3,311 research outputs found
Strategic tradeoffs in competitor dynamics on adaptive networks
Recent empirical work highlights the heterogeneity of social competitions
such as political campaigns: proponents of some ideologies seek debate and
conversation, others create echo chambers. While symmetric and static network
structure is typically used as a substrate to study such competitor dynamics,
network structure can instead be interpreted as a signature of the competitor
strategies, yielding competition dynamics on adaptive networks. Here we
demonstrate that tradeoffs between aggressiveness and defensiveness (i.e.,
targeting adversaries vs. targeting like-minded individuals) creates
paradoxical behaviour such as non-transitive dynamics. And while there is an
optimal strategy in a two competitor system, three competitor systems have no
such solution; the introduction of extreme strategies can easily affect the
outcome of a competition, even if the extreme strategies have no chance of
winning. Not only are these results reminiscent of classic paradoxical results
from evolutionary game theory, but the structure of social networks created by
our model can be mapped to particular forms of payoff matrices. Consequently,
social structure can act as a measurable metric for social games which in turn
allows us to provide a game theoretical perspective on online political
debates.Comment: 20 pages (11 pages for the main text and 9 pages of supplementary
material
Semistable reduction for overconvergent F-isocrystals, III: Local semistable reduction at monomial valuations
We resolve the local semistable reduction problem for overconvergent
F-isocrystals at monomial valuations (Abhyankar valuations of height 1 and
residue transcendence degree 0). We first introduce a higher-dimensional
analogue of the generic radius of convergence for a p-adic differential module,
which obeys a convexity property. We then combine this convexity property with
a form of the p-adic local monodromy theorem for so-called fake annuli.Comment: 36 pages; v3: refereed version; adds appendix with two example
A VLA search for young protostars embedded in dense cores
Four dense cores, L1582A, L1689A, B133 and B68, classified as prestellar in
terms of the absence of detectable NIR emission, are observed at radio
wavelengths to investigate whether they nurture very young protostars. No
definite young protostars were discovered in any of the four cores observed. A
few radio sources were discovered close to the observed cores, but these are
most likely extragalactic sources or YSOs unrelated to the cores observed. In
L1582A we discovered a weak radio source near the centre of the core with radio
characteristics and offset from the peak of the submillimeter emission similar
to that of the newly discovered protostar in the core L1014, indicating a
possible protostellar nature for this source. This needs to be confirmed with
near- and/or mid-infrared observations (e.g. with Spitzer). Hence based on the
current observations we are unable to confirm unequivocally that L1582A is
starless. In L1689A a possible 4.5-sigma radio source was discovered at the
centre of the core, but needs to be confirmed with future observations. In B133
a weak radio source, possibly a protostar, was discovered at the edge of the
core on a local peak of the core submm emission, but no source was detected at
the centre of the core. Thus, B133 is probably starless, but may have a
protostar at its edge. In B68 no radio sources were discovered inside or at the
edge of the core, and thus B68 is indeed starless. Four more radio sources with
spectral indices characteristic of young protostars were discovered outside the
cores but within the extended clouds in which these cores reside. Conclusions:
We conclude that the number of cores misclassified as prestellar is probably
very small and does not significantly alter the estimated lifetime of the
prestellar phase.Comment: Accepted by A&
The Spitzer c2d Survey of Nearby Dense Cores: VI. The Protostars of Lynds Dark Nebula 1221
Observations of Lynds Dark Nebula 1221 from the Spitzer Space Telescope are
presented. These data show three candidate protostars towards L1221, only two
of which were previously known. The infrared observations also show signatures
of outflowing material, an interpretation which is also supported by radio
observations with the Very Large Array. In addition, molecular line maps from
the Five College Radio Astronomy Observatory are shown.
One-dimensional dust continuum modelling of two of these protostars, IRS1 and
IRS3, is described. These models show two distinctly different protostars
forming in very similar environments. IRS1 shows a higher luminosity and larger
inner radius of the envelope than IRS3. The disparity could be caused by a
difference in age or mass, orientation of outflow cavities, or the impact of a
binary in the IRS1 core.Comment: accepted for publication in Ap
The acidic domain of the endothelial membrane protein GPIHBP1 stabilizes lipoprotein lipase activity by preventing unfolding of its catalytic domain.
GPIHBP1 is a glycolipid-anchored membrane protein of capillary endothelial cells that binds lipoprotein lipase (LPL) within the interstitial space and shuttles it to the capillary lumen. The LPL•GPIHBP1 complex is responsible for margination of triglyceride-rich lipoproteins along capillaries and their lipolytic processing. The current work conceptualizes a model for the GPIHBP1•LPL interaction based on biophysical measurements with hydrogen-deuterium exchange/mass spectrometry, surface plasmon resonance, and zero-length cross-linking. According to this model, GPIHBP1 comprises two functionally distinct domains: (1) an intrinsically disordered acidic N-terminal domain; and (2) a folded C-terminal domain that tethers GPIHBP1 to the cell membrane by glycosylphosphatidylinositol. We demonstrate that these domains serve different roles in regulating the kinetics of LPL binding. Importantly, the acidic domain stabilizes LPL catalytic activity by mitigating the global unfolding of LPL's catalytic domain. This study provides a conceptual framework for understanding intravascular lipolysis and GPIHBP1 and LPL mutations causing familial chylomicronemia
Structure of an Enclosed Dimer Formed by the Drosophila Period Protein
Period (PER) is the major transcription inhibitor in metazoan circadian clocks and lies at the center of several feedback loops that regulate gene expression. Dimerization of Drosophila PER influences nuclear translocation, repressor activity, and behavioral rhythms. The structure of a central, 346-residue PER fragment reveals two associated PAS (Per-Arnt-Sim) domains followed by a protruding α-helical extension (αF). A closed, pseudosymmetric
dimer forms from a cross handshake interaction of the N-terminal
PAS domain with αF of the opposing subunit. Strikingly, a shift of αF against the PAS β-sheet generates two alternative subunit interfaces in the dimer. Taken together with a previously reported PER structure in which αF extends, these data indicate that αF unlatches to switch association of PER with itself to its partner Timeless. The variable positions of the αF helix provide snapshots of a helix dissociation mechanism that has relevance to other PAS protein systems. Conservation of PER interaction residues among
a family of PAS-AB-containing transcription factors suggests that contacts mediating closed PAS-AB dimers serve a general function
Properties of the Youngest Protostars in Perseus, Serpens, and Ophiuchus
We present an unbiased census of deeply embedded protostars in Perseus,
Serpens, and Ophiuchus, assembled by combining large-scale 1.1 mm Bolocam
continuum and Spitzer Legacy surveys. We identify protostellar candidates based
on their mid-infrared properties, correlate their positions with 1.1 mm core
positions, and construct well-sampled SEDs using our extensive wavelength
coverage (lam=1.25-1100 micron). Source classification based on the bolometric
temperature yields a total of 39 Class 0 and 89 Class I sources in the three
cloud sample. We compare to protostellar evolutionary models using the
bolometric temperature-luminosity diagram, finding a population of low
luminosity Class I sources that are inconsistent with constant or monotonically
decreasing mass accretion rates. This result argues strongly for episodic
accretion during the Class I phase, with more than 50% of sources in a
``sub-Shu'' (dM/dt < 1e-6 Msun/yr) accretion state. Average spectra are
compared to protostellar radiative transfer models, which match the observed
spectra fairly well in Stage 0, but predict too much near-IR and too little
mid-IR flux in Stage I. Finally, the relative number of Class 0 and Class I
sources are used to estimate the lifetime of the Class 0 phase; the three cloud
average yields a Class 0 lifetime of 1.7e5 yr, ruling out an extremely rapid
early accretion phase. Correcting photometry for extinction results in a
somewhat shorter lifetime (1.1e5 yr). In Ophiuchus, however, we find very few
Class 0 sources (N(Class0)/N(ClassI)=0.1-0.2), similar to previous studies of
that cloud. The observations suggest a consistent picture of nearly constant
average accretion rate through the entire embedded phase, with accretion
becoming episodic by at least the Class I stage, and possibly earlier.Comment: 31 pages, 19 figures, 8 tables; accepted for publication in Ap
- …