650 research outputs found
Turbulence and its effect on protostellar disk formation
We analyse simulations of turbulent, magnetised molecular cloud cores
focussing on the formation of Class 0 stage protostellar discs and the physical
conditions in their surroundings. We show that for a wide range of initial
conditions Keplerian discs are formed in the Class 0 stage already.
Furthermore, we show that the accretion of mass and angular momentum in the
surroundings of protostellar discs occurs in a highly anisotropic manner, by
means of a few narrow accretion channels. The magnetic field structure in the
vicinity of the discs is highly disordered, revealing field reversals up to
distances of 1000 AU. These findings demonstrate that as soon as even mild
turbulent motions are included, the classical disc formation scenario of a
coherently rotating environment and a well-ordered magnetic field breaks down.Comment: Invited contribution to the NIC proceedings 2016 for the John von
Neumann-Institut f\"ur Computing (NIC) Symposium 201
The structure of self-gravitating clouds
To study the interaction of star-formation and turbulent molecular cloud
structuring, we analyse numerical models and observations of self-gravitating
clouds using the Delta-variance as statistical measure for structural
characteristics. In the models we resolve the transition from purely
hydrodynamic turbulence to gravitational collapse associated with the formation
and mass growth of protostellar cores. We compare models of driven and freely
decaying turbulence with and without magnetic fields. Self-gravitating
supersonic turbulence always produces a density structure that contains most
power on the smallest scales provided by collapsed cores as soon as local
collapse sets in. This is in contrast to non-self-gravitating hydrodynamic
turbulence where the Delta-variance is dominated by large scale structures. To
detect this effect in star-forming regions observations have to resolve the
high density contrast of protostellar cores with respect to their ambient
molecular cloud. Using the 3mm continuum map of a star-forming cluster in
Serpens we show that the dust emission traces the full density evolution. On
the contrary, the density range accessible by molecular line observations is
insufficient for this analysis. Only dust emission and dust extinction
observations are able to to determine the structural parameters of star-forming
clouds following the density evolution during the gravitational collapse.Comment: 12 pages, 9 figures, A&A in pres
Accretion and magnetic field morphology around Class 0 stage protostellar discs
We analyse simulations of turbulent, magnetised molecular cloud cores
focussing on the formation of Class 0 stage protostellar discs and the physical
conditions in their surroundings. We show that for a wide range of initial
conditions Keplerian discs are formed in the Class 0 stage already. In
particular, we show that even subsonic turbulent motions reduce the magnetic
braking efficiency sufficiently in order to allow rotationally supported discs
to form. We therefore suggest that already during the Class 0 stage the
fraction of Keplerian discs is significantly higher than 50%, consistent with
recent observational trends but significantly higher than predictions based on
simulations with misaligned magnetic fields, demonstrating the importance of
turbulent motions for the formation of Keplerian discs. We show that the
accretion of mass and angular momentum in the surroundings of protostellar
discs occurs in a highly anisotropic manner, by means of a few narrow accretion
channels. The magnetic field structure in the vicinity of the discs is highly
disordered, revealing field reversals up to distances of 1000 AU. These
findings demonstrate that as soon as even mild turbulent motions are included,
the classical disc formation scenario of a coherently rotating environment and
a well-ordered magnetic field breaks down. Hence, it is highly questionable to
assess the magnetic braking efficiency based on non-turbulent collapse
simulation. We strongly suggest that, in addition to the global magnetic field
properties, the small-scale accretion flow and detailed magnetic field
structure have to be considered in order to assess the likelihood of Keplerian
discs to be present.Comment: 14 pages, 6 figures, accepted for publication in MNRAS, updated to
final versio
Gravoturbulent Star Formation: Effects of the Equation of State on Stellar Masses
Stars form by gravoturbulent fragmentation of interstellar gas clouds. The
supersonic turbulence ubiquitously observed in Galactic molecular gas generates
strong density fluctuations with gravity taking over in the densest and most
massive regions. Collapse sets in to build up stars and star clusters.
Turbulence plays a dual role. On global scales it provides support, while at
the same time it can promote local collapse. Stellar birth is thus intimately
linked to the dynamic behavior of parental gas clouds, which governs when and
where protostellar cores form, and how they contract and grow in mass via
accretion from the surrounding cloud material to build up stars. The equation
of state plays a pivotal role in the fragmentation process. Under typical cloud
conditions, massive stars form as part of dense clusters following the "normal"
mass function observed, e.g. in the solar neighborhood. However, for gas with
an effective polytropic index greater than unity star formation becomes biased
towards isolated massive stars. This is relevant for understanding the
properties of zero-metallicity stars (Population III) or stars that form under
extreme environmental conditions like in the Galactic center or in luminous
starbursts.Comment: 9 pages, 4 figure, to be published in the Proceedings of the IAU
Colloquium No. 227, 2005, "Massive Star Birth: A Crossroads of Astrophysics
The relation between the true and observed fractal dimensions of turbulent clouds
Observations of interstellar gas clouds are typically limited to
two-dimensional (2D) projections of the intrinsically three-dimensional (3D)
structure of the clouds. In this study, we present a novel method for relating
the 2D projected fractal dimension () to the 3D fractal
dimension () of turbulent clouds. We do this by
computing the fractal dimension of clouds over two orders of magnitude in
turbulent Mach number , corresponding to seven orders of
magnitude in spatial scales within the clouds. This provides us with the data
to create a new empirical relation between and
. The proposed relation is
, where the minimum 3D fractal dimension,
, the minimum projected fractal
dimension, , , , and . The minimum 3D fractal dimension, , indicates that in the high limit the 3D clouds
are dominated by planar shocks. The relation between
and of molecular clouds may be a useful tool for
those who are seeking to understand the 3D structures of molecular clouds,
purely based upon 2D projected data and shows promise for relating the physics
of the turbulent clouds to the fractal dimension.Comment: 14 pages, 7 figures. Accepted 2019 May 1
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