221 research outputs found
Deux ou trois choses que l'on savait dans la France de l'Ouest, en 2001, a propos de la nature de certaines structures funéraires dites monumentales, ainsi que leur ordonnancement chronologique au long des Ve et IVe millenaires
On donne ici des résultats récents sur les monuments mégalithiques du Morbihan (site de
Lannec Er Gadouer), et quelques idées sur la monumentalité funéraire et le Néolithique des régiones atlantiques
Sels de mer, sels de terre. Indices et preuves de fabrication du sel sur le rivages de l'Europe Occidentale, du Ve au IIIe millénaire
On donne ici quelques reflexions sur l'exploitation de sel dès le Néolithique et sa valeur
comme source de richesse et prestige. Sont analysés quelques exemples etnographiques et historiques d'obtention
de ce produit, en les comparant avec quelques traces archéologiques dans différents lieux de l'Europe
Amplification of MHD waves in swirling astrophysical flows
Recently it was found that helical magnetized flows efficiently amplify
Alfv\'en waves (Rogava et al. 2003, A&A, v.399, p.421). This robust and
manifold nonmodal effect was found to involve regimes of transient algebraic
growth (for purely ejectional flows), and exponential instabilities of both
usual and parametric nature. However the study was made in the incompressible
limit and an important question remained open - whether this amplification is
inherent to swirling MHD flows per se and what is the degree of its dependence
on the incompressibility condition. In this paper, in order to clear up this
important question, we consider full compressible spectrum of MHD modes:
Alfv\'en waves (AW), slow magnetosonic waves (SMW) and fast magnetosonic waves
(FMW). We find that helical flows inseparably blend these waves with each other
and make them unstable, creating the efficient energy transfer from the mean
flow to the waves. The possible role of these instabilities for the onset of
the MHD turbulence, self-heating of the flow and the overall dynamics of
astrophysical flows are discussed.Comment: 8 pages, 9 figures, accepted for publication (18.03.2003) in the
"Astronomy and Astrophysics
Vertically Self-Gravitating ADAFs in the Presence of Toroidal Magnetic Field
Force due to the self-gravity of the disc in the vertical direction is
considered to study its possible effects on the structure of a magnetized
advection-dominated accretion disc. We present steady-sate self similar
solutions for the dynamical structure of such a type of the accretion flows.
Our solutions imply reduced thickness of the disc because of the self-gravity.
It also imply that the thickness of the disc will increase by adding the
magnetic field strength.Comment: Accepted for publication in Astrophysics and Space Science
On type-I migration near opacity transitions. A generalized Lindblad torque formula for planetary population synthesis
We give an expression for the Lindblad torque acting on a low-mass planet
embedded in a protoplanetary disk that is valid even at locations where the
surface density or temperature profile cannot be approximated by a power law,
such as an opacity transition. At such locations, the Lindblad torque is known
to suffer strong deviation from its standard value, with potentially important
implications for type I migration, but the full treatment of the tidal
interaction is cumbersome and not well suited to models of planetary population
synthesis. The expression that we propose retains the simplicity of the
standard Lindblad torque formula and gives results that accurately reproduce
those of numerical simulations, even at locations where the disk temperature
undergoes abrupt changes. Our study is conducted by means of customized
numerical simulations in the low-mass regime, in locally isothermal disks, and
compared to linear torque estimates obtained by summing fully analytic torque
estimates at each Lindblad resonance. The functional dependence of our modified
Lindblad torque expression is suggested by an estimate of the shift of the
Lindblad resonances that mostly contribute to the torque, in a disk with sharp
gradients of temperature or surface density, while the numerical coefficients
of the new terms are adjusted to seek agreement with numerics. As side results,
we find that the vortensity related corotation torque undergoes a boost at an
opacity transition that can counteract migration, and we find evidence from
numerical simulations that the linear corotation torque has a non-negligible
dependency upon the temperature gradient, in a locally isothermal disk.Comment: Appeared in special issue of "Celestial Mechanics and Dynamical
Astronomy" on Extrasolar Planetary System
Sub-arcsec imaging of the AB Aur molecular disk and envelope at millimeter wavelengths: a non Keplerian disk
We present sub-arcsecond images of AB Auriga obtained with the IRAM Plateau
de Bure interferometer in the isotopologues of CO, and in continuum at 3 and
1.3 mm. Instead of being centrally peaked, the continuum emission is dominated
by a bright, asymmetric (spiral-like) feature at about 140 AU from the central
star. The large scale molecular structure suggests the AB Aur disk is inclined
between 23 and 43 degrees, but the strong asymmetry of the continuum and
molecular emission prevents an accurate determination of the inclination of the
inner parts. We find significant non-Keplerian motion, with a best fit exponent
for the rotation velocity law of 0.41 +/- 0.01, but no evidence for radial
motions. The disk has an inner hole about 70 AU in radius. The disk is warm and
shows no evidence of depletion of CO. The dust properties suggest the dust is
less evolved than in typical T Tauri disks. Both the spiral-like feature and
the departure from purely Keplerian motions indicates the AB Aur disk is not in
quasi-equilibrium. Disk self-gravity is insufficient to create the
perturbation. This behavior may be related either to an early phase of star
formation in which the Keplerian regime is not yet fully established and/or to
a disturbance of yet unknown origin. An alternate, but unproven, possibility is
that of a low mass companion located about 40 AU from AB Aur.Comment: 10 pages, 5 figures, accepted for publication in Astronomy &
Astrophysic
A VLA search for young protostars embedded in dense cores
Four dense cores, L1582A, L1689A, B133 and B68, classified as prestellar in
terms of the absence of detectable NIR emission, are observed at radio
wavelengths to investigate whether they nurture very young protostars. No
definite young protostars were discovered in any of the four cores observed. A
few radio sources were discovered close to the observed cores, but these are
most likely extragalactic sources or YSOs unrelated to the cores observed. In
L1582A we discovered a weak radio source near the centre of the core with radio
characteristics and offset from the peak of the submillimeter emission similar
to that of the newly discovered protostar in the core L1014, indicating a
possible protostellar nature for this source. This needs to be confirmed with
near- and/or mid-infrared observations (e.g. with Spitzer). Hence based on the
current observations we are unable to confirm unequivocally that L1582A is
starless. In L1689A a possible 4.5-sigma radio source was discovered at the
centre of the core, but needs to be confirmed with future observations. In B133
a weak radio source, possibly a protostar, was discovered at the edge of the
core on a local peak of the core submm emission, but no source was detected at
the centre of the core. Thus, B133 is probably starless, but may have a
protostar at its edge. In B68 no radio sources were discovered inside or at the
edge of the core, and thus B68 is indeed starless. Four more radio sources with
spectral indices characteristic of young protostars were discovered outside the
cores but within the extended clouds in which these cores reside. Conclusions:
We conclude that the number of cores misclassified as prestellar is probably
very small and does not significantly alter the estimated lifetime of the
prestellar phase.Comment: Accepted by A&
Evolution of oxygen isotopic composition in the inner solar nebula
Changes in the chemical and isotopic composition of the solar nebula with
time are reflected in the properties of different constituents that are
preserved in chondritic meteorites. CR carbonaceous chondrites are among the
most primitive of all chondrite types and must have preserved solar nebula
records largely unchanged. We have analyzed the oxygen and magnesium isotopes
in a range of the CR constituents of different formation temperatures and ages,
including refractory inclusions and chondrules of various types. The results
provide new constraints on the time variation of the oxygen isotopic
composition of the inner (<5 AU) solar nebula - the region where refractory
inclusions and chondrules most likely formed. A chronology based on the decay
of short-lived 26Al (t1/2 ~ 0.73 Ma) indicates that the inner solar nebula gas
was 16O-rich when refractory inclusions formed, but less than 0.8 Ma later, gas
in the inner solar nebula became 16O-poor and this state persisted at least
until CR chondrules formed ~1-2 Myr later. We suggest that the inner solar
nebula became 16O-poor because meter-size icy bodies, which were enriched in
17,18O due to isotopic self-shielding during the ultraviolet photo dissociation
of CO in the protosolar molecular cloud or protoplanetary disk, agglomerated
outside the snowline, drifted rapidly towards the Sun, and evaporated at the
snowline. This led to significant enrichment in 16O-depleted water, which then
spread through the inner solar system. Astronomical studies of the spatial
and/or temporal variations of water abundance in protoplanetary disks may
clarify these processes.Comment: 27 pages, 5 figure
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