1,128 research outputs found
Multiscale Analysis of the Gradient of Linear Polarisation
We propose a new multiscale method to calculate the amplitude of the gradient
of the linear polarisation vector using a wavelet-based formalism. We
demonstrate this method using a field of the Canadian Galactic Plane Survey
(CGPS) and show that the filamentary structure typically seen in gradients of
linear polarisation maps depends strongly on the instrumental resolution. Our
analysis reveals that different networks of filaments are present on different
angular scales. The wavelet formalism allows us to calculate the power spectrum
of the fluctuations seen in gradients of linear polarisation maps and to
determine the scaling behaviour of this quantity. The power spectrum is found
to follow a power law with gamma ~ 2.1. We identify a small drop in power
between scales of 80 < l < 300 arcmin, which corresponds well to the overlap in
the u-v plane between the Effelsberg 100-m telescope and the DRAO 26-m
telescope data. We suggest that this drop is due to undersampling present in
the 26-m telescope data. In addition, the wavelet coefficient distributions
show higher skewness on smaller scales than at larger scales. The spatial
distribution of the outliers in the tails of these distributions creates a
coherent subset of filaments correlated across multiple scales, which trace the
sharpest changes in the polarisation vector P within the field. We suggest that
these structures may be associated with highly compressive shocks in the
medium. The power spectrum of the field excluding these outliers shows a
steeper power law with gamma ~ 2.5.Comment: 12 pages, 12 figure
Principles and practices for the application of systems engineering to heterogeneous research partnerships
A heterogeneous research partnership (HRP) is one in which industry, academia and government collaborate to conduct research, typically of national importance. Whilst most HRPs complete their planned duration and deliver their agreed tasks, it is not uncommon for participants to be left feeling somewhat dissatisfied, suggesting that the requirements which are being met are incomplete. There is an opportunity to improve the success of HRPs by establishing principles and practices for the application of systems engineering in their development.
The thesis reviews literature drawn from a broad body of work covering three main areas: the context of HRPs themselves, systems engineering and related disciplines, and research methodology. The research adopts an interpretive approach, initially applying Soft Systems Methodology in a pilot case study and subsequently conducting a qualitative analysis of sixteen HRP case studies in order to develop and refine generic models which are relevant to HRPs. Drawing from the commentary of interviewees, published sources and other evidence, major themes across the case studies are integrated in order to develop ten principles and ten practices for the application of systems engineering to HRPs.
The importance of consistency between the research context, systems approach and research methodology is emphasised, and the thesis highlights a significant philosophical challenge facing system of systems research as the discipline seeks to use a range of hard and soft systems approaches which are fundamentally rooted in different paradigms
Compaction à sec et saturée de renforts textiles utilisés pour la fabrication de pièces composites
Modèles du comportement mécanique en compaction d'assemblages homogènes de fibres d'orientations aléatoires ou arbitraires -- Modèles du comportement mécanique en compaction de renforts utilisés pour la production de pièces composites -- Compaction of textile reinforcements for composites manufacturing : -- review of experimental results -- Compaction of textile reinforcements for composites manufacturing : -- compaction and relaxation of dry and H[indice inférieur 2] O -- saturated woven reinforcements -- Compaction of textile reinforcements for composites manufacturing : -- reorganization of the fiber network -- Modélisation géométrique des renforts textiles plans
A Water Maser and Ammonia Survey of GLIMPSE Extended Green Objects (EGOs)
We present the results of a Nobeyama 45-m water maser and ammonia survey of
all 94 northern GLIMPSE Extended Green Objects (EGOs), a sample of massive
young stellar objects (MYSOs) identified based on their extended 4.5 micron
emission. We observed the ammonia (1,1), (2,2), and (3,3) inversion lines, and
detect emission towards 97%, 63%, and 46% of our sample, respectively (median
rms ~50 mK). The water maser detection rate is 68% (median rms ~0.11 Jy). The
derived water maser and clump-scale gas properties are consistent with the
identification of EGOs as young MYSOs. To explore the degree of variation among
EGOs, we analyze subsamples defined based on MIR properties or maser
associations. Water masers and warm dense gas, as indicated by emission in the
higher-excitation ammonia transitions, are most frequently detected towards
EGOs also associated with both Class I and II methanol masers. 95% (81%) of
such EGOs are detected in water (ammonia(3,3)), compared to only 33% (7%) of
EGOs without either methanol maser type. As populations, EGOs associated with
Class I and/or II methanol masers have significantly higher ammonia linewidths,
column densities, and kinetic temperatures than EGOs undetected in methanol
maser surveys. However, we find no evidence for statistically significant
differences in water maser properties (such as maser luminosity) among any EGO
subsamples. Combining our data with the 1.1 mm continuum Bolocam Galactic Plane
Survey, we find no correlation between isotropic water maser luminosity and
clump number density. Water maser luminosity is weakly correlated with clump
(gas) temperature and clump mass.Comment: Astrophysical Journal, accepted. Emulateapj, 24 pages including 24
figures, plus 9 tables (including full content of online-only tables
The infrared dust bubble N22: an expanding HII region and the star formation around it
Aims. To increase the observational samples of star formation around
expanding Hii regions, we analyzed the interstellar medium and star formation
around N22.
Methods. We used data extracted from the seven large-scale surveys from
infrared to radio wavelengths. In addition we used the JCMT observations of the
J = 3-2 line of 12CO emission data released on CADC and the 12CO J = 2-1 and J
=3-2 lines observed by the KOSMA 3 m telescope. We performed a multiwavelength
study of bubble N22.
Results. A molecular shell composed of several clumps agrees very well with
the border of N22, suggesting that its expansion is collecting the surrounding
material. The high integrated 12CO line intensity ratio (ranging from 0.7 to
1.14) implies that shocks have driven into the molecular clouds. We identify
eleven possible O-type stars inside the Hii region, five of which are located
in projection inside the cavity of the 20 cm radio continuum emission and are
probably the exciting-star candidates of N22. Twenty-nine YSOs (young stellar
objects) are distributed close to the dense cores of N22. We conclude that star
formation is indeed active around N22; the formation of most of YSOs may have
been triggered by the expanding of the Hii region. After comparing the
dynamical age of N22 and the fragmentation time of the molecular shell, we
suggest that radiation-driven compression of pre-existing dense clumps may be
ongoing.Comment: accepted in A&A 30/05/2012. arXiv admin note: text overlap with
arXiv:1010.5430 by other author
Characterizing star formation activity in infrared dark cloud MSXDC G048.65-00.29
Infrared Dark Clouds (IRDCs), condensed regions of the ISM with high column
densities, low temperatures and high masses, are suspected sites of star
formation. Thousands of IRDCs have already been identified. To date, it has not
been resolved whether IRDCs always show star formation activity and, if so, if
massive star formation (> 8 solar masses) is the rule or the exception in
IRDCs. Previous analysis of sub-millimeter cores in the cloud MSXDC
G048.65-00.29 (G48.65) indicates embedded star formation activity. To
characterize this activity in detail, mid-infrared photometry (3-30 micron) has
been obtained with the Spitzer Space Telescope. This paper analyzes the point
sources seen in the 24 micron band, combined with counterparts or upper limits
at shorter and longer wavelengths. Data points in wavelength bands ranging from
1 up to 850 micron are used to compare each 24 micron source to a set of
Spectral Energy Distributions of Young Stellar Object (YSO) models. By
assessing the models that fit the data, an attempt is made to identify YSOs as
such and determine their evolutionary stages and stellar masses. A total of 17
sources are investigated, 13 of which are classified as YSOs, primarily - but
not exclusively - in an early embedded phase of star formation. The modeled
masses of the central stellar objects range from sub-solar to ~8 solar masses.
Every YSO is at less than 1 pc projected distance from its nearest YSO
neighbor. We conclude that IRDC G48.65 is a region of active star formation. We
find YSOs in various evolutionary phases, indicating that the star formation in
this cloud is not an instantaneous process. The inferred masses of the central
objects suggest that this IRDC hosts only low to intermediate mass YSOs and
none with masses exceeding ~8 solar masses.Comment: 10 pages, 6 figures; v2: minor editorial changes to match published
versio
Pre-main sequence stars with disks in the Eagle Nebula observed in scattered light
NGC6611 and its parental cloud, the Eagle Nebula (M16), are well-studied
star-forming regions, thanks to their large content of both OB stars and stars
with disks and the observed ongoing star formation. We identified 834
disk-bearing stars associated with the cloud, after detecting their excesses in
NIR bands from J band to 8.0 micron. In this paper, we study in detail the
nature of a subsample of disk-bearing stars that show peculiar characteristics.
They appear older than the other members in the V vs. V-I diagram, and/or they
have one or more IRAC colors at pure photospheric values, despite showing NIR
excesses, when optical and infrared colors are compared. We confirm the
membership of these stars to M16 by a spectroscopic analysis. The physical
properties of these stars with disks are studied by comparing their spectral
energy distributions (SEDs) with the SEDs predicted by models of T-Tauri stars
with disks and envelopes. We show that the age of these stars estimated from
the V vs. V-I diagram is unreliable since their V-I colors are altered by the
light scattered by the disk into the line of sight. Only in a few cases their
SEDs are compatible with models with excesses in V band caused by optical
veiling. Candidate members with disks and photospheric IRAC colors are selected
by the used NIR disk diagnostic, which is sensitive to moderate excesses, such
as those produced by disks with low masses. In 1/3 of these cases, scattering
of stellar flux by the disks can also be invoked. The photospheric light
scattered by the disk grains into the line of sight can affect the derivation
of physical parameters of ClassII stars from photometric optical and NIR data.
Besides, the disks diagnostic we defined are useful for selecting stars with
disks, even those with moderate excesses or whose optical colors are altered by
veiling or photospheric scattered light.Comment: Accepted for publication in A&
The pre-ZAMS nature of Mol160/IRAS23385+6053 confirmed by Spitzer
Determining the timeline for the formation of massive YSOs requires the
identification and characterisation of all the phases that a massive forming
YSO undergoes. It is of particular interest to verify the observability of the
phase in which the object is rapidly accreting while not yet igniting the
fusion of hydrogen that marks the arrival on the ZAMS. One of the candidate
prototypical objects for this phase is Mol160/IRAS23385+6053, which previous
studies suggest it could be in a pre-Hot Core stage. We further investigate
this issue by means of Spitzer imaging and spectroscopy in the 5-70 micron
range. The dense core of Mol160/IRAS23385+6053, which up to now had only been
detected at submm and mm wavelenghts has been revealed for the first time at 24
and 70 micron by Spitzer. The complete 24 micron -3.4 mm continuum cannot be
fitted with a standard model of a Zero-Age Main-Sequence (ZAMS) star embedded
in an envelope. A simple greybody fit yields a mass of 220 solar masses. The
luminosity is slightly in excess of 3000 solar luminosities, which is a factor
of 5 less than previous estimates when only IRAS fluxes were available between
20 and 100 micron. The source is under-luminous by the same factor with respect
to UCHII regions or Hot-Cores of similar circumstellar mass, and simple models
show that this is compatible with an earlier evolutionary stage. Spectroscopy
between 5 and 40 microns revelas typical PDR/PIR conditions, where the required
UV illumination may be provided by other sources revealed at 24 microns in the
same region, and which can be plausibly modeled as moderately embedded
intermediate-mass ZAMS stars. Our results strengthen the suggestion that the
central core in Mol160/IRAS23385+6053 is a massive YSO actively accreting from
its circumstellar envelope and which did not yet begin hydrogen fusion.Comment: Accepted by A&
Sky maps without anisotropies in the cosmic microwave background are a better fit to WMAP's uncalibrated time ordered data than the official sky maps
The purpose of this reanalysis of the WMAP uncalibrated time ordered data
(TOD) was two fold. The first was to reassess the reliability of the detection
of the anisotropies in the official WMAP sky maps of the cosmic microwave
background (CMB). The second was to assess the performance of a proposed
criterion in avoiding systematic error in detecting a signal of interest. The
criterion was implemented by testing the null hypothesis that the uncalibrated
TOD was consistent with no anisotropies when WMAP's hourly calibration
parameters were allowed to vary. It was shown independently for all 20 WMAP
channels that sky maps with no anisotropies were a better fit to the TOD than
those from the official analysis. The recently launched Planck satellite should
help sort out this perplexing result.Comment: 11 pages with 1 figure and 2 tables. Extensively rewritten to explain
the research bette
Galaxy Formation with local photoionisation feedback I. Methods
We present a first study of the effect of local photoionising radiation on
gas cooling in smoothed particle hydrodynamics simulations of galaxy formation.
We explore the combined effect of ionising radiation from young and old stellar
populations. The method computes the effect of multiple radiative sources using
the same tree algorithm used for gravity, so it is computationally efficient
and well resolved. The method foregoes calculating absorption and scattering in
favour of a constant escape fraction for young stars to keep the calculation
efficient enough to simulate the entire evolution of a galaxy in a cosmological
context to the present day. This allows us to quantify the effect of the local
photoionisation feedback through the whole history of a galaxy`s formation. The
simulation of a Milky Way like galaxy using the local photoionisation model
forms ~ 40 % less stars than a simulation that only includes a standard uniform
background UV field. The local photoionisation model decreases star formation
by increasing the cooling time of the gas in the halo and increasing the
equilibrium temperature of dense gas in the disc. Coupling the local radiation
field to gas cooling from the halo provides a preventive feedback mechanism
which keeps the central disc light and produces slowly rising rotation curves
without resorting to extreme feedback mechanisms. These preliminary results
indicate that the effect of local photoionising sources is significant and
should not be ignored in models of galaxy formation.Comment: Accepted for Publication in MNRAS, 13 pages, 13 figure
- …