3,830 research outputs found
Search for L5 Earth Trojans with DECam
Most of the major planets in the Solar system support populations of co-orbiting bodies, known as Trojans, at their L4 and L5 Lagrange points. In contrast, Earth has only one known co-orbiting companion. This paper presents the results from a search for Earth Trojans (ETs) using the DECam instrument on the Blanco Telescope at CTIO. This search found no additional Trojans in spite of greater coverage compared to previous surveys of the L5 point. Therefore, the main result of this work is to place the most stringent constraints to date on the population of ETs. These constraints depend on assumptions regarding the underlying population properties, especially the slope of the magnitude distribution (which in turn depends on the size and albedo distributions of the objects). For standard assumptions, we calculate upper limits to a 90 perâcent confidence limit on the L5 population of N_(ET) < 1 for magnitude H < 15.5, N_(ET) = 60â85 for H < 19.7, and N_(ET) = 97 for H = 20.4. This latter magnitude limit corresponds to Trojans âź300 m in size for albedo 0.15. At H = 19.7, these upper limits are consistent with previous L4 ET constraints and significantly improve L5 constraints
Linear Sigma Models with Torsion
Gauged linear sigma models with (0,2) supersymmetry allow a larger choice of
couplings than models with (2,2) supersymmetry. We use this freedom to find a
fully linear construction of torsional heterotic compactifications, including
models with branes. As a non-compact example, we describe a family of metrics
which correspond to deformations of the heterotic conifold by turning on
H-flux. We then describe compact models which are gauge-invariant only at the
quantum level. Our construction gives a generalization of symplectic reduction.
The resulting spaces are non-Kahler analogues of familiar toric spaces like
complex projective space. Perturbatively conformal models can be constructed by
considering intersections.Comment: 40 pages, LaTeX, 1 figure; references added; a new section on
supersymmetry added; quantization condition revisite
Heterotic Sigma Models with N=2 Space-Time Supersymmetry
We study the non-linear sigma model realization of a heterotic vacuum with
N=2 space-time supersymmetry. We examine the requirements of (0,2) + (0,4)
world-sheet supersymmetry and show that a geometric vacuum must be described by
a principal two-torus bundle over a K3 manifold.Comment: 20 pages, uses xy-pic; v3: typos corrected, reference added,
discussion of constraints on Hermitian form modifie
Search for L5 Earth Trojans with DECam
Most of the major planets in the Solar system support populations of co-orbiting bodies, known as Trojans, at their L4 and L5 Lagrange points. In contrast, Earth has only one known co-orbiting companion. This paper presents the results from a search for Earth Trojans (ETs) using the DECam instrument on the Blanco Telescope at CTIO. This search found no additional Trojans in spite of greater coverage compared to previous surveys of the L5 point. Therefore, the main result of this work is to place the most stringent constraints to date on the population of ETs. These constraints depend on assumptions regarding the underlying population properties, especially the slope of the magnitude distribution (which in turn depends on the size and albedo distributions of the objects). For standard assumptions, we calculate upper limits to a 90 perâcent confidence limit on the L5 population of N_(ET) < 1 for magnitude H < 15.5, N_(ET) = 60â85 for H < 19.7, and N_(ET) = 97 for H = 20.4. This latter magnitude limit corresponds to Trojans âź300 m in size for albedo 0.15. At H = 19.7, these upper limits are consistent with previous L4 ET constraints and significantly improve L5 constraints
Supersymmetric (non-)Abelian Bundles in the Type I and SO(32) Heterotic String
We discuss perturbative four-dimensional compactifications of both the SO(32)
heterotic and the Type I string on smooth Calabi-Yau manifolds endowed with
general non-abelian and abelian bundles. We analyse the generalized
Green-Schwarz mechanism for multiple anomalous U(1) factors and derive the
generically non-universal one-loop threshold corrections to the gauge kinetic
function as well as the one-loop corrected Fayet-Iliopoulos terms. The latter
can be interpreted as a stringy one-loop correction to the
Donaldson-Uhlenbeck-Yau condition. Applying S-duality, for the Type I string we
obtain the perturbative Pi-stability condition for non-abelian bundles on
curved spaces. Some simple examples are given, and we qualitatively discuss
some generic phenomenological aspects of this kind of string vacua. In
particular, we point out that in principle an intermediate string scale
scenario with TeV scale large extra dimensions might be possible for the
heterotic string.Comment: LaTeX, 32 pages, v2: refs adde
GLSMs for non-Kahler Geometries
We identify a simple mechanism by which H-flux satisfying the modified
Bianchi identity arises in garden-variety (0,2) gauged linear sigma models.
Taking suitable limits leads to effective gauged linear sigma models with
Green-Schwarz anomaly cancellation. We test the quantum-consistency of a class
of such effective theories by constructing an off-shell superconformal algebra,
providing evidence that these models run to good CFTs in the deep IR.Comment: 37 pages, Minor updates for v
Population analysis of open clusters: radii and mass segregation
Aims: Based on our well-determined sample of open clusters in the all-sky
catalogue ASCC-2.5 we derive new linear sizes of some 600 clusters, and
investigate the effect of mass segregation of stars in open clusters. Methods:
Using statistical methods, we study the distribution of linear sizes as a
function of spatial position and cluster age. We also examine statistically the
distribution of stars of different masses within clusters as a function of the
cluster age. Results: No significant dependence of the cluster size on location
in the Galaxy is detected for younger clusters (< 200 Myr), whereas older
clusters inside the solar orbit turned out to be, on average, smaller than
outside. Also, small old clusters are preferentially found close to the
Galactic plane, whereas larger ones more frequently live farther away from the
plane and at larger Galactocentric distances. For clusters with (V - M_V) <
10.5, a clear dependence of the apparent radius on age has been detected: the
cluster radii decrease by a factor of about 2 from an age of 10 Myr to an age
of 1 Gyr. A detailed analysis shows that this observed effect can be explained
by mass segregation and does not necessarily reflect a real decrease of cluster
radii. We found evidence for the latter for the majority of clusters older than
30 Myr. Among the youngest clusters (between 5 and 30 Myr), there are some
clusters with a significant grade of mass segregation, whereas some others show
no segregation at all. At a cluster age between 50 and 100 Myrs, the
distribution of stars of different masses becomes more regular over cluster
area. In older clusters the evolution of the massive stars is the most
prominent effect we observe.Comment: 14 pages, 12 figures, accepted for publication in Astronomy &
Astrophysic
Crystal structure of monomeric human β-2- microglobulin reveals clues to its amyloidogenic properties
Dissociation of human β-2-microglobulin (β(2)m) from the heavy chain of the class I HLA complex is a critical first step in the formation of amyloid fibrils from this protein. As a consequence of renal failure, the concentration of circulating monomeric β(2)m increases, ultimately leading to deposition of the protein into
amyloid fibrils and development of the disorder, dialysis-related amyloidosis. Here we present the crystal structure of a monomeric form of human β(2)m determined at 1.8-Ă
resolution that reveals remarkable structural changes relative to the HLA-bound protein. These involve the restructuring of a β bulge that separates two
short β strands to form a new six-residue β strand at one edge of this β sandwich protein. These structural changes remove key features proposed to have evolved to protect β sheet proteins from aggregation [Richardson, J.&Richardson, D. (2002) Proc. Natl. Acad.
Sci. USA 99, 2754â2759] and replaces them with an aggregationcompetent surface. In combination with solution studies using (1)H NMR, we show that the crystal structure presented here represents a rare species in solution that could provide important clues about the mechanism of amyloid formation from the normally highly
soluble native protein
The Age of the Oldest Stars in the Local Galactic Disk From Hipparcos Parallaxes of G and K Subgiants
We review the history of the discovery of field subgiant stars and their
importance in the age dating of the Galactic disk. We use the cataloged data
from the Hipparcos satellite in this latter capacity. Based on Hipparcos
parallaxes accurate to 10% or better, the absolute magnitude of the lower
envelope of the nearly horizontal subgiant sequence for field stars in the H-R
diagram for B-V colors between 0.85 and 1.05 is measured to be M_V = 4.03 +/-
0.06. The age of the field stars in the solar neighborhood is found to be 7.9
+/- 0.7 Gyr by fitting the theoretical isochrones for [Fe/H] = +0.37 to the
lower envelope of the Hipparcos subgiants. The same grid of isochrones yields
ages, in turn, of 4.0 +/- 0.2 Gyr, 6.2 +/- 0.5 Gyr, and 7.5 to 10 Gyr for the
old Galactic clusters M67, NGC188, and NGC6791. The ages of both the Galactic
disk in the solar neighborhood and of NGC6791 are, nevertheless, likely between
3 and 5 Gyr younger than the oldest halo globular clusters, which have ages of
13.5 Gyr. The most significant results are (1) the supermetallicity of the
oldest local disk stars, and (2) the large age difference between the most
metal-poor component of the halo and the thick and thin disk in the solar
neighborhood. These facts are undoubtedly related and pose again the problem of
the proper scenario for the timing of events in the formation of the halo and
the Galactic disk in the solar neighborhood. [Abstract Abridged]Comment: 44 pages, 12 Figures; accepted for publication in PASP; high
resolution versions of Figures 1, 2, 6 and 9 available at
http://bubba.ucdavis.edu/~lubin/Sandage
Novel Branches of (0,2) Theories
We show that recently proposed linear sigma models with torsion can be
obtained from unconventional branches of conventional gauge theories. This
observation puts models with log interactions on firm footing. If non-anomalous
multiplets are integrated out, the resulting low-energy theory involves log
interactions of neutral fields. For these cases, we find a sigma model geometry
which is both non-toric and includes brane sources. These are heterotic sigma
models with branes. Surprisingly, there are massive models with compact complex
non-Kahler target spaces, which include brane/anti-brane sources. The simplest
conformal models describe wrapped heterotic NS5-branes. We present examples of
both types.Comment: 36 pages, LaTeX, 2 figures; typo in Appendix fixed; references added
and additional minor change
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