2,257 research outputs found
The Eye of the Tornado - an isolated, high mass young stellar object near the Galactic centre
We present infrared (AAT, UKIRT) and radio (VLA, SEST) observations of the
Eye of the Tornado, a compact source apparently near the head of the Tornado
Nebula. The near-infrared Br-gamma and He I lines are broad (FWHM 40 and 30
km/s, respectively) and have a line centre at Vlsr = -205 km/s. This
corresponds to a feature at the same velocity in the 12CO J=1-0 line profile.
The kinematic velocity derived from Galactic rotation places the Eye at the
distance of the Galactic Centre (i.e. 8.5 kpc) and separated (probably
foreground) from the Tornado Nebula. Four knots of emission are seen in the
Br-gamma line and at 6 and 20 cm. Together with the flat radio spectral index,
we confirm that the Eye contains ionized gas, but that this is embedded within
a dense molecular core. The spectral energy distribution can be modelled as a
two-component blackbody + greybody, peaking at far-IR wavelengths. The knots
are UC HII regions, and the core contains a luminous (2 x 10^4 Lsun), embedded,
massive young stellar source. We also propose a geometrical model for the Eye
to account for both its spectral energy distribution and its morphology.Comment: 25 pages, including 5 figures. Accepted by Monthly Notices of the
Royal Astronomical Society on 27/10/0
Multiwavelength observations of the supernova remnant G349.7+02 interacting with a molecular cloud
We present molecular-line observations at millimetre, centimetre and infrared
wavelengths of the region containing OH(1720 MHz) masers in the supernova
remnant (SNR) G349.7+0.2, using the Australia Telescope (AT) Mopra antenna, the
Swedish-ESO Submillimeter Telescope, the AT Compact Array and the UNSW Infrared
Fabry-Perot narrow-band filter installed on the Anglo-Australian Telescope.
Several molecular transitions were observed between 1.6 and 3 mm to constrain
the physical parameters of the molecular cloud interacting with the SNR and to
investigate the effects of the SNR shock on the gas chemistry. We detected
shock-excited near-infrared H2 emission towards the centre of the SNR,
revealing highly clumped molecular gas and a good correlation with published
mid-infrared images from the Spitzer Space Telescope. An excellent correlation
between the H2 clumps and OH(1720 MHz) maser positions supports the shock
excitation of the OH(1720 MHz) maser emission. Furthermore, we detected OH
absorption at 1665 and 1667 MHz which shows a good correlation with the shocked
H2 emission and the masers. We found maser emission at 1665 MHz near the
OH(1720 MHz) masers in this SNR, which is found to be associated with a GLIMPSE
source SSTGLMC G349.7294+00.1747. We also detected 1665 and 1667 MHz OH masers,
and weak 4.8 GHz H2CO absorption towards the ultracompact HII region IRAS
17147-3725 located to the southeast of the SNR. We found no 4.7- or 6-GHz
excited-state OH masers or 6-GHz CH3OH maser towards either the SNR or the HII
region.Comment: 25 pages, 13 figures, published in MNRA
Navigating the structural landscape of de Novo α-helical bundles
The
association of amphipathic α helices in water leads to
α-helical-bundle protein structures. However, the driving force
for thisthe hydrophobic effectis not specific and
does not define the number or the orientation of helices in the associated
state. Rather, this is achieved through deeper sequence-to-structure
relationships, which are increasingly being discerned. For example,
for one structurally extreme but nevertheless ubiquitous class of
bundlethe α-helical coiled coilsrelationships
have been established that discriminate between all-parallel dimers,
trimers, and tetramers. Association states above this are known, as
are antiparallel and mixed arrangements of the helices. However, these
alternative states are less well understood. Here, we describe a synthetic-peptide
system that switches between parallel hexamers and various up–down–up–down
tetramers in response to single-amino-acid changes and solution conditions.
The main accessible states of each peptide variant are characterized
fully in solution and, in most cases, to high resolution with X-ray
crystal structures. Analysis and inspection of these structures helps
rationalize the different states formed. This navigation of the structural
landscape of α-helical coiled coils above the dimers and trimers
that dominate in nature has allowed us to design rationally a well-defined
and hyperstable antiparallel coiled-coil tetramer (apCC-Tet). This
robust de novo protein provides another scaffold for further structural
and functional designs in protein engineering and synthetic biology
Hardness of porous nanocrystalline Co-Ni electrodeposits
The Hall-Petch relationship can fail when the grain size is below a critical value of tens of nanometres. This occurs particularly for coatings having porous surfaces. In this study, electrodeposited nanostructured Co-Ni coatings from four different nickel electroplating baths having grain sizes in the range of 11-23 nm have been investigated. The finest grain size, approximately 11 nm, was obtained from a coating developed from the nickel sulphate bath. The Co-Ni coatings have a mixed face centred cubic and hexagonal close-packed structures with varying surface morphologies and different porosities. A cluster-pore mixture model has been proposed by considering no contribution from pores to the hardness. As the porosity effect was taken into consideration, the calculated pore-free hardness is in agreement with the ordinary Hall-Petch relationship even when the grain size is reduced to 11 nm for the Co-Ni coatings with 77±2 at% cobalt. The present model was applied to other porous nanocrystalline coatings, and the Hall-Petch relationship was maintained. © 2013 The Korean Institute of Metals and Materials and Springer Science+Business Media Dordrecht. © KIM and Springer
The RMS Survey: 13CO observations of candidate massive YSOs in the southern hemisphere
Abridged: The Red MSX Source (RMS) survey is an ongoing multi-wavelength
observational programme designed to return a large, well-selected sample of
massive young stellar objects (MYSOs). Here we present 13CO observations made
towards 854 MYSOs candidates located in the 3rd and 4th quadrants. We detected
13CO emission towards a total of 751 of the 854 RMS sources observed (~88%). In
total 2185 emission components are detected above 3 level. Multiple
emission profiles are observed towards the majority of these sources - 455
sources (~60%) - with an average of ~4 molecular clouds along the line of
sight. These multiple emission features make it difficult to assign a kinematic
velocity to many of our sample. We have used archival CS (J=2-1) and maser
velocities to resolved the component multiplicity towards 82 sources and have
derived a criterion which is used to identify the most likely component for a
further 202 multiple component sources. Combined with the single component
detections we have obtained unambiguous kinematic velocities towards 580
sources (~80% of the detections). The 171 sources for which we have not been
able to determine the kinematic velocity will require additional line data.
Using the rotation curve of Brand and Blitz (1993) and their radial velocities
we calculate kinematic distances for all components detected.Comment: Summitted to A&A, the resolution of figure 1 has been reduced,
samples of Table 3 and Figure 11 are now included but the full version will
only be available in the online version of the journa
The Giant Pillars of the Carina Nebula
Results are presented from a multi-wavelength study of the giant pillars
within the Carina Nebula. Using near-IR data from 2MASS, mid-IR data from MSX,
843MHz radio continuum maps from the MOST, and molecular line and continuum
observations from the SEST, we investigate the nature of the pillars and search
for evidence of ongoing star formation within them. Photodissociation regions
(PDRs) exist across the whole nebula and trace the giant pillars, as well as
many ridges, filaments, and condensations (Av > 7 mag). Morphological
similarities between emission features at 21um and 843MHz adjacent to the PDRs,
suggests that the molecular material has been carved by the intense stellar
winds and UV radiation from the nearby massive stars. In addition, star forming
cores are found at the tips of several of the pillars. Using a stellar density
distribution, several candidate embedded clusters are also found. One is
clearly seen in the 2MASS images and is located within a dense core
(G287.84-0.82). A search for massive young stellar objects and compact HII
regions using mid-IR colour criteria, reveal twelve candidates across the
complex. Grey-body fits to SEDs for four of these objects are suggestive of
OB-stars. We find that massive star formation in the Carina Nebula is occurring
across the whole complex and confirm it has been continuous over the past 3
Myrs.Comment: 14 pages, 10 figures (low resolution), accepted by A&
Comparison of techniques for handling missing covariate data within prognostic modelling studies: a simulation study
Background: There is no consensus on the most appropriate approach to handle missing covariate data within prognostic modelling studies. Therefore a simulation study was performed to assess the effects of different missing data techniques on the performance of a prognostic model.
Methods: Datasets were generated to resemble the skewed distributions seen in a motivating breast cancer example. Multivariate missing data were imposed on four covariates using four different mechanisms; missing completely at random (MCAR), missing at random (MAR), missing not at random (MNAR) and a combination of all three mechanisms. Five amounts of incomplete cases from 5% to 75% were considered. Complete case analysis (CC), single imputation (SI) and five multiple imputation (MI) techniques available within the R statistical software were investigated: a) data augmentation (DA) approach assuming a multivariate normal distribution, b) DA assuming a general location model, c) regression switching imputation, d) regression switching with predictive mean matching (MICE-PMM) and e) flexible additive imputation models. A Cox proportional hazards model was fitted and appropriate estimates for the regression coefficients and model performance measures were obtained.
Results: Performing a CC analysis produced unbiased regression estimates, but inflated standard errors, which affected the significance of the covariates in the model with 25% or more missingness. Using SI, underestimated the variability; resulting in poor coverage even with 10% missingness. Of the MI approaches, applying MICE-PMM produced, in general, the least biased estimates and better coverage for the incomplete covariates and better model performance for all mechanisms. However, this MI approach still produced biased regression coefficient estimates for the incomplete skewed continuous covariates when 50% or more cases had missing data imposed with a MCAR, MAR or combined mechanism. When the missingness depended on the incomplete covariates, i.e. MNAR, estimates were biased with more than 10% incomplete cases for all MI approaches.
Conclusion: The results from this simulation study suggest that performing MICE-PMM may be the preferred MI approach provided that less than 50% of the cases have missing data and the missing data are not MNAR
12mm line survey of the dense molecular gas towards the W28 field TeV gamma-ray sources
We present 12mm Mopra observations of dense molecular gas towards the W28
supernova remnant (SNR) field. The focus is on the dense molecular gas towards
the TeV gamma-ray sources detected by the H.E.S.S. telescopes, which likely
trace the cosmic-rays from W28 and possibly other sources in the region. Using
the NH3 inversion transitions we reveal several dense cores inside the
molecular clouds, the majority of which coincide with high-mass star formation
and HII regions, including the energetic ultra-compact HII region G5.89-0.39. A
key exception to this is the cloud north east of W28, which is well-known to be
disrupted as evidenced by clusters of 1720MHz OH masers and broad CO line
emission. Here we detect broad NH3, up to the (9,9) transition, with linewidths
up to 16 km/s. This broad NH3 emission spatially matches well with the TeV
source HESS J1801-233 and CO emission, and its velocity dispersion distribution
suggests external disruption from the W28 SNR direction. Other lines are
detected, such as HC3N and HC5N, H2O masers, and many radio recombination
lines, all of which are primarily found towards the southern high-mass star
formation regions. These observations provide a new view onto the internal
structures and dynamics of the dense molecular gas towards the W28 SNR field,
and in tandem with future higher resolution TeV gamma-ray observations will
offer the chance to probe the transport of cosmic-rays into molecular clouds.Comment: 18 pages, 10 figures, 5 tables, accepted for publication in MNRAS.
Online appendices containing additional molecular line: fit parameters, maps,
PV plots & spectra, will be available through MNRA
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