31 research outputs found
The Role of Turbulence and Magnetic Fields in Simulated Filamentary Structure
We use numerical simulations of turbulent cluster-forming regions to study
the nature of dense filamentary structures in star formation. Using four
hydrodynamic and magnetohydrodynamic simulations chosen to match observations,
we identify filaments in the resulting column density maps and analyze their
properties. We calculate the radial column density profiles of the filaments
every 0.05 Myr and fit the profiles with the modified isothermal and pressure
confined isothermal cylinder models, finding reasonable fits for either model.
The filaments formed in the simulations have similar radial column density
profiles to those observed. Magnetic fields provide additional pressure support
to the filaments, making `puffier' filaments less prone to fragmentation than
in the pure hydrodynamic case, which continue to condense at a slower rate. In
the higher density simulations, the filaments grow faster through the increased
importance of gravity. Not all of the filaments identified in the simulations
will evolve to form stars: some expand and disperse. Given these different
filament evolutionary paths, the trends in bulk filament width as a function of
time, magnetic field strength, or density, are weak, and all cases are
reasonably consistent with the finding of a constant filament width in
different star-forming regions. In the simulations, the mean FWHM lies between
0.06 and 0.26 pc for all times and initial conditions, with most lying between
0.1 to 0.15 pc; the range in FWHMs are, however, larger than seen in typical
Herschel analyses. Finally, the filaments display a wealth of substructure
similar to the recent discovery of filament bundles in Taurus.Comment: 18 pages, 8 figures, 7 tables. Accepted for publication in Ap
Simulating the Formation of Massive Protostars: I. Radiative Feedback and Accretion Disks
We present radiation hydrodynamic simulations of collapsing protostellar
cores with initial masses of 30, 100, and 200 M. We follow their
gravitational collapse and the formation of a massive protostar and
protostellar accretion disk. We employ a new hybrid radiative feedback method
blending raytracing techniques with flux-limited diffusion for a more accurate
treatment of the temperature and radiative force. In each case, the disk that
forms becomes Toomre-unstable and develops spiral arms. This occurs between
0.35 and 0.55 freefall times and is accompanied by an increase in the accretion
rate by a factor of 2-10. Although the disk becomes unstable, no other stars
are formed. In the case of our 100 and 200 M simulation, the star
becomes highly super-Eddington and begins to drive bipolar outflow cavities
that expand outwards. These radiatively-driven bubbles appear stable, and
appear to be channeling gas back onto the protostellar accretion disk.
Accretion proceeds strongly through the disk. After 81.4 kyr of evolution, our
30 M simulation shows a star with a mass of 5.48 M and a
disk of mass 3.3 M, while our 100 M simulation forms a 28.8
M mass star with a 15.8 M disk over the course of 41.6 kyr,
and our 200 M simulation forms a 43.7 M star with an 18
M disk in 21.9 kyr. In the absence of magnetic fields or other forms
of feedback, the masses of the stars in our simulation do not appear limited by
their own luminosities.Comment: 24 pages, 14 figures. Accepted to The Astrophysical Journa
A general hybrid radiation transport scheme for star formation simulations on an adaptive grid
Radiation feedback plays a crucial role in the process of star formation. In
order to simulate the thermodynamic evolution of disks, filaments, and the
molecular gas surrounding clusters of young stars, we require an efficient and
accurate method for solving the radiation transfer problem. We describe the
implementation of a hybrid radiation transport scheme in the adaptive
grid-based FLASH general magnetohydrodynamics code. The hybrid scheme splits
the radiative transport problem into a raytracing step and a diffusion step.
The raytracer captures the first absorption event, as stars irradiate their
environments, while the evolution of the diffuse component of the radiation
field is handled by a flux-limited diffusion (FLD) solver. We demonstrate the
accuracy of our method through a variety of benchmark tests including the
irradiation of a static disk, subcritical and supercritical radiative shocks,
and thermal energy equilibration. We also demonstrate the capability of our
method for casting shadows and calculating gas and dust temperatures in the
presence of multiple stellar sources. Our method enables radiation-hydrodynamic
studies of young stellar objects, protostellar disks, and clustered star
formation in magnetized, filamentary environments.Comment: 16 pages, 15 figures, accepted to Ap
Simulating protostellar evolution and radiative feedback in the cluster environment
Radiative feedback is among the most important consequences of clustered star formation inside molecular clouds. At the onset of star formation, radiation from massive stars heats the surrounding gas, which suppresses the formation of many low‐mass stars. When simulating pre‐main‐sequence stars, their stellar properties must be defined by a pre‐stellar model. Different approaches to pre‐stellar modelling may yield quantitatively different results. In this paper, we compare two existing pre‐stellar models under identical initial conditions to gauge whether the choice of model has any significant effects on the final population of stars. The first model treats stellar radii and luminosities with a zero‐age main‐sequence (ZAMS) model, while separately estimating the accretion luminosity by interpolating to published pre‐stellar tracks. The second, more accurate pre‐stellar model self‐consistently evolves the radius and luminosity of each star under highly variable accretion conditions. Each is coupled to a raytracing‐based radiative feedback code that also treats ionization. The impact of the self‐consistent model is less ionizing radiation and less heating during the early stages of star formation. This may affect final mass distributions. We noted a peak stellar mass reduced by 8 per cent from 47.3 to 43.5 M⊙ in the evolutionary model, relative to the track‐fit model. Also, the difference in mass between the two largest stars in each case is reduced from 14 to 7.5 M⊙. The H ii regions produced by these massive stars were also seen to flicker on time‐scales down to the limit imposed by our time‐step (<560 yr), rapidly changing in size and shape, confirming previous cluster simulations using ZAMS‐based estimates for pre‐stellar ionizing flu
The origins and spread of domestic horses from the Western Eurasian steppes
This is the final version. Available on open access from Nature Research via the DOI in this recordData availability: All collapsed and paired-end sequence data for samples sequenced in this study are available in compressed fastq format through the European Nucleotide Archive under accession number PRJEB44430, together with rescaled and trimmed bam sequence alignments against both the nuclear and mitochondrial horse reference genomes. Previously published ancient data used in this study are available under accession numbers PRJEB7537, PRJEB10098, PRJEB10854, PRJEB22390 and PRJEB31613, and detailed in Supplementary Table 1. The genomes of ten modern horses, publicly available, were also accessed as indicated in their corresponding original publications57,61,85-87.NOTE: see the published version available via the DOI in this record for the full list of authorsDomestication of horses fundamentally transformed long-range mobility and warfare. However, modern domesticated breeds do not descend from the earliest domestic horse lineage associated with archaeological evidence of bridling, milking and corralling at Botai, Central Asia around 3500 BC. Other longstanding candidate regions for horse domestication, such as Iberia and Anatolia, have also recently been challenged. Thus, the genetic, geographic and temporal origins of modern domestic horses have remained unknown. Here we pinpoint the Western Eurasian steppes, especially the lower Volga-Don region, as the homeland of modern domestic horses. Furthermore, we map the population changes accompanying domestication from 273 ancient horse genomes. This reveals that modern domestic horses ultimately replaced almost all other local populations as they expanded rapidly across Eurasia from about 2000 BC, synchronously with equestrian material culture, including Sintashta spoke-wheeled chariots. We find that equestrianism involved strong selection for critical locomotor and behavioural adaptations at the GSDMC and ZFPM1 genes. Our results reject the commonly held association between horseback riding and the massive expansion of Yamnaya steppe pastoralists into Europe around 3000 BC driving the spread of Indo-European languages. This contrasts with the scenario in Asia where Indo-Iranian languages, chariots and horses spread together, following the early second millennium BC Sintashta culture
Connecting Planetary Composition with Formation
The rapid advances in observations of the different populations of
exoplanets, the characterization of their host stars and the links to the
properties of their planetary systems, the detailed studies of protoplanetary
disks, and the experimental study of the interiors and composition of the
massive planets in our solar system provide a firm basis for the next big
question in planet formation theory. How do the elemental and chemical
compositions of planets connect with their formation? The answer to this
requires that the various pieces of planet formation theory be linked together
in an end-to-end picture that is capable of addressing these large data sets.
In this review, we discuss the critical elements of such a picture and how they
affect the chemical and elemental make up of forming planets. Important issues
here include the initial state of forming and evolving disks, chemical and dust
processes within them, the migration of planets and the importance of planet
traps, the nature of angular momentum transport processes involving turbulence
and/or MHD disk winds, planet formation theory, and advanced treatments of disk
astrochemistry. All of these issues affect, and are affected by the chemistry
of disks which is driven by X-ray ionization of the host stars. We discuss how
these processes lead to a coherent end-to-end model and how this may address
the basic question.Comment: Invited review, accepted for publication in the 'Handbook of
Exoplanets', eds. H.J. Deeg and J.A. Belmonte, Springer (2018). 46 pages, 10
figure
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The role of magnetic fields in star formation
Stars are born in turbulent, magnetized filamentary molecular clouds, typically as members of star clusters. Several remarkable technical advances enable observations of magnetic structure and field strengths across many physical scales, from galactic scales on which giant molecular clouds (GMCs) are assembled, down to the surfaces of magnetized accreting young stars. These are shedding new light on the role of magnetic fields in star formation. Magnetic fields affect the gravitational fragmentation and formation of filamentary molecular clouds, the formation and fragmentation of magnetized disks, and finally to the shedding of excess angular momentum in jets and outflows from both the disks and young stars. Magnetic fields play a particularly important role in angular momentum transport on all of these scales. Numerical simulations have provided an important tool for tracking the complex process of the collapse and evolution of protostellar gas since several competing physical processes are at play - turbulence, gravity, MHD, and radiation fields. This paper focuses on the role of magnetic fields in three crucial regimes of star formation: the formation of star clusters emphasizing fragmentation, disk formation and the origin of early jets and outflows, to processes that control the spin evolution of young stars.Peer reviewed: YesNRC publication: N