403 research outputs found
Magnetic Fields in Stellar Jets
Although several lines of evidence suggest that jets from young stars are
driven magnetically from accretion disks, existing observations of field
strengths in the bow shocks of these flows imply that magnetic fields play only
a minor role in the dynamics at these locations. To investigate this apparent
discrepancy we performed numerical simulations of expanding magnetized jets
with stochastically variable input velocities with the AstroBEAR MHD code.
Because the magnetic field B is proportional to the density n within
compression and rarefaction regions, the magnetic signal speed drops in
rarefactions and increases in the compressed areas of velocity-variable flows.
In contrast, B ~ n^0.5 for a steady-state conical flow with a toroidal field,
so the Alfven speed in that case is constant along the entire jet. The
simulations show that the combined effects of shocks, rarefactions, and
divergent flow cause magnetic fields to scale with density as an intermediate
power 1 > p > 0.5. Because p > 0.5, the Alfven speed in rarefactions decreases
on average as the jet propagates away from the star. This behavior is extremely
important to the flow dynamics because it means that a typical Alfven velocity
in the jet close to the star is significantly larger than it is in the
rarefactions ahead of bow shocks at larger distances, the one place where the
field is a measurable quantity. We find that the observed values of weak fields
at large distances are consistent with strong fields required to drive the
observed mass loss close to the star. For a typical stellar jet the crossover
point inside which velocity perturbations of 30 - 40 km/s no longer produce
shocks is ~ 300 AU from the source
Driving spiral arms in the circumstellar disks of HD 100546 and HD 141569A
With 2D hydrodynamical simulations of disks perturbed externally by stars,
brown dwarfs or planets we investigate possible scenarios that can account for
the spiral structure in circumstellar disks. We consider two scenarios, spiral
structure driven by an external bound planet or low mass star and that excited
by a previous stellar close encounter or flyby. We find that both scenarios
produce morphology similar to that observed in the outer disks of HD 141569A
and HD 100546; moderately open 2-armed outer spiral structure. The outer
two-armed spiral structure observed in the disk of HD 141569A is qualitatively
reproduced with tidal perturbations from its companion binary HD 141569B,C on a
prograde orbit near periapse. Our simulation accounts for the outer spiral
arms, but is less successful than the secular model of Augereau and Papaloizou
at matching the lopsidedness or asymmetry of the disk edge at 300AU. The disk
has been previously truncated by the tidal force from the binary. A bound
object (stellar or planetary) is unlikely to explain the spiral structure in HD
100546. A co-eval planet or brown dwarf in the disk of sufficient mass to
account for the amplitude of the spiral structure would be detectable in NICMOS
and STIS images, however existing images reveal no such object. A previous
encounter could explain the observed structure, provided that the encounter
occurred less than a few thousand year ago. The object responsible for causing
the spiral structure must then be within a few arcminutes of the star. However,
the USNO-B proper motion survey reveals no candidate object. Moreover, the
probability that a field star encountered HD 100546 in the past few thousand
years is very low.Comment: accepted to A
On the Planet and the Disk of CoKuTau/4
Spitzer observations of the young star CoKuTau/4 reveal a disk with a 10 AU
hole that is most likely caused by a newly formed planet. Assuming that the
planet opened a gap in the viscous disk, we estimate that the planet mass is
greater than 0.1 Jupiter masses. This estimate depends on a lower limit to the
disk viscosity derived from the time scale needed to accrete the inner disk,
creating the now detectable hole. The planet migration time scale must at least
modestly exceed the time for the spectrally inferred hole to clear. The
proximity of the planet to the disk edge implied by our limits suggests that
the latter is perturbed by the nearby planet and may exhibit a spiral pattern
rotating with the planet. This pattern might be resolved with current ground
based mid-infrared cameras and optical cameras on the Hubble Space Telescope.
The required sub-Myr planet formation may challenge core accretion formation
models. However, we find that only if the planet mass is larger than about 10
Jupiter masses, allowing for a high enough surface density without inducing
migration, would formation by direct gravitational instability be possible.Comment: Submitted to ApJ
The Evolution of Protoplanetary Disk Edges
We investigate gap formation in gaseous protostellar disks by a planet in a
circular orbit in the limit of low disk viscosity. This regime may be
appropriate to an aging disk after the epoch of planet formation. We find that
the distance of planet to the gap outer boundary can be between the location of
the and outer Lindblad resonances. This distance is weakly
dependent upon both the planet's mass and disk viscosity. We find that the
evolution of the disk edge takes place on two timescales. The first timescale
is set by the spiral density waves driven by the planet. The second timescale
depends on the viscosity of the disk. The disk approaches a state where the
outward angular momentum flux caused by the disk viscosity is balanced by the
dissipation of spiral density waves which are driven at the Lindblad
resonances. This occurs inefficiently however because of the extremely low gas
density near the planet. We find that the distance between the planet and the
peak density at the disk outer edge is only weakly dependent on the viscosity
and planet mass, however the ratio of the gas density near the planet to that
in the disk (or the slope of density along the disk edge) is strongly dependent
upon both quantities. We find that the disk density profile along the edge
scales approximately with disk viscosity divided by the square of the planet
mass. We account for this behavior with a simple scenario in which the
dissipation of angular momentum from the spiral density waves is balanced
against diffusion in the steep edge of the disk.Comment: Accepted for publication in ApJ, 11 figures, 13 page
Generating worst-case scenarios by randomly distributing loads for risk assessment in low voltage residential electricity grids
In order to assess the capacity of low voltage electricity grids different grid operation cases are usually analyzed. These cases are used to identify weaknesses in the grid, evaluate the risks involved and subsequently facilitate the integration of new loads such as electric vehicles or heat pumps which are joining these grids in an increasing degree. This study suggests a random load allocation algorithm to create realistic worst-case scenarios for grid operation without the need for historical load data or reverting to load profiles. This is achieved by distributing loads asymmetrically across all three phases so that they comply with grid codes and burden the local transformer moderately. In this way, a multitude of feasible load scenarios is generated and evaluated. A metric is proposed to select those scenarios which lead to a critical operation state of the grid. The generated worst-case scenarios can be used to evaluate the potential capacity and risks of integrating new consumers into grids. This is demonstrated in a use case where electric vehicles are integrated into the investigated grid at half of all connection points. The Analysis shows that the grid is additionally stressed and the reinforcement of cables or charge management would be required to facilitate the safe operation of the grid with additional loads
Driving Habits, Cognition, and Health-Related Quality of Life in Middle-Aged and Older Adults with HIV
Cognitive impairment is known to increase with aging in people living with HIV (PLWH). Impairment in cognitive domains required for safe driving may put PLWH at risk for poor driving outcomes, decreased mobility, and health-related quality of life (HRQoL). This study described the driving behaviors of middle-aged and older PLWH and examined correlations between driving behaviors and cognitive functioning (Aim 1), and driving behaviors and HRQoL domains (Aim 2). A sample of 260 PLWH ages 40 and older completed a comprehensive assessment including a battery of cognitive tests, an HRQoL measure, and a measure of self-reported driving habits. Associations between driving habits, cognitive function, and HRQoL domains were examined. While 212 (81.54%) participants reported currently driving, only 166 (63.85%) possessed a driver\u27s license. Several significant correlations emerged between driving habits and both cognitive and HRQoL variables, with a general pattern suggesting that current greater driving exposure was associated with better cognitive functioning and HRQoL. Given consistent associations that emerged between the social functioning HRQoL domain and several driving habits, multivariable regression was conducted to examine the unique association between an index of greater driving exposure (i.e., days driven per week) and social functioning, adjusting for potential confounders (race, income, education, depression, and global cognition). Results showed that more days driven per week was a significant, independent correlate of higher social functioning. Understanding the factors underlying driving behaviors in PLWH may contribute to interventions to promote better mobility and improved access to care
The Grizzly, October 23, 1987
Middle States to Evaluate the Ursinus Community • Gilmore Tops Successful Homecoming • Greek Life Threatened • Letters: Pledging Restrictions Destructive?; Professor Supports Religious Understanding • Hot Entertainment to Hit Ursinus • As We Begin the World • Dance Forum Dazzles Audience • Scholars Offer Variety • Old Wine in New Bottles • Speech Exam Offered • Surf\u27s Up for the Lady Bears • Ursinus Athletes and Coach Win at Homecoming • This Run\u27s for you Vince • Athlete of the Week: Football\u27s Glenn Worgan • Grizzlies to Hang Tough • Choral Groups to Entertain Ursinus • WVOU is Back! • Ritter Center: A Beehive of Busyness the Busie Body to Premiere Oct. 29 • Owners Strike Backhttps://digitalcommons.ursinus.edu/grizzlynews/1197/thumbnail.jp
The Freshman, vol. 4, no. 1
The Freshman was a weekly, student newsletter issued on Mondays throughout the academic year. The newsletter included calendar notices, coverage of campus social events, lectures, and athletic teams. The intent of the publication was to create unity, a sense of community, and class spirit among first year students. The Class of 1937 run of The Freshman featured original cover art by sketch artist Jack Frost (John Edward Frost, 1915-1997), who was born in Eastport, Maine. He attended the University of Maine for only a single academic year before moving to Massachusetts to work for the Boston Herald. Frost later became a columnist and illustrator for the Boston Post
Unknowable bodies, unthinkable sexualities: lesbian and transgender legal invisibility in the Toronto women's bathhouse raid
Although litigation involving sexual orientation and gender identity discrimination claims has generated considerable public attention in recent years, lesbian and transgender bodies and sexualities still remain largely invisible in Anglo-American courts. While such invisibility is generally attributed to social norms that fail to recognize lesbian and transgender experiences, the capacity to 'not see' or 'not know' queer bodies and sexualities also involves wilful acts of ignorance. Drawing from R. v Hornick (2002) a Canadian case involving the police raid of a women's bathhouse, this article explores how lesbian and transgender bodies and sexualities are actively rendered invisible via legal knowledge practices, norms and rationalities. It argues that limited knowledge and limited thinking not only regulate the borders of visibility and belonging, but play an active part in shaping identities, governing conduct and producing subjectivity
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