834 research outputs found
The Kinematics of Molecular Cloud Cores in the Presence of Driven and Decaying Turbulence: Comparisons with Observations
In this study we investigate the formation and properties of prestellar and
protostellar cores using hydrodynamic, self-gravitating Adaptive Mesh
Refinement simulations, comparing the cases where turbulence is continually
driven and where it is allowed to decay. We model observations of these cores
in the CO, NH, and NH lines, and from
the simulated observations we measure the linewidths of individual cores, the
linewidths of the surrounding gas, and the motions of the cores relative to one
another. Some of these distributions are significantly different in the driven
and decaying runs, making them potential diagnostics for determining whether
the turbulence in observed star-forming clouds is driven or decaying. Comparing
our simulations with observed cores in the Perseus and Ophiuchus clouds
shows reasonably good agreement between the observed and simulated core-to-core
velocity dispersions for both the driven and decaying cases. However, we find
that the linewidths through protostellar cores in both simulations are too
large compared to the observations. The disagreement is noticably worse for the
decaying simulation, in which cores show highly supersonic infall signatures in
their centers that decrease toward their edges, a pattern not seen in the
observed regions. This result gives some support to the use of driven
turbulence for modeling regions of star formation, but reaching a firm
conclusion on the relative merits of driven or decaying turbulence will require
more complete data on a larger sample of clouds as well as simulations that
include magnetic fields, outflows, and thermal feedback from the protostars.Comment: 18 pages, 12 figures, accepted to A
The Protostellar Luminosity Function
The protostellar luminosity function (PLF) is the present-day luminosity
function of the protostars in a region of star formation. It is determined
using the protostellar mass function (PMF) in combination with a stellar
evolutionary model that provides the luminosity as a function of instantaneous
and final stellar mass. As in McKee & Offner (2010), we consider three main
accretion models: the Isothermal Sphere model, the Turbulent Core model, and an
approximation of the Competitive Accretion model. We also consider the effect
of an accretion rate that tapers off linearly in time and an accelerating star
formation rate. For each model, we characterize the luminosity distribution
using the mean, median, maximum, ratio of the median to the mean, standard
deviation of the logarithm of the luminosity, and the fraction of very low
luminosity objects. We compare the models with bolometric luminosities observed
in local star forming regions and find that models with an approximately
constant accretion time, such as the Turbulent Core and Competitive Accretion
models, appear to agree better with observation than those with a constant
accretion rate, such as the Isothermal Sphere model. We show that observations
of the mean protostellar luminosity in these nearby regions of low-mass star
formation suggest a mean star formation time of 0.30.1 Myr. Such a
timescale, together with some accretion that occurs non-radiatively and some
that occurs in high-accretion, episodic bursts, resolves the classical
"luminosity problem" in low-mass star formation, in which observed protostellar
luminosities are significantly less than predicted. An accelerating star
formation rate is one possible way of reconciling the observed star formation
time and mean luminosity.Comment: 22 pages, 9 figures, accepted to Ap
Can magnetized turbulence set the mass scale of stars?
Understanding the evolution of self-gravitating, isothermal, magnetized gas is crucial for star formation, as these physical processes have been postulated to set the initial mass function (IMF). We present a suite of isothermal magnetohydrodynamic (MHD) simulations using the GIZMO code that follow the formation of individual stars in giant molecular clouds (GMCs), spanning a range of Mach numbers found in observed GMCs (M∼10−50). As in past works, the mean and median stellar masses are sensitive to numerical resolution, because they are sensitive to low-mass stars that contribute a vanishing fraction of the overall stellar mass. The mass-weighted median stellar mass M₅₀ becomes insensitive to resolution once turbulent fragmentation is well resolved. Without imposing Larson-like scaling laws, our simulations find M₅₀∝∼M₀M⁻³α_(turb)SFE^(1/3) for GMC mass M₀, sonic Mach number M, virial parameter α_(turb), and star formation efficiency SFE = M⋆/M₀. This fit agrees well with previous IMF results from the RAMSES, ORION2, and SPHNG codes. Although M₅₀ has no significant dependence on the magnetic field strength at the cloud scale, MHD is necessary to prevent a fragmentation cascade that results in non-convergent stellar masses. For initial conditions and SFE similar to star-forming GMCs in our Galaxy, we predict M₅₀ to be >20M⊙, an order of magnitude larger than observed (∼2M⊙), together with an excess of brown dwarfs. Moreover, M₅₀ is sensitive to initial cloud properties and evolves strongly in time within a given cloud, predicting much larger IMF variations than are observationally allowed. We conclude that physics beyond MHD turbulence and gravity are necessary ingredients for the IMF
Radiation-Hydrodynamic Simulations of the Formation of Orion-Like Star Clusters I. Implications for the Origin of the Initial Mass Function
One model for the origin of typical galactic star clusters such as the Orion
Nebula Cluster (ONC) is that they form via the rapid, efficient collapse of a
bound gas clump within a larger, gravitationally-unbound giant molecular cloud.
However, simulations in support of this scenario have thus far have not
included the radiation feedback produced by the stars; radiative simulations
have been limited to significantly smaller or lower density regions. Here we
use the ORION adaptive mesh refinement code to conduct the first ever
radiation-hydrodynamic simulations of the global collapse scenario for the
formation of an ONC-like cluster. We show that radiative feedback has a
dramatic effect on the evolution: once the first ~10-20% of the gas mass is
incorporated into stars, their radiative feedback raises the gas temperature
high enough to suppress any further fragmentation. However, gas continues to
accrete onto existing stars, and, as a result, the stellar mass distribution
becomes increasingly top-heavy, eventually rendering it incompatible with the
observed IMF. Systematic variation in the location of the IMF peak as star
formation proceeds is incompatible with the observed invariance of the IMF
between star clusters, unless some unknown mechanism synchronizes the IMFs in
different clusters by ensuring that star formation is always truncated when the
IMF peak reaches a particular value. We therefore conclude that the global
collapse scenario, at least in its simplest form, is not compatible with the
observed stellar IMF. We speculate that processes that slow down star
formation, and thus reduce the accretion luminosity, may be able to resolve the
problem.Comment: 17 pages, 13 figures, emulateapj format, ApJ in press; simulation
movies available at http://www.ucolick.org/~krumholz/publications.htm
The Origin and Universality of the Stellar Initial Mass Function
We review current theories for the origin of the Stellar Initial Mass
Function (IMF) with particular focus on the extent to which the IMF can be
considered universal across various environments. To place the issue in an
observational context, we summarize the techniques used to determine the IMF
for different stellar populations, the uncertainties affecting the results, and
the evidence for systematic departures from universality under extreme
circumstances. We next consider theories for the formation of prestellar cores
by turbulent fragmentation and the possible impact of various thermal,
hydrodynamic and magneto-hydrodynamic instabilities. We address the conversion
of prestellar cores into stars and evaluate the roles played by different
processes: competitive accretion, dynamical fragmentation, ejection and
starvation, filament fragmentation and filamentary accretion flows, disk
formation and fragmentation, critical scales imposed by thermodynamics, and
magnetic braking. We present explanations for the characteristic shapes of the
Present-Day Prestellar Core Mass Function and the IMF and consider what
significance can be attached to their apparent similarity. Substantial
computational advances have occurred in recent years, and we review the
numerical simulations that have been performed to predict the IMF directly and
discuss the influence of dynamics, time-dependent phenomena, and initial
conditions.Comment: 24 pages, 6 figures. Accepted for publication as a chapter in
Protostars and Planets VI, University of Arizona Press (2014), eds. H.
Beuther, R. S. Klessen, C. P. Dullemond, Th. Hennin
Influence of Portosystemic Shunt on Liver Regeneration after Hepatic Resection in Pigs
Objective. The minimal amount of liver mass necessary for regeneration is still a matter of debate. The aim of the study was to analyze liver regeneration factors after extended resection with or without portosystemic shunt. Methods. An extended left hemihepatectomy was performed in 25 domestic pigs, in 15 cases after a portosystemic H-shunt. The expression of Ki-67, VEGF, TGF-α, FGF, and CK-7 was analyzed in paraffin-embedded tissue sections.
Results. The volume of the remnant liver increased about 2.5-fold at the end of the first week after resection. With 19 cells/10 Glisson fields versus 4/10, Ki-67-expression was significantly higher in the H-shunt group. VEGF- and CK-7-expressions were significantly higher in the control group. No significant change was found in FGF-expression. The expression of TGF-α was higher, but not significantly, in the control group. Conclusions. The expression of Ki-67, and therefore hepatocyte regeneration, was increased in the shunt group. The expression of CK-7 on biliary epithelium and the expression of VEGF, however, were stronger in the control group
The Cop Number of the One-Cop-Moves Game on Planar Graphs
Cops and robbers is a vertex-pursuit game played on graphs. In the classical
cops-and-robbers game, a set of cops and a robber occupy the vertices of the
graph and move alternately along the graph's edges with perfect information
about each other's positions. If a cop eventually occupies the same vertex as
the robber, then the cops win; the robber wins if she can indefinitely evade
capture. Aigner and Frommer established that in every connected planar graph,
three cops are sufficient to capture a single robber. In this paper, we
consider a recently studied variant of the cops-and-robbers game, alternately
called the one-active-cop game, one-cop-moves game or the lazy-cops-and-robbers
game, where at most one cop can move during any round. We show that Aigner and
Frommer's result does not generalise to this game variant by constructing a
connected planar graph on which a robber can indefinitely evade three cops in
the one-cop-moves game. This answers a question recently raised by Sullivan,
Townsend and Werzanski.Comment: 32 page
- …