1,178 research outputs found
Determining Star Formation Thresholds from Observations
Most gas in giant molecular clouds is relatively low-density and forms star
inefficiently, converting only a small fraction of its mass to stars per
dynamical time. However, star formation models generally predict the existence
of a threshold density above which the process is efficient and most mass
collapses to stars on a dynamical timescale. A number of authors have proposed
observational techniques to search for a threshold density above which star
formation is efficient, but it is unclear which of these techniques, if any,
are reliable. In this paper we use detailed simulations of turbulent,
magnetised star-forming clouds, including stellar radiation and outflow
feedback, to investigate whether it is possible to recover star formation
thresholds using current observational techniques. Using mock observations of
the simulations at realistic resolutions, we show that plots of projected star
formation efficiency per free-fall time can detect the
presence of a threshold, but that the resolutions typical of current dust
emission or absorption surveys are insufficient to determine its value. In
contrast, proposed alternative diagnostics based on a change in the slope of
the gas surface density versus star formation rate surface density
(Kennicutt-Schmidt relation) or on the correlation between young stellar object
counts and gas mass as a function of density are ineffective at detecting
thresholds even when they are present. The signatures in these diagnostics
sometimes taken as indicative of a threshold in observations, which we
generally reproduce in our mock observations, do not prove to correspond to
real physical features in the 3D gas distribution.Comment: 10 pages, 7 figures, Accepted for publication in MNRA
Comprehensive analysis of nanodiamond evidence relating to the Younger Dryas Impact Hypothesis
Conflicting evidence for the role of JNK as a target in breast cancer cell proliferation: comparisons between pharmacological inhibition and selective shRNA knockdown approaches
As a target, the JNK pathway has been implicated in roles including cell death, proliferation, and inflammation in variety of contexts which span cardiovascular disease, neurodegenerative pathologies, and cancer. JNK1 and JNK2 have recently been demonstrated to function independently, highlighting a new parameter in the study of the JNK pathway. In order for JNK1 and JNK2-specific roles to be defined, better tools need to be employed. Previous studies have relied upon the broad spectrum JNK inhibitor, SP600125, to characterize the role of JNK signaling in a number of cell lines, including the breast cancer cell line MCF-7. In line with previous literature, our study has demonstrated that SP600125 treatment inhibited c-Jun and JNK phosphorylation and MCF-7 proliferation. However, in addition to targeting JNK1, JNK2, and JNK3, SP600125 has been previously demonstrated to suppress the activity of a number of other serine/threonine kinases, making SP600125 an inadequate tool for JNK isoform-specific roles to be determined. In this study, lentiviral shRNA was employed to selectively knockdown JNK1, JNK2, and JNK1/2 in MCF-7 cells. Using this approach, JNK phosphorylation was fully inhibited following stable knockdown of respective JNK isoforms. Interestingly, despite suppression of JNK phosphorylation, MCF-7 cell proliferation, cell cycle progression, or cell death remained unaffected. These findings raise the question of whether JNK phosphorylation really is pivotal in MCF-7 cell growth and death or if suppression of these events is a result of one of the many off-targets cited for SP600125
Complete Genome Sequence and Comparative Metabolic Profiling of the Prototypical Enteroaggregative Escherichia coli Strain 042
Background \ud
Escherichia coli can experience a multifaceted life, in some cases acting as a commensal while in other cases causing intestinal and/or extraintestinal disease. Several studies suggest enteroaggregative E. coli are the predominant cause of E. coli-mediated diarrhea in the developed world and are second only to Campylobacter sp. as a cause of bacterial-mediated diarrhea. Furthermore, enteroaggregative E. coli are a predominant cause of persistent diarrhea in the developing world where infection has been associated with malnourishment and growth retardation. \ud
\ud
Methods \ud
In this study we determined the complete genomic sequence of E. coli 042, the prototypical member of the enteroaggregative E. coli, which has been shown to cause disease in volunteer studies. We performed genomic and phylogenetic comparisons with other E. coli strains revealing previously uncharacterised virulence factors including a variety of secreted proteins and a capsular polysaccharide biosynthetic locus. In addition, by using Biologβ’ Phenotype Microarrays we have provided a full metabolic profiling of E. coli 042 and the non-pathogenic lab strain E. coli K-12. We have highlighted the genetic basis for many of the metabolic differences between E. coli 042 and E. coli K-12. \ud
\ud
Conclusion \ud
This study provides a genetic context for the vast amount of experimental and epidemiological data published thus far and provides a template for future diagnostic and intervention strategies
Optical response and band structure of LiCoO2 including electron-hole interaction effects
The optical response functions and band structures of LiCoO are studied
at different levels of approximation, from density functional theory (DFT) in
the generalized gradient approximation (GGA) to quasiparticle self-consistent
QS (with for Green's function and for screened Coulomb interaction)
without and with ladder diagrams (QS) and the Bethe Salpeter Equation
(BSE) approach. The QS method is found to strongly overestimate the band
gap and electron-hole or excitonic effects are found to be important. They
lower the quasiparticle gap by only about 11~\% but the lowest energy peaks in
absorption are found to be excitonic in nature. The contributions from
different band to band transitions and the relation of excitons to band-to-band
transitions are analyzed. The excitons are found to be strongly localized. A
comparison to experimental data is presented.Comment: 10 pages, 9 figure
Comparison of a Quick Drinking Screen with the Timeline Followback for Individuals with Alcohol Problems
Objective: Two major strategies have typically been used to assess recent drinking: (1) Daily Estimation (DE) measures such as the Timeline Followback (TLFB) and (2) Quantity-Frequency (QF) summary measures. Although QF measures provide a quick and easy measure of consumption, they have been criticized as not being able to capture sporadic and unpatterned drinking (e.g., days that reflect important social and/or health risks). The TLFB, a psychometrically sound drinking assessment method, is able to capture all drinking, including sporadic heavy days and unpatterned drinking. In some situations, however, recall of daily drinking may not be possible or practical (e.g., limited time; no resources). This article compares results obtained by using a QF measure and a DE measure to assess problem drinkersβ pretreatment drinking.
Method: The current study, part of a large community mail intervention with 825 alcohol abusers, compared results from two drinking measures covering the same time interval that were administered on two different occasions approximately 2.5 weeks apart. Both measures, the Quick Drinking Screen (QDS; a QF summary measure that collected data by telephone) and the TLFB (a self-administered daily estimation measure), collected drinking data for the year prior to the interview.
Results: Although the QDS and the TLFB are very different drinking measures, remarkably similar aggregate drinking data were obtained for five drinking variables.
Conclusions: When it is not necessary or possible to gather detailed drinking data, the QDS produces reliable brief summary measures of drinking, at least for not severely alcohol dependent individuals. Also, respondents do not appear to use a repetitive response pattern when completing the TLFB
Minimum Information about a Neuroscience Investigation (MINI) Electrophysiology
This module represents the formalized opinion of the authors and the CARMEN consortium, which identifies the minimum information required to report the use of electrophysiology in a neuroscience study, for submission to the CARMEN system (www.carmen.org.uk).

Radiation-Hydrodynamic Simulations of the Formation of Orion-Like Star Clusters I. Implications for the Origin of the Initial Mass Function
One model for the origin of typical galactic star clusters such as the Orion
Nebula Cluster (ONC) is that they form via the rapid, efficient collapse of a
bound gas clump within a larger, gravitationally-unbound giant molecular cloud.
However, simulations in support of this scenario have thus far have not
included the radiation feedback produced by the stars; radiative simulations
have been limited to significantly smaller or lower density regions. Here we
use the ORION adaptive mesh refinement code to conduct the first ever
radiation-hydrodynamic simulations of the global collapse scenario for the
formation of an ONC-like cluster. We show that radiative feedback has a
dramatic effect on the evolution: once the first ~10-20% of the gas mass is
incorporated into stars, their radiative feedback raises the gas temperature
high enough to suppress any further fragmentation. However, gas continues to
accrete onto existing stars, and, as a result, the stellar mass distribution
becomes increasingly top-heavy, eventually rendering it incompatible with the
observed IMF. Systematic variation in the location of the IMF peak as star
formation proceeds is incompatible with the observed invariance of the IMF
between star clusters, unless some unknown mechanism synchronizes the IMFs in
different clusters by ensuring that star formation is always truncated when the
IMF peak reaches a particular value. We therefore conclude that the global
collapse scenario, at least in its simplest form, is not compatible with the
observed stellar IMF. We speculate that processes that slow down star
formation, and thus reduce the accretion luminosity, may be able to resolve the
problem.Comment: 17 pages, 13 figures, emulateapj format, ApJ in press; simulation
movies available at http://www.ucolick.org/~krumholz/publications.htm
- β¦