1,011 research outputs found
Preliminary galaxy extraction from DENIS images
The extragalactic applications of NIR surveys are summarized with a focus on
the ability to map the interstellar extinction of our Galaxy. Very preliminary
extraction of galaxies on a set of 180 consecutive images is presented, and the
results illustrate some of the pitfalls in attempting an homogeneous extraction
of galaxies from these wide-angle and shallow surveys.Comment: Invited talk at "The Impact of Large-Scale Near-IR Sky Surveys",
meeting held in Tenerife, Spain, April 1996. 10 pages LaTeX with style file
and 4 PS files include
The Role of Column Density in the Formation of Stars and Black Holes
The stellar mass in disk galaxies scales approximately with the fourth power
of the rotation velocity, and the masses of the central black holes in galactic
nuclei scale approximately with the fourth power of the bulge velocity
dispersion. It is shown here that these relations can be accounted for if, in a
forming galaxy with an isothermal mass distribution, gas with a column density
above about 8 Msun/pc^2 goes into stars while gas with a column density above
about 2 g/cm^2 (10^4 Msun/pc^2) goes into a central black hole. The lower
critical value is close to the column density of about 10 Msun/pc^2 at which
atomic gas becomes molecular, and the upper value agrees approximately with the
column density of about 1 g/cm^2 at which the gas becomes optically thick to
its cooling radiation. These results are plausible because molecule formation
is evidently necessary for star formation, and because the onset of a high
optical depth in a galactic nucleus may suppress continuing star formation and
favour the growth of a central black hole.Comment: Accepted by Nature Physic
Fundamental Aspects of the ISM Fractality
The ubiquitous clumpy state of the ISM raises a fundamental and open problem
of physics, which is the correct statistical treatment of systems dominated by
long range interactions. A simple solvable hierarchical model is presented
which explains why systems dominated by gravity prefer to adopt a fractal
dimension around 2 or less, like the cold ISM and large scale structures. This
has direct relation with the general transparency, or blackness, of the
Universe.Comment: 6 pages, LaTeX2e, crckapb macro, no figure, uuencoded compressed tar
file. To be published in the proceeedings of the "Dust-Morphology"
conference, Johannesburg, 22-26 January, 1996, D. Block (ed.), (Kluwer
Dordrecht
Protostellar Jet and Outflow in the Collapsing Cloud Core
We investigate the driving mechanism of outflows and jets in star formation
process using resistive MHD nested grid simulations. We found two distinct
flows in the collapsing cloud core: Low-velocity outflows (sim 5 km/s) with a
wide opening angle, driven from the first adiabatic core, and high-velocity
jets (sim 50 km/s) with good collimation, driven from the protostar.
High-velocity jets are enclosed by low-velocity outflow. The difference in the
degree of collimation between the two flows is caused by the strength of the
magnetic field and configuration of the magnetic field lines. The magnetic
field around an adiabatic core is strong and has an hourglass configuration.
Therefore, the low-velocity outflow from the adiabatic core are driven mainly
by the magnetocentrifugal mechanism and guided by the hourglass-like field
lines. In contrast, the magnetic field around the protostar is weak and has a
straight configuration owing to Ohmic dissipation in the high-density gas
region. Therefore, high-velocity jet from the protostar are driven mainly by
the magnetic pressure gradient force and guided by straight field lines.
Differing depth of the gravitational potential between the adiabatic core and
the protostar cause the difference of the flow speed. Low-velocity outflows
correspond to the observed molecular outflows, while high-velocity jets
correspond to the observed optical jets. We suggest that the protostellar
outflow and the jet are driven by different cores (the first adiabatic core and
protostar), rather than that the outflow being entrained by the jet.Comment: To appear in the proceedings of the "Protostellar Jets in Context"
conference held on the island of Rhodes, Greece (7-12 July 2008
Efficient Mixing at low Reynolds numbers using polymer additives
Mixing in fluids is a rapidly developing field of fluid mechanics
\cite{Sreen,Shr,War}, being an important industrial and environmental problem.
The mixing of liquids at low Reynolds numbers is usually quite weak in simple
flows, and it requires special devices to be efficient. Recently, the problem
of mixing was solved analytically for a simple case of random flow, known as
the Batchelor regime \cite{Bat,Kraich,Fal,Sig,Fouxon}. Here we demonstrate
experimentally that very viscous liquids at low Reynolds number, . Here we
show that very viscous liquids containing a small amount of high molecular
weight polymers can be mixed quite efficiently at very low Reynolds numbers,
for a simple flow in a curved channel. A polymer concentration of only 0.001%
suffices. The presence of the polymers leads to an elastic instability
\cite{LMS} and to irregular flow \cite{Ours}, with velocity spectra
corresponding to the Batchelor regime \cite{Bat,Kraich,Fal,Sig,Fouxon}. Our
detailed observations of the mixing in this regime enable us to confirm sevearl
important theoretical predictions: the probability distributions of the
concentration exhibit exponential tails \cite{Fal,Fouxon}, moments of the
distribution decay exponentially along the flow \cite{Fouxon}, and the spatial
correlation function of concentration decays logarithmically.Comment: 11 pages, 5 figure
Secular Evolution of Galaxy Morphologies
Today we have numerous evidences that spirals evolve dynamically through
various secular or episodic processes, such as bar formation and destruction,
bulge growth and mergers, sometimes over much shorter periods than the standard
galaxy age of 10-15 Gyr. This, coupled to the known properties of the Hubble
sequence, leads to a unique sense of evolution: from Sm to Sa. Linking this to
the known mass components provides new indications on the nature of dark matter
in galaxies. The existence of large amounts of yet undetected dark gas appears
as the most natural option. Bounds on the amount of dark stars can be given
since their formation is mostly irreversible and requires obviously a same
amount of gas.Comment: 8 pages, Latex2e, crckapb.sty macros, 1 Postscript figure, replaced
with TeX source; To be published in the proceeedings of the "Dust-Morphology"
conference, Johannesburg, 22-26 January, 1996, D. Block (ed.), (Kluwer
Dordrecht
The effects of a Variable IMF on the Chemical Evolution of the Galaxy
In this work we explore the effects of adopting an initial mass function
(IMF) variable in time on the chemical evolution of the Galaxy. In order to do
that we adopt a chemical evolution model which assumes two main infall episodes
for the formation of the Galaxy. We study the effects on such a model of
different IMFs. First, we use a theoretical one based on the statistical
description of the density field arising from random motions in the gas. This
IMF is a function of time as it depends on physical conditions of the site of
star formation. We also investigate the behaviour of the model predictions
using other variable IMFs, parameterized as a function of metallicity. Our
results show that the theoretical IMF when applied to our model depends on time
but such time variation is important only in the early phases of the Galactic
evolution, when the IMF is biased towards massive stars. We also show that the
use of an IMF which is a stronger function of time does not lead to a good
agreement with the observational constraints suggesting that if the IMF varied
this variation should have been small. Our main conclusion is that the G-dwarf
metallicity distribution is best explained by infall with a large timescale and
a constant IMF, since it is possible to find variable IMFs of the kind studied
here, reproducing the G-dwarf metallicity but this worsens the agreement with
other observational constraints.Comment: 7 pages, to appear in "The Chemical Evolution of the Milky Way: Stars
vs Clusters", Vulcano, September 1999, F. Giovannelli and F. Matteucci eds.
(Kluwer, Dordrecht) in pres
Fundamental Strings, Holography, and Nonlinear Superconformal Algebras
We discuss aspects of holography in the AdS_3 \times S^p near string geometry
of a collection of straight fundamental heterotic strings. We use anomalies and
symmetries to determine general features of the dual CFT. The symmetries
suggest the appearance of nonlinear superconformal algebras, and we show how
these arise in the framework of holographic renormalization methods. The
nonlinear algebras imply intricate formulas for the central charge, and we show
that in the bulk these correspond to an infinite series of quantum gravity
corrections. We also makes some comments on the worldsheet sigma-model for
strings on AdS_3\times S^2, which is the holographic dual geometry of parallel
heterotic strings in five dimensions.Comment: 25 page
Stellar Disk Truncations: Where do we stand ?
In the light of several recent developments we revisit the phenomenon of
galactic stellar disk truncations. Even 25 years since the first paper on outer
breaks in the radial light profiles of spiral galaxies, their origin is still
unclear. The two most promising explanations are that these 'outer edges'
either trace the maximum angular momentum during the galaxy formation epoch, or
are associated with global star formation thresholds. Depending on their true
physical nature, these outer edges may represent an improved size
characteristic (e.g., as compared to D_25) and might contain fossil evidence
imprinted by the galaxy formation and evolutionary history. We will address
several observational aspects of disk truncations: their existence, not only in
normal HSB galaxies, but also in LSB and even dwarf galaxies; their detailed
shape, not sharp cut-offs as thought before, but in fact demarcating the start
of a region with a steeper exponential distribution of starlight; their
possible association with bars; as well as problems related to the
line-of-sight integration for edge-on galaxies (the main targets for truncation
searches so far). Taken together, these observations currently favour the
star-formation threshold model, but more work is necessary to implement the
truncations as adequate parameters characterising galactic disks.Comment: LaTeX, 10 pages, 6 figures, presented at the "Penetrating Bars
through Masks of Cosmic Dust" conference in South Africa, proceedings
published by Kluwer, and edited by Block, D.L., Freeman, K.C., Puerari, I., &
Groess, R; v3 to match published versio
The Formation of the First Low-Mass Stars From Gas With Low Carbon and Oxygen Abundances
The first stars in the Universe are predicted to have been much more massive
than the Sun. Gravitational condensation accompanied by cooling of the
primordial gas due to molecular hydrogen, yields a minimum fragmentation scale
of a few hundred solar masses. Numerical simulations indicate that once a gas
clump acquires this mass, it undergoes a slow, quasi-hydrostatic contraction
without further fragmentation. Here we show that as soon as the primordial gas
- left over from the Big Bang - is enriched by supernovae to a carbon or oxygen
abundance as small as ~0.01-0.1% of that found in the Sun, cooling by
singly-ionized carbon or neutral oxygen can lead to the formation of low-mass
stars. This mechanism naturally accommodates the discovery of solar mass stars
with unusually low (10^{-5.3} of the solar value) iron abundance but with a
high (10^{-1.3} solar) carbon abundance. The minimum stellar mass at early
epochs is partially regulated by the temperature of the cosmic microwave
background. The derived critical abundances can be used to identify those
metal-poor stars in our Milky Way galaxy with elemental patterns imprinted by
the first supernovae.Comment: 14 pages, 2 figures (appeared today in Nature
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