Filamentary Fragments Research Articles

Structure of a Coulomb cluster in the cusp magnetic trap under microgravity conditions

The results of the experiment on the formation of a cluster in a cusp magnetic trap under microgravity conditions and its destruction at a gradual increase of its electric potential are discussed. It is shown that if a cluster is formed by initially uncharged particles, then the auto‐adhesion forces exert an important influence on its shape, structure, and subsequent decay. The structure of the cluster is discussed, and it is shown that it is formed by filamentary fragments.

Filamentary Fragmentation and Accretion in High-mass Star-forming Molecular Clouds

Abstract Filamentary structures are ubiquitous in high-mass star-forming molecular clouds. Their relation with high-mass star formation is still to be understood. Here we report interferometric observations toward eight filamentary high-mass star-forming clouds. A total of 50 dense cores are identified in these clouds, most of which present signatures of high-mass star formation. Five of them are not associated with any star formation indicators and hence are prestellar core candidates. Evolutionary phases of these cores and their line widths, temperatures, abundances, and virial parameters are found to be correlated. In a subsample of four morphologically well-defined filaments, we find that their fragmentation cannot be solely explained by thermal or turbulence pressure support. We also investigate distributions of gas temperatures and nonthermal motions along the filaments and find a spatial correlation between nonthermal line widths and star formation activities. We find evidence of gas flows along these filaments and derive an accretion rate along filaments of ∼10−4 . These results suggest a strong relationship between massive filaments and high-mass star formation, through (i) filamentary fragmentation in very early evolutionary phases to form dense cores, (ii) accretion flows along filaments that are important for the growth of dense cores and protostars, and (iii) enhancement of nonthermal motion in the filaments by the feedback or accretion during star formation.

Filamentary fragmentation in a turbulent medium

We present the results of smoothed particle hydrodynamic simulations investigating the evolution and fragmentation of filaments that are accreting from a turbulent medium. We show that the presence of turbulence, and the resulting inhomogeneities in the accretion flow, play a significant role in the fragmentation process. Filaments which experience a weakly turbulent accretion flow fragment in a two-tier hierarchical fashion, similar to the fragmentation pattern seen in the Orion Integral Shaped Filament. Increasing the energy in the turbulent velocity field results in more sub-structure within the filaments, and one sees a shift from gravity-dominated fragmentation to turbulence-dominated fragmentation. The sub-structure formed in the filaments is elongated and roughly parallel to the longitudinal axis of the filament, similar to the fibres seen in observations of Taurus, and suggests that the fray and fragment scenario is a possible mechanism for the production of fibres. We show that the formation of these fibre-like structures is linked to the vorticity of the velocity field inside the filament and the filament's accretion from an inhomogeneous medium. Moreover, we find that accretion is able to drive and sustain roughly sonic levels of turbulence inside the filaments, but is not able to prevent radial collapse once the filaments become supercritical. However, the supercritical filaments which contain fibre-like structures do not collapse radially, suggesting that fibrous filaments may not necessarily become radially unstable once they reach the critical line-density.

THE FORMATION OF THE PRIMITIVE STAR SDSS J102915+172927: EFFECT OF THE DUST MASS AND THE GRAIN-SIZE DISTRIBUTION

ABSTRACT Understanding the formation of the extremely metal-poor star SDSS J102915+172927 is of fundamental importance to improve our knowledge on the transition between the first and second generation of stars in the universe. In this paper, we perform three-dimensional cosmological hydrodynamical simulations of dust-enriched halos during the early stages of the collapse process including a detailed treatment of the dust physics. We employ the astrochemistry package krome coupled with the hydrodynamical code enzo assuming grain-size distributions produced by the explosion of core-collapse supernovae (SNe) of 20 and 35 M ⊙ primordial stars, which are suitable to reproduce the chemical pattern of the SDSS J102915+172927 star. We find that the dust mass yield produced from Population III SNe explosions is the most important factor that drives the thermal evolution and the dynamical properties of the halos. Hence, for the specific distributions relevant in this context, the composition, the dust optical properties, and the size range have only minor effects on the results due to similar cooling functions. We also show that the critical dust mass to enable fragmentation provided by semi-analytical models should be revised, as we obtain values one order of magnitude larger. This determines the transition from disk fragmentation to a more filamentary fragmentation mode, and suggests that likely more than one single SN event or efficient dust growth should be invoked to get such high dust content.

Magnetohydrodynamic stability of broad line region clouds

Hydrodynamic stability has been a longstanding issue for the cloud model of the broad line region in active galactic nuclei. We argue that the clouds may be gravitationally bound to the supermassive black hole. If true, stabilisation by thermal pressure alone becomes even more difficult. We further argue that if magnetic fields should be present in such clouds at a level that could affect the stability properties, they need to be strong enough to compete with the radiation pressure on the cloud. This would imply magnetic field values of a few Gauss for a sample of Active Galactic Nuclei we draw from the literature. We then investigate the effect of several magnetic configurations on cloud stability in axi-symmetric magnetohydrodynamic simulations. For a purely azimuthal magnetic field which provides the dominant pressure support, the cloud first gets compressed by the opposing radiative and gravitational forces. The pressure inside the cloud then increases, and it expands vertically. Kelvin-Helmholtz and column density instability lead to a filamentary fragmentation of the cloud. This radiative dispersion continues until the cloud is shredded down to the resolution level. For a helical magnetic field configuration, a much more stable cloud core survives with a stationary density histogram which takes the form of a power law. Our simulated clouds develop sub-Alfvenic internal motions on the level of a few hundred km/s.

The effects of mixing on tracer relationships in the polar vortices

We review recent analyses of tracer observations within the Arctic vortex, showing anomalous tracer‐tracer relationships which indicate the mixing of outside air into the vortex. Similar anomalous relationships have previously been observed in vortex fragments, following breakdown of the Arctic vortex, and interpreted as indicative of mixing within the filamentary fragments. In the light of other observations we conclude that this interpretation is incorrect: the mixing took place into the vortex prior to breakdown. We discuss the effects of mixing on tracer relationships within the vortex, illustrated by results both from a simple conceptual model and from two three‐dimensional chemical transport models. These models indicate that continuous mixing across the vortex edge produces entirely changed tracer relations inside the vortex and that separate, compact vortex relationships can develop. In general, tracer relationships should not be used to estimate denitrification or chemical ozone loss unless the effects of mixing can be quantified; this may be possible using information from other tracers, but one cannot assume that the effects of anomalous mixing are manifested as simple mixing lines on tracer‐tracer diagrams.