287 research outputs found
On the unique possibility to increase significantly the contrast of dark resonances on D1 line of Rb
We propose and study, theoretically and experimentally, a new scheme of
excitation of a coherent population trapping resonance for D1 line of alakli
atoms with nuclear spin by bichromatic linearly polarized light ({\em
lin}{\em lin} field) at the conditions of spectral resolution of the
excited state. The unique properties of this scheme result in a high contrast
of dark resonance for D1 line of Rb.Comment: 9 pages, 7 figures. This material has been partially presented on
ICONO-2005, 14 May 2005, St. Petersburg, Russia. v2 references added; text is
changed a bi
Evolutionary models for low-mass stars and brown dwarfs: uncertainties and limits at very young ages
We analyse pre-Main Sequence evolutionary tracks for low mass stars with
masses m \le 1.4 \msol based on the Baraffe et al. (1998) input physics. We
also extend the recent Chabrier et al. (2000) evolutionary models based on
dusty atmosphere to young brown dwarfs down to one mass of Jupiter. We analyse
current theoretical uncertainties due to molecular line lists, convection and
initial conditions. Simple tests on initial conditions show the high
uncertainties of models at ages \simle 1 Myr. We find a significant
sensitivity of atmosphere profiles to the treatment of convection at low
gravity and \te < 4000 K, whereas it vanishes as gravity increases. This
effect adds another source of uncertainty on evolutionary tracks at very early
phases. We show that at low surface gravity (\log g \simle 3.5,) the common
picture of vertical Hayashi lines with constant \te is oversimplified. The
effect of a variation of initial deuterium abundance is studied. We compare our
models with evolutionary tracks available in the literature and discuss the
main differences. We finally analyse to which extent current observations of
young systems provide a good test for pre-Main Sequence tracks.Comment: 12 pages, Latex file, uses aa.cls, accepted for publication in A&
Evidence for a fundamental stellar upper mass limit from clustered star formation
The observed masses of the most massive stars do not surpass about 150Msun.
This may either be a fundamental upper mass limit which is defined by the
physics of massive stars and/or their formation, or it may simply reflect the
increasing sparsity of such very massive stars so that observing even
higher-mass stars becomes unlikely in the Galaxy and the Magellanic Clouds. It
is shown here that if the stellar initial mass function (IMF) is a power-law
with a Salpeter exponent (alpha=2.35) for massive stars then the richest very
young cluster R136 seen in the Large Magellanic Cloud (LMC) should contain
stars with masses larger than 750Msun. If, however, the IMF is formulated by
consistently incorporating a fundamental upper mass limit then the observed
upper mass limit is arrived at readily even if the IMF is invariant. An
explicit turn-down or cutoff of the IMF near 150Msun is not required; our
formulation of the problem contains this implicitly. We are therefore led to
conclude that a fundamental maximum stellar mass near 150Msun exists, unless
the true IMF has alpha>2.8.Comment: MNRAS, accepted, 6 page
High-resolution study of a star-forming cluster in the Cep-A HW2 region
Due to its relatively small distance (725 pc), the Cepheus A East
star-forming region is an ideal laboratory to study massive star formation
processes. Based on its morphology, it has been suggested that the flattened
molecular gas distribution around the YSO HW2 may be a 350-AU-radius massive
protostellar disk. Goal of our work is to ascertain the nature of this
structure. We have employed the Plateau de Bure Interferometer to acquire
(sub-)arcsecond-resolution imaging of high-density and shock tracers, such as
methyl cyanide (CH3CN) and silicon monoxide (SiO), towards the HW2 position. On
the 1-arcsecond (about 725 AU) scale, the flattened distribution of molecular
gas around HW2 appears to be due to the projected superposition, on the plane
of the sky, of at least three protostellar objects, of which at least one is
powering a molecular outflow at a small angle with respect to the line of
sight. The presence of a protostellar disk around HW2 is not ruled out, but
such structure is likely to be detected on a smaller spatial scale, or using
different molecular tracers.Comment: 6 pages, 5 figures, accepted for publication in Astronomy &
Astrophysic
Trapped Protostellar Winds and their Breakout
Observations show that high-velocity jets stem from deeply embedded young
stars, which may still be experiencing infall from their parent cloud cores.
Yet theory predicts that, early in this buildup, any outgoing wind is trapped
by incoming material of low angular momentum. As collapse continues and brings
in more rapidly rotating gas, the wind can eventually break out. Here we model
this transition by following the motion of the shocked shell created by impact
of the wind and a rotating, collapsing envelope. We first demonstrate, both
analytically and numerically, that our previous, quasi-static solutions are
dynamically unstable. Our present, fully time-dependent calculations include
cases both where the wind is driven back by infall to the stellar surface, and
where it erupts as a true outflow. For the latter, we find that the time of
breakout is sim 50,000 yr for wind speeds of 200 km/s. The reason for the delay
is that the shocked material, including the swept-up infall, must be able to
climb out of the star's gravitational potential well.
We explore the critical wind speed necessary for breakout as a function of
the mass transport rates in the wind and infall, as well as the cloud rotation
rate Omega0 and time since the start of infall. Breakout does occur for
realistic parameter choices. The actual breakout times would change if we
relaxed the assumption of perfect mixing between the wind and infall material.
Our expanding shells do not exhibit the collimation of observed jets, but
continue to expand laterally. To halt this expansion, the density in the
envelope must fall off less steeply than in our model.Comment: 44 pages, 10 figures, accepted to Ap
On the Influence of Uncertainties in Chemical Reaction Rates on Results of the Astrochemical Modelling
With the chemical reaction rate database UMIST95 (Millar et al. 1997) we
analyze how uncertainties in rate constants of gas-phase chemical reactions
influence the modelling of molecular abundances in the interstellar medium.
Random variations are introduced into the rate constants to estimate the
scatter in theoretical abundances. Calculations are performed for dark and
translucent molecular clouds where gas phase chemistry is adequate. Similar
approach was used by Pineau des Forets & Roueff (2000) for the study of
chemical bistability. All the species are divided into 6 sensitivity groups
according to the value of the scatter in their model abundances computed with
varied rate constants. It is shown that the distribution of species within
these groups depends on the number of atoms in a molecule and on the adopted
physical conditions. The simple method is suggested which allows to single out
reactions that are most important for the evolution of a given species.Comment: 4 pages. To appear in the proceedings of the 4th Cologne-Bonn Zermatt
Symposiu
Pre-galactic metal enrichment - The chemical signatures of the first stars
The emergence of the first sources of light at redshifts of z ~ 10-30
signaled the transition from the simple initial state of the Universe to one of
increasing complexity. We review recent progress in our understanding of the
formation of the first stars and galaxies, starting with cosmological initial
conditions, primordial gas cooling, and subsequent collapse and fragmentation.
We emphasize the important open question of how the pristine gas was enriched
with heavy chemical elements in the wake of the first supernovae. We conclude
by discussing how the chemical abundance patterns conceivably allow us to probe
the properties of the first stars and subsequent stellar generations, and allow
us to test models of early metal enrichment.Comment: 52 pages, 20 figures, clarifications, references added, accepted for
publication in the Reviews of Modern Physic
A large, massive, rotating disk around an isolated young stellar object
We present multi-wavelengths observations and a radiative transfer model of a
newly discovered massive circumstellar disk of gas and dust which is one of the
largest disks known today. Seen almost edge-on, the disk is resolved in
high-resolution near-infrared (NIR) images and appears as a dark lane of high
opacity intersecting a bipolar reflection nebula. Based on molecular line
observations we estimate the distance to the object to be 3.5 kpc. This leads
to a size for the dark lane of ~10500 AU but due to shadowing effects the true
disk size could be smaller. In Spitzer/IRAC 3.6 micron images the elongated
shape of the bipolar reflection nebula is still preserved and the bulk of the
flux seems to come from disk regions that can be detected due to the slight
inclination of the disk. At longer IRAC wavelengths, the flux is mainly coming
from the central regions penetrating directly through the dust lane.
Interferometric observations of the dust continuum emission at millimeter
wavelengths with the SMA confirm this finding as the peak of the unresolved
mm-emission coincides perfectly with the peak of the Spitzer/IRAC 5.8 micron
flux and the center of the dark lane seen in the NIR images. Simultaneously
acquired CO data reveal a molecular outflow along the northern part of the
reflection nebula which seems to be the outflow cavity. An elongated gaseous
disk component is also detected and shows signs of rotation. The emission is
perpendicular to the molecular outflow and thus parallel to but even more
extended than the dark lane in the NIR images. Based on the dust continuum and
the CO observations we estimate a disk mass of up to a few solar masses
depending on the underlying assumptions. Whether the disk-like structure is an
actual accretion disk or rather a larger-scale flattened envelope or pseudodisk
is difficult to discriminate with the current dataset (abridged).Comment: Accepted for publication in ApJ, 29 pages preprint style incl. 10
Figure
Ultrafast Electronic Band Gap Control in an Excitonic Insulator
We report on the nonequilibrium dynamics of the electronic structure of the layered semiconductor Ta2NiSe5 investigated by time- and angle-resolved photoelectron spectroscopy. We show that below the critical excitation density of FC=0.2 mJ cm-2, the band gap narrows transiently, while it is enhanced above FC. Hartree-Fock calculations reveal that this effect can be explained by the presence of the low-temperature excitonic insulator phase of Ta2NiSe5, whose order parameter is connected to the gap size. This work demonstrates the ability to manipulate the band gap of Ta2NiSe5 with light on the femtosecond time scale
The Structure and Evolution of Magnetized Cloud Cores in a Zero--Density Background
Molecular-line observations of star-forming cloud cores indicate that they
are not the flattened structures traditionally considered by theory. Rather,
they are elongated, perhaps in the direction of their internal magnetic field.
We are thus motivated to consider the structure and evolution of axisymmetric,
magnetized clouds that start from a variety of initial states, both flattened
(oblate) and elongated (prolate). We devise a new technique, dubbed the
-method, that allows us to construct magnetostatic equilibria of any
specified shape. We find, in agreement with previous authors, that the field
lines in oblate clouds bend inward. However, those in prolate clouds bow
outward, confining the structures through magnetic tension.
We next follow the quasi-static evolution of these clouds via ambipolar
diffusion, under the assumption of constant core mass. An oblate cloud either
relaxes to a magnetically force-free sphere or, if sufficiently massive,
flattens along its polar axis as its central density runs away. A prolate cloud
always relaxes to a sphere of modest central density. We finally consider the
evolution of an initially spherical cloud subject to the tidal gravity of
neighboring bodies. Although the structure constricts equatorially, it also
shortens along the pole, so that it ultimately flattens on the way to collapse.
In summary, none of our initial states can evolve to the point of collapse
while maintaining an elongated shape. We speculate that this situation will
change once we allow the cloud to gain mass from its environment.Comment: 19 pages, plus 20 postscript figures. Accepted by Ap
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