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'Sisters under the skin'? Anglo-Saxon nuns and nunneries in southern England
Ionisation Feedback in Star and Cluster Formation Simulations
Feedback from photoionisation may dominate on parsec scales in massive
star-forming regions. Such feedback may inhibit or enhance the star formation
efficiency and sustain or even drive turbulence in the parent molecular cloud.
Photoionisation feedback may also provide a mechanism for the rapid expulsion
of gas from young clusters' potentials, often invoked as the main cause of
'infant mortality'. There is currently no agreement, however, with regards to
the efficiency of this process and how environment may affect the direction
(positive or negative) in which it proceeds. The study of the photoionisation
process as part of hydrodynamical simulations is key to understanding these
issues, however, due to the computational demand of the problem, crude
approximations for the radiation transfer are often employed. We will briefly
review some of the most commonly used approximations and discuss their major
drawbacks. We will then present the results of detailed tests carried out using
the detailed photoionisation code MOCASSIN and the SPH+ionisation code iVINE
code, aimed at understanding the error introduced by the simplified
photoionisation algorithms. This is particularly relevant as a number of new
codes have recently been developed along those lines. We will finally propose a
new approach that should allow to efficiently and self-consistently treat the
photoionisation problem for complex radiation and density fields.Comment: Invited review presented at the IAU Symposium 270: Computational Star
Formation held in Barcelona (May 31st- June 4th 2010) - Refereed paper
version; 8 Pages, 4 Figure
Hubble Space Telescope NICMOS Polarization Observations of Three Edge-on Massive YSOs
Massive young stellar objects (YSOs), like low-mass YSOs, appear to be
surrounded by optically thick envelopes and/or disks and have regions, often
bipolar, that are seen in polarized scattered light at near-infrared
wavelengths. We are using the 0.2'' spatial resolution of NICMOS on Hubble
Space Telescope to examine the structure of the disks and outflow regions of
massive YSOs in star-forming regions within a few kpc of the Sun. Here we
report on 2 micron polarimetry of NGC 6334 V and S255 IRS1. NGC 6334 V consists
of a double-lobed bright reflection nebula seen against a dark region, probably
an optically thick molecular cloud. Our polarization measurements show that the
illuminating star lies ~ 2'' south of the line connecting the two lobes; we do
not detect this star at 2 micron, but there are a small radio source and a
mid-infrared source at this location. S255 IRS1 consists of two YSOs (NIRS1 and
NIRS3) with overlapping scattered light lobes and luminosities corresponding to
early B stars. Included in IRS1 is a cluster of stars from whose polarization
we determine the local magnetic field direction. Neither YSO has its scattered
light lobes aligned with this magnetic field. The line connecting the scattered
light lobes of NIRS1 is twisted symmetrically around the star; the best
explanation is that the star is part of a close binary and the outflow axis of
NIRS1 is precessing as a result of non-coplanar disk and orbit. The star NIRS3
is also offset from the line connecting its two scattered light lobes. We
suggest that all three YSOs show evidence of episodic ejection of material as
they accrete from dense, optically thick envelopes.Comment: 39 pages, 7 figures, 4 tables To be published in The Astrophysical
Journa
Massive star formation and feedback in W49A: The source of our Galaxy's most luminous water maser outflow
We present high spatial resolution mid-IR images of the ring of UCHII regions
in W49A obtained at Gemini North, allowing us to identify the driving source of
its powerful H2O maser outflow. These data also confirm our previous report
that several radio sources in the ring are undetected in the mid-IR because
they are embedded deep inside the cloud core. We locate the source of the water
maser outflow at the position of the compact mid-IR peak of source G (source
G:IRS1). This IR source is not coincident with any identified compact radio
continuum source, but is coincident with a hot molecular core, so we propose
that G:IRS1 is a hot core driving an outflow analogous to the wide-angle
bipolar outflow in OMC-1. G:IRS1 is at the origin of a larger bipolar cavity
and CO outflow. The water maser outflow is orthogonal to the bipolar CO cavity,
so the masers probably reside near its waist in the cavity walls. Models of the
IR emission require a massive protostar of 45Msun, 3e5Lsun, and an effective
envelope accretion rate of 1e-3Msun/yr. Feedback from the central star could
potentially drive the H2O maser outflow, but it has insufficient radiative
momentum to have driven the large-scale CO outflow, requiring that this massive
star had an active accretion disk over the past 10^4 yr. Combined with the
spatialy resolved morphology in IR images, G:IRS1 in W49 provides compelling
evidence for a massive protostar that formed by accreting from a disk,
accompanied by a bipolar outflow.Comment: 14 pages, MNRAS accepte
"The Book of Negroesâ illustrated edition: circulating African-Canadian history through the Middlebrow"
This article examines the 2009 deluxe illustrated edition of Lawrence Hillâs Commonwealth Writersâ Prizeâ and Canada Readsâwinning novel The Book of Negroes, originally published in 2007. It relates the story of Aminata, a West African girl kidnapped and sold into slavery, and her experiences on an indigo plantation in the American south, followed by further displacements to Charleston, Nova Scotia, Sierra Leone, and London. In New York, as the Revolutionary War comes to a close, Aminata becomes the scribe for the Book of Negroes, documenting the Black Loyalists, as well as the slaves and indentured servants of white Loyalists, granted passage by the British to Canada. Hill has commented that the Book of Negroes is an important document about which Canadians are largely ignorant. This desire to circulate knowledge about African-Canadian history through the novel is particularly manifest in the illustrated edition of 2009, where a photograph of the Book of Negroes features prominently, along with countless other images and captions which supplement and interrupt Hillâs narrative. This article considers the significance and implications of this âkeepsakeâ or âsouvenirâ edition, particularly its circulation of knowledge about African-Canadian history through visual pleasure
Ionisation feedback in star formation simulations: The role of diffuse fields
We compare the three-dimensional gas temperature distributions obtained by a
dedicated radiative transfer and photoionisation code, MOCASSIN, against those
obtained by the recently-developed Smooth Particle Hydrodynamics (SPH) plus
ionisation code iVINE for snapshots of an hydrodynamical simulation of a
turbulent interstellar medium (ISM) irradiated by a nearby O star. Our tests
demonstrate that the global ionisation properties of the region are correctly
reproduced by iVINE, hence validating further application of this code to the
study of feedback in star forming regions. However we highlight potentially
important discrepancies in the detailed temperature distribution. In particular
we show that in the case of highly inhomogenous density distributions the
commonly employed on-the-spot (OTS) approximation yields unrealistically sharp
shadow regions which can affect the dynamical evolution of the system. We
implement a simple strategy to include the effects of the diffuse field in
future calculations, which makes use of physically motivated temperature
calibrations of the diffuse-field dominated regions and can be readily applied
to similar codes. We find that while the global qualitative behaviour of the
system is captured by simulations with the OTS approximation, the inclusion of
the diffuse field in iVINE (called DiVINE) results in a stronger confinement of
the cold gas, leading to denser and less coherent structures. This in turn
leads to earlier triggering of star formation. We confirm that turbulence is
being driven in simulations that include the diffuse field, but the efficiency
is slightly lower than in simulations that use the OTS approximation.Comment: 11 pages, 10 Figures, Accepted for publication in MNRA
On the theory of disc photoevaporation
We discuss a hydrodynamical model for the dispersal of protoplanetary discs
around young, low mass (<1.5 M_sun) stars by photoevaporation from the central
object's energetic radiation, which considers the far-ultraviolet as well as
the X-ray component of the radiation field. We present analytical scaling
relations and derive estimates for the total mass-loss rates, as well as
discussing the existence of similarity solutions for flows from primordial
discs and discs with inner holes. Furthermore, we perform numerical
calculations, which span a wide range of parameter space and allow us to
provide accurate scalings of the mass-loss rates with the physical parameters
of the systems (X-ray and FUV luminosity, stellar mass, disc mass, disc
temperature and inner hole radius).
The model suggest that the X-ray component dominates the photoevaporative
mass-loss rates from the inner disc. The mass-loss rates have values in the
range from 10e-11 to 10e-7 M_sun/yr and scale linearly with X-ray luminosity,
with only a weak dependence on the other parameters explored. However, in the
case of high FUV to X-ray (L_FUV/L_X>100) luminosity ratios, the FUV constricts
the X-ray flow and may dominate the mass-loss.
Simulations of low mass discs with inner holes demonstrate a further stage of
disc clearing, which we call `thermal sweeping'. This process occurs when the
mid-plane pressure drops to sufficiently low values. At this stage a bound,
warm, X-ray heated region becomes sufficiently large and unstable, such that
the remaining disc material is cleared on approximately dynamical time-scales.
This process significantly reduces the time taken to clear the outer regions of
the disc, resulting in an expected transition disc population that will be
dominated by accreting objects, as indicated by recent observations.Comment: Accepted MNRAS, 25 page