197,469 research outputs found
Confirming what we know: Understanding questionable research practices in intro physics labs
Many institutions are changing the focus of their introductory physics labs
from verifying physics content towards teaching students about the skills and
nature of science. As instruction shifts, so too will the ways students
approach and behave in the labs. In this study, we evaluated students' lab
notes from an early activity in an experimentation-focused lab course. We found
that about 30% of student groups (out of 107 groups at three institutions)
recorded questionable research practices in their lab notes, such as subjective
interpretations of results or manipulating equipment and data. The large
majority of these practices were associated with confirmatory goals, which we
suspect stem from students' prior exposure to verification labs. We propose
ways for experimentation-focused labs to better engage students in the
responsible conduct of research and authentic scientific practice.Comment: 4 pages, 4 figure
Interacting Supernovae: Types IIn and Ibn
Supernovae (SNe) that show evidence of strong shock interaction between their
ejecta and pre-existing, slower circumstellar material (CSM) constitute an
interesting, diverse, and still poorly understood category of explosive
transients. The chief reason that they are extremely interesting is because
they tell us that in a subset of stellar deaths, the progenitor star may become
wildly unstable in the years, decades, or centuries before explosion. This is
something that has not been included in standard stellar evolution models, but
may significantly change the end product and yield of that evolution, and
complicates our attempts to map SNe to their progenitors. Another reason they
are interesting is because CSM interaction is an efficient engine for making
bright transients, allowing super-luminous transients to arise from normal SN
explosion energies, and allowing transients of normal SN luminosities to arise
from sub-energetic explosions or low radioactivity yield. CSM interaction
shrouds the fast ejecta in bright shock emission, obscuring our normal view of
the underlying explosion, and the radiation hydrodynamics of the interaction is
challenging to model. The CSM interaction may also be highly non-spherical,
perhaps linked to binary interaction in the progenitor system. In some cases,
these complications make it difficult to definitively tell the difference
between a core-collapse or thermonuclear explosion, or to discern between a
non-terminal eruption, failed SN, or weak SN. Efforts to uncover the physical
parameters of individual events and connections to possible progenitor stars
make this a rapidly evolving topic that continues to challenge paradigms of
stellar evolution.Comment: Final draft of a chapter in the "SN Handbook". Accepted. 25 pages, 3
fig
A search for rapid optical variability in radio-quiet quasars
The detection of rapid variability on a time-scale of hours in radio-quiet
quasars (RQQSOs) could be a powerful discriminator between starburst, accretion
disc and relativistic jet models of these sources. This paper contains an
account of a dedicated search for rapid optical variability in RQQSOs. The
technique used differential photometry between the RQQSO and stars in the same
field of view of the CCD. The 23 RQQSOs that were observed all have high
luminosities (-27 1.
The total amount of observation time was about 60 hours and these observations
are part of an ongoing programme, started in September 1990, to search for
rapid variability in RQQSOs. No evidence for short-term variability greater
than about 0.1 magnitudes was detected in any of the 23 sources, however
long-term variability was recorded for the radio-quiet quasar PG 2112+059. The
finding charts are included here because they identify the RQQSO and the
reference stars used in the photometry, and hence are available for use by
other observers.Comment: Accepted for publication in A&AS. 10 pages, 3 figures. Figure 1
(finding charts) available by anonymous ftp from:
bermuda.ucd.ie:/pub/outgoing/charts.eps.g
Selective advantage for multicellular replicative strategies: A two-cell example
This paper develops a quasispecies model where cells can adopt a two-cell
survival strategy. Within this strategy, pairs of cells join together, at which
point one of the cells sacrifices its own replicative ability for the sake of
the other cell. We develop a simplified model for the evolutionary dynamics of
this process, allowing us to solve for the steady-state using standard
approaches from quasispecies theory. We find that our model exhibits two
distinct regimes of behavior: At low concentrations of limiting resource, the
two-cell strategy outcompetes the single-cell survival strategy, while at high
concentrations of limiting resource, the single-cell survival strategy
dominates. Associated with the two solution regimes of our model is a
localization to delocalization transition over the portion of the genome coding
for the multicell strategy, analogous to the error catastrophe in standard
quasispecies models. The existence of such a transition indicates that
multicellularity can emerge because natural selection does not act on specific
cells, but rather on replicative strategies. Within this framework, individual
cells become the means by which replicative strategies are propagated. Such a
framework is therefore consistent with the concept that natural selection does
not act on individuals, but rather on populations.Comment: 4 pages, 2 figures, to be submitted to Physical Review Letter
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Geometrical Comparison of Conventional and Gerotor-Type Positive Displacement Screw Machines
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