5,421 research outputs found
Leadership and the Research Productivity of University Departments
Much of human knowledge is produced in the world's university departments. There is little scientific evidence, however, about how those hundreds of thousands of departments are best organized and led. This study hand-collects longitudinal data on departmental chairpersons in 58 US universities over a 15-year period. There is one robust predictor of a department's future research output. After adjustment for a range of personal and institutional characteristics, departmental research productivity improves when the incoming department Chair's publications are highly cited. A one SD increase in citations is associated with a 0.5 SD later rise in departmental productivity. By contrast, the quality-weighted publication record per se of the incoming Chair has no predictive power
The First Stars
We review recent theoretical results on the formation of the first stars in
the universe, and emphasize related open questions. In particular, we discuss
the initial conditions for Population III star formation, as given by variants
of the cold dark matter cosmology. Numerical simulations have investigated the
collapse and the fragmentation of metal-free gas, showing that the first stars
were predominantly very massive. The exact determination of the stellar masses,
and the precise form of the primordial initial mass function, is still hampered
by our limited understanding of the accretion physics and the protostellar
feedback effects. We address the importance of heavy elements in bringing about
the transition from an early star formation mode dominated by massive stars, to
the familiar mode dominated by low mass stars, at later times. We show how
complementary observations, both at high redshifts and in our local cosmic
neighborhood, can be utilized to probe the first epoch of star formation.Comment: 38 pages, 10 figures, draft version for 2004 Annual Reviews of
Astronomy and Astrophysics, high-resolution version available at
http://cfa-www.harvard.edu/~vbromm
The effect of medications associated with drug-induced pancreatitis on pancreatic cancer risk : a nested case-control study of routine Scottish data
Funding: This work was supported by Cancer Research UK (reference C37316/A25535). Acknowledgements: We wish to thank PCCIUR, University of Aberdeen, especially Artur Wozniak, for extracting the data and performing case-control matching.Peer reviewedPublisher PD
How Reasoning Aims at Truth
Many hold that theoretical reasoning aims at truth. In this paper, I ask what it is for reasoning to be thus aim-directed. Standard answers to this question explain reasoningâs aim-directedness in terms of intentions, dispositions, or rule-following. I argue that, while these views contain important insights, they are not satisfactory. As an alternative, I introduce and defend a novel account: reasoning aims at truth in virtue of being the exercise of a distinctive kind of cognitive power, one that, unlike ordinary dispositions, is capable of fully explaining its own exercises. I argue that this account is able to avoid the difficulties plaguing standard accounts of the relevant sort of mental teleology
Morphological and molecular evidence supports specific recognition of the recently extinct Bettongia anhydra (Marsupialia: Macropodidae)
In 1933, geologist and explorer Michael Terry collected the skull of a small macropodid captured by members of his party near Lake Mackay, western Northern Territory. In 1957, this skull was described as the sole exemplar of a distinct subspecies, Bettongia penicillata anhydra, but was later synonymized with B. lesueur and thereafter all but forgotten. We use a combination of craniodental morphology and ancient mitochondrial DNA to confirm that the Lake Mackay specimen is taxonomically distinct from all other species of Bettongia and recognize an additional specimen from a Western Australian Holocene fossil accumulation. B. anhydra is morphologically and genetically most similar to B. lesueur but differs in premolar shape, rostrum length, dentary proportions, and molar size gradient. In addition, it has a substantial mitochondrial cytochrome b pairwise distance of 9.6â12% relative to all other bettongs. The elevation of this recently extinct bettong to species status indicates that Australiaâs mammal extinction record over the past 2 centuries is even worse than currently accepted. Like other bettongs, B. anhydra probably excavated much of its food and may have performed valuable ecological services that improved soil structure and water infiltration and retention, as well as playing an important role in the dispersal of seeds and mycorrhizal fungal spores. All extant species of Bettongia have experienced extensive range contractions since European colonization and some now persist only on island refugia. The near total loss of these ecosystem engineers from the Australian landscape has far-reaching ecological implications
An x ray archive on your desk: The Einstein CD-ROM's
Data from the Einstein Observatory imaging proportional counter (IPC) and high resolution imager (HRI) were released on several CD-ROM sets. The sets released so far include pointed IPC and HRI observations in both simple image and detailed photon event list format, as well as the IPC slew survey. With the data on these CD-ROMS's the user can perform spatial analysis (e.g., surface brightness distributions), spectral analysis (with the IPC event lists), and timing analysis (with the IPC and HRI event lists). The next CD-ROM set will contain IPC unscreened data, allowing the user to perform custom screening to recover, for instance, data during times of lost aspect data or high particle background rates
p53-mediated delayed NF-ÎșB activity enhances etoposide-induced cell death in medulloblastoma
Medulloblastoma (MB) is an embryonic brain tumour that arises in the cerebellum. Using several MB cell lines, we have demonstrated that the chemotherapeutic drug etoposide induces a p53- and caspase-dependent cell death. We have observed an additional caspase-independent cell death mechanism involving delayed nuclear factor ÎșB (NF-ÎșB) activity. The delayed induction was controlled by a p53-dependent transcription step and the production of death receptors (especially CD95/Fas). We further demonstrated that in both MB and glioblastoma (GM) cell lines, in which the p53 pathway was not functional, no p65 activation could be detected upon etoposide treatment. MB cell lines that have mutations in p53 or NF-ÎșB are either less sensitive (NF-ÎșB mutant) or even completely resistant (p53 mutant) to chemotherapeutic intervention. The optimal cell death was only achieved when both p53 and NF-ÎșB were switched on. Taken together, our results shed light on the mechanism of NF-ÎșB activation by etoposide in brain tumours and show that the genetic background of MB and GM cells determines their sensitivity to chemotherapy and has to be taken into account for efficient therapeutic intervention
Cosmic Renaissance: The First Sources of Light
I review recent progress in understanding the formation of the first stars
and quasars. The initial conditions for their emergence are given by the now
firmly established model of cosmological structure formation. Numerical
simulations of the collapse and fragmentation of primordial gas indicate that
the first stars formed at redshifts z ~ 20 - 30, and that they were
predominantly very massive, with M_* > 100 M_sun. Important uncertainties,
however, remain. Paramount among them is the accretion process, which builds up
the final stellar mass by incorporating part of the diffuse, dust-free envelope
into the central protostellar core. The first quasars, on the other hand, are
predicted to have formed later on, at z ~ 10, in more massive dark matter
halos, with total masses, ~ 10^8 M_sun, characteristic of dwarf galaxies.Comment: 16 pages, 7 figures, invited review, to appear in PASP, Feb. 200
The Formation of the First Stars. I. The Primordial Star Forming Cloud
To constrain the nature of the very first stars, we investigate the collapse
and fragmentation of primordial, metal-free gas clouds. We explore the physics
of primordial star formation by means of three-dimensional simulations of the
dark matter and gas components, using smoothed particle hydrodynamics, under a
wide range of initial conditions, including the initial spin, the total mass of
the halo, the redshift of virialization, the power spectrum of the DM
fluctuations, the presence of HD cooling, and the number of particles employed
in the simulation. We find characteristic values for the temperature, T ~ a few
100 K, and the density, n ~ 10^3-10^4 cm^-3, characterising the gas at the end
of the initial free-fall phase. These values are rather insensitive to the
initial conditions. The corresponding Jeans mass is M_J ~ 10^3 M_sun. The
existence of these characteristic values has a robust explanation in the
microphysics of H2 cooling, connected to the minimum temperature that can be
reached with the H2 coolant, and to the critical density at which the
transition takes place betweeb levels being populated according to NLTE, and
according to LTE.
In all cases, the gas dissipatively settles into an irregular, central
configuration which has a filamentary and knotty appearance. The fluid regions
with the highest densities are the first to undergo runaway collapse due to
gravitational instability, and to form clumps with initial masses ~ 10^3 M_sun,
close to the characteristic Jeans scale. These results suggest that the first
stars might have been quite massive, possibly even very massive with M_star >
100 M_sun.Comment: Minor revisions. 26 pages, including 24 figures and 5 tables. ApJ, in
press. To appear in the Dec. 20, 2001 issue (v563
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