200 research outputs found
Near-Infrared Imaging Polarimetry of Young Stellar Objects in rho-Ophiuchi
The results of a near-infrared (J H K LP) imaging linear polarimetry survey
of 20 young stellar objects (YSOs) in rho Ophiuchi are presented. The majority
of the sources are unresolved, with K-band polarizations, P_K < 6 per cent.
Several objects are associated with extended reflection nebulae. These objects
have centrosymmetric vector patterns with polarization discs over their cores;
maximum polarizations of P_K > 20 per cent are seen over their envelopes.
Correlations are observed between the degree of core polarization and the
evolutionary status inferred from the spectral energy distribution. K-band core
polarizations >6 per cent are only observed in Class I YSOs. A 3D Monte Carlo
model with oblate grains aligned with a magnetic field is used to investigate
the flux distributions and polarization structures of three of the rho Oph YSOs
with extended nebulae. A rho proportional to r^(-1.5) power law for the density
is applied throughout the envelopes. The large-scale centrosymmetric
polarization structures are due to scattering. However, the polarization
structure in the bright core of the nebula appears to require dichroic
extinction by aligned non-spherical dust grains. The position angle indicates a
toroidal magnetic field in the inner part of the envelope. Since the measured
polarizations attributed to dichroic extinction are usually <10 per cent, the
grains must either be nearly spherical or very weakly aligned. The higher
polarizations observed in the outer parts of the reflection nebulae require
that the dust grains responsible for scattering have maximum grain sizes <=1.05
microns.Comment: 26 pages. Accepted by MNRAS. Available as online early versio
Near Infrared polarimetry of a sample of YSOs
Our goal is to study the physical properties of the circumstellar environment
of young stellar objetcs (YSOs). In particular, the determination of the
scattering mechanism can help to constrain the optical depth of the disk and/or
envelope in the near infrared. We used the IAGPOL imaging polarimeter along
with the CamIV infrared camera at the LNA observatory to obtain near infrared
polarimetry measurements at the H band of a sample of optically visible YSOs,
namely, eleven T Tauri stars and eight Herbig Ae/Be stars. An independent
determination of the disk (or jet) orientation was obtained for twelve objects
from the literature. The circumstellar optical depth could be then estimated
comparing the integrated polarization position angle (PA) with the direction of
the major axis of the disk projected in the plane of the sky. In general,
optically thin disks have polarization PA perpendicular to the disk plane. In
contrast, optically thick disks produce polarization PA parallel to the disks.
Among the T Tauri stars, three are consistent with optically thin disks (AS
353A, RY Tau and UY Aur) and five with optically thick disks (V536 Aql, DG Tau,
DO Tau, HL Tau and LkHalpha 358). Among the Herbig Ae/Be stars, two stars show
evidence of optically thin disk (Hen 3-1191 and VV Ser) and two of optically
thick disks (PDS 453 and MWC 297). Our results seem consistent with the fact
that optically thick disks at near infrared bands are associated more likely
with younger YSOs. Marginal evidence of polarization reversal is found in RY
Tau, RY Ori, WW Vul, and UY Aur. On the first three cases this feature can be
associated to the UXOR phenomenon. Correlations with the IRAS colours and the
spectral index yielded evidence of an evolutionary segregation with the disks
tend to be optically thin when they are older.Comment: 15 pages, 15 figures, accepted in Astronomy and Astrophysic
Troubling the exclusive privileges of citizenship: mobile solidarities, asylum seekers, and the right to work
This article discusses asylum seekers and the right to work in the UK. Differential access to the labour market is one of the ways in which the state maintains a distinction between British citizens, who ‘belong’, and non-citizens who do not. While such a policy approach garners widespread support amongst the general public of citizens, it does not go uncontested. This article discusses a UK-based campaign, ‘Let Them Work’, which has sought to influence the government in extending the right to work to asylum seekers. In doing so, it demonstrates the ways in which the stratified regime of citizenship rights is contested politically, and explores how such contestation troubles the exclusive privileges of citizenship by enacting mobile solidarities from marginalised spaces
Privatisation of agro‐industrial parastatals and Anglophone opposition in Cameroon
This article focuses on the regional anglophone opposition in Cameroon which arose after 15 July 1994, when the government was forced by international donors to announce the privatization of 15 public enterprises, notably in the transport and agroindustrial sectors. The most prominent among them was the Cameroon Development Corporation (CDC), founded in 1946/1947. The author argues that the strong resistance of anglophones in general and the Bakweri in particular to the privatization of the CDC can only be fully understood in the context of the 'anglophone problem'. Privatization of the CDC was perceived as a further step by the francophone-dominated State towards destruction of the anglophone cultural and economic heritage. This perception was strengthened by the fact that the CDC has the reputation of being one of the rare parastatals in Cameroon which from its inception has played a significant role in regional development and which had a relatively good performance record until the economic crisis. Moreover, the Bakweri, the owners of the CDC lands, were not consulted.ASC – Publicaties niet-programma gebonde
High resolution imaging polarimetry of HL Tau and magnetic field structure
We present high quality near infrared imaging polarimetry of HL Tau at 0.4 to
0.6 arcsec resolution, obtained with Subaru/CIAO and UKIRT/IRCAM. 3-D Monte
Carlo modelling with aligned oblate grains is used to probe the structure of
the circumstellar envelope and the magnetic field, as well as the dust
properties. At J band the source shows a centrosymmetric pattern dominated by
scattered light. In the H and K bands the central source becomes visible and
its polarisation appears to be dominated by dichroic extinction, with a
position angle inclined by ~40 degrees to the disc axis. The polarisation
pattern of the environs on scales up to 200 AU is consistent with the same
dichroic extinction signature superimposed on the centrosymmetric scattering
pattern. These data can be modelled with a magnetic field which is twisted on
scales from tens to hundreds of AU, or alternatively by a field which is
globally misaligned with the disc axis. A unique solution to the field
structure will require spatially resolved circular polarisation data. The best
fit Monte Carlo model indicates a shallow near infrared extinction law. When
combined with the observed high polarisation and non-negligible albedo these
constraints can be fitted with a grain model involving dirty water ice mantles
in which the largest particles have radii slightly in excess of 1 um. The best
fit model has an envelope structure which is slightly flattened on scales up to
several hundred AU. Both lobes of the bipolar outflow cavity contain a
substantial optical depth of dust (not just within the cavity walls). Curved,
approximately parabolic, cavity walls fit the data better than a conical
cavity. The small inner accretion disc observed at millimetre wavelengths is
not seen at this spatial resolution.Comment: Accepted by MNRAS, 21 pages, 10 figure
The new enclosures: critical perspectives on corporate land deals
The contributions to this collection use the tools of agrarian political economy to
explore the rapid growth and complex dynamics of large-scale land deals in recent
years, with a special focus on the implications of big land deals for property and
labour regimes, labour processes and structures of accumulation. The first part of
this introductory essay examines the implications of this agrarian political
economy perspective. First we explore the continuities and contrasts between
historical and contemporary land grabs, before examining the core underlying
debate around large- versus small-scale farming futures. Next, we unpack the
diverse contexts and causes of land grabbing today, highlighting six overlapping
mechanisms. The following section turns to assessing the crisis narratives that
frame the justifications for land deals, and the flaws in the argument around there
being excess, empty or idle land available. Next the paper turns to an examination
of the impacts of land deals, and the processes of inclusion and exclusion at play,
before looking at patterns of resistance and constructions of alternatives. The
final section introduces the papers in the collection.ESR
Disks and Outflows in CO Rovibrational Emission from Embedded, Low-Mass Young Stellar Objects
Young circumstellar disks that are still embedded in dense molecular
envelopes may differ from their older counterparts, but are historically
difficult to study because emission from a disk can be confused with envelope
or outflow emission. CO fundamental emission is a potentially powerful probe of
the disk/wind structure within a few AU of young protostars. In this paper, we
present high spectral (R=90,000) and spatial (0.3") resolution VLT/CRIRES
M-band spectra of 18 low-mass young stellar objects (YSOs) with dense envelopes
in nearby star-froming regions to explore the utility of CO fundamental 4.6
micron emission as a probe of very young disks. CO fundamental emission is
detected from 14 of the YSOs in our sample. The emission line profiles show a
range of strengths and shapes, but can generally be classified into a broad,
warm component and a narrow, cool component. The broad CO emission is detected
more frequently from YSOs with bolometric luminosities of <15 Lsun than those
with >15 Lsun, and as with CO emission from CTTSs is attributed to the warm
(~1000 K) inner AU of the disk. The CO emission from objects with high
bolometric luminosity is produced in cooler (~320 K), narrow lines in 12CO and
in rarer isotopologues. From some objects, the narrow lines are blueshifted by
up to ~10 km/s, indicating a slow wind origin. For other sources the lines are
located at the systemic velocity of the star and likely arise in the disk. For
a few YSOs, spatially-extended CO and H2 S(9) emission is detected up to 2"
from the central source and is attributed to interactions between the wind and
surrounding molecular material. Warm CO absorption is detected in the wind of
six objects with velocities up to 100 km/s, often in discrete velocity
components. That the wind is partially molecular where it is launched favors
ejection in a disk wind rather than a coronal or chromospheric wind.Comment: 26 pages, accepted by A&
Epigenomic characterization of Clostridioides difficile finds a conserved DNA methyltransferase that mediates sporulation and pathogenesis
Clostridioides (formerly Clostridium) difficile is a leading cause of healthcare-associated infections. Although considerable progress has been made in the understanding of its genome, the epigenome of C. difficile and its functional impact has not been systematically explored. Here, we perform a comprehensive DNA methylome analysis of C. difficile using 36 human isolates and observe a high level of epigenomic diversity. We discovered an orphan DNA methyltransferase with a well-defined specificity, the corresponding gene of which is highly conserved across our dataset and in all of the approximately 300 global C. difficile genomes examined. Inactivation of the methyltransferase gene negatively impacts sporulation, a key step in C. difficile disease transmission, and these results are consistently supported by multiomics data, genetic experiments and a mouse colonization model. Further experimental and transcriptomic analyses suggest that epigenetic regulation is associated with cell length, biofilm formation and host colonization. These findings provide a unique epigenetic dimension to characterize medically relevant biological processes in this important pathogen. This study also provides a set of methods for comparative epigenomics and integrative analysis, which we expect to be broadly applicable to bacterial epigenomic studies
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