717 research outputs found
Carbon Dioxide Fluxes and Plants Interactions across the Succession of a Created Brackish Marsh in Southwestern Louisiana
The purpose of this study was to examine: 1) carbon dioxide (CO2) fluxes in different plant communities in a created and natural brackish marsh and 2) the role of an early colonizing species, Spartina alterniflora, in facilitating or suppressing a later colonizer and higher marsh species, Spartina patens. CO2 fluxes were measured in four key successional stages: 1) non-vegetated sediment of a newly created marsh; 2) Spartina alterniflora-dominated areas of a newly created marsh; 3) S. alterniflora-dominated natural marsh; and 4) Spartina patens- dominated natural marsh. Spartina alterniflora-dominated areas were sinks for CO2 in both created and natural marsh with an average CO2 uptake rate of 7.0 (± 1.0) μmol m-2s-1. Bare sediment in the created marsh and S. patens in the natural marsh had approximately net neutral CO2 fluxes with bare sediment averaging 0.2 (± 1.0) μmol m-2s-1 of CO2 efflux and S. patens areas averaging 0.2 (± 1.0) μmol m-2s-1 of CO2 uptake. CO2 fluxes were found to be mainly determined by plant community type and significantly related to aboveground biomass. Because vegetation develops much faster than soil properties in created marshes, these findings indicate that CO2 fluxes can reach similar levels to that of natural marshes relatively quickly following marsh creation. To determine S. alterniflora interactions with S. patens in developing created marshes, S. patens were transplanted into either bare sediment or within S. alterniflora clones. Spartina patens transplanted into the bare sediment expanded at a rate of 33.0 (± 9.0) cm-2month-1, and had a stem density of 2115 (±180) stems m-2. Spartina patens transplanted within S. alterniflora contracted in area at a rate of -3.0 (± 3.0) cm-2month-1, and had a stem density of 1362.5 (±164) stems m-2. These results indicate that S. alterniflora presence impedes S. patens establishment in a created marsh. The created marsh was found to have a large reserve of available nitrogen in the bare sediment possibly favoring S. alterniflora. Together these studies illustrate that created marsh plant community development is important for CO2 fluxes but plant iterations may differ in a young created marsh from a natural marsh
The Nature of the Dense Core Population in the Pipe Nebula: Thermal Cores Under Pressure
In this paper we present the results of a systematic investigation of an
entire population of starless dust cores within a single molecular cloud.
Analysis of extinction data shows the cores to be dense objects characterized
by a narrow range of density. Analysis of C18O and NH3 molecular-line
observations reveals very narrow lines. The non-thermal velocity dispersions
measured in both these tracers are found to be subsonic for the large majority
of the cores and show no correlation with core mass (or size). Thermal pressure
is thus the dominate source of internal gas pressure and support for most of
the core population. The total internal gas pressures of the cores are found to
be roughly independent of core mass over the entire range of the core mass
function (CMF) indicating that the cores are in pressure equilibrium with an
external source of pressure. This external pressure is most likely provided by
the weight of the surrounding Pipe cloud within which the cores are embedded.
Most of the cores appear to be pressure confined, gravitationally unbound
entities whose nature, structure and future evolution are determined by only a
few physical factors which include self-gravity, the fundamental processes of
thermal physics and the simple requirement of pressure equilibrium with the
surrounding environment. The observed core properties likely constitute the
initial conditions for star formation in dense gas. The entire core population
is found to be characterized by a single critical Bonnor-Ebert mass. This mass
coincides with the characteristic mass of the Pipe CMF indicating that most
cores formed in the cloud are near critical stability. This suggests that the
mass function of cores (and the IMF) has its origin in the physical process of
thermal fragmentation in a pressurized medium.Comment: To appear in the Astrophysical Journa
The Initial Mass Function of the Orion Nebula Cluster across the H-burning limit
We present a new census of the Orion Nebula Cluster (ONC) over a large field
of view (>30'x30'), significantly increasing the known population of stellar
and substellar cluster members with precisely determined properties. We develop
and exploit a technique to determine stellar effective temperatures from
optical colors, nearly doubling the previously available number of objects with
effective temperature determinations in this benchmark cluster. Our technique
utilizes colors from deep photometry in the I-band and in two medium-band
filters at lambda~753 and 770nm, which accurately measure the depth of a
molecular feature present in the spectra of cool stars. From these colors we
can derive effective temperatures with a precision corresponding to better than
one-half spectral subtype, and importantly this precision is independent of the
extinction to the individual stars. Also, because this technique utilizes only
photometry redward of 750nm, the results are only mildly sensitive to optical
veiling produced by accretion. Completing our census with previously available
data, we place some 1750 sources in the Hertzsprung-Russel diagram and assign
masses and ages down to 0.02 solar masses. At faint luminosities, we detect a
large population of background sources which is easily separated in our
photometry from the bona fide cluster members. The resulting initial mass
function of the cluster has good completeness well into the substellar mass
range, and we find that it declines steeply with decreasing mass. This suggests
a deficiency of newly formed brown dwarfs in the cluster compared to the
Galactic disk population.Comment: 16 pages, 18 figures. Accepted for publication in The Astrophysical
Journa
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
The nature of the dense core population in the pipe nebula: core and cloud kinematics from C18O observations
We present molecular-line observations of 94 dark cloud cores identified in
the Pipe nebula through near-IR extinction mapping. Using the Arizona Radio
Observatory 12m telescope, we obtained spectra of these cores in the J=1-0
transition of C18O. We use the measured core parameters, i.e., antenna
temperature, linewidth, radial velocity, radius and mass, to explore the
internal kinematics of these cores as well as their radial motions through the
larger molecular cloud. We find that the vast majority of the dark extinction
cores are true cloud cores rather than the superposition of unrelated
filaments. While we identify no significant correlations between the core's
internal gas motions and the cores' other physical parameters, we identify
spatially correlated radial velocity variations that outline two main kinematic
components of the cloud. The largest is a 15pc long filament that is
surprisingly narrow both in spatial dimensions and in radial velocity.
Beginning in the Stem of the Pipe, this filament displays uniformly small C18O
linewidths (dv~0.4kms-1) as well as core to core motions only slightly in
excess of the gas sound speed. The second component outlines what appears to be
part of a large (2pc; 1000 solar mass) ring-like structure. Cores associated
with this component display both larger linewidths and core to core motions
than in the main cloud. The Pipe Molecular Ring may represent a primordial
structure related to the formation of this cloud.Comment: Accepted to ApJ. 14 pages, 11 figures. Complete table at end of
documen
The nature of the dense core population in the Pipe Nebula: A survey of NH3, CCS, and HC5N molecular line emission
Recent extinction studies of the Pipe Nebula (d=130 pc) reveal many cores
spanning a range in mass from 0.2 to 20.4 Msun. These dense cores were
identified via their high extinction and comprise a starless population in a
very early stage of development. Here we present a survey of NH3 (1,1), NH3
(2,2), CCS (2_1,1_0), and HC5N (9,8) emission toward 46 of these cores. An
atlas of the 2MASS extinction maps is also presented. In total, we detect 63%
of the cores in NH3 (1,1) 22% in NH3 (2,2), 28% in CCS, and 9% in HC5N
emission. We find the cores are associated with dense gas (~10^4 cm-3) with 9.5
< T_k < 17 K. Compared to C18O, we find the NH3 linewidths are systematically
narrower, implying that the NH3 is tracing the dense component of the gas and
that these cores are relatively quiescent. We find no correlation between core
linewidth and size. The derived properties of the Pipe cores are similar to
cores within other low-mass star-forming regions: the only differences are that
the Pipe cores have weaker NH3 emision and most show no current star formation
as evidenced by the lack of embedded infrared sources. Such weak NH3 emission
could arise due to low column densities and abundances or reduced excitation
due to relatively low core volume densities. Either alternative implies that
the cores are relatively young. Thus, the Pipe cores represent an excellent
sample of dense cores in which to study the initial conditions for star
formation and the earliest stages of core formation and evolution.Comment: 35 pages, 10 figures (excluding the appendix). For the complete
appendix contact [email protected]. Accepted for publication in ApJ
The peroxisomal multifunctional protein interacts with cortical microtubules in plant cells
BACKGROUND: The plant peroxisomal multifunctional protein (MFP) possesses up to four enzymatic activities that are involved in catalyzing different reactions of fatty acid β-oxidation in the peroxisome matrix. In addition to these peroxisomal activities, in vitro assays revealed that rice MFP possesses microtubule- and RNA-binding activities suggesting that this protein also has important functions in the cytosol. RESULTS: We demonstrate that MFP is an authentic microtubule-binding protein, as it localized to the cortical microtubule array in vivo, in addition to its expected targeting to the peroxisome matrix. MFP does not, however, interact with the three mitotic microtubule arrays. Microtubule co-sedimentation assays of truncated versions of MFP revealed that multiple microtubule-binding domains are present on the MFP polypeptide. This indicates that these regions function together to achieve high-affinity binding of the full-length protein. Real-time imaging of a transiently expressed green fluorescent protein-MFP chimera in living plant cells illustrated that a dynamic, spatial interaction exits between peroxisomes and cortical microtubules as peroxisomes move along actin filaments or oscillate at fixed locations. CONCLUSION: Plant MFP is associated with the cortical microtubule array, in addition to its expected localization in the peroxisome. This observation, coupled with apparent interactions that frequently occur between microtubules and peroxisomes in the cell cortex, supports the hypothesis that MFP is concentrated on microtubules in order to facilitate the regulated import of MFP into peroxisomes
Chemical Differentiation toward the Pipe Nebula Starless Cores
We used the new IRAM 30-m FTS backend to perform an unbiased ~15 GHz wide
survey at 3 mm toward the Pipe Nebula young diffuse starless cores. We found an
unexpectedly rich chemistry. We propose a new observational classification
based on the 3 mm molecular line emission normalized by the core visual
extinction (Av). Based on this classification, we report a clear
differentiation in terms of chemical composition and of line emission
properties, which served to define three molecular core groups. The "diffuse"
cores, Av<~15, show poor chemistry with mainly simple species (e.g. CS and
CCH). The "oxo-sulfurated" cores, Av~15--22, appear to be abundant in species
like SO and SO2, but also in HCO, which seem to disappear at higher densities.
Finally, the "deuterated" cores, Av>~22, show typical evolved chemistry prior
to the onset of the star formation process, with nitrogenated and deuterated
species, as well as carbon chain molecules. Based on these categories, one of
the "diffuse" cores (Core 47) has the spectral line properties of the
"oxo-sulfurated" ones, which suggests that it is a possible failed core.Comment: Accepted for publication in A&A. 5 pages, 2 figure
Identifying Primordial Substructure in NGC 2264
We present new Spitzer Space Telescope observations of the young cluster
NGC2264. Observations at 24 micron with the Multiband Imaging Photometer has
enabled us to identify the most highly embedded and youngest objects in
NGC2264. This letter reports on one particular region of NGC2264 where bright
24 micron sources are spatially configured in curious linear structures with
quasi-uniform separations. The majority of these sources (~60% are found to be
protostellar in nature with Class I spectral energy distributions. Comparison
of their spatial distribution with sub-millimeter data from Wolf-Chase (2003)
and millimeter data from Peretto et al. (2005) shows a close correlation
between the dust filaments and the linear spatial configurations of the
protostars, indicating that star formation is occurring primarily within dense
dusty filaments. Finally, the quasi-uniform separations of the protostars are
found to be comparable in magnitude to the expected Jeans length suggesting
thermal fragmentation of the dense filamentary material.Comment: Accepted for publication in ApJL, 5 pages, 4 figures. Color version
available from the following webpages: http://cfa-www.harvard.edu/~pteixeir/
and http://cfa-www.harvard.edu/~clada
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