3,348 research outputs found
Lowering of surface melting temperature in atomic clusters with a nearly closed shell structure
We investigate the interplay of particle number, N, and structural properties
of selected clusters with N=12 up to N=562 by employing Gupta potentials
parameterized for Aluminum and extensive Monte-Carlo simulations. Our analysis
focuses on closed shell structures with extra atoms. The latter can put the
cluster under a significant stress and we argue that typically such a strained
system exhibits a reduced energy barrier for (surface) diffusion of cluster
atoms. Consequently, also its surface melting temperature, T_S, is reduced, so
that T_S separates from and actually falls well below the bulk value. The
proposed mechanism may be responsible for the suppression of the surface
melting temperature observed in a recent experiments.Comment: 9 pages, 7 figures, 1 table, REVTeX 4; submitted to Phys.Rev.
Chronic exposure to fluoxetine (Prozac) causes developmental delays in Rana pipiens larvae
Selective serotonin reuptake inhibitors (SSRIs), such as fluoxetine, are among the many pharmaceuticals detected in aquatic ecosystems. Although the acute effects of SSRIs on select organisms have been reported, little is understood about the chronic effects of these drugs on amphibians, which are particularly sensitive to environmental pollutants. Serotonin plays important roles in many physiological functions, including a wide array of developmental processes. Exposure to SSRIs during development may cause developmental complications in a variety of organisms, but little is known about the degree of exposure necessary to cause deleterious effects. Here, we sought to gain a better understanding of the effects of SSRIs on amphibian development by use of a combined laboratory and outdoor mesocosm study. Tadpoles in a laboratory setting were exposed to a low (0.029â”g/L) and a high (0.29â”g/L) concentration of the common SSRI fluoxetine from stages 21 and 22 through completion of metamorphosis. Tadpoles in outdoor mesocosms were exposed to fluoxetine concentrations ranging from 0.1 to 0.3â”g/L. Exposed tadpoles in the laboratory showed delayed development compared with controls when stage was assessed throughout the experiment. Control tadpoles also gained weight faster than treatment tadpoles, which may be explained by reduced food intake. Mesocosm tadpoles exhibited similar trends, but no significant differences were detected. These results indicate that ecologically relevant levels of fluoxetine may cause developmental delays in amphibians. Environ. Toxicol. Chem. 2010;29:2845â2850. © 2010 SETACPeer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/78304/1/345_ftp.pd
Extreme AGN Feedback and Cool Core Destruction in the X-ray Luminous Galaxy Cluster MACS J1931.8-2634
We report on a deep, multiwavelength study of the galaxy cluster MACS
J1931.8-2634 using Chandra X-ray, Subaru optical, and VLA 1.4 GHz radio data.
This cluster (z=0.352) harbors one of the most X-ray luminous cool cores yet
discovered, with an equivalent mass cooling rate within the central 50 kpc is
approximately 700 solar masses/yr. Unique features observed in the central core
of MACSJ1931.8-2634 hint to a wealth of past activity that has greatly
disrupted the original cool core. We observe a spiral of relatively cool,
dense, X-ray emitting gas connected to the cool core, as well as highly
elongated intracluster light (ICL) surrounding the cD galaxy. Extended radio
emission is observed surrounding the central AGN, elongated in the east-west
direction, spatially coincident with X-ray cavities. The power input required
to inflate these `bubbles' is estimated from both the X-ray and radio emission
to reside between 4 and 14e45 erg/s, putting it among the most powerful jets
ever observed. This combination of a powerful AGN outburst and bulk motion of
the cool core have resulted in two X-ray bright ridges to form to the north and
south of the central AGN at a distance of approximately 25 kpc. The northern
ridge has spectral characteristics typical of cool cores and is consistent with
being a remnant of the cool core after it was disrupted by the AGN and bulk
motions. It is also the site of H-alpha filaments and young stars. The X-ray
spectroscopic cooling rate associated with this ridge is approximately 165
solar masses/yr, which agrees with the estimate of the star formation rate from
broad-band optical imaging (170 solar masses/yr). MACS J1931.8-2634 appears to
harbor one of most profoundly disrupted low entropy cores observed in a
cluster, and offers new insights into the survivability of cool cores in the
context of hierarchical structure formation.Comment: 19 pages, 15 figures, 5 tables. Accepted by MNRAS for publication
September 30 201
Probing the extreme realm of AGN feedback in the massive galaxy cluster, RX J1532.9+3021
We present a detailed Chandra, XMM-Newton, VLA and HST analysis of one of the
strongest cool core clusters known, RX J1532.9+3021 (z=0.3613). Using new, deep
90 ks Chandra observations, we confirm the presence of a western X-ray cavity
or bubble, and report on a newly discovered eastern X-ray cavity. The total
mechanical power associated with these AGN-driven outflows is (22+/-9)*10^44
erg/s, and is sufficient to offset the cooling, indicating that AGN feedback
still provides a viable solution to the cooling flow problem even in the
strongest cool core clusters. Based on the distribution of the optical
filaments, as well as a jet-like structure seen in the 325 MHz VLA radio map,
we suggest that the cluster harbours older outflows along the north to south
direction. The jet of the central AGN is therefore either precessing or
sloshing-induced motions have caused the outflows to change directions. There
are also hints of an X-ray depression to the north aligned with the 325 MHz
jet-like structure, which might represent the highest redshift ghost cavity
discovered to date. We further find evidence of a cold front (r=65kpc) that
coincides with the outermost edge of the western X-ray cavity and the edge of
the radio mini-halo. The common location of the cold front with the edge of the
radio mini-halo supports the idea that the latter originates from electrons
being reaccelerated due to sloshing induced turbulence. Alternatively, its
coexistence with the edge of the X-ray cavity may be due to cool gas being
dragged out by the outburst. We confirm that the central AGN is highly
sub-Eddington and conclude that a >10^10M_Sun or a rapidly spinning black hole
is favoured to explain both the radiative-inefficiency of the AGN and the
powerful X-ray cavities.Comment: Accepted for publication to ApJ (minor corrections), 16 pages, 16
figures, 5 tables. Full resolution at http://www.stanford.edu/~juliehl/M1532
Ram pressure stripping of the cool core of the Ophiuchus Cluster
(abridged) We report results from a Chandra study of the central regions of
the nearby, X-ray bright, Ophiuchus Cluster (z = 0.03), the second-brightest
cluster in the sky. Our study reveals a dramatic, close-up view of the
stripping and potential destruction of a cool core within a rich cluster. The
X-ray emission from the Ophiuchus Cluster core exhibits a comet-like morphology
extending to the north, driven by merging activity, indicative of ram-pressure
stripping caused by rapid motion through the ambient cluster gas. A cold front
at the southern edge implies a velocity of 1000200 km/s (M~0.6). The X-ray
emission from the cluster core is sharply peaked. As previously noted, the peak
is offset by 4 arcsec (~2 kpc) from the optical center of the associated cD
galaxy, indicating that ram pressure has slowed the core, allowing the
relatively collisionless stars and dark matter to carry on ahead. The cluster
exhibits the strongest central temperature gradient of any massive cluster
observed to date: the temperature rises from 0.7 keV within 1 kpc of the
brightness peak, to 10 keV by 30 kpc. A strong metallicity gradient is also
observed within the same region. This supports a picture in which the outer
parts of the cool core have been stripped by ram-pressure due to its rapid
motion. The cooling time of the innermost gas is very short, ~5
yrs. Within the central 10 kpc radius, multiple small-scale fronts and a
complex thermodynamic structure are observed, indicating significant motions.
Beyond the central 50 kpc, and out to a radius ~150 kpc, the cluster appears
relatively isothermal and has near constant metallicity. The exception is a
large, coherent ridge of enhanced metallicity observed to trail the cool core,
and which is likely to have been stripped from it.Comment: Accepted to MNRAS. 11 pages, 9 figure
A Spitzer IRAC Imaging Survey for T Dwarf Companions Around M, L, and T Dwarfs: Observations, Results, and Monte Carlo Population Analyses
We report observational techniques, results, and Monte Carlo population
analyses from a Spitzer Infrared Array Camera imaging survey for substellar
companions to 117 nearby M, L, and T dwarf systems (median distance of 10 pc,
mass range of 0.6 to \sim0.05 M\odot). The two-epoch survey achieves typical
detection sensitivities to substellar companions of [4.5 {\mu}m] \leq 17.2 mag
for angular separations between about 7" and 165". Based on common proper
motion analysis, we find no evidence for new substellar companions. Using Monte
Carlo orbital simulations (assuming random inclination, random eccentricity,
and random longitude of pericenter), we conclude that the observational
sensitivities translate to an ability to detect 600-1100K brown dwarf
companions at semimajor axes greater than ~35 AU, and to detect 500-600K
companions at semimajor axes greater than ~60 AU. The simulations also estimate
a 600-1100K T dwarf companion fraction of < 3.4% for 35-1200 AU separations,
and < 12.4% for the 500-600K companions, for 60-1000 AU separations.Comment: 35 pages, 6 figure
ALMA observation of the disruption of molecular gas in M87
We present the results from Atacama Large Millimeter Array (ALMA) observations centred 40 arcsec (3 kpc in projection) south-east of the nucleus of M87. We report the detection of extended CO (2-1) line emission with a total flux of (5.5 ± 0.6) Ă 10-18 erg s-1 cm-2 and corresponding molecular gas mass M_{H_2}=(4.7 ± 0.4) Ă 10^5 M_{â}, assuming a Galactic CO to H2 conversion factor. ALMA data indicate a line-of-sight velocity of -129 ± 3 km s-1, in good agreement with measurements based on the [C II] and H α+[N II] lines, and a velocity dispersion of Ï = 27 ± 3 km s-1. The CO (2-1) emission originates only outside the radio lobe of the active galactic nucleus (AGN) seen in the 6 cm Very Large Array image, while the filament prolongs further inwards at other wavelengths. The molecular gas in M87 appears to be destroyed or excited by AGN activity, either by direct interaction with the radio plasma, or by the shock driven by the lobe into the X-ray emitting atmosphere. This is an important piece of the puzzle in understanding the impact of the central AGN on the amount of the coldest gas from which star formation can proceed
Feedback under the microscope II: heating, gas uplift, and mixing in the nearest cluster core
Using a combination of deep 574ks Chandra data, XMM-Newton high-resolution
spectra, and optical Halpha+NII images, we study the nature and spatial
distribution of the multiphase plasma in M87. Our results provide direct
observational evidence of `radio mode' AGN feedback in action, stripping the
central galaxy of its lowest entropy gas and preventing star-formation. This
low entropy gas was entrained with and uplifted by the buoyantly rising
relativistic plasma, forming long "arms". These arms are likely oriented within
15-30 degrees of our line-of-sight. The mass of the uplifted gas in the arms is
comparable to the gas mass in the approximately spherically symmetric 3.8 kpc
core, demonstrating that the AGN has a profound effect on its immediate
surroundings. The coolest X-ray emitting gas in M87 has a temperature of ~0.5
keV and is spatially coincident with Halpha+NII nebulae, forming a multiphase
medium where the cooler gas phases are arranged in magnetized filaments. We
place strong upper limits of 0.06 Msun/yr on the amount of plasma cooling
radiatively from 0.5 keV and show that a uniform, volume-averaged heating
mechanism could not be preventing the cool gas from further cooling. All of the
bright Halpha filaments appear in the downstream region of the <3 Myr old shock
front, at smaller radii than ~0.6'. We suggest that shocks induce shearing
around the filaments, thereby promoting mixing of the cold gas with the ambient
hot ICM via instabilities. By bringing hot thermal particles into contact with
the cool, line-emitting gas, mixing can supply the power and ionizing particles
needed to explain the observed optical spectra. Mixing of the coolest X-ray
emitting plasma with the cold optical line emitting filamentary gas promotes
efficient conduction between the two phases, allowing non-radiative cooling
which could explain the lack of X-ray gas with temperatures under 0.5 keV.Comment: to appear in MNRA
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