85 research outputs found
Long-Term Stability of Planets in Binary Systems
A simple question of celestial mechanics is investigated: in what regions of
phase space near a binary system can planets persist for long times? The
planets are taken to be test particles moving in the field of an eccentric
binary system. A range of values of the binary eccentricity and mass ratio is
studied, and both the case of planets orbiting close to one of the stars, and
that of planets outside the binary orbiting the system's center of mass, are
examined. From the results, empirical expressions are developed for both 1) the
largest orbit around each of the stars, and 2) the smallest orbit around the
binary system as a whole, in which test particles survive the length of the
integration (10^4 binary periods). The empirical expressions developed, which
are roughly linear in both the mass ratio mu and the binary eccentricity e, are
determined for the range 0.0 <= e <= 0.7-0.8 and 0.1 <= mu <= 0.9 in both
regions, and can be used to guide searches for planets in binary systems. After
considering the case of a single low-mass planet in binary systems, the
stability of a mutually-interacting system of planets orbiting one star of a
binary system is examined, though in less detail.Comment: 19 pages, 5 figures, 7 tables, accepted by the Astronomical Journa
A Search for Exozodiacal Dust and Faint Companions Near Sirius, Procyon, and Altair with the NICMOS Coronagraph
We observed Sirius, Altair, and Procyon with the NICMOS Coronagraph on the
Hubble Space Telescope to look for scattered light from exozodiacal dust and
faint companions within 10 AU from these stars. We did not achieve enough
dynamic range to surpass the upper limits set by IRAS on the amount of
exo-zodiacal dust in these systems, but we did set strong upper limits on the
presence of nearby late-type and sub-stellar companions.Comment: 10 pages, 4 figure
A Candidate Protoplanet in the Taurus Star Forming Region
HST/NICMOS images of the class I protostar TMR-1 (IRAS04361+2547) reveal a
faint companion with 10.0" = 1400 AU projected separation. The central
protostar is itself resolved as a close binary with 0.31" = 42 AU separation,
surrounded by circumstellar reflection nebulosity. A long narrow filament seems
to connect the protobinary to the faint companion TMR-1C, suggesting a physical
association. If the sources are physically related then we hypothesize that
TMR-1C has been ejected by the protobinary. If TMR-1C has the same age and
distance as the protobinary then current models indicate its flux is consistent
with a young giant planet of several Jovian masses.Comment: 16 pages, 1 figure, Accepted by Astrophysical Journal Letters,
Related information is available at http://www.extrasolar.co
Planetary Dynamics and Habitable Planet Formation In Binary Star Systems
Whether binaries can harbor potentially habitable planets depends on several
factors including the physical properties and the orbital characteristics of
the binary system. While the former determines the location of the habitable
zone (HZ), the latter affects the dynamics of the material from which
terrestrial planets are formed (i.e., planetesimals and planetary embryos), and
drives the final architecture of the planets assembly. In order for a habitable
planet to form in a binary star system, these two factors have to work in
harmony. That is, the orbital dynamics of the two stars and their interactions
with the planet-forming material have to allow terrestrial planet formation in
the habitable zone, and ensure that the orbit of a potentially habitable planet
will be stable for long times. We have organized this chapter with the same
order in mind. We begin by presenting a general discussion on the motion of
planets in binary stars and their stability. We then discuss the stability of
terrestrial planets, and the formation of potentially habitable planets in a
binary-planetary system.Comment: 56 pages, 29 figures, chapter to appear in the book: Planets in
Binary Star Systems (Ed. N. Haghighipour, Springer publishing company
Licensed human natural killer cells aid dendritic cell maturation via TNFSF14/LIGHT
Interactions between natural killer (NK) cells and dendritic cells (DC) aid DC
maturation and promote T cell responses. Here, we have analysed the response of
human NK cells to tumor cells and we identify a pathway by which NK-DC
interactions occur. Gene expression profiling of tumor-responsive NK cells identified
the very rapid induction of TNFSF14 (also known as LIGHT), a cytokine implicated
in the enhancement of anti-tumor responses. TNFSF14 protein expression was
induced by three primary mechanisms of NK cell activation, namely via the
engagement of CD16, by the synergistic activity of multiple target cell-sensing NK
cell activation receptors and by the cytokines IL-2 and IL-15. For anti-tumor
responses, TNFSF14 was preferentially produced by the licensed NK cell population,
defined by the expression of inhibitory receptors specific for self-MHC class I
molecules. In contrast, IL-2 and IL-15 treatment induced TNFSF14 production by
both licensed and unlicensed NK cells, reflecting the ability of pro-inflammatory
conditions to override the licensing mechanism. Importantly, both tumor and cytokine
activated NK cells induced DC maturation in a TNFSF14-dependent manner. The
coupling of TNFSF14 production to tumor-sensing NK cell activation receptors links
the tumor immune surveillance function of NK cells to DC maturation and adaptive
immunity. Furthermore, regulation by NK cell licensing helps to safeguard against
TNFSF14 production in response to healthy tissues
The Time Scale of Escape from Star Clusters
In this paper a cluster is modelled as a smooth potential (due to the cluster
stars) plus the steady tidal field of the Galaxy. In this model there is a
minimum energy below which stars cannot escape. Above this energy, however, the
time scale on which stars escape varies with the orbital parameters of the star
(mainly its energy) in a way which we attempt to quantify, with both
theoretical arguments and computer simulations. Within the limitations of the
model we show that the time scale is long enough to complicate the
interpretation of full N-body simulations of clusters, and that stars above the
escape energy may remain bound to the cluster for about a Hubble time.Comment: 15 pages, 11 figures, plain TeX, submitted to MNRA
Spatio-temporal Models of Lymphangiogenesis in Wound Healing
Several studies suggest that one possible cause of impaired wound healing is
failed or insufficient lymphangiogenesis, that is the formation of new
lymphatic capillaries. Although many mathematical models have been developed to
describe the formation of blood capillaries (angiogenesis), very few have been
proposed for the regeneration of the lymphatic network. Lymphangiogenesis is a
markedly different process from angiogenesis, occurring at different times and
in response to different chemical stimuli. Two main hypotheses have been
proposed: 1) lymphatic capillaries sprout from existing interrupted ones at the
edge of the wound in analogy to the blood angiogenesis case; 2) lymphatic
endothelial cells first pool in the wound region following the lymph flow and
then, once sufficiently populated, start to form a network. Here we present two
PDE models describing lymphangiogenesis according to these two different
hypotheses. Further, we include the effect of advection due to interstitial
flow and lymph flow coming from open capillaries. The variables represent
different cell densities and growth factor concentrations, and where possible
the parameters are estimated from biological data. The models are then solved
numerically and the results are compared with the available biological
literature.Comment: 29 pages, 9 Figures, 6 Tables (39 figure files in total
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