11,943 research outputs found
Do we need to know the temperature in prestellar cores?
Molecular line observations of starless (prestellar) cores combined with a
chemical evolution modeling and radiative transfer calculations are a powerful
tool to study the earliest stages of star formation. However, conclusions drawn
from such a modeling may noticeably depend on the assumed thermal structure of
the cores. The assumption of isothermality, which may work well in
chemo-dynamical studies, becomes a critical factor in molecular line formation
simulations. We argue that even small temperature variations, which are likely
to exist in starless cores, can have a non-negligible effect on the
interpretation of molecular line data and derived core properties. In
particular, ``chemically pristine'' isothermal cores (low depletion) can have
centrally peaked CO and CS radial intensity profiles, while
having ring-like intensity distributions in models with a colder center and/or
warmer envelope assuming the same underlying chemical structure. Therefore,
derived molecular abundances based on oversimplified thermal models may lead to
a mis-interpretation of the line data.Comment: ApJL, accepte
A rotating disk around the very young massive star AFGL 490
We observed the embedded, young 8--10 Msun star AFGL 490 at subarcsecond
resolution with the Plateau de Bure Interferometer in the C17O (2--1)
transition and found convincing evidence that AFGL 490 is surrounded by a
rotating disk. Using two-dimensional modeling of the physical and chemical disk
structure coupled to line radiative transfer, we constrain its basic
parameters. We obtain a relatively high disk mass of 1 Msun and a radius of ~
1500 AU. A plausible explanation for the apparent asymmetry of the disk
morphology is given.Comment: 4 pages, 5 figure
The Star-Forming Region NGC 346 in the Small Magellanic Cloud with Hubble Space Telescope ACS Observations I. Photometry
We present a photometric study of the star-forming region NGC 346 and its
surrounding field in the Small Magellanic Cloud, using data taken with the
Advanced Camera for Surveys (ACS) on board the Hubble Space Telescope (HST).
The data set contains both short and long exposures for increased dynamic
range, and photometry was performed using the ACS module of the stellar
photometry package DOLPHOT. We detected almost 100,000 stars over a magnitude
range of V ~ 11 to V ~ 28 mag, including all stellar types from the most
massive young stars to faint lower main sequence and pre-main sequence stars.
We find that this region, which is characterized by a plethora of stellar
systems and interesting objects, is an outstanding example of mixed stellar
populations. We take into account different features of the color-magnitude
diagram of all the detected stars to distinguish the two dominant stellar
systems: The stellar association NGC 346 and the old spherical star cluster BS
90. These observations provide a complete stellar sample of a field about 5
arcmin x 5 arcmin around the most active star-forming region in this galaxy.
Considering the importance of these data for various investigations in the
area, we provide the full stellar catalog from our photometry. This paper is
the first part of an ongoing study to investigate in detail the two dominant
stellar systems in the area and their surrounding field.Comment: 8 pages, 5 figures, Accepted for publication in The Astrophysical
Journal Supplement Series, A high-resolution version can be found at:
http://www.astro.uni-bonn.de/~dgoulier/Science/Gouliermis+_NGC346_I.pd
Comprehensive experimental analysis of nonlinear dynamics in an optically-injected semiconductor laser
We present the first comprehensive experimental study, to our knowledge, of the routes between nonlinear dynamics induced in a semiconductor laser under external optical injection based on an analysis of time-averaged measurements of the optical and RF spectra and phasors of real-time series of the laser output. The different means of analysis are compared for several types of routes and the benefits of each are discussed in terms of the identification and mapping of the nonlinear dynamics. Finally, the results are presented in a novel audio/video format that describes the evolution of the dynamics with the injection parameters. © 2011 Author(s)
Dust extinction and absorption: the challenge of porous grains
In many models of dusty objects in space the grains are assumed to be
composite or fluffy. However, the computation of the optical properties of such
particles is still a very difficult problem. We analyze how the increase of
grain porosity influences basic features of cosmic dust -- interstellar
extinction, dust temperature, infrared bands and millimeter opacity. Porous
grains can reproduce the flat extinction across the 3 - 8 \mkm wavelength
range measured for several lines of sight by {\it ISO} and {\it Spitzer}.
Porous grains are generally cooler than compact grains. At the same time, the
temperature of very porous grains becomes slightly larger in the case of the
EMT-Mie calculations in comparison with the results found from the
layered-sphere model. The layered-sphere model predicts a broadening of
infrared bands and a shift of the peak position to larger wavelengths as
porosity grows. In the case of the EMT-Mie model variations of the feature
profile are less significant. It is also shown that the millimeter mass
absorption coefficients grow as porosity increases with a faster growth
occurring for particles with Rayleigh/non-Rayleigh inclusions. As a result, for
very porous particles the coefficients given by two models can differ by a
factor of about 3.
It is found that an increase of porosity leads to an increase of extinction
cross sections at some wavelengths and a decrease at others depending on the
grain model. However, this behaviour is sufficient to reproduce the extinction
curve in the direction of the star Sco using current solar abundances.
In the case of the star Oph our model requires larger amounts of carbon
and iron in the dust-phase than is available.Comment: Astronomy and Astrophysics (accepted; 11 pages, 11 figures
Ferromagnetism and non-local correlations in the Hubbard model
We study the possibility and stability of band-ferromagnetism in the
single-band Hubbard model for the simple cubic (SC) lattice. A non-local
self-energy is derived within a modified perturbation theory. Results for the
spectral density and quasiparticle density of states are shown with special
attention to the effects of k-dependence. The importance of non-local
correlations for the fulfillment of the Mermin-Wagner theorem is our main
result. A phase digram showing regions of ferromagnetic order is calculated for
the three dimensional lattice. Besides, we show results for the optical
conductivity and prove that already the renormalized one-loop contribution to
the conductivity cancels the Drude peak exactly in case of a local self-energy
which is not anymore true for a non-local self-energy.Comment: 11 pages, 10 figures, accepted for publication in PR
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