771 research outputs found
Disc orientations in pre-main-sequence multiple systems. A study in southern star formation regions
Classical T Tauri stars are encircled by accretion discs most of the time
unresolved by conventional imaging observation. However, numerical simulations
show that unresolved aperture linear polarimetry can be used to extract
information about the geometry of the immediate circumstellar medium that
scatter the starlight. Monin, Menard & Duchene (1998) previously suggested that
polarimetry can be used to trace the relative orientation of discs in young
binary systems in order to shed light on the stellar and planet formation
process. In this paper, we report on new VLT/FORS1 optical linear polarisation
measurements of 23 southern binaries spanning a range of separation from 0.8''
to 10''. In each field, the polarisation of the central binary is extracted, as
well as the polarisation of nearby stars in order to estimate the local
interstellar polarisation. We find that, in general, the linear polarisation
vectors of individual components in binary systems tend to be parallel to each
other. The amplitude of their polarisations are also correlated. These findings
are in agreement with our previous work and extend the trend to smaller
separations. They are also similar to other studies, e.g., Donar et al. 1999;
Jensen et al. 2000, 2004; Wolf et al. 2001. However, we also find a few systems
showing large differences in polarisation level, possibly indicating different
inclinations to the line-of-sight for their discs.Comment: 13 pages, 11 figures, accepted in Astronomy and Astrophysics.
accepted in Astronomy and Astrophysics (A&A
Decoupled and unidirectional asymptotic models for the propagation of internal waves
We study the relevance of various scalar equations, such as inviscid
Burgers', Korteweg-de Vries (KdV), extended KdV, and higher order equations (of
Camassa-Holm type), as asymptotic models for the propagation of internal waves
in a two-fluid system. These scalar evolution equations may be justified with
two approaches. The first method consists in approximating the flow with two
decoupled, counterpropagating waves, each one satisfying such an equation. One
also recovers homologous equations when focusing on a given direction of
propagation, and seeking unidirectional approximate solutions. This second
justification is more restrictive as for the admissible initial data, but
yields greater accuracy. Additionally, we present several new coupled
asymptotic models: a Green-Naghdi type model, its simplified version in the
so-called Camassa-Holm regime, and a weakly decoupled model. All of the models
are rigorously justified in the sense of consistency
Asymptotic models for the generation of internal waves by a moving ship, and the dead-water phenomenon
This paper deals with the dead-water phenomenon, which occurs when a ship
sails in a stratified fluid, and experiences an important drag due to waves
below the surface. More generally, we study the generation of internal waves by
a disturbance moving at constant speed on top of two layers of fluids of
different densities. Starting from the full Euler equations, we present several
nonlinear asymptotic models, in the long wave regime. These models are
rigorously justified by consistency or convergence results. A careful
theoretical and numerical analysis is then provided, in order to predict the
behavior of the flow and in which situations the dead-water effect appears.Comment: To appear in Nonlinearit
Wave operator bounds for 1-dimensional Schr\"odinger operators with singular potentials and applications
Boundedness of wave operators for Schr\"odinger operators in one space
dimension for a class of singular potentials, admitting finitely many Dirac
delta distributions, is proved. Applications are presented to, for example,
dispersive estimates and commutator bounds.Comment: 16 pages, 0 figure
Molecular excitation in the Eagle nebula's fingers
Context: The M16 nebula is a relatively nearby Hii region, powered by O stars
from the open cluster NGC 6611, which borders to a Giant Molecular Cloud.
Radiation from these hot stars has sculpted columns of dense obscuring material
on a few arcmin scales. The interface between these pillars and the hot ionised
medium provides a textbook example of a Photodissociation Region (PDR).
Aims: To constrain the physical conditions of the atomic and molecular
material with submillimeter spectroscopic observations.
Methods: We used the APEX submillimeter telescope to map a ~3'x3' region in
the CO J=3-2, 4-3 and 7-6 rotational lines, and a subregion in atomic carbon
lines. We also observed C18O(3-2) and CO(7-6) with longer integrations on five
peaks found in the CO(3-2) map. The large scale structure of the pillars is
derived from the molecular lines' emission distribution. We estimate the
magnitude of the velocity gradient at the tips of the pillars and use LVG
modelling to constrain their densities and temperatures. Excitation
temperatures and carbon column densities are derived from the atomic carbon
lines.
Results: The atomic carbon lines are optically thin and excitation
temperatures are of order 60 K to 100 K, well consistent with observations of
other Hii region-molecular cloud interfaces. We derive somewhat lower
temperatures from the CO line ratios, of order 40 K. The Ci/CO ratio is around
0.1 at the fingers tips.Comment: 4 pages, APEX A&A special issue, accepte
Multiwavelength studies of the gas and dust disc of IRAS 04158+2805
We present a study of the circumstellar environment of IRAS 04158+2805 based
on multi-wavelength observations and models. Images in the optical and
near-infrared, a polarisation map in the optical, and mid-infrared spectra were
obtained with VLT-FORS1, CFHT-IR, and Spitzer-IRS. Additionally we used an
X-ray spectrum observed with Chandra. We interpret the observations in terms of
a central star surrounded by an axisymmetric circumstellar disc, but without an
envelope, to test the validity of this simple geometry. We estimate the
structural properties of the disc and its gas and dust content. We modelled the
dust disc with a 3D continuum radiative transfer code, MCFOST, based on a
Monte-Carlo method that provides synthetic scattered light images and
polarisation maps, as well as spectral energy distributions. We find that the
disc images and spectral energy distribution narrowly constrain many of the
disc model parameters, such as a total dust mass of 1.0-1.75x10^-4 sollar
masses and an inclination of 62-63 degrees. The maximum grain size required to
fit all available data is of the order of 1.6-2.8 microns although the upper
end of this range is loosely constrained. The observed optical polarisation map
is reproduced well by the same disc model, suggesting that the geometry we find
is adequate and the optical properties are representative of the visible dust
content. We compare the inferred dust column density to the gas column density
derived from the X-ray spectrum and find a gas-to-dust ratio along the line of
sight that is consistent with the ISM value. To our knowledge, this measurement
is the first to directly compare dust and gas column densities in a
protoplanetary disc.Comment: 8 figures, 11 pages, accepted by A&
The XMM-Newton Extended Survey of the Taurus Molecular Cloud (XEST)
(abridged:) The XMM-Newton Extended Survey of the Taurus Molecular Cloud
(XEST) surveys the most populated ~5 square degrees of the Taurus star
formation region, using the XMM-Newton X-ray observatory to study the thermal
structure, variability, and long-term evolution of hot plasma, to investigate
the magnetic dynamo, and to search for new potential members of the
association. Many targets are also studied in the optical, and high-resolution
X-ray grating spectroscopy has been obtained for selected bright sources. The
X-ray spectra have been coherently analyzed with two different thermal models
(2-component thermal model, and a continuous emission measure distribution
model). We present overall correlations with fundamental stellar parameters
that were derived from the previous literature. A few detections from Chandra
observations have been added. The present overview paper introduces the project
and provides the basic results from the X-ray analysis of all sources detected
in the XEST survey.Comprehensive tables summarize the stellar properties of all
targets surveyed. The survey goes deeper than previous X-ray surveys of Taurus
by about an order of magnitude and for the first time systematically accesses
very faint and strongly absorbed TMC objects. We find a detection rate of 85%
and 98% for classical and weak-line T Tau stars (CTTS resp. WTTS), and identify
about half of the surveyed protostars and brown dwarfs. Overall, 136 out of 169
surveyed stellar systems are detected. We describe an X-ray luminosity vs. mass
correlation, discuss the distribution of X-ray-to-bolometric luminosity ratios,
and show evidence for lower X-ray luminosities in CTTS compared to WTTS.
Detailed analysis (e.g., variability, rotation-activity relations, influence of
accretion on X-rays) will be discussed in a series of accompanying papers.Comment: 75 pg, 77 figs. Accepted by A&A, to appear in a special section/issue
dedicated to the XMM-Newton Extended Survey of the Taurus Molecular Cloud
(XEST). V2: ASCII Table 14 added. Version with higher resolution figures at
http://www.issibern.ch/teams/Taurus/papers.html or
http://www.astro.phys.ethz.ch/papers/guedel/guedel_p_nf.htm
Spatially resolved H2 emission from the disk around T Tau N
We report the detection of quiescent H2 emission in a spatially resolved
ring-like structure within 100 AU of T Tau N. We present evidence to show that
the emission most likely arises from shocks in the atmosphere of a nearly
face-on disk around T Tau N. Using high spatial resolution 3D spectroscopic
K-band data, we trace the spatial distribution of several H2 NIR rovibrational
lines in the vicinity of T Tau N. We detect weak H2 emission from the v=1-0
S(0), S(1), Q(1) lines and the v=2-1 S(1) line in a ring-like structure around
T Tau N between 0.1'' (~15 AU) and 0.7'' (~100AU) from the star. The v=1-0 S(0)
and v=2-1 S(1) lines are detected only in the outer parts of the ring
structure. Closer to the star, the strong continuum limits our sensitivity to
these lines. The total flux of the v=1-0 S(1) line is 1.8 *10^{-14} ergs
s^{-1}cm^{-2}, similar to previous measurements of H2 in circumstellar disks.
The velocity of the H2 emitting gas around T Tau N is consistent with the rest
velocity of the star, and the H2 does not seem to be part of a collimated
outflow. Both shocks impinging on the surface of a disk and irradiation of a
disk by UV-photons and X-rays from the central star are plausible candidates
for the H2 excitation mechanism. However, irradiation should not create a large
degree of excitation at radii larger than 20 AU. Most likely the H2 emission
arises in the atmosphere of a flared disk with radius 85-100 AU and mass
0.005-0.5Msun, where the gas is excited by shocks created when a wide-angle
wind impinges on the disk. The H2 emission could also originate from shock
excitation in the cavity walls of an envelope, but this requires an unusually
high velocity of the wide-angle wind from T Tau N.Comment: Accepted by A&
Resolved Debris Discs Around A Stars in the Herschel DEBRIS Survey
The majority of debris discs discovered so far have only been detected
through infrared excess emission above stellar photospheres. While disc
properties can be inferred from unresolved photometry alone under various
assumptions for the physical properties of dust grains, there is a degeneracy
between disc radius and dust temperature that depends on the grain size
distribution and optical properties. By resolving the disc we can measure the
actual location of the dust. The launch of Herschel, with an angular resolution
superior to previous far-infrared telescopes, allows us to spatially resolve
more discs and locate the dust directly. Here we present the nine resolved
discs around A stars between 20 and 40 pc observed by the DEBRIS survey. We use
these data to investigate the disc radii by fitting narrow ring models to
images at 70, 100 and 160 {\mu}m and by fitting blackbodies to full spectral
energy distributions. We do this with the aim of finding an improved way of
estimating disc radii for unresolved systems. The ratio between the resolved
and blackbody radii varies between 1 and 2.5. This ratio is inversely
correlated with luminosity and any remaining discrepancies are most likely
explained by differences to the minimum size of grain in the size distribution
or differences in composition. We find that three of the systems are well fit
by a narrow ring, two systems are borderline cases and the other four likely
require wider or multiple rings to fully explain the observations, reflecting
the diversity of planetary systems.Comment: 19 pages, 13 figures, 6 tables. Accepted for publication in MNRA
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