Stars with Large Gaps
We have identified four circumstellar disks with a deficit of dust emission
from their inner 15-50 AU. All four stars have F-G spectral type, and were
uncovered as part of the Spitzer Space Telescope ``Cores to Disks'' Legacy
Program Infrared Spectrograph (IRS) first look survey of ~100 pre-main sequence
stars. Modeling of the spectral energy distributions indicates a reduction in
dust density by factors of 100-1000 from disk radii between ~0.4 and 15-50 AU,
but with massive gas-rich disks at larger radii. This large contrast between
the inner and outer disk has led us to use the term `cold disks' to distinguish
these unusual systems. However, hot dust [0.02-0.2 Mmoon] is still present
close to the central star (R ~0.8 AU). We introduce the 30/13 micron, flux
density ratio as a new diagnostic for identifying cold disks. The mechanisms
for dust clearing over such large gaps are discussed. Though rare, cold disks
are likely in transition from an optically thick to an optically thin state,
and so offer excellent laboratories for the study of planet formation.Comment: 13 pages, 3 figures, accepted to ApJ
Characterizing HR3549B using SPHERE
Aims. In this work, we characterize the low mass companion of the A0 field
star HR3549. Methods. We observed HR3549AB in imaging mode with the the NIR
branch (IFS and IRDIS) of SPHERE@VLT, with IFS in YJ mode and IRDIS in the H
band. We also acquired a medium resolution spectrum with the IRDIS long slit
spectroscopy mode. The data were reduced using the dedicated SPHERE GTO
pipeline, purposely designed for this instrument. We employed algorithms such
as PCA and TLOCI to reduce the speckle noise. Results. The companion was
clearly visible both with IRDIS and IFS.We obtained photometry in four
different bands as well as the astrometric position for the companion. Based on
our astrometry, we confirm that it is a bound object and put constraints on its
orbit. Although several uncertainties are still present, we estimate an age of
~100-150 Myr for this system, yielding a most probable mass for the companion
of 40-50MJup and T_eff ~300-2400 K. Comparing with template spectra points to a
spectral type between M9 and L0 for the companion, commensurate with its
position on the color-magnitude diagram.Comment: Accepted by A&A, 13 pages, 10 Figures (Figures 9 and 10 degraded to
reduce the dimension
The GAPS Programme with HARPS-N@TNG VI: The Curious Case of TrES-4b
We revisit the TrES-4 system parameters based on high-precision HARPS-N
radial-velocity measurements and new photometric light curves. A combined
spectroscopic and photometric analysis allows us to determine a spectroscopic
orbit with an amplitude m s. The derived mass of TrES-4b is
found to be , significantly lower than
previously reported. Combined with the large radius () inferred from our analysis, TrES-4b becomes
the second-lowest density transiting hot Jupiter known. We discuss several
scenarios to explain the puzzling discrepancy in the mass of TrES-4b in the
context of the exotic class of highly inflated transiting giant planets.Comment: 5 pages, 4 figures, Letter accepted for publication in Astronomy and
Astrophysic
The disruption of nearby galaxies by the Milky Way
Interactions between galaxies are common and are an important factor in
determining their physical properties such as position along the Hubble
sequence and star-formation rate. There are many possible galaxy interaction
mechanisms, including merging, ram-pressure stripping, gas compression,
gravitational interaction and cluster tides. The relative importance of these
mechanisms is often not clear, as their strength depends on poorly known
parameters such as the density, extent and nature of the massive dark halos
that surround galaxies. A nearby example of a galaxy interaction where the
mechanism is controversial is that between our own Galaxy and two of its
neighbours -- the Large and Small Magellanic Clouds. Here we present the first
results of a new HI survey which provides a spectacular view of this
interaction. In addition to the previously known Magellanic Stream, which
trails 100 degrees behind the Clouds, the new data reveal a counter-stream
which lies in the opposite direction and leads the motion of the Clouds. This
result supports the gravitational model in which leading and trailing streams
are tidally torn from the body of the Magellanic Clouds.Comment: 17 pages with 5 figures in gif format, scheduled for publication in
the August 20th, 1998 issue of Natur
Detection of He I \AA{} absorption on HD 189733 b with CARMENES high-resolution transmission spectroscopy
We present three transit observations of HD 189733 b obtained with the
high-resolution spectrograph CARMENES at Calar Alto. A strong absorption signal
is detected in the near-infrared He I triplet at 10830 \AA{} in all three
transits. During mid-transit, the mean absorption level is %
measured in a 10 km s range at a net blueshift of km
s (10829.84--10830.57 \AA{}). The absorption signal exhibits radial
velocities of km s and km s during
ingress and egress, respectively; measured in the planetary rest frame. We show
that stellar activity related pseudo-signals interfere with the planetary
atmospheric absorption signal. They could contribute as much as 80% of the
observed signal and might also affect the radial velocity signature, but
pseudo-signals are very unlikely to explain the entire signal. The observed
line ratio between the two unresolved and the third line of the He I triplet is
, which strongly deviates from the value expected for an optically
thin atmospheres. When interpreted in terms of absorption in the planetary
atmosphere, this favors a compact helium atmosphere with an extent of only 0.2
planetary radii and a substantial column density on the order of cm. The observed radial velocities can be understood either in
terms of atmospheric circulation with equatorial superrotation or as a sign of
an asymmetric atmospheric component of evaporating material. We detect no clear
signature of ongoing evaporation, like pre- or post-transit absorption, which
could indicate material beyond the planetary Roche lobe, or radial velocities
in excess of the escape velocity. These findings do not contradict planetary
evaporation, but only show that the detected helium absorption in HD 189733 b
does not trace the atmospheric layers that show pronounced escape signatures.Comment: 13 pages, 12 figures, accepted for publication in A&
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