4,009 research outputs found
Radiation Transfer of Models of Massive Star Formation. III. The Evolutionary Sequence
We present radiation transfer (RT) simulations of evolutionary sequences of
massive protostars forming from massive dense cores in environments of high
surface densities. The protostellar evolution is calculated with a detailed
multi-zone model, with the accretion rate regulated by feedback from an
evolving disk-wind outflow cavity. Disk and envelope evolutions are calculated
self-consistently. In this framework, an evolutionary track is determined by
three environmental initial conditions: the initial core mass M_c, the mean
surface density of the ambient star-forming clump Sigma_cl, and the
rotational-to-gravitational energy ratio of the initial core, beta_c.
Evolutionary sequences with various M_c, Sigma_cl, beta_c are constructed. We
find that in a fiducial model with M_c=60Msun, Sigma_cl=1 g/cm^2 and
beta_c=0.02, the final star formation efficiency >~0.43. For each evolutionary
track, RT simulations are performed at selected stages, with temperature
profiles, SEDs, and images produced. At a given stage the envelope temperature
is highly dependent on Sigma_cl, but only weakly dependent on M_c. The SED and
MIR images depend sensitively on the evolving outflow cavity, which gradually
wides as the protostar grows. The fluxes at <~100 microns increase
dramatically, and the far-IR peaks move to shorter wavelengths. We find that,
despite scatter caused by different M_c, Sigma_cl, beta, and inclinations,
sources at a given evolutionary stage appear in similar regions on color-color
diagrams, especially when using colors at >~ 70 microns, where the scatter due
to the inclination is minimized, implying that such diagrams can be useful
diagnostic tools of evolutionary stages of massive protostars. We discuss how
intensity profiles along or perpendicular to the outflow axis are affected by
environmental conditions and source evolution.Comment: 28 pages, 26 figures. Accepted for publication in Ap
The Impact of Feedback in Massive Star Formation. II. Lower Star Formation Efficiency at Lower Metallicity
We conduct a theoretical study of the formation of massive stars over a wide
range of metallicities from 1e-5 to 1Zsun and evaluate the star formation
efficiencies (SFEs) from prestellar cloud cores taking into account multiple
feedback processes. Unlike for simple spherical accretion, in the case of disk
accretion feedback processes do not set upper limits on stellar masses. At
solar metallicity, launching of magneto-centrifugally-driven outflows is the
dominant feedback process to set SFEs, while radiation pressure, which has been
regarded to be pivotal, has only minor contribution even in the formation of
over-100Msun stars. Photoevaporation becomes significant in over-20Msun star
formation at low metallicities of <1e-2Zsun, where dust absorption of ionizing
photons is inefficient. We conclude that if initial prestellar core properties
are similar, then massive stars are rarer in extremely metal-poor environments
of 1e-5 - 1e-3Zsun. Our results give new insight into the high-mass end of the
initial mass function and its potential variation with galactic and
cosmological environments.Comment: 13 pages, 9 figures, accepted for publication in The Astrophysical
Journa
Variable Accretion Rates and Fluffy First Stars
We combine the output of hydrodynamical simulations of Population III star
cluster formation with stellar evolution models, and calculate the evolution of
protostars experiencing variable mass accretion rates due to interactions
within a massive disk. We find that the primordial protostars are extended
'fluffy' objects for the bulk of their pre-main-sequence lifetimes. Accretion
luminosity feedback from such objects is high, but as shown in previous work,
has a minimal effect on the star cluster. The extended radii of the protostars,
combined with the observation of close encounters in the simulations, suggests
that mergers will occur in such systems. Furthermore, mass transfer between
close protostellar binaries with extended radii could lead to massive tight
binaries, which are a possible progenitor of gamma ray bursts.Comment: 7 pages, 6 figures, 2 tables. To be published in MNRA
High frequency dynamics in liquid nickel: an IXS study
Owing to their large relatively thermal conductivity, peculiar,
non-hydrodynamic features are expected to characterize the acoustic-like
excitations observed in liquid metals. We report here an experimental study of
collective modes in molten nickel, a case of exceptional geophysical interest
for its relevance in Earth interior science. Our result shed light on
previously reported contrasting evidences: in the explored energy-momentum
region no deviation from the generalized hydrodynamic picture describing non
conductive fluids are observed. Implications for high frequency transport
properties in metallic fluids are discussed.Comment: 6 pages, 4 figures, to appear in "Journal of Chemical Physics
Does the bonding rule break down in AsSe glass?
The local coordination numbers of AsSe glass were determined by a
combination of anomalous x-ray scattering experiments, reverse Monte Carlo
calculations, and {\it ab initio} molecular dynamics simulations. The
well-known `8- bonding rule' proposed by Mott breaks down around the As
atoms, exceeding the rule by 7--26%. An experimental prediction based on
mean-field theory agrees with the present experimental and theoretical results.
The fourfold coordinated As atoms likely form As-As wrong bond chains rather
than ethan-like configurations, which is identified as the origin for the
breakdown of the `8- bonding rule'.Comment: 6 pages, 6figures, 1table, submitted to Europhysics Letter
Structural and dynamical properties of liquid Si. An orbital-free molecular dynamics study
Several static and dynamic properties of liquid silicon near melting have
been determined from an orbital free {\em ab-initio} molecular dynamics
simulation. The calculated static structure is in good agreement with the
available X-ray and neutron diffraction data. The dynamical structure shows
collective density excitations with an associated dispersion relation which
closely follows recent experimental data. It is found that liquid silicon can
not sustain the propagation of shear waves which can be related to the power
spectrum of the velocity autocorrelation function. Accurate estimates have also
been obtained for several transport coefficients. The overall picture is that
the dynamic properties have many characteristics of the simple liquid metals
although some conspicuous differences have been found.Comment: 12 pages, 11 figure
W49A: A starburst triggered by expanding shells
W49A is a giant molecular cloud which harbors some of the most luminous
embedded clusters in the Galaxy. However, the explanation for this
starburst-like phenomenon is still under debate. Methods. We investigated
large-scale Spitzer mid-infrared images together with a Galatic Ring Survey
13CO J = 1-0 image, complemented with higher resolution (~ 11 arcsec) 13CO J =
2-1 and C18O J = 2-1 images over a ~ 15 x 13 pc^2 field obtained with the IRAM
30m telescope. Two expanding shells have been identified in the mid-infrared
images, and confirmed in the position-velocity diagrams made from the 13CO J =
2-1 and C18O J = 2-1 data. The mass of the averaged expanding shell, which has
an inner radius of ~ 3.3 pc and a thickness of ~ 0.41 pc, is about 1.9 x 10^4
M*. The total kinetic energy of the expanding shells is estimated to be ~ 10^49
erg which is probably provided by a few massive stars, whose radiation pressure
and/or strong stellar winds drive the shells. The expanding shells are likely
to have a common origin close to the two ultracompact Hii regions (source O and
source N), and their expansion speed is estimated to be ~ 5 km/s, resulting in
an age of ~ 3-7 x 10^5 years. In addition, on larger (~ 35 x 50 pc^2) scales,
remnants of two gas ejections have been identified in the 13CO J = 1 - 0 data.
Both ejections seem to have the same center as the expanding shells with a
total energy of a few times 10^50 erg. The main driving mechanism for the gas
ejections is unclear, but likely related to the mechanism which triggers the
starburst in W49A
Measuring non-extensitivity parameters in a turbulent Couette-Taylor flow
We investigate probability density functions of velocity differences at
different distances r measured in a Couette-Taylor flow for a range of Reynolds
numbers Re. There is good agreement with the predictions of a theoretical model
based on non-extensive statistical mechanics (where the entropies are
non-additive for independent subsystems). We extract the scale-dependent
non-extensitivity parameter q(r, Re) from the laboratory data.Comment: 8 pages, 5 figure
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