47 research outputs found
Outflow forces of low mass embedded objects in Ophiuchus: a quantitative comparison of analysis methods
The outflow force of molecular bipolar outflows is a key parameter in
theories of young stellar feedback on their surroundings. The focus of many
outflow studies is the correlation between the outflow force, bolometric
luminosity and envelope mass. However, it is difficult to combine the results
of different studies in large evolutionary plots over many orders of magnitude
due to the range of data quality, analysis methods and corrections for
observational effects such as opacity and inclination. We aim to determine the
outflow force for a sample of low luminosity embedded sources. We will quantify
the influence of the analysis method and the assumptions entering the
calculation of the outflow force. We use the James Clerk Maxwell Telescope to
map 12CO J=3-2 over 2'x2' regions around 16 Class I sources of a well-defined
sample in Ophiuchus at 15" resolution. The outflow force is then calculated
using seven different methods differing e.g. in the use of intensity-weighted
emission and correction factors for inclination. The results from the analysis
methods differ from each other by up to a factor of 6, whereas observational
properties and choices in the analysis procedure affect the outflow force by up
to a factor of 4. For the sample of Class I objects, bipolar outflows are
detected around 13 sources including 5 new detections, where the three
non-detections are confused by nearby outflows from other sources. When
combining outflow forces from different studies, a scatter by up to a factor of
5 can be expected. Although the true outflow force remains unknown, the
separation method (separate calculation of dynamical time and momentum) is
least affected by the uncertain observational parameters. The correlations
between outflow force, bolometric luminosity and envelope mass are further
confirmed down to low luminosity sources.Comment: 24 pages, 13 figures, Accepted by A&
The RMS Survey: The Bolometric Fluxes and Luminosity Distributions of Young Massive Stars
Context: The Red MSX Source (RMS) survey is returning a large sample of
massive young stellar objects (MYSOs) and ultra-compact (UC) \HII{} regions
using follow-up observations of colour-selected candidates from the MSX point
source catalogue. Aims: To obtain the bolometric fluxes and, using kinematic
distance information, the luminosities for young RMS sources with far-infrared
fluxes. Methods: We use a model spectral energy distribution (SED) fitter to
obtain the bolometric flux for our sources, given flux data from our work and
the literature. The inputs to the model fitter were optimised by a series of
investigations designed to reveal the effect varying these inputs had on the
resulting bolometric flux. Kinematic distances derived from molecular line
observations were then used to calculate the luminosity of each source.
Results: Bolometric fluxes are obtained for 1173 young RMS sources, of which
1069 have uniquely constrained kinematic distances and good SED fits. A
comparison of the bolometric fluxes obtained using SED fitting with trapezium
rule integration and two component greybody fits was also undertaken, and
showed that both produce considerable scatter compared to the method used here.
Conclusions: The bolometric flux results allowed us to obtain the luminosity
distributions of YSOs and UC\HII{} regions in the RMS sample, which we find to
be different. We also find that there are few MYSOs with L
10\lsol{}, despite finding many MYSOs with 10\lsol{} L
10\lsol{}.Comment: 12 pages, 12 figures, 3 tables, accepted to A&A. The full versions of
tables 1 and 2 will be available via the CDS upon publicatio
CO in Protostars (COPS): -SPIRE Spectroscopy of Embedded Protostars
We present full spectral scans from 200-670m of 26 Class 0+I
protostellar sources, obtained with -SPIRE, as part of the
"COPS-SPIRE" Open Time program, complementary to the DIGIT and WISH Key
programs. Based on our nearly continuous, line-free spectra from 200-670
m, the calculated bolometric luminosities () increase by 50%
on average, and the bolometric temperatures () decrease by 10% on
average, in comparison with the measurements without Herschel. Fifteen
protostars have the same Class using and /. We identify rotational transitions of CO lines from J=4-3 to J=13-12,
along with emission lines of CO, HCO, HO, and [CI]. The ratios
of CO to CO indicate that CO emission remains optically
thick for < 13. We fit up to four components of temperature from
the rotational diagram with flexible break points to separate the components.
The distribution of rotational temperatures shows a primary population around
100 K with a secondary population at 350 K. We quantify the correlations
of each line pair found in our dataset, and find the strength of correlation of
CO lines decreases as the difference between -level between two CO lines
increases. The multiple origins of CO emission previously revealed by
velocity-resolved profiles are consistent with this smooth distribution if each
physical component contributes to a wide range of CO lines with significant
overlap in the CO ladder. We investigate the spatial extent of CO emission and
find that the morphology is more centrally peaked and less bipolar at high-
lines. We find the CO emission observed with SPIRE related to outflows, which
consists two components, the entrained gas and shocked gas, as revealed by our
rotational diagram analysis as well as the studies with velocity-resolved CO
emission.Comment: 50 pages, 18 figures, accepted to ApJS. Revised for Table 6 and
Figure
Weak-anchoring effects in a thin pinned ridge of nematic liquid crystal
A theoretical investigation of weak-anchoring effects in a thin
two-dimensional pinned static ridge of nematic liquid crystal resting on a flat
solid substrate in an atmosphere of passive gas is performed. Specifically, we
solve a reduced version of the general system of governing equations recently
derived by Cousins et al. [Proc. Roy. Soc. A}, 478(2259):20210849, 2022] valid
for a symmetric thin ridge under the one-constant approximation of the
Frank--Oseen bulk elastic energy with pinned contact lines to determine the
shape of the ridge and the behaviour of the director within it. Numerical
investigations covering a wide range of parameter values indicate that the
energetically-preferred solutions can be classified in terms of the
Jenkins--Barratt--Barbero--Barberi critical thickness into five qualitatively
different types of solution. In particular, the theoretical results suggest
that anchoring breaking occurs close to the contact lines. The theoretical
predictions are supported by the results of physical experiments for a ridge of
the nematic 4'-pentyl-4-biphenylcarbonitrile (5CB). In particular, these
experiments show that the homeotropic anchoring at the gas--nematic interface
is broken close to the contact lines by the stronger rubbed planar anchoring at
the nematic--substrate interface. A comparison between the experimental values
of and the theoretical predictions for the effective refractive index of the
ridge gives a first estimate of the anchoring strength of an interface between
air and 5CB to be at a temperature
of C
The RMS Survey: Critical Tests of Accretion Models for the Formation of Massive Stars
There is currently no accepted theoretical framework for the formation of the
most massive stars, and the manner in which protostars continue to accrete and
grow in mass beyond \sim10Msun is still a controversial topic. In this study we
use several prescriptions of stellar accretion and a description of the
Galactic gas distribution to simulate the luminosities and spatial distribution
of massive protostellar population of the Galaxy. We then compare the
observables of each simulation to the results of the Red MSX Source (RMS)
survey, a recently compiled database of massive young stellar objects. We find
that the observations are best matched by accretion rates which increase as the
protostar grows in mass, such as those predicted by the turbulent core and
competitive accretion (i.e. Bondi-Hoyle) models. These 'accelerating accretion'
models provide very good qualitative and quantitative fits to the data, though
we are unable to distinguish between these two models on our simulations alone.
We rule out models with accretion rates which are constant with time, and those
which are initially very high and which fall away with time, as these produce
results which are quantitatively and/or qualitatively incompatible with the
observations. To simultaneously match the low- and high-luminosity YSO
distribution we require the inclusion of a 'swollen-star' pre-main-sequence
phase, the length of which is well-described by the Kelvin-Helmholz timescale.
Our results suggest that the lifetime of the YSO phase is \sim 10^5yrs, whereas
the compact Hii-region phase lasts between \sim 2 - 4 \times 10^5yrs depending
on the final mass of the star. Finally, the absolute numbers of YSOs are best
matched by a globally averaged star-formation rate for the Galaxy of
1.5-2Msun/yr.Comment: 22 pages, 24 figures. Accepted for publication in MNRA
High-mass star formation
Pamela D Klaassen and Joseph Mottram report on a workshop at the University of Leiden which discussed the formation of high-mass stars from large to small scales in the era of Herschel and ALMA
Weak-anchoring effects in a thin pinned ridge of nematic liquid crystal
A theoretical investigation of weak-anchoring effects in a thin two-dimensional pinned static ridge of nematic liquid crystal resting on a flat solid substrate in an atmosphere of passive gas is performed. Specifically, we solve a reduced version of the general system of governing equations recently derived by Cousins et al. [Proc. R. Soc. A 478, 20210849 (2022)] valid for a symmetric thin ridge under the one-constant approximation of the Frank-Oseen bulk elastic energy with pinned contact lines to determine the shape of the ridge and the behavior of the director within it. Numerical investigations covering a wide range of parameter values indicate that the energetically preferred solutions can be classified in terms of the Jenkins-Barratt-Barbero-Barberi critical thickness into five qualitatively different types of solution. In particular, the theoretical results suggest that anchoring breaking occurs close to the contact lines. The theoretical predictions are supported by the results of physical experiments for a ridge of the nematic 4 ′ -pentyl-4-biphenylcarbonitrile (5CB). In particular, these experiments show that the homeotropic anchoring at the gas-nematic interface is broken close to the contact lines by the stronger rubbed planar anchoring at the nematic-substrate interface. A comparison between the experimental values of and the theoretical predictions for the effective refractive index of the ridge gives a first estimate of the anchoring strength of an interface between air and 5CB to be (9.80 ± 1.12) × 10^(− 6) Nm^(−1) at a temperature of (22 ± 1.5)∘C
The JCMT BISTRO Survey: The Magnetic Field Strength in the Orion A Filament
We determine the magnetic field strength in the OMC 1 region of the Orion A filament via a new implementation of the Chandrasekhar-Fermi method using observations performed as part of the James Clerk Maxwell Telescope (JCMT) B-Fields In Star-Forming Region Observations (BISTRO) survey with the POL-2 instrument. We combine BISTRO data with archival SCUBA-2 and HARP observations to find a plane-of-sky magnetic field strength in OMC 1 of B_pos=6.6±4.7 mG, where δB_pos=4.7 mG represents a predominantly systematic uncertainty. We develop a new method for measuring angular dispersion, analogous to unsharp masking. We find a magnetic energy density of ~1.7×10^-7 Jm^-3 in OMC 1, comparable both to the gravitational potential energy density of OMC 1 (~10^-7 Jm^-3), and to the energy density in the Orion BN/KL outflow (~10^-7 Jm^-3). We find that neither the Alfvén velocity in OMC 1 nor the velocity of the super-Alfvénic outflow ejecta is sufficiently large for the BN/KL outflow to have caused large-scale distortion of the local magnetic field in the ~500-year lifetime of the outflow. Hence, we propose that the hour-glass field morphology in OMC 1 is caused by the distortion of a primordial cylindrically-symmetric magnetic field by the gravitational fragmentation of the filament and/or the gravitational interaction of the BN/KL and S clumps. We find that OMC 1 is currently in or near magnetically-supported equilibrium, and that the current large-scale morphology of the BN/KL outflow is regulated by the geometry of the magnetic field in OMC 1, and not vice versa
Guidelines for the use and interpretation of assays for monitoring autophagy (3rd edition)
In 2008 we published the first set of guidelines for standardizing research in autophagy. Since then, research on this topic has continued to accelerate, and many new scientists have entered the field. Our knowledge base and relevant new technologies have also been expanding. Accordingly, it is important to update these guidelines for monitoring autophagy in different organisms. Various reviews have described the range of assays that have been used for this purpose. Nevertheless, there continues to be confusion regarding acceptable methods to measure autophagy, especially in multicellular eukaryotes. For example, a key point that needs to be emphasized is that there is a difference between measurements that monitor the numbers or volume of autophagic elements (e.g., autophagosomes or autolysosomes) at any stage of the autophagic process versus those that measure fl ux through the autophagy pathway (i.e., the complete process including the amount and rate of cargo sequestered and degraded). In particular, a block in macroautophagy that results in autophagosome accumulation must be differentiated from stimuli that increase autophagic activity, defi ned as increased autophagy induction coupled with increased delivery to, and degradation within, lysosomes (inmost higher eukaryotes and some protists such as Dictyostelium ) or the vacuole (in plants and fungi). In other words, it is especially important that investigators new to the fi eld understand that the appearance of more autophagosomes does not necessarily equate with more autophagy. In fact, in many cases, autophagosomes accumulate because of a block in trafficking to lysosomes without a concomitant change in autophagosome biogenesis, whereas an increase in autolysosomes may reflect a reduction in degradative activity. It is worth emphasizing here that lysosomal digestion is a stage of autophagy and evaluating its competence is a crucial part of the evaluation of autophagic flux, or complete autophagy. Here, we present a set of guidelines for the selection and interpretation of methods for use by investigators who aim to examine macroautophagy and related processes, as well as for reviewers who need to provide realistic and reasonable critiques of papers that are focused on these processes. These guidelines are not meant to be a formulaic set of rules, because the appropriate assays depend in part on the question being asked and the system being used. In addition, we emphasize that no individual assay is guaranteed to be the most appropriate one in every situation, and we strongly recommend the use of multiple assays to monitor autophagy. Along these lines, because of the potential for pleiotropic effects due to blocking autophagy through genetic manipulation it is imperative to delete or knock down more than one autophagy-related gene. In addition, some individual Atg proteins, or groups of proteins, are involved in other cellular pathways so not all Atg proteins can be used as a specific marker for an autophagic process. In these guidelines, we consider these various methods of assessing autophagy and what information can, or cannot, be obtained from them. Finally, by discussing the merits and limits of particular autophagy assays, we hope to encourage technical innovation in the field