90 research outputs found
On the cohomological spectrum and support varieties for infinitesimal unipotent supergroup schemes
We show that if is an infinitesimal elementary supergroup scheme of
height , then the cohomological spectrum of is naturally
homeomorphic to the variety of supergroup homomorphisms
from a certain (non-algebraic) affine
supergroup scheme into . In the case , we further
identify the cohomological support variety of a finite-dimensional
-supermodule as a subset of . We then discuss how our
methods, when combined with recently-announced results by Benson, Iyengar,
Krause, and Pevtsova, can be applied to extend the homeomorphism
to arbitrary infinitesimal unipotent supergroup
schemes.Comment: Fixed some algebra misidentifications, primarily in Sections 1.3 and
3.3. Simplified the proof of Proposition 3.3.
Identification of Globular Cluster Stars in RAVE data II: Extended tidal debris around NGC 3201
We report the identification of extended tidal debris potentially associated with the globular cluster NGC 3201, using the RAdial Velocity Experiment (RAVE) catalogue. We find the debris stars are located at a distance range of 1–7 kpc based on the forthcoming RAVE distance estimates. The derived space velocities and integrals of motion show interesting connections to NGC 3201, modulo uncertainties in the proper motions. Three stars, which are among the four most likely candidates for NGC 3201 tidal debris, are separated by 80° on the sky yet are well matched by the 12 Gyr, [Fe/H] = −1.5 isochrone appropriate for the cluster. This is the first time tidal debris around this cluster has been reported over such a large spatial extent, with implications for the cluster's origin and dynamical evolution
Beryllium anomalies in solar-type field stars
We present a study of beryllium (Be) abundances in a large sample of field
solar-type dwarfs and sub-giants spanning a large range of effective
temperatures. The analysis shows that Be is severely depleted for F stars, as
expected by the light-element depletion models. However, we also show that
Beryllium abundances decrease with decreasing temperature for stars cooler than
6000 K, a result that cannot be explained by current theoretical models
including rotational mixing, but that is, at least in part, expected from the
models that take into account internal wave physics. In particular, the light
element abundances of the coolest and youngest stars in our sample suggest that
Be, as well as lithium (Li), has already been burned early during their
evolution. Furthermore, we find strong evidence for the existence of a Be-gap
for solar-temperature stars. The analysis of Li and Be abundances in the
sub-giants of our sample also shows the presence of one case that has still
detectable amounts of Li, while Be is severely depleted. Finally, we compare
the derived Be abundances with Li abundances derived using the same set of
stellar parameters. This gives us the possibility to explore the temperatures
for which the onset of Li and Be depletion occurs.Comment: 16 pages, 13 figures, accepted for publication in Astronomy &
Astrophysic
The realizability of operations on homotopy groups concentrated in two degrees
The homotopy groups of a space are endowed with homotopy operations which
define the \Pi-algebra of the space. An Eilenberg-MacLane space is the
realization of a \Pi-algebra concentrated in one degree. In this paper, we
provide necessary and sufficient conditions for the realizability of a
\Pi-algebra concentrated in two degrees. We then specialize to the stable case,
and list infinite families of such \Pi-algebras that are not realizable.Comment: Version 2: Some minor corrections. A few changes to the exposition.
To appear in the Journal of Homotopy and Related Structure
Another thread in the tapestry of stellar feedback: X-ray binaries
We consider X-ray binaries (XBs) as potential sources of stellar feedback.
XBs observationally appear able to deposit a high fraction of their power
output into their local interstellar medium, which may make them a
non-negligible source of energy input. The formation rate of the most luminous
XBs rises with decreasing metallicity, which should increase their significance
during galaxy formation in the early universe. We also argue that stochastic
effects are important to XB feedback (XBF) and may dominate the systematic
changes due to metallicity in many cases. Large stochastic variation in the
magnitude of XBF at low absolute star formation rates provides a natural reason
for diversity in the evolution of dwarf galaxies which were initially almost
identical, with several percent of such halos experiencing energy input from
XBs roughly two orders of magnitude above the most likely value. These
probability distributions suggest that the effect of XBF is most commonly
significant for total stellar masses between ~10^7 and 10^8 Msun, which might
resolve a current problem with modelling populations of such galaxies. We
explain how XBs might inject energy before luminous supernovae (SNe) contribute
significantly to feedback and how XBs can assist in keeping gas hot long after
the last core-collapse SN has exploded. [...] XBF could be especially important
to some dwarf galaxies, potentially heating gas without expelling it; the
properties of XBF also match those previously derived as allowing episodic star
formation. We also argue that the efficiency of SN feedback (SNF) might be
reduced when XBF has had the opportunity to act first. In addition, we note
that the effect of SNF is unlikely to be scale-free; galaxies smaller than ~100
pc might well experience less effective SNF. (Slightly abbreviated to fit arXiv
size limit.)Comment: Very belatedly updated to include a note added in proof and
additional reference. The definitive version is at:
mnras.oxfordjournals.org/content/423/2/164
Feedback and the Formation of Dwarf Galaxy Stellar Halos
Stellar population studies show that low mass galaxies in all environments
exhibit stellar halos that are older and more spherically distributed than the
main body of the galaxy. In some cases, there is a significant intermediate age
component that extends beyond the young disk. We examine a suite of Smoothed
Particle Hydrodynamic (SPH) simulations and find that elevated early star
formation activity combined with supernova feedback can produce an extended
stellar distribution that resembles these halos for model galaxies ranging from
= 15 km s to 35 km s, without the need for accretion of
subhalos.Comment: 15 pages, 15 figures, accepted MNRA
Dust and Ionized Gas Association in E/S0 Galaxies with Dust Lanes: Clues to their Origin
We present results from an on-going programme to study the dust and ionized
gas in E/S0 galaxies with dust lanes. Our data, together with results from
previous studies of E/S0 galaxies, are used to demonstrate the tight
relationship between these two components. This relationship is discussed in
light of our current understanding of the nature and origin of the interstellar
medium (ISM), and in particular in the context of the interplay between the
different multi-temperature components. We show that focusing on dust obscured
regions as tracers of the ISM, and on their properties, serves as independent
evidence for the external origin of the dust and ionized gas.Comment: 11 pages, 5 figures, 3 tables. MNRAS in pres
Evaluating digital diabetic retinopathy screening in people aged 90 years and over
To evaluate the effectiveness of digital diabetic retinopathy screening in patients aged 90 years and over.MethodsThis is a retrospective analysis of 200 randomly selected patients eligible for diabetic retinopathy screening aged 90 years and over within the Birmingham, Solihull, and Black Country Screening Programme.ResultsOne hundred and seventy-nine (90%) patients attended screening at least once. 133 (74%) annual screening after their first screen, of whom 59% had no detectable diabetic retinopathy; 38 (21%) were referred for ophthalmology clinical assessment-36 for nondiabetic retinopathy reasons and two for diabetic maculopathy. Cataract accounted for 50% of all referrals for ophthalmology clinical assessment. Of the 133 patients placed on annual screening, 93 (70%) were screened at least once more. In terms of level of diabetic retinopathy, assessability or other ocular pathologies, 8 improved, 51 remained stable, and 31 deteriorated. Of the latter, 19 patients were referred for ophthalmology clinical assessment; none of these for diabetic retinopathy.ConclusionsScreening provides opportunistic identification of important nondiabetic retinopathy eye conditions. However, in view of the low identification rate of sight-threatening diabetic retinopathy in patients aged 90 years and over, and the current mission statement of the NHS Diabetic Eye Screening Programme, systematic annual diabetic retinopathy screening may not be justified in this age group of patients, but rather be performed in optometric practice
Dusty Planetary Systems
Extensive photometric stellar surveys show that many main sequence stars show
emission at infrared and longer wavelengths that is in excess of the stellar
photosphere; this emission is thought to arise from circumstellar dust. The
presence of dust disks is confirmed by spatially resolved imaging at infrared
to millimeter wavelengths (tracing the dust thermal emission), and at optical
to near infrared wavelengths (tracing the dust scattered light). Because the
expected lifetime of these dust particles is much shorter than the age of the
stars (>10 Myr), it is inferred that this solid material not primordial, i.e.
the remaining from the placental cloud of gas and dust where the star was born,
but instead is replenished by dust-producing planetesimals. These planetesimals
are analogous to the asteroids, comets and Kuiper Belt objects (KBOs) in our
Solar system that produce the interplanetary dust that gives rise to the
zodiacal light (tracing the inner component of the Solar system debris disk).
The presence of these "debris disks" around stars with a wide range of masses,
luminosities, and metallicities, with and without binary companions, is
evidence that planetesimal formation is a robust process that can take place
under a wide range of conditions. This chapter is divided in two parts. Part I
discusses how the study of the Solar system debris disk and the study of debris
disks around other stars can help us learn about the formation, evolution and
diversity of planetary systems by shedding light on the frequency and timing of
planetesimal formation, the location and physical properties of the
planetesimals, the presence of long-period planets, and the dynamical and
collisional evolution of the system. Part II reviews the physical processes
that affect dust particles in the gas-free environment of a debris disk and
their effect on the dust particle size and spatial distribution.Comment: 68 pages, 25 figures. To be published in "Solar and Planetary
Systems" (P. Kalas and L. French, Eds.), Volume 3 of the series "Planets,
Stars and Stellar Systems" (T.D. Oswalt, Editor-in-chief), Springer 201
Multimodal imaging findings in a case of severe Central Serous Chorioretinopathy in an uncomplicated pregnancy
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