4,640 research outputs found
Stellar Velocity Dispersion of the Leo A Dwarf Galaxy
We measure the first stellar velocity dispersion of the Leo A dwarf galaxy,
\sigma = 9.3 +- 1.3 km/s. We derive the velocity dispersion from the radial
velocities of ten young B supergiants and two HII regions in the central region
of Leo A. We estimate a projected mass of 8 +- 2.7 x10^7 solar masses within a
radius of 2 arcmin, and a mass to light ratio of at least 20 +- 6 M_sun/L_sun.
These results imply Leo A is at least ~80% dark matter by mass.Comment: 6 pages, accepted to Ap
The TAOS Project: Upper Bounds on the Population of Small KBOs and Tests of Models of Formation and Evolution of the Outer Solar System
We have analyzed the first 3.75 years of data from TAOS, the Taiwanese
American Occultation Survey. TAOS monitors bright stars to search for
occultations by Kuiper Belt Objects (KBOs). This dataset comprises 5e5
star-hours of multi-telescope photometric data taken at 4 or 5 Hz. No events
consistent with KBO occultations were found in this dataset. We compute the
number of events expected for the Kuiper Belt formation and evolution models of
Pan & Sari (2005), Kenyon & Bromley (2004), Benavidez & Campo Bagatin (2009),
and Fraser (2009). A comparison with the upper limits we derive from our data
constrains the parameter space of these models. This is the first detailed
comparison of models of the KBO size distribution with data from an occultation
survey. Our results suggest that the KBO population is comprised of objects
with low internal strength and that planetary migration played a role in the
shaping of the size distribution.Comment: 18 pages, 16 figures, Aj submitte
Circumbinary Chaos: Using Pluto's Newest Moon to Constrain the Masses of Nix & Hydra
The Pluto system provides a unique local laboratory for the study of binaries
with multiple low mass companions. In this paper, we study the orbital
stability of P4, the most recently discovered moon in the Pluto system. This
newfound companion orbits near the plane of the Pluto-Charon binary, roughly
halfway between the two minor moons Nix and Hydra. We use a suite of few body
integrations to constrain the masses of Nix and Hydra, and the orbital
parameters of P4. For the system to remain stable over the age of the Solar
System, the masses of Nix and Hydra likely do not exceed 5e16 kg and 9e16 kg,
respectively. These upper limits assume a fixed mass ratio between Nix and
Hydra at the value implied by their median optical brightness. Our study finds
that stability is more sensitive to their total mass and that a downward
revision of Charon's eccentricity (from our adopted value of 0.0035) is
unlikely to significantly affect our conclusions. Our upper limits are an order
of magnitude below existing astrometric limits on the masses of Nix and Hydra.
For a density at least that of ice, the albedos of Nix and Hydra would exceed
0.3. This constraint implies they are icy, as predicted by giant impact models.
Even with these low masses, P4 only remains stable if its eccentricity e <
0.02. The 5:1 commensurability with Charon is particularly unstable, Combining
stability constraints with the observed mean motion places the preferred orbit
for P4 just exterior to the 5:1 resonance. These predictions will be tested
when the New Horizons satellite visits Pluto. Based on the results for the
Pluto-Charon system, we expect that circumbinary, multi-planet systems will be
more widely spaced than their singleton counterparts. Further, circumbinary
exoplanets close to the three-body stability boundary, such as those found by
Kepler, are less likely to have other companions nearby.Comment: 12 pages, 7 figure
Collisional Cascades in Planetesimal Disks II. Embedded Planets
We use a multiannulus planetesimal accretion code to investigate the growth
of icy planets in the outer regions of a planetesimal disk. In a quiescent
minimum mass solar nebula, icy planets grow to sizes of 1000--3000 km on a
timescale t = 15-20 Myr (a/30 AU)^3 where a is the distance from the central
star. Planets form faster in more massive nebulae. Newly-formed planets stir up
leftover planetesimals along their orbits and produce a collisional cascade
where icy planetesimals are slowly ground to dust.
The dusty debris of planet formation has physical characteristics similar to
those observed in beta Pic, HR 4796A, and other debris disks. We derive dust
masses for small particles, 1 mm and smaller, and large particles, 1 mm and
larger, as a function of the initial conditions in the planetesimal disk. The
dust luminosities derived from these masses are similar to those observed in
Vega, HR 4796A, and other debris disks. The calculations produce bright rings
and dark gaps. Bright rings occur where 1000 km and larger planets have
recently formed. Dark gaps are regions where planets have cleared out dust or
shadows where planets have yet to form.Comment: to be published in the Astronomical Journal, January 2004; 7 pages of
text; 17 figures at
http://cfa-www.harvard.edu/~kenyon/pf/emb-planet-figures.pdf; 2 animations at
http://cfa-www.harvard.edu/~kenyon/pf/emb-planet-movies.htm
Resident Corneal Cells Communicate with Neutrophils Leading to the Production of IP-10 during the Primary Inflammatory Response to HSV-1 Infection
In this study we show that murine and human neutrophils are capable of secreting IP-10 in response to communication from the HSV-1 infected cornea and that they do so in a time frame associated with the recruitment of CD8+ T cells and CXCR3-expressing cells. Cellular markers were used to establish that neutrophil influx corresponded in time to peak IP-10 production, and cellular depletion confirmed neutrophils to be a significant source of IP-10 during HSV-1 corneal infection in mice. A novel ex vivo model for human corneal tissue infection with HSV-1 was used to confirm that cells resident in the cornea are also capable of stimulating neutrophils to secrete IP-10. Our results support the hypothesis that neutrophils play a key role in T-cell recruitment and control of viral replication during HSV-1 corneal infection through the production of the T-cell recruiting chemokine IP-10
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Design and Benchmark Testing for Open Architecture Reconfigurable Mobile Spirometer and Exhaled Breath Monitor with GPS and Data Telemetry.
Portable and wearable medical instruments are poised to play an increasingly important role in health monitoring. Mobile spirometers are available commercially, and are used to monitor patients with advanced lung disease. However, these commercial monitors have a fixed product architecture determined by the manufacturer, and researchers cannot easily experiment with new configurations or add additional novel sensors over time. Spirometry combined with exhaled breath metabolite monitoring has the potential to transform healthcare and improve clinical management strategies. This research provides an updated design and benchmark testing for a flexible, portable, open access architecture to measure lung function, using common Arduino/Android microcontroller technologies. To demonstrate the feasibility and the proof-of-concept of this easily-adaptable platform technology, we had 43 subjects (healthy, and those with lung diseases) perform three spirometry maneuvers using our reconfigurable device and an office-based commercial spirometer. We found that our system compared favorably with the traditional spirometer, with high accuracy and agreement for forced expiratory volume in 1 s (FEV1) and forced vital capacity (FVC), and gas measurements were feasible. This provides an adaptable/reconfigurable open access "personalized medicine" platform for researchers and patients, and new chemical sensors and other modular instrumentation can extend the flexibility of the device in the future
Vacancy localization in the square dimer model
We study the classical dimer model on a square lattice with a single vacancy
by developing a graph-theoretic classification of the set of all configurations
which extends the spanning tree formulation of close-packed dimers. With this
formalism, we can address the question of the possible motion of the vacancy
induced by dimer slidings. We find a probability 57/4-10Sqrt[2] for the vacancy
to be strictly jammed in an infinite system. More generally, the size
distribution of the domain accessible to the vacancy is characterized by a
power law decay with exponent 9/8. On a finite system, the probability that a
vacancy in the bulk can reach the boundary falls off as a power law of the
system size with exponent 1/4. The resultant weak localization of vacancies
still allows for unbounded diffusion, characterized by a diffusion exponent
that we relate to that of diffusion on spanning trees. We also implement
numerical simulations of the model with both free and periodic boundary
conditions.Comment: 35 pages, 24 figures. Improved version with one added figure (figure
9), a shift s->s+1 in the definition of the tree size, and minor correction
Accretion in the Early Kuiper Belt II. Fragmentation
We describe new planetesimal accretion calculations in the Kuiper Belt that
include fragmentation and velocity evolution. All models produce two power law
cumulative size distributions, N_C propto r^{-q}, with q = 2.5 for radii less
than 0.3-3 km and q = 3 for radii exceeding 1-3 km. The power law indices are
nearly independent of the initial mass in the annulus, the initial eccentricity
of the planetesimal swarm, and the initial size distribution of the
planetesimal swarm. The transition between the two power laws moves to larger
radii as the initial eccentricity increases. The maximum size of objects
depends on their intrinsic tensile strength; Pluto formation requires a
strength exceeding 300 erg per gram. Our models yield formation timescales for
Pluto-sized objects of 30-40 Myr for a minimum mass solar nebula. The
production of several `Plutos' and more than 10^5 50 km radius Kuiper Belt
objects leaves most of the initial mass in 0.1-10 km radius objects that can be
collisionally depleted over the age of the solar system. These results resolve
the puzzle of large Kuiper Belt objects in a small mass Kuiper Belt.Comment: to appear in the Astronomical Journal (July 1999); 54 pages including
7 tables and 13 figure
The Rise and Fall of Debris Disks: MIPS Observations of h and chi Persei and the Evolution of Mid-IR Emission from Planet Formation
We describe Spitzer/MIPS observations of the double cluster, h and
Persei, covering a 0.6 square-degree area surrounding the cores of both
clusters. The data are combined with IRAC and 2MASS data to investigate
616 sources from 1.25-24 . We use the long-baseline -[24] color
to identify two populations with IR excess indicative of circumstellar
material: Be stars with 24 excess from optically-thin free free
emission and 17 fainter sources (J 14-15) with [24] excess consistent
with a circumstellar disk. The frequency of IR excess for the fainter sources
increases from 4.5 through 24 . The IR excess is likely due to
debris from the planet formation process. The wavelength-dependent behavior is
consistent with an inside-out clearing of circumstellar disks. A comparison of
the 24 excess population in h and Per sources with results for
other clusters shows that 24 emission from debris disks 'rises' from 5
to 10 Myr, peaks at 10-15 Myr, and then 'falls' from 15/20 Myr to
1 Gyr.Comment: 48 pages, 15 figures, accepted for publication in The Astrophysical
Journa
First Results From The Taiwanese-American Occultation Survey (TAOS)
Results from the first two years of data from the Taiwanese-American
Occultation Survey (TAOS) are presented. Stars have been monitored
photometrically at 4 Hz or 5 Hz to search for occultations by small (~3 km)
Kuiper Belt Objects (KBOs). No statistically significant events were found,
allowing us to present an upper bound to the size distribution of KBOs with
diameters 0.5 km < D < 28 km.Comment: 5 pages, 5 figure, accepted in Ap
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