103 research outputs found
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
Correlations Between Central Massive Objects And Their Host Galaxies: From Bulgeless Spirals to Ellipticals
Recent observations by Ferrarese et al. (2006) and Wehner et al. (2006)
reveal that a majority of galaxies contain a central massive object (CMO),
either a supermassive black hole (SMBH) or a compact stellar nucleus,
regardless of the galaxy mass or morphological type, and that there is a tight
relation between the masses of CMOs and those of the host galaxies. Several
recent studies show that feedback from black holes can successfully explain the
\msigma correlation in massive elliptical galaxies that contain SMBHs.
However, puzzles remain in spirals or dwarf spheroids that do not appear to
have black holes but instead harbor a compact central stellar cluster. Here we
use three-dimensional, smoothed particle hydrodynamics simulations of isolated
galaxies to study the formation and evolution of CMOs in bulgeless disk
galaxies, and simulations of merging galaxies to study the transition of the
CMO--host mass relation from late-type bulgeless spirals to early-type
ellipticals. Our results suggest that the observed correlations may be
established primarily by the depletion of gas in the central region by
accretion and star-formation, and may hold for all galaxy types. A systematic
search for CMOs in the nuclei of bulgeless disk galaxies would offer a test of
this conclusion. (Abridged)Comment: 11 pages, 8 figures, accepted to Ap
Protein Folding Database (PFD 2.0): an online environment for the International Foldeomics Consortium
The Protein Folding Database (PFD) is a publicly accessible repository of thermodynamic and kinetic protein folding data. Here we describe the first major revision of this work, featuring extensive restructuring that conforms to standards set out by the recently formed International Foldeomics Consortium. The database now adopts standards for data acquisition, analysis and reporting proposed by the consortium, which will facilitate the comparison of folding rates, energies and structure across diverse sets of proteins. Data can now be easily deposited using a rich set of deposition tools. Enhanced search tools allow sophisticated searching and graphical data analysis affords simple data analysis online. PFD can be accessed freely at
On the evolution of local material properties and residual stress in a three-pass SA508 steel weld
The PAndAS Field of Streams: stellar structures in the Milky Way halo toward Andromeda and Triangulum
We reveal the highly structured nature of the Milky Way stellar halo within
the footprint of the PAndAS photometric survey from blue main sequence and main
sequence turn-off stars. We map no fewer than five stellar structures within a
heliocentric range of ~5 to 30 kpc. Some of these are known (the Monoceros
Ring, the Pisces/Triangulum globular cluster stream), but we also uncover three
well-defined stellar structures that could be, at least partly, responsible for
the so-called Triangulum/Andromeda and Triangulum/Andromeda 2 features. In
particular, we trace a new faint stellar stream located at a heliocentric
distance of ~17 kpc. With a surface brightness of \Sigma_V ~ 32-32.5
mag/arcsec^2, it follows an orbit that is almost parallel to the Galactic plane
north of M31 and has so far eluded surveys of the Milky Way halo as these tend
to steer away from regions dominated by the Galactic disk. Investigating our
follow-up spectroscopic observations of PAndAS, we serendipitously uncover a
radial velocity signature from stars that have colors and magnitudes compatible
with the stream. From the velocity of eight likely member stars, we show that
this stellar structure is dynamically cold, with an unresolved velocity
dispersion that is lower than 7.1 km/s at the 90-percent confidence level.
Along with the width of the stream (300-650 pc), its dynamics points to a
dwarf-galaxy-accretion origin. The numerous stellar structures we can map in
the Milky Way stellar halo between 5 and 30 kpc and their varying morphology is
a testament to the complex nature of the stellar halo at these intermediate
distances.Comment: 11 pages, 8 figures, accepted for publication in the ApJ, Figure 3 is
the money plo
Bondi-Hoyle Accretion in a Turbulent Medium
The Bondi-Hoyle formula gives the approximate accretion rate onto a point
particle accreting from a uniform medium. However, in many situations accretion
onto point particles occurs from media that are turbulent rather than uniform.
In this paper, we give an approximate solution to the problem of a point
particle accreting from an ambient medium of supersonically turbulent gas.
Accretion in such media is bimodal, at some points resembling classical
Bondi-Hoyle flow, and in other cases being closer to the vorticity-dominated
accretion flows recently studied by Krumholz, McKee, & Klein. Based on this
observation, we develop a theoretical prediction for the accretion rate, and
confirm that our predictions are highly consistent with the results of
numerical simulations. The distribution of accretion rates is lognormal, and
the mean accretion rate in supersonically turbulent gas can be substantially
enhanced above the value that would be predicted by a naive application of the
Bondi-Hoyle formula. However, it can also be suppressed by the vorticity, just
as Krumholz, McKee, & Klein found for non-supersonic vorticity-dominated flows.
Magnetic fields, which we have not included in these models, may further
inhibit accretion. Our results have significant implications for a number
astrophysical problems, ranging from star formation to the black holes in
galactic centers. In particular, there are likely to be significant errors in
results that assume that accretion from turbulent media occurs at the
unmodified Bondi-Hoyle rate, or that are based on simulations that do not
resolve the Bondi-Hoyle radius of accreting objects.Comment: Accepted for publication in ApJ; 13 pages, 9 figures, emulateap
Major substructure in the M31 outer halo: the South-West Cloud
We undertake the first detailed analysis of the stellar population and spatial properties of a diffuse substructure in the outer halo ofM31. The South-West Cloud lies at a projected distance of Ì100 kpc from the centre of M31 and extends for at least Ì
The PAndAS field of streams: Stellar structures in the milky way halo toward andromeda and triangulum
We reveal the highly structured nature of the Milky Way (MW) stellar halo within the footprint of the Pan-Andromeda Archaeological Survey (PAndAS) photometric survey from blue main sequence (MS) and MS turn-off stars. We map no fewer than five stellar structures within a heliocentric range of 5-30 kpc. Some of these are known (the Monoceros Ring, the Pisces/Triangulum globular cluster stream), but we also uncover three well-defined stellar structures that could be, at least partly, responsible for the so-called Triangulum/Andromeda and Triangulum/Andromeda 2 features. In particular, we trace a new faint stellar stream located at a heliocentric distance of 17 kpc. With a surface brightness of ÎŁ V 32-32.5 mag arcsec-2, it follows an orbit that is almost parallel to the Galactic plane north of M31 and has so far eluded surveys of the MW halo as these tend to steer away from regions dominated by the Galactic disk. Investigating our follow-up spectroscopic observations of PAndAS, we serendipitously uncover a radial velocity signature from stars that have colors and magnitudes compatible with the stream. From the velocity of eight likely member stars, we show that this stellar structure is dynamically cold, with an unresolved velocity dispersion that is lower than 7.1 km s-1 at the 90% confidence level. Along with the width of the stream (300-650 pc), its dynamics point to a dwarf-galaxy-accretion origin. The numerous stellar structures we can map in the MW stellar halo between 5 and 30 kpc and their varying morphology is a testament to the complex nature of the stellar halo at these intermediate distances
Star Formation in Isolated Disk Galaxies. II. Schmidt Laws and Efficiency of Gravitational Collapse
(Abridged). We model gravitational instability in a wide range of isolated
disk galaxies, using GADGET, a three-dimensional, smoothed particle
hydrodynamics code. The model galaxies include a dark matter halo and a disk of
stars and isothermal gas. The global Schmidt law observed in disk galaxies is
quantitatively reproduced by our models. We find that the surface density of
star formation rate directly correlates with the strength of local
gravitational instability. The local Schmidt laws of individual galaxies in our
models show clear evidence of star formation thresholds. Our results suggest
that the non-linear development of gravitational instability determines the
local and global Schmidt laws, and the star formation thresholds.Comment: 21 pages, 15 figures, to appear on ApJ March 2006. (Paper I is ApJ
626, 823, 2005, or astro-ph/0501022
Do case-only designs yield consistent results across design and different databases? A case study of hip fractures and benzodiazepines.
BACKGROUND: The case-crossover (CXO) and self-controlled case series (SCCS) designs are increasingly used in pharmacoepidemiology. In both, relative risk estimates are obtained within persons, implicitly controlling for time-fixed confounding variables. OBJECTIVES: To examine the consistency of relative risk estimates of hip/femur fractures (HFF) associated with the use of benzodiazepines (BZD) across case-only designs in two databases (DBs), when a common protocol was applied. METHODS: CXO and SCCS studies were conducted in BIFAP (Spain) and CPRD (UK). Exposure to BZD was divided into non-use, current, recent and past use. For CXO, odds ratios (OR; 95%CI) of current use versus non-use/past were estimated using conditional logistic regression adjusted for co-medications (AOR). For the SCCS, conditional Poisson regression was used to estimate incidence rate ratios (IRR; 95%CI) of current use versus non/past-use, adjusted for age. To investigate possible event-exposure dependence the relative risk in the 30 days prior to first BZD exposure was also evaluated. RESULTS: In the CXO current use of BZD was associated with an increased risk of HFF in both DBs, AORBIFAP = 1.47 (1.29-1.67) and AORCPRD = 1.55 (1.41-1.70). In the SCCS, IRRs for current exposure was 0.79 (0.72-0.86) in BIFAP and 1.21 (1.13-1.30) in CPRD. However, when we considered separately the 30-day pre-exposure period, the IRR for current period was 1.43 (1.31-1.57) in BIFAP and 1.37 (1.27-1.47) in CPRD. CONCLUSIONS: CXO designs yielded consistent results across DBs, while initial SCCS analyses did not. Accounting for event-exposure dependence, estimates derived from SCCS were more consistent across DBs and designs
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