2,049 research outputs found
The efficiency of star formation in clustered and distributed regions
We investigate the formation of both clustered and distributed populations of
young stars in a single molecular cloud. We present a numerical simulation of a
10,000 solar mass elongated, turbulent, molecular cloud and the formation of
over 2500 stars. The stars form both in stellar clusters and in a distributed
mode which is determined by the local gravitational binding of the cloud. A
density gradient along the major axis of the cloud produces bound regions that
form stellar clusters and unbound regions that form a more distributed
population. The initial mass function also depends on the local gravitational
binding of the cloud with bound regions forming full IMFs whereas in the
unbound, distributed regions the stellar masses cluster around the local Jeans
mass and lack both the high-mass and the low-mass stars. The overall efficiency
of star formation is ~ 15 % in the cloud when the calculation is terminated,
but varies from less than 1 % in the the regions of distributed star formation
to ~ 40 % in regions containing large stellar clusters. Considering that large
scale surveys are likely to catch clouds at all evolutionary stages, estimates
of the (time-averaged) star formation efficiency for the giant molecular cloud
reported here is only ~ 4 %. This would lead to the erroneous conclusion of
'slow' star formation when in fact it is occurring on a dynamical timescale.Comment: 9 pages, 8 figures, MNRAS in pres
Validated prediction of weld residual stresses in austenitic steel pipe girth welds before and after thermal ageing, Part 2: modelling and validation
Radiation Driven Implosion and Triggered Star Formation
We present simulations of initially stable isothermal clouds exposed to
ionizing radiation from a discrete external source, and identify the conditions
that lead to radiatively driven implosion and star formation. We use the
Smoothed Particle Hydrodynamics code SEREN (Hubber et al. 2010) and the
HEALPix-based photoionization algorithm described in Bisbas et al. (2009). We
find that the incident ionizing flux is the critical parameter determining the
evolution: high fluxes simply disperse the cloud, whereas low fluxes trigger
star formation. We find a clear connection between the intensity of the
incident flux and the parameters of star formation.Comment: 4 pages, 2 figures, conference proceedings, IAU Symposium 270 (eds.
Alves, Elmegreen, Girart, Trimble
Gravitational Collapse in Turbulent Molecular Clouds. II. Magnetohydrodynamical Turbulence
Hydrodynamic supersonic turbulence can only prevent local gravitational
collapse if the turbulence is driven on scales smaller than the local Jeans
lengths in the densest regions, a very severe requirement (Paper I). Magnetic
fields have been suggested to support molecular clouds either magnetostatically
or via magnetohydrodynamic (MHD) waves. Whereas the first mechanism would form
sheet-like clouds, the second mechanism not only could exert a pressure onto
the gas counteracting the gravitational forces, but could lead to a transfer of
turbulent kinetic energy down to smaller spatial scales via MHD wave
interactions. This turbulent magnetic cascade might provide sufficient energy
at small scales to halt local collapse.
We test this hypothesis with MHD simulations at resolutions up to 256^3
zones, done with ZEUS-3D. We first derive a resolution criterion for
self-gravitating, magnetized gas: in order to prevent collapse of
magnetostatically supported regions due to numerical diffusion, the minimum
Jeans length must be resolved by four zones. Resolution of MHD waves increases
this requirement to roughly six zones. We then find that magnetic fields cannot
prevent local collapse unless they provide magnetostatic support. Weaker
magnetic fields do somewhat delay collapse and cause it to occur more uniformly
across the supported region in comparison to the hydrodynamical case. However,
they still cannot prevent local collapse for much longer than a global
free-fall time.Comment: 32 pages, 14 figures, accepted by Ap
Substellar companions and isolated planetary mass objects from protostellar disc fragmentation
Self-gravitating protostellar discs are unstable to fragmentation if the gas
can cool on a time scale that is short compared to the orbital period. We use a
combination of hydrodynamic simulations and N-body orbit integrations to study
the long term evolution of a fragmenting disc with an initial mass ratio to the
star of M_disc/M_star = 0.1. For a disc which is initially unstable across a
range of radii, a combination of collapse and subsequent accretion yields
substellar objects with a spectrum of masses extending (for a Solar mass star)
up to ~0.01 M_sun. Subsequent gravitational evolution ejects most of the lower
mass objects within a few million years, leaving a small number of very massive
planets or brown dwarfs in eccentric orbits at moderately small radii. Based on
these results, systems such as HD 168443 -- in which the companions are close
to or beyond the deuterium burning limit -- appear to be the best candidates to
have formed via gravitational instability. If massive substellar companions
originate from disc fragmentation, while lower-mass planetary companions
originate from core accretion, the metallicity distribution of stars which host
massive substellar companions at radii of ~1 au should differ from that of
stars with lower mass planetary companions.Comment: 5 pages, accepted for publication in MNRA
Gravitational Collapse in Turbulent Molecular Clouds. I. Gasdynamical Turbulence
Observed molecular clouds often appear to have very low star formation
efficiencies and lifetimes an order of magnitude longer than their free-fall
times. Their support is attributed to the random supersonic motions observed in
them. We study the support of molecular clouds against gravitational collapse
by supersonic, gas dynamical turbulence using direct numerical simulation.
Computations with two different algorithms are compared: a particle-based,
Lagrangian method (SPH), and a grid-based, Eulerian, second-order method
(ZEUS). The effects of both algorithm and resolution can be studied with this
method. We find that, under typical molecular cloud conditions, global collapse
can indeed be prevented, but density enhancements caused by strong shocks
nevertheless become gravitationally unstable and collapse into dense cores and,
presumably, stars. The occurance and efficiency of local collapse decreases as
the driving wave length decreases and the driving strength increases. It
appears that local collapse can only be prevented entirely with unrealistically
short wave length driving, but observed core formation rates can be reproduced
with more realistic driving. At high collapse rates, cores are formed on short
time scales in coherent structures with high efficiency, while at low collapse
rates they are scattered randomly throughout the region and exhibit
considerable age spread. We suggest that this naturally explains the observed
distinction between isolated and clustered star formation.Comment: Minor revisions in response to referee, thirteen figures, accepted to
Astrophys.
A sensory 3-D map of the odor description space derived from a comparison of numeric odor profile databases
Many authors have proposed different schemes of odor classification, which are useful to aid the complex task of describing smells. However, reaching a consensus on a particular classification seems difficult because our psychophysical space of odor description is a continuum and is not clustered into well-defined categories. An alternative approach is to describe the perceptual space of odors as a low-dimensional coordinate system. This idea was first proposed by Crocker and Henderson in 1927, who suggested using numeric profiles based on 4 dimensions: fragrant, acid, burnt, and caprylic. In the present work, the odor profiles of 144 aroma chemicals were compared by means of statistical regression with comparable numeric odor profiles obtained from 2 databases, enabling a plausible interpretation of the 4 dimensions. Based on the results and taking into account comparable 2D sensory maps of odor descriptors from the literature, a 3D sensory map (odor cube) has been drawn up to improve understanding of the similarities and dissimilarities of the odor descriptors most frequently used in fragrance chemistry.Zarzo Castelló, M. (2015). A sensory 3-D map of the odor description space derived from a comparison of numeric odor profile databases. Chemical Senses. 40(5):305-313. doi:10.1093/chemse/bjv012S30531340
Chaotic star formation and the alignment of stellar rotation with disc and planetary orbital axes
We investigate the evolution of the relative angle between the stellar
rotation axis and the circumstellar disc axis of a star that forms in a stellar
cluster from the collapse of a turbulent molecular cloud. This is an inherently
chaotic environment with variable accretion, both in terms of rate and the
angular momentum of the material, and dynamical interactions between stars. We
find that the final stellar rotation axis and disc spin axis can be strongly
misaligned, but this occurs primarily when the disc is truncated by a dynamical
encounter so that the final disc rotation axis depends simply on what fell in
last. This may lead to planetary systems with orbits that are misaligned with
the stellar rotation axis, but only if the final disc contains enough mass to
form planets. We also investigate the time variability of the inner disc spin
axis, which is likely to determine the direction of a protostellar jet. We find
that the jet direction varies more strongly for lighter discs, such as those
that have been truncated by dynamical interactions or have suffered a period of
rapid accretion. Finally, we note that variability of the angular momentum of
the material accreting by a star implies that the internal velocity field of
such stars may be more complicated than that of aligned differential rotation.Comment: Accepted for publication in MNRAS, 11 pages, 6 figure
Binary Formation in Star-Forming Clouds with Various Metallicities
Cloud evolution for various metallicities is investigated by
three-dimensional nested grid simulations, in which the initial ratio of
rotational to gravitational energy of the host cloud \beta_0 (=10^-1 - 10^-6)
and cloud metallicity Z (=0 - Z_\odot) are parameters. Starting from a central
number density of n = 10^4 cm^-3, cloud evolution for 48 models is calculated
until the protostar is formed (n \simeq 10^23 cm^-3) or fragmentation occurs.
The fragmentation condition depends both on the initial rotational energy and
cloud metallicity. Cloud rotation promotes fragmentation, while fragmentation
tends to be suppressed in clouds with higher metallicity. Fragmentation occurs
when \beta_0 > 10^-3 in clouds with solar metallicity, while fragmentation
occurs when \beta_0 > 10^-5 in the primordial gas cloud. Clouds with lower
metallicity have larger probability of fragmentation, which indicates that the
binary frequency is a decreasing function of cloud metallicity. Thus, the
binary frequency at the early universe (or lower metallicity environment) is
higher than at present day (or higher metallicity environment). In addition,
binary stars born from low-metallicity clouds have shorter orbital periods than
those from high-metallicity clouds. These trends are explained in terms of the
thermal history of the collapsing cloud.Comment: 11 pages, 2 figures, Submitted to ApJL, For high resolution figures
see http://astro3.sci.hokudai.ac.jp/~machida/binary-metal.pd
What does a universal IMF imply about star formation?
We show that the same initial mass function (IMF) can result from very
different modes of star formation from very similar underlying core and/or
system mass functions. In particular, we show that the canonical IMF can be
recovered from very similar system mass functions, but with very different mass
ratio distributions within those systems. This is a consequence of the
basically log-normal shapes of all of the distributions. We also show that the
relationships between the shapes of the core, system, and stellar mass
functions may not be trivial. Therefore, different star formation in different
regions could still result in the same IMF.Comment: 6 pages, 4 figures. MNRAS, in pres
- …