1,463 research outputs found
Aligator.jl - A Julia Package for Loop Invariant Generation
We describe the Aligator.jl software package for automatically generating all
polynomial invariants of the rich class of extended P-solvable loops with
nested conditionals. Aligator.jl is written in the programming language Julia
and is open-source. Aligator.jl transforms program loops into a system of
algebraic recurrences and implements techniques from symbolic computation to
solve recurrences, derive closed form solutions of loop variables and infer the
ideal of polynomial invariants by variable elimination based on Gr\"obner basis
computation
Toxicological Assessment of Zinc Oxide and Titanium Dioxide on Staghorn Coral, Acropora cervicornis
Coral reefs are one of the most diverse and biologically significant ecosystems on the planet. However, anthropogenic impacts have led to a global decline in overall reef health in recent years. To address concerns that organic ultraviolet filters in sunscreen may contribute to reduced coral health, the mineral UV filters, zinc oxide (ZnO) and titanium dioxide (TiO2), have become more common in sunscreen formulas as they are often considered “reef safe” despite limited information on the toxicological effects of these compounds in corals. This study evaluated the effects of exposure to unmodified forms of zinc oxide or titanium dioxide compared to the commercial products ZinClear IM® 50CCT (ZnO 50%) and UV Cut (TiO2 40%) on the threatened Atlantic scleractinian coral, Acropora cervicornis. Exposure-associated stress on coral fragments was determined utilizing measurements of photosynthetic efficiency, behavioral responses, and calcification in 48-h exposure assays. Corals exposed to TiO2 were observed to have mild to moderate response during the 48 h exposure but ultimately returned to normal behavior at the end of the 48-h exposure. Corals exposed to ZnO had moderate to severe responses, with a high percentage of tissue mortality at concentrations \u3e 190.7 µg/L. Exposure to ZinClearIM® 50CCT or UV Cut resulted in no significant response and no mortality. Corals had a significantly greater response to ZnO in comparison to TiO2
Three-dimensional modeling of the HI kinematics of NGC 2915
The nearby blue compact dwarf, NGC 2915, has its stellar disc embedded in a
large, extended (~ 22 B-band scale-lengths) HI disc. New high-resolution HI
synthesis observations of NGC 2915 have been obtained with the Australia
Telescope Compact Array. These observations provide evidence of extremely
complex HI kinematics within the immediate vicinity of the galaxy's
star-forming core. We identify and quantify double-peaked HI line profiles near
the centre of the galaxy and show that the HI energetics can be accounted for
by the mechanical energy output of the central high-mass stellar population
within time-scales of 10^6-10^7 yr. Full three-dimensional models of the HI
data cube are generated and compared to the observations to test various
physical scenarios associated with the high-mass star-forming core of NGC 2915.
Purely circular HI kinematics are ruled out together with the possibility of a
high-velocity-dispersion inter-stellar medium at inner radii. Radial velocities
of ~ 30 km/s are required to describe the central-most HI kinematics of the
system. Our results lend themselves to the simple physical scenario in which
the young stellar core of the galaxy expels the gas outwards from the centre of
the disc, thereby creating a central HI under-density. These kinematics should
be thought of as being linked to a central HI outflow rather than a large-scale
galactic blow-out or wind.Comment: 11 pages, 6 figures, accepted for publication in MNRA
Star Formation in Violent and Normal Evolutionary Phases
Mergers of massive gas-rich galaxies trigger violent starbursts that - over
timescales of Myr and regions kpc - form massive and compact
star clusters comparable in mass and radii to Galactic globular clusters. The
star formation efficiency is higher by 1 - 2 orders of magnitude in these
bursts than in undisturbed spirals, irregulars or even BCDs. We ask the
question if star formation in these extreme regimes is just a scaled-up version
of the normal star formation mode of if the formation of globular clusters
reveals fundamentally different conditions.Comment: 4 pages To appear in The Evolution of Galaxies. II. Basic building
blocks, eds. M. Sauvage, G. Stasinska, L. Vigroux, D. Schaerer, S. Madde
A Gaseous Group with Unusual Remote Star Formation
We present VLA 21-cm observations of the spiral galaxy ESO 481-G017 to
determine the nature of remote star formation traced by an HII region found 43
kpc and ~800 km s^-1 from the galaxy center (in projection). ESO 481-G017 is
found to have a 120 kpc HI disk with a mass of 1.2x10^10 Msun and UV GALEX
images reveal spiral arms extending into the gaseous disk. Two dwarf galaxies
with HI masses close to 10^8 Msun are detected at distances of ~200 kpc from
ESO 481-G017 and a HI cloud with a mass of 6x10^7 Msun is found near the
position and velocity of the remote HII region. The HII region is somewhat
offset from the HI cloud spatially and there is no link to ESO 481-G017 or the
dwarf galaxies. We consider several scenarios for the origin of the cloud and
HII region and find the most likely is a dwarf galaxy that is undergoing ram
pressure stripping. The HI mass of the cloud and Halpha luminosity of the HII
region (10^38.1 erg s^-1) are consistent with dwarf galaxy properties, and the
stripping can trigger the star formation as well as push the gas away from the
stars.Comment: 19 pages, 4 figures, accepted by PAS
On the Escape of Ionizing Radiation from Starbursts
Far-ultraviolet spectra obtained with show that the strong
1036 interstellar absorption-line is essentially black in five of
the UV-brightest local starburst galaxies. Since the opacity of the neutral ISM
below the Lyman-edge will be significantly larger than in the line, these
data provide strong constraints on the escape of ionizing radiation from these
starbursts. Interpreted as a a uniform absorbing slab, the implied optical
depth at the Lyman edge is huge (). Alternatively, the areal
covering factor of opaque material is typically 94%. Thus, the fraction
of ionizing stellar photons that escape the ISM of each galaxy is small: our
conservative estimates typically yield . Inclusion of
extinction due to dust will further decrease . An analogous analysis
of the rest-UV spectrum of the star-forming galaxy at =2.7
leads to similar constraints on . These new results agree with the
constraints provided by direct observations below the Lyman edge in a few other
local starbursts. However, they differ from the recently reported properties of
star-forming galaxies at 3. We assess the idea that the strong
galactic winds seen in many powerful starbursts clear channels through their
neutral ISM. We show empirically that such outflows may be a necessary - but
not sufficient - part of the process for creating a relatively porous ISM. We
note that observations will soon document the cosmic evolution in the
contribution of star-forming galaxies to the metagalactic ionizing background,
with important implications for the evolution of the IGM.Comment: 17 pages; ApJ, in pres
The Metal-Enriched Outer Disk of NGC 2915
We present optical emission-line spectra for outlying HII regions in the
extended neutral gas disk surrounding the blue compact dwarf galaxy NGC 2915.
Using a combination of strong-line R23 and direct oxygen abundance
measurements, we report a flat, possibly increasing, metallicity gradient out
to 1.2 times the Holmberg radius. We find the outer-disk of NGC 2915 to be
enriched to a metallicity of 0.4 Z_solar. An analysis of the metal yields shows
that the outer disk of NGC 2915 is overabundant for its gas fraction, while the
central star-foming core is similarly under-abundant for its gas fraction. Star
formation rates derived from very deep ~14 ks GALEX FUV exposures indicate that
the low-level of star formation observed at large radii is not sufficient to
have produced the measured oxygen abundances at these galactocentric distances.
We consider 3 plausible mechanisms that may explain the metal-enriched outer
gaseous disk of NGC 2915: radial redistribution of centrally generated metals,
strong galactic winds with subsequent fallback, and galaxy accretion. Our
results have implications for the physical origin of the mass-metallicity
relation for gas-rich dwarf galaxies.Comment: 11 pages, 4 figures, accepted to ApJ April 8th, 201
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