1,492 research outputs found
The Effects of Cyclone Hudah on the Forest of Masoala Peninsula, Madagascar
Cyclones regularly impact the east coast of Madagascar but
almost nothing is known about their effects on Malagasy
ecosystems. On 2 April 2000 the powerful winds of Cyclone
Hudah struck the humid forests in the northern part of Masoala
Peninsula. An analysis of satellite images revealed that 3 % of
the forest here was severely damaged or 22 % when just the
forest of the northeast was considered. In 2001 the vegetation
in this area was studied using fifteen 0.1 ha plots located in
differently impacted sites. The cyclone had caused tree death
(up to 53 % of the trunks within a plot); a reduction in trunk
basal area (to 64 %); uprooting (to 46 %); snapping of trunks (to 51 %); reduction of crown volume (to mean of 83 %); an increase in herbs, liana and woody pioneer species and decrease in saplings of primary forest species; an increase in vegetation in the âshrubâ layer and decrease in vegetation in the âcanopyâ layer. Among a sample of 340 trees killed by the cyclone, 39 % were uprooted, 37 % snapped, and 24 % were still standing. Mortality was relatively low for trees with either small or large trunk diameter compared to trees with intermediate size. In 2003 the plots were re - examined. Six had been impacted by human activities (fire or timber exploitation). A comparison between the vegetation in 2001 and 2003 in 6 plots that had been moderately or seriously impacted by Cyclone Hudah but not impacted by human activity revealed a further decrease in trunk density, an increase in the frequency of herbs and woody pioneers, a decrease in the frequency of liana and saplings of primary forest trees in the âshrub layerâ, and an increase of vegetation in the âshrub layerâ but decrease of vegetation in the âcanopy layerâ. The lack of recovery of this vegetation towards its pre - cyclone state may be explained by the impact of Cyclone Ihary, in 2002. Recovery may yet occur provided the vegetation is not impacted by human activities, particularly burning, that can derail this process. It is recommended that following cyclones conservation managers prioritize fire control
Day - time feeding ecology of Eulemur cinereiceps in the Agnalazaha Forest, Mahabo - Mananivo, Madagascar
The Agnalazaha Forest, a degraded fragment of littoral forest in southeast Madagascar, contains a small population of the endangered Eulemur cinereiceps To better conserve this species its feeding ecology was described by habituating two groups and recording their activities, the food types and species exploited, and the location of food trees by focal animal sampling. The lemursâ environment was also described by measuring forest structure, and monitoring climate and phenology. In total, the groups were observed for 498 hours over 11 months. Monthly time spent feeding averaged 9.6 % of total observation time. The species was highly frugivorous (93 % of total time spent feeding). 55 different plant species were exploited for food. Time spent feeding and diet were not simply related to rainfall and temperature nor to food type availability. The two groupsâ home ranges were 54.9 ha and 58.4 ha and showed a 40 % overlap. The overlap occurred in the swamp forest, which is rich in food plants. To improve the conservation of E. cinereiceps at the Agnalazaha Forest, it is recommended that: The swamp forest be included within the zone of strict conservation; important lemur food plants used for restoration; and alternative sources of timber and fuel wood provided for the local population, thereby allowing greater forest regeneration
The effect of poloidal velocity shear on the local development of current-driven instabilities
We perform a local (short-wavelength) linear stability analysis of an
axisymmetric column of magnetized plasma with a nearly toroidal magnetic field
and a smooth poloidal velocity shear by perturbing the equations of
relativistic magnetohydrodynamics. We identify two types of unstable modes,
which we call 'exponential' and 'overstable', respectively. The exponential
modes are present in the static equilibria and their growth rates decrease with
increasing velocity shear. The overstable modes are driven by the effects of
velocity shear and dominate the exponential modes for sufficiently high shear
values. We argue that these local instabilities can provide an important energy
dissipation mechanism in astrophysical relativistic jets. Strong co-moving
velocity shear arises naturally in the magnetic acceleration mechanism,
therefore it may play a crucial role in converting Poynting-flux-dominated jets
into matter-dominated jets, regulating the global acceleration and collimation
processes, and producing the observed emission of blazars and gamma-ray bursts.Comment: 8 pages, 5 figures, submitted to MNRA
New insights into the evolution of the FR I radio galaxy 3C 270 (NGC 4261) from VLA and GMRT radio observations
We present Giant Metrewave Radio Telescope (GMRT) 240 MHz observations of the
nearby luminous FR I radio source 3C 270, in the group-central elliptical NGC
4261. Combining these data with reprocessed Very Large Array (VLA) 1.55 and 4.8
GHz observations, we produce spectral index maps that reveal a constant
spectral index along the jets and a gradual steepening from the ends of the
jets through the lobes towards the nucleus. A Jaffe & Perola (JP) model fitted
to the integrated spectrum of the source gives an asymptotic low-frequency
index of , while JP models fitted to the
observed spectral index trend along the lobes allow us to estimate radiative
ages of Myr and Myr for the west and east lobes respectively.
Our age estimates are a factor of two lower than the 75-Myr upper limit derived
from X-ray data (O'Sullivan et al. 2011). We find unlikely the scenario of an
early supersonic phase in which the lobe expanded into the ISM at approximately
Mach 6 (3500 km s), and suggest that either the source underwent
multiple AGN outbursts with possible large changes in jet power, or possibly
that the source age that we find is due to a backflow that transports young
electrons from the jet tips through the lobes toward the nucleus relatively
quickly. We calculate that in the lobes the energy ratio of non-radiating to
radiating particles is indicating significant gas entrainment. If
the lobes are in pressure balance with their surroundings, the total energy
required to heat the entrained material is erg, 40% of the
total enthalpy of the lobes.Comment: 14 pages, 11 figures, 8 tables. Accepted for publication by MNRAS.
Revised throughout in response to referee's comment
A Merger Scenario for the Dynamics of Abell 665
We present new redshift measurements for 55 galaxies in the vicinity of the
rich galaxy cluster Abell 665. When combined with results from the literature,
we have good velocity measurements for a sample of 77 confirmed cluster members
from which we derive the cluster's redshift z=0.1829 +/- 0.0005 and
line-of-sight velocity dispersion of 1390 +/- 120 km/s. Our analysis of the
kinematical and spatial data for the subset of galaxies located within the
central 750 kpc reveals only subtle evidence for substructure and
non-Gaussianity in the velocity distribution. We find that the brightest
cluster member is not moving significantly relative to the other galaxies near
the center of the cluster. On the other hand, our deep ROSAT high resolution
image of A665 shows strong evidence for isophotal twisting and centroid
variation, thereby confirming previous suggestions of significant substructure
in the hot X-ray--emitting intracluster gas. In light of this evident
substructure, we have compared the optical velocity data with N-body
simulations of head-on cluster mergers. We find that a merger of two similar
mass subclusters (mass ratios of 1:1 or 1:2) seen close to the time of
core-crossing produces velocity distributions that are consistent with that
observed.Comment: 30 pages and 7 figures. Accepted by the Astrophysical Journal Full
resoultion figures 1 and 3 available in postscript at
http://www.physics.rutgers.edu/~percy/A665paper.htm
Approche expĂ©rimentale de lâutilisation de glyphosate dans le contrĂŽle de Melaleuca quinquenervia (Myrtaceae), une espĂšce envahissante dans la rĂ©serve communautaire de la forĂȘt dâAnalalava-Foulpointe (Madagascar)
Le Niaouli Melaleuca quinquenervia est une des espĂšces envahissantes les plus agressives Ă Madagascar et elle gagne de plus en plus de terrain en colonisant des milieux marĂ©cageux autour de la Nouvelle Aire ProtĂ©gĂ©e de la forĂȘt dâAnalalava Foulpointe. LâinquiĂ©tude sur son envahissement augmente, et mĂȘme si lâĂ©radication paraĂźt dâores et dĂ©jĂ impossible, il convient de trouver dĂšs maintenant une solution efficace pour contrĂŽler son expansion jusquâĂ lâaire protĂ©gĂ©e. Seul le contrĂŽle chimique peut ĂȘtre envisagĂ© pour cette espĂšce qui fait preuve dâune grande rĂ©silience face Ă la coupe et au feu. Cette Ă©tude vise Ă identifier le meilleur protocole dâutilisation de glyphosate dans le contrĂŽle de lâinvasion de M. quinquenervia dans la forĂȘt dâAnalalava Foulpointe. Cinq solutions de glyphosate de concentrations diffĂ©rentes, S1 (0 g /l), S2 (90 g/l), S3 (180 g/l), S4 (270g/l) et S5 (360g/l) ont Ă©tĂ© testĂ©es sur 200 individus, soit 40 individus pour chaque solution. Les solutions de glyphosate sur la surface coupĂ©e du tronc dâun individu ont Ă©tĂ© appliquĂ©es soit au pinceau, soit au pulvĂ©risateur. Des suivis ont Ă©tĂ© faits pendant quatre mois, observant lâĂ©tat des individus traitĂ©s et lâimpact du traitement sur les plantes ligneuses environnantes. La plupart des individus traitĂ©s sont morts, prĂ©sentant plus de 85% de taux de mortalitĂ©. Certaines plantes non ciblĂ©es ont Ă©tĂ© touchĂ©es (19 % des individus recensĂ©s), quel que soit la concentration en ingrĂ©dient actif de glyphosate et lâĂ©quipement utilisĂ©s. De cette expĂ©rience, le meilleur protocole dâutilisation de glyphosate est la concentration de 90 g/l de lâingrĂ©dient actif appliquĂ©e sur la surface coupĂ©e dâun tronc traitĂ© en utilisant un pinceau.Melaleuca quinquenervia, one of the most aggressive invasive species in Madagascar, gains more and more ground and colonizes wetland environments around the New Protected Area of the Analalava forest in Foulpointe. Concerns over its invasion increase and even if eradication seems already impossible, we must now find an effective solution to control its expansion towards the protected area. Only chemical control may be considered for this species, given its resilience to cutting and fire. This study aims at identifying the best protocol for the use of glyphosate in controlling the invasion of M. quinquenervia in the Analalava forest in Foulpointe. Five solutions of different concentrations of glyphosate, S1 (0 g/l), S2 (90 g/l), S3 (180 g/l), S4 (270 g/l), and S5 (360 g/l) were tested on 200 individuals, or 40 individuals for each solution. Glyphosate solutions were applied on the cut surface of the trunk of an individual using either a brush or a sprayer. The controls were done for four months by observing the status on treated individuals and the impacts of the treatment on non - target plants. Most individuals treated are dead, showing an 85 % rate mortality. Some non - target plants were affected (19 % of the surveyed ones), regardless of the concentration of active ingredient glyphosate and of the equipment used. From these experiments, the best protocol for the glyphosate use is a concentration of 90 g/l of the active ingredient applied to the cut surface of the trunk treaty using a brush
The Dynamical State fo the Starless Dense Core FeSt 1-457: A Pulsating Globule?
High resolution molecular line observations of CS, HCO+, C18O and N2H+ were
obtained toward the starless globule FeSt 1-457 in order to investigate its
kinematics and chemistry. The HCO+ and CS spectra show clear self-reversed and
asymmetric profiles across the face of the globule. The sense of the observed
asymmetry is indicative of the global presence of expansion motions in the
outer layers of the globule. These motions appear to be subsonic and
significantly below the escape velocity of the globule. Comparison of our
observations with near-infrared extinction data indicate that the globule is
gravitationally bound. Taken together these considerations lead us to suggest
that the observed expansion has its origin in an oscillatory motion of the
outer layers of the globule which itself is likely in a quasi-stable state near
hydrostatic equilibrium. Analysis of the observed linewidths of CO and N2H+
confirm that thermal pressure is the dominant component of the cloud's internal
support. A simple calculation suggests that the dominant mode of pulsation
would be an l = 2 mode with a period of 0.3 Myr. Deformation of the globule due
to the large amplitude l = 2 oscillation may be responsible for the
double-peaked structure of the core detected in high resolution extinction
maps. Detailed comparison of the molecular-line observations and extinction
data provides evidence for significant depletion of C18O and perhaps HCO+ while
N2H+ may be undepleted to a cloud depth of about 40 magnitudes of visual
extinction.Comment: to appear in ApJ vol 665 20 August 2007
Hydrodynamical simulations of the Sunyaev--Zel'dovich effect
We use a hydrodynamical N-body code to generate simulated maps, of size one
square degree, of the thermal SZ effect. We study three different cosmologies;
the currently-favoured low-density model with a cosmological constant, a
critical-density model and a low-density open model. We stack simulation boxes
corresponding to different redshifts in order to include contributions to the
Compton y-parameter out to the highest necessary redshifts. Our main results
are:
1. The mean y-distortion is around for low-density
cosmologies, and for critical density. These are below
current limits, but not by a wide margin in the former case.
2. In low-density cosmologies, the mean y-distortion comes from a broad range
of redshifts, the bulk coming from and a tail out to . For
critical-density models, most of the contribution comes from .
3. The number of SZ sources above a given depends strongly on instrument
resolution. For a one arcminute beam, there is around 0.1 sources per square
degree with in a critical-density Universe, and around 8 such
sources per square degree in low-density models. Low-density models with and
without a cosmological constant give very similar results.
4. We estimate that the {\sc Planck} satellite will be able to see of order
25000 SZ sources if the Universe has a low density, or around 10000 if it has
critical density.Comment: 9 pages LaTeX file with eleven figures (including four in colour)
incorporated (uses mn.sty and epsf). Further colour images and animations at
http://star-www.cpes.susx.ac.uk/~andrewl/sz/sz.html Updated to match
published versio
Quasar Proper Motions and Low-Frequency Gravitational Waves
We report observational upper limits on the mass-energy of the cosmological
gravitational-wave background, from limits on proper motions of quasars.
Gravitational waves with periods longer than the time span of observations
produce a simple pattern of apparent proper motions over the sky, composed
primarily of second-order transverse vector spherical harmonics. A fit of such
harmonics to measured motions yields a 95%-confidence limit on the mass-energy
of gravitational waves with frequencies <2e-9 Hz, of <0.11/h*h times the
closure density of the universe.Comment: 15 pages, 1 figure. Also available at
http://charm.physics.ucsb.edu:80/people/cgwinn/cgwinn_group/index.htm
The Brightest Cluster Galaxy in Abell 85: The Largest Core Known so far
We have found that the brightest cluster galaxy (BCG) in Abell~85, Holm 15A,
displays the largest core so far known. Its cusp radius, kpc (), is more than 18 times
larger than the mean for BCGs, and kpc larger than A2261-BCG, hitherto
the largest-cored BCG (Postman, Lauer, Donahue, et al. 2012) Holm 15A hosts the
luminous amorphous radio source 0039-095B and has the optical signature of a
LINER. Scaling laws indicate that this core could host a supermassive black
hole (SMBH) of mass . We
suggest that cores this large represent a relatively short phase in the
evolution of BCGs, whereas the masses of their associated SBMH might be set by
initial conditions.Comment: 14 pages, 3 figure, 2 tables, accepted for publication in ApJ Letters
on October 6th, 2014, replacement of previous manuscript submitted on May
30th, 2014 to astro-p
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