16,384 research outputs found
The Origin of the Initial Mass Function
We review recent advances in our understanding of the origin of the initial
mass function (IMF). We emphasize the use of numerical simulations to
investigate how each physical process involved in star formation affects the
resulting IMF. We stress that it is insufficient to just reproduce the IMF, but
that any successful model needs to account for the many observed properties of
star forming regions including clustering, mass segregation and binarity.
Fragmentation involving the interplay of gravity, turbulence, and thermal
effects is probably responsible for setting the characteristic stellar mass.
Low-mass stars and brown dwarfs can form through the fragmentation of dense
filaments and disks, possibly followed by early ejection from these dense
environments which truncates their growth in mass. Higher-mass stars and the
Salpeter-like slope of the IMF are most likely formed through continued
accretion in a clustered environment. The effects of feedback and magnetic
fields on the origin of the IMF are still largely unclear. Lastly, we discuss a
number of outstanding problems that need to be addressed in order to develop a
complete theory for the origin of the IMF.Comment: PPV conference paper, 16 pages, 11 figur
Travelling to exotic places with cavity QED systems
Recent theoretical schemes for utilizing cavity QED models as quantum
simulators are reviewed. By considering a quadrature representation for the
fields, it is shown how Jahn-Teller models, effective Abelian or non-Abelian
gauge potentials, transverse Hall currents, and relativistic effects naturally
arise in these systems. Some of the analytical predictions are verified
numerically using realistic experimental parameters taking into account for
system losses. Thereby demonstrating their feasibility with current
experimental setups.Comment: 5 pages, 3 figure
Thermal and Fragmentation Properties of Star-forming Clouds in Low-metallicity Environments
The thermal and chemical evolution of star-forming clouds is studied for
different gas metallicities, Z, using the model of Omukai (2000), updated to
include deuterium chemistry and the effects of cosmic microwave background
(CMB) radiation. HD-line cooling dominates the thermal balance of clouds when Z
\~ 10^{-5}-10^{-3} Z_sun and density ~10^{5} cm^{-3}. Early on, CMB radiation
prevents the gas temperature to fall below T_CMB, although this hardly alters
the cloud thermal evolution in low-metallicity gas. From the derived
temperature evolution, we assess cloud/core fragmentation as a function of
metallicity from linear perturbation theory, which requires that the core
elongation E := (b-a)/a > E_NL ~ 1, where a (b) is the short (long) core axis
length. The fragment mass is given by the thermal Jeans mass at E = E_NL. Given
these assumptions and the initial (gaussian) distribution of E we compute the
fragment mass distribution as a function of metallicity. We find that: (i) For
Z=0, all fragments are very massive, > 10^{3}M_sun, consistently with previous
studies; (ii) for Z>10^{-6} Z_sun a few clumps go through an additional high
density (> 10^{10} cm^{-3}) fragmentation phase driven by dust-cooling, leading
to low-mass fragments; (iii) The mass fraction in low-mass fragments is
initially very small, but at Z ~ 10^{-5}Z_sun it becomes dominant and continues
to grow as Z is increased; (iv) as a result of the two fragmentation modes, a
bimodal mass distribution emerges in 0.01 0.1Z_sun,
the two peaks merge into a singly-peaked mass function which might be regarded
as the precursor of the ordinary Salpeter-like IMF.Comment: 38 pages, 16 figures, ApJ in pres
The Nature of the Dense Core Population in the Pipe Nebula: Thermal Cores Under Pressure
In this paper we present the results of a systematic investigation of an
entire population of starless dust cores within a single molecular cloud.
Analysis of extinction data shows the cores to be dense objects characterized
by a narrow range of density. Analysis of C18O and NH3 molecular-line
observations reveals very narrow lines. The non-thermal velocity dispersions
measured in both these tracers are found to be subsonic for the large majority
of the cores and show no correlation with core mass (or size). Thermal pressure
is thus the dominate source of internal gas pressure and support for most of
the core population. The total internal gas pressures of the cores are found to
be roughly independent of core mass over the entire range of the core mass
function (CMF) indicating that the cores are in pressure equilibrium with an
external source of pressure. This external pressure is most likely provided by
the weight of the surrounding Pipe cloud within which the cores are embedded.
Most of the cores appear to be pressure confined, gravitationally unbound
entities whose nature, structure and future evolution are determined by only a
few physical factors which include self-gravity, the fundamental processes of
thermal physics and the simple requirement of pressure equilibrium with the
surrounding environment. The observed core properties likely constitute the
initial conditions for star formation in dense gas. The entire core population
is found to be characterized by a single critical Bonnor-Ebert mass. This mass
coincides with the characteristic mass of the Pipe CMF indicating that most
cores formed in the cloud are near critical stability. This suggests that the
mass function of cores (and the IMF) has its origin in the physical process of
thermal fragmentation in a pressurized medium.Comment: To appear in the Astrophysical Journa
Modeling a high mass turn down in the stellar initial mass function
Statistical sampling from the stellar initial mass function (IMF) for all
star-forming regions in the Galaxy would lead to the prediction of ~1000 Msun
stars unless there is a rapid turn-down in the IMF beyond several hundred solar
masses. Such a turndown is not necessary for dense clusters because the number
of stars sampled is always too small. Here we explore several mechanisms for an
upper mass cutoff, including an exponential decline of the star formation
probability after a turbulent crossing time. The results are in good agreement
with the observed IMF over the entire stellar mass range, and they give a
gradual turn down compared to the Salpeter function above ~100 Msun for normal
thermal Jeans mass, M_J. The upper mass turn down should scale with M_J in
different environments. A problem with the models is that they cannot give both
the observed power-law IMF out to the high-mass sampling limit in dense
clusters, as well as the observed lack of supermassive stars in whole galaxy
disks. Either there is a sharper upper-mass cutoff in the IMF, perhaps from
self-limitation, or the IMF is different for dense clusters than for the
majority of star formation that occurs at lower density. Dense clusters seem to
have an overabundance of massive stars relative to the average IMF in a galaxy.Comment: 19 pages, 2 figures, Astrophysical Journal, Vol 539, August 10, 200
Statistics of Velocity from Spectral Data: Modified Velocity Centroids
We address the problem of studying interstellar turbulence using spectral
line data. We find a criterion when the velocity centroids may provide
trustworthy velocity statistics. To enhance the scope of centroids
applications, we construct a measure that we term ``modified velocity
centroids'' (MVCs) and derive an analytical solution that relates the 2D
spectra of the modified centroids with the underlying 3D velocity spectrum. We
test our results using synthetic maps constructed with data obtained through
simulations of compressible magnetohydrodynamical (MHD) turbulence. We show
that the modified velocity centroids (MVCs) are complementary to the the
Velocity Channel Analysis (VCA) technique. Employed together, they make
determining of the velocity spectral index more reliable and for wider variety
of astrophysical situations.Comment: 4 pages, 1 figure, Accepted for publication in ApJ Letters. minor
change
Flows, Fragmentation, and Star Formation. I. Low-mass Stars in Taurus
The remarkably filamentary spatial distribution of young stars in the Taurus
molecular cloud has significant implications for understanding low-mass star
formation in relatively quiescent conditions. The large scale and regular
spacing of the filaments suggests that small-scale turbulence is of limited
importance, which could be consistent with driving on large scales by flows
which produced the cloud. The small spatial dispersion of stars from gaseous
filaments indicates that the low-mass stars are generally born with small
velocity dispersions relative to their natal gas, of order the sound speed or
less. The spatial distribution of the stars exhibits a mean separation of about
0.25 pc, comparable to the estimated Jeans length in the densest gaseous
filaments, and is consistent with roughly uniform density along the filaments.
The efficiency of star formation in filaments is much higher than elsewhere,
with an associated higher frequency of protostars and accreting T Tauri stars.
The protostellar cores generally are aligned with the filaments, suggesting
that they are produced by gravitational fragmentation, resulting in initially
quasi-prolate cores. Given the absence of massive stars which could strongly
dominate cloud dynamics, Taurus provides important tests of theories of
dispersed low-mass star formation and numerical simulations of molecular cloud
structure and evolution.Comment: 32 pages, 9 figures: to appear in Ap
CLEF 2017 NewsREEL Overview: Offline and Online Evaluation of Stream-based News Recommender Systems
The CLEF NewsREEL challenge allows researchers to evaluate news
recommendation algorithms both online (NewsREEL Live) and offline (News-
REEL Replay). Compared with the previous year NewsREEL challenged participants
with a higher volume of messages and new news portals. In the 2017
edition of the CLEF NewsREEL challenge a wide variety of new approaches have
been implemented ranging from the use of existing machine learning frameworks,
to ensemble methods to the use of deep neural networks. This paper gives an
overview over the implemented approaches and discusses the evaluation results.
In addition, the main results of Living Lab and the Replay task are explained
Direct numerical simulations for non-Newtonian rheology of concentrated particle dispersions
The non-Newtonian behavior of a monodisperse concentrated dispersion of
spherical particles was investigated using a direct numerical simulation
method, that takes into account hydrodynamic interactions and thermal
fluctuations accurately. Simulations were performed under steady shear flow
with periodic boundary conditions in the three directions. The apparent shear
viscosity of the dispersions was calculated at volume fractions ranging from
0.31 to 0.56. Shear-thinning behavior was clearly observed at high volume
fractions. The low- and high-limiting viscosities were then estimated from the
apparent viscosity by fitting these data into a semi-empirical formula.
Furthermore, the short-time motions were examined for Brownian particles
fluctuating in concentrated dispersions, for which the fluid inertia plays an
important role. The mean square displacement was monitored in the vorticity
direction at several different Peclet numbers and volume fractions so that the
particle diffusion coefficient is determined from the long-time behavior of the
mean square displacement. Finally, the relationship between the non-Newtonian
viscosity of the dispersions and the structural relaxation of the dispersed
Brownian particles is examined
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