34 research outputs found
Oval Orbiting Stellar Winds
Stars typically eject a continuous supersonic flow of gas known as a stellar wind. Most stars are in multiple systems with two or more members, and their orbital motions affect the wind morphology. The Wilkin and Hausner (2017) orbiting wind model made the assumption that the wind was initially isotropic at launch. Here, we generalize this wind to a non-isotropic launch that is concentrated to the poles or equator of the star. This paper presents a self-consistent solution to this problem for the wind velocity and density structure
Modeling Nonaxisymmetric Bow Shocks: Solution Method and Exact Analytic Solutions
A new solution method is presented for steady-state, momentum-conserving,
non-axisymmetric bow shocks and colliding winds in the thin-shell limit. This
is a generalization of previous formulations to include a density gradient in
the pre-shock ambient medium, as well as anisotropy in the pre-shock wind. For
cases where the wind is unaccelerated, the formalism yields exact, analytic
solutions.
Solutions are presented for two bow shock cases: (1) that due to a star
moving supersonically with respect to an ambient medium with a density gradient
perpendicular to the stellar velocity, and (2) that due to a star with a
misaligned, axisymmetric wind moving in a uniform medium. It is also shown
under quite general circumstances that the total rate of energy thermalization
in the bow shock is independent of the details of the wind asymmetry, including
the orientation of the non-axisymmetric driving wind, provided the wind is
non-accelerating and point-symmetric. A typical feature of the solutions is
that the region near the standoff point is tilted, so that the star does not
lie along the bisector of a parabolic fit to the standoff region. The principal
use of this work is to infer the origin of bow shock asymmetries, whether due
to the wind or ambient medium, or both.Comment: 26 pages and 6 figures accepted to ap
Spin vectors in the Koronis family: V. Resolving the ambiguous rotation period of (3032) Evans
A sidereal rotation counting approach is demonstrated by resolving an
ambiguity in the synodic rotation period of Koronis family member (3032) Evans,
whose rotation lightcurves' features did not easily distinguish between doubly-
and quadruply-periodic. It confirms that Evans's spin rate does not exceed the
rubble-pile spin barrier and thus presents no inconsistency with being a ~14-km
reaccumulated object. The full spin vector solution for Evans is comparable to
those for the known prograde low-obliquity comparably-fast rotators in the
Koronis family, consistent with having been spun up by YORP thermal radiation
torques.Comment: 8 pages, 6 figures, accepted for publication in Icaru
Demonstrating the Principles of Aperture Synthesis with the Very Small Radio Telescope
We have developed a set of college-level, table-top labs for teaching the basics of radio interferometry and aperture synthesis. These labs are performed with the Very Small Radio Telescope (VSRT), an interferometer using satellite TV electronics as detectors and compact fluorescent light bulbs as microwave signal sources. The hands-on experience provided by the VSRT in these labs allows students to gain a conceptual understanding of radio interferometry and aperture synthesis without the rigorous mathematical background traditionally required
Trapped Protostellar Winds and their Breakout
Observations show that high-velocity jets stem from deeply embedded young
stars, which may still be experiencing infall from their parent cloud cores.
Yet theory predicts that, early in this buildup, any outgoing wind is trapped
by incoming material of low angular momentum. As collapse continues and brings
in more rapidly rotating gas, the wind can eventually break out. Here we model
this transition by following the motion of the shocked shell created by impact
of the wind and a rotating, collapsing envelope. We first demonstrate, both
analytically and numerically, that our previous, quasi-static solutions are
dynamically unstable. Our present, fully time-dependent calculations include
cases both where the wind is driven back by infall to the stellar surface, and
where it erupts as a true outflow. For the latter, we find that the time of
breakout is sim 50,000 yr for wind speeds of 200 km/s. The reason for the delay
is that the shocked material, including the swept-up infall, must be able to
climb out of the star's gravitational potential well.
We explore the critical wind speed necessary for breakout as a function of
the mass transport rates in the wind and infall, as well as the cloud rotation
rate Omega0 and time since the start of infall. Breakout does occur for
realistic parameter choices. The actual breakout times would change if we
relaxed the assumption of perfect mixing between the wind and infall material.
Our expanding shells do not exhibit the collimation of observed jets, but
continue to expand laterally. To halt this expansion, the density in the
envelope must fall off less steeply than in our model.Comment: 44 pages, 10 figures, accepted to Ap
Community engagement to enhance trust between Gypsy/Travellers, and maternity, early yearsâ and child dental health services: protocol for a multimethod exploratory study
Gypsy/Travellers have poor health and experience discrimination alongside structural and cultural barriers when accessing health services and consequently may mistrust those services. Our study aims to investigate which approaches to community engagement are most likely to be effective at enhancing trust between Gypsy/Travellers and mainstream health services.
Methods
This multi-method 30-month study, commenced in June 2015, and comprises four stages.
1.
Three related reviews: a) systematic review of Gypsy/Travellersâ access to health services; b) systematic review of reviews of how trust has been conceptualised within healthcare; c) realist synthesis of community engagement approaches to enhance trust and increase Gypsy/Travellersâ participation in health services. The reviews will consider any economic literature;
2.
Online consultation with health and social care practitioners, and civil society organisations on existing engagement activities, including perceptions of barriers and good practice;
3.
Four in-depth case studies of different Gypsy/Traveller communities, focusing on maternity, early years and child dental health services. The case studies include the views of 32â48 mothers of pre-school children, 32â40 healthcare providers and 8â12 informants from third sector organisations.
4.
Two stakeholder workshops exploring whether policy options are realistic, sustainable and replicable.
Case study data will be analysed thematically informed by the evaluative framework derived from the realist synthesis in stage one.
The main outputs will be: a) an evaluative framework of Gypsy/Travellersâ engagement with health services; b) recommendations for policy and practice; c) evidence on which to base future implementation strategies including estimation of costs.
Discussion
Our novel multi-method study seeks to provide recommendations for policy and practice that have potential to improve uptake and delivery of health services, and to reduce lifetime health inequalities for Gypsy/Travellers. The findings may have wider resonance for other marginalised populations. Strengths and limitations of the study are discussed
Macrofossil evidence for a rapid and severe CretaceousâPaleogene mass extinction in Antarctica
Debate continues about the nature of the CretaceousâPaleogene (KâPg) mass extinction event. An abrupt crisis triggered by a bolide impact contrasts with ideas of a more gradual extinction involving flood volcanism or climatic changes. Evidence from high latitudes has also been used to suggest that the severity of the extinction decreased from low latitudes towards the poles. Here we present a record of the KâPg extinction based on extensive assemblages of marine macrofossils (primarily new data from benthic molluscs) from a highly expanded CretaceousâPaleogene succession: the LoÌpez de Bertodano Formation of Seymour Island, Antarctica. We show that the extinction was rapid and severe in Antarctica, with no significant biotic decline during the latest Cretaceous, contrary to previous studies. These data are consistent with a catastrophic driver for the extinction, such as bolide impact, rather than a significant contribution from Deccan Traps volcanism during the late Maastrichtian