673 research outputs found
Should we be afraid of the dark? Dark trading and market quality
© 2016 We exploit a unique natural experiment—recent restrictions of dark trading in Canada and Australia—and proprietary trade-level data to analyze the effects of dark trading. Disaggregating two types of dark trading, we find that dark limit order markets are beneficial to market quality, reducing quoted, effective, and realized spreads and increasing informational efficiency. In contrast, we do not find consistent evidence that dark midpoint crossing systems significantly affect market quality. Our results support recent theory that dark limit order markets encourage aggressive competition in liquidity provision. We discuss implications for the regulation of dark trading and tick sizes
The Core of the Participatory Budgeting Problem
In participatory budgeting, communities collectively decide on the allocation
of public tax dollars for local public projects. In this work, we consider the
question of fairly aggregating the preferences of community members to
determine an allocation of funds to projects. This problem is different from
standard fair resource allocation because of public goods: The allocated goods
benefit all users simultaneously. Fairness is crucial in participatory decision
making, since generating equitable outcomes is an important goal of these
processes. We argue that the classic game theoretic notion of core captures
fairness in the setting. To compute the core, we first develop a novel
characterization of a public goods market equilibrium called the Lindahl
equilibrium, which is always a core solution. We then provide the first (to our
knowledge) polynomial time algorithm for computing such an equilibrium for a
broad set of utility functions; our algorithm also generalizes (in a
non-trivial way) the well-known concept of proportional fairness. We use our
theoretical insights to perform experiments on real participatory budgeting
voting data. We empirically show that the core can be efficiently computed for
utility functions that naturally model our practical setting, and examine the
relation of the core with the familiar welfare objective. Finally, we address
concerns of incentives and mechanism design by developing a randomized
approximately dominant-strategy truthful mechanism building on the exponential
mechanism from differential privacy
Mapping Exoplanets
The varied surfaces and atmospheres of planets make them interesting places
to live, explore, and study from afar. Unfortunately, the great distance to
exoplanets makes it impossible to resolve their disk with current or near-term
technology. It is still possible, however, to deduce spatial inhomogeneities in
exoplanets provided that different regions are visible at different
times---this can be due to rotation, orbital motion, and occultations by a
star, planet, or moon. Astronomers have so far constructed maps of thermal
emission and albedo for short period giant planets. These maps constrain
atmospheric dynamics and cloud patterns in exotic atmospheres. In the future,
exo-cartography could yield surface maps of terrestrial planets, hinting at the
geophysical and geochemical processes that shape them.Comment: Updated chapter for Handbook of Exoplanets, eds. Deeg & Belmonte. 17
pages, including 6 figures and 4 pages of reference
Voluntary exercise can strengthen the circadian system in aged mice
Consistent daily rhythms are important to healthy aging according to studies linking disrupted circadian rhythms with negative health impacts. We studied the effects of age and exercise on baseline circadian rhythms and on the circadian system's ability to respond to the perturbation induced by an 8 h advance of the light:dark (LD) cycle as a test of the system's robustness. Mice (male, mPer2luc/C57BL/6) were studied at one of two ages: 3.5 months (n = 39) and >18 months (n = 72). We examined activity records of these mice under entrained and shifted conditions as well as mPER2::LUC measures ex vivo to assess circadian function in the suprachiasmatic nuclei (SCN) and important target organs. Age was associated with reduced running wheel use, fragmentation of activity, and slowed resetting in both behavioral and molecular measures. Furthermore, we observed that for aged mice, the presence of a running wheel altered the amplitude of the spontaneous firing rate rhythm in the SCN in vitro. Following a shift of the LD cycle, both young and aged mice showed a change in rhythmicity properties of the mPER2::LUC oscillation of the SCN in vitro, and aged mice exhibited longer lasting internal desynchrony. Access to a running wheel alleviated some age-related changes in the circadian system. In an additional experiment, we replicated the effect of the running wheel, comparing behavioral and in vitro results from aged mice housed with or without a running wheel (>21 months, n = 8 per group, all examined 4 days after the shift). The impact of voluntary exercise on circadian rhythm properties in an aged animal is a novel finding and has implications for the health of older people living with environmentally induced circadian disruption
Eta Carinae and the Luminous Blue Variables
We evaluate the place of Eta Carinae amongst the class of luminous blue
variables (LBVs) and show that the LBV phenomenon is not restricted to
extremely luminous objects like Eta Car, but extends luminosities as low as
log(L/Lsun) = 5.4 - corresponding to initial masses ~25 Msun, and final masses
as low as ~10-15 Msun. We present a census of S Doradus variability, and
discuss basic LBV properties, their mass-loss behaviour, and whether at maximum
light they form pseudo-photospheres. We argue that those objects that exhibit
giant Eta Car-type eruptions are most likely related to the more common type of
S Doradus variability. Alternative atmospheric models as well as
sub-photospheric models for the instability are presented, but the true nature
of the LBV phenomenon remains as yet elusive. We end with a discussion on the
evolutionary status of LBVs - highlighting recent indications that some LBVs
may be in a direct pre-supernova state, in contradiction to the standard
paradigm for massive star evolution.Comment: 27 pages, 6 figures, Review Chapter in "Eta Carinae and the supernova
imposters" (eds R. Humphreys and K. Davidson) new version submitted to
Springe
Ectopic internal carotid artery presenting as an oropharyngeal mass
Ectopic internal carotid artery (ICA) is a very rare variation. The major congenital abnormalities of the ICA can be classified as agenesis, aplasia and hypoplasia, and they can be unilateral or bilateral. Anomalies of the neck artery may be vascular neoplasms or ectopic position. Carotid angiograms provide absolute confirmation of an aberrant carotid artery, while EcoColorDoppler (ECD) gives also important information about the evaluation of carotid vassels. Nevertheless Computed Tomography (CT) and Magnetic Resonance Imaging (MRI) of the neck provide spatial information about the adjacent pharyngeal anatomy and are less invasive than angiogram. Injuries to the ICA during simple pharyngeal surgical procedures can be catastrophic due to the risk of massive bleeding. We report a case of a 56 year-old male patient suffering from dysphagia associated with aberrant ICA manifesting itself as a pulsative protruding of the left lateral wall of the oropharynx
Horizontal Gene Transfer of a ColV Plasmid Has Resulted in a Dominant Avian Clonal Type of Salmonella enterica Serovar Kentucky
Salmonella enterica continues to be a significant cause of foodborne gastrointestinal illness in humans. A wide variety of Salmonella serovars have been isolated from production birds and from retail poultry meat. Recently, though, S. enterica subsp. enterica serovar Kentucky has emerged as one of the prominent Salmonella serovars isolated from broiler chickens. Recent work suggests that its emergence apparently coincides with its acquisition of a ColV virulence plasmid. In the present study, we examined 902 Salmonella isolates belonging to 59 different serovars for the presence of this plasmid. Of the serovars examined, the ColV plasmid was found only among isolates belonging to the serovars Kentucky (72.9%), Typhimurium (15.0%) and Heidelberg (1.7%). We demonstrated that a single PFGE clonal type of S. Kentucky harbors this plasmid, and acquisition of this plasmid by S. Kentucky significantly increased its ability to colonize the chicken cecum and cause extraintestinal disease. Comparison of the completed sequences of three ColV plasmids from S. Kentucky isolated from different geographical locales, timepoints and sources revealed a nearly identical genetic structure with few single nucleotide changes or insertions/deletions. Overall, it appears that the ColV plasmid was recently acquired by a single clonal type S. Kentucky and confers to its host enhanced colonization and fitness capabilities. Thus, the potential for horizontal gene transfer of virulence and fitness factors to Salmonella from other enteric bacteria exists in poultry, representing a potential human health hazard
An optical supernova associated with the X-ray flash XRF 060218
Long-duration gamma-ray bursts (GRBs) are associated with type Ic supernovae
that are more luminous than average and that eject material at very high
velocities. Less-luminous supernovae were not hitherto known to be associated
with GRBs, and therefore GRB-supernovae were thought to be rare events. Whether
X-ray flashes - analogues of GRBs, but with lower luminosities and fewer
gamma-rays - can also be associated with supernovae, and whether they are
intrinsically 'weak' events or typical GRBs viewed off the axis of the burst,
is unclear. Here we report the optical discovery and follow-up observations of
the type Ic supernova SN 2006aj associated with X-ray flash XRF 060218.
Supernova 2006aj is intrinsically less luminous than the GRB-supernovae, but
more luminous than many supernovae not accompanied by a GRB. The ejecta
velocities derived from our spectra are intermediate between these two groups,
which is consistent with the weakness of both the GRB output and the supernova
radio flux. Our data, combined with radio and X-ray observations, suggest that
XRF 060218 is an intrinsically weak and soft event, rather than a classical GRB
observed off-axis. This extends the GRB-supernova connection to X-ray flashes
and fainter supernovae, implying a common origin. Events such as XRF 060218 are
probably more numerous than GRB-supernovae.Comment: Final published versio
Observational and Physical Classification of Supernovae
This chapter describes the current classification scheme of supernovae (SNe).
This scheme has evolved over many decades and now includes numerous SN Types
and sub-types. Many of these are universally recognized, while there are
controversies regarding the definitions, membership and even the names of some
sub-classes; we will try to review here the commonly-used nomenclature, noting
the main variants when possible. SN Types are defined according to
observational properties; mostly visible-light spectra near maximum light, as
well as according to their photometric properties. However, a long-term goal of
SN classification is to associate observationally-defined classes with specific
physical explosive phenomena. We show here that this aspiration is now finally
coming to fruition, and we establish the SN classification scheme upon direct
observational evidence connecting SN groups with specific progenitor stars.
Observationally, the broad class of Type II SNe contains objects showing strong
spectroscopic signatures of hydrogen, while objects lacking such signatures are
of Type I, which is further divided to numerous subclasses. Recently a class of
super-luminous SNe (SLSNe, typically 10 times more luminous than standard
events) has been identified, and it is discussed. We end this chapter by
briefly describing a proposed alternative classification scheme that is
inspired by the stellar classification system. This system presents our
emerging physical understanding of SN explosions, while clearly separating
robust observational properties from physical inferences that can be debated.
This new system is quantitative, and naturally deals with events distributed
along a continuum, rather than being strictly divided into discrete classes.
Thus, it may be more suitable to the coming era where SN numbers will quickly
expand from a few thousands to millions of events.Comment: Extended final draft of a chapter in the "SN Handbook". Comments most
welcom
- …