1,976 research outputs found
Design of a five-axis ultra-precision micro-milling machine—UltraMill. Part 2: Integrated dynamic modelling, design optimisation and analysis
Using computer models to predict the dynamic performance of ultra-precision machine tools can help manufacturers to substantially reduce the lead time and cost of developing new machines. However, the use of electronic drives on such machines is becoming widespread, the machine dynamic performance depending not only on the mechanical structure and components but also on the control system and electronic drives. Bench-top ultra-precision machine tools are highly desirable for the micro-manufacturing of high-accuracy micro-mechanical components. However, the development is still at the nascent stage and hence lacks standardised guidelines. Part 2 of this two-part paper proposes an integrated approach, which permits analysis and optimisation of the entire machine dynamic performance at the early design stage. Based on the proposed approach, the modelling and simulation process of a novel five-axis bench-top ultra-precision micro-milling machine tool—UltraMill—is presented. The modelling and simulation cover the dynamics of the machine structure, the moving components, the control system and the machining process and are used to predict the entire machine performance of two typical configurations
Molecular phylogeny of brachiopods and phoronids based on nuclear-encoded small subunit ribosomal RNA gene sequences
Brachiopod and phoronid phylogeny is inferred from SSU rDNA sequences of 28 articulate and nine inarticulate brachiopods, three phoronids, two ectoprocts and various outgroups, using gene trees reconstructed by weighted parsimony, distance and maximum likelihood methods. Of these sequences, 33 from brachiopods, two from phoronids and one each from an ectoproct and a priapulan are newly determined. The brachiopod sequences belong to 31 different genera and thus survey about 10% of extant genus-level diversity. Sequences determined in different laboratories and those from closely related taxa agree well, but evidence is presented suggesting that one published phoronid sequence (GenBank accession UO12648) is a brachiopod-phoronid chimaera, and this sequence is excluded from the analyses. The chiton, Acanthopleura, is identified as the phenetically proximal outgroup; other selected outgroups were chosen to allow comparison with recent, non-molecular analyses of brachiopod phylogeny. The different outgroups and methods of phylogenetic reconstruction lead to similar results, with differences mainly in the resolution of weakly supported ancient and recent nodes, including the divergence of inarticulate brachiopod sub-phyla, the position of the rhynchonellids in relation to long- and short-looped articulate brachiopod clades and the relationships of some articulate brachiopod genera and species. Attention is drawn to the problem presented by nodes that are strongly supported by non-molecular evidence but receive only low bootstrap resampling support. Overall, the gene trees agree with morphology-based brachiopod taxonomy, but novel relationships are tentatively suggested for thecideidine and megathyrid brachiopods. Articulate brachiopods are found to be monophyletic in all reconstructions, but monophyly of inarticulate brachiopods and the possible inclusion of phoronids in the inarticulate brachiopod clade are less strongly established. Phoronids are clearly excluded from a sister-group relationship with articulate brachiopods, this proposed relationship being due to the rejected, chimaeric sequence (GenBank UO12648). Lineage relative rate tests show no heterogeneity of evolutionary rate among articulate brachiopod sequences, but indicate that inarticulate brachiopod plus phoronid sequences evolve somewhat more slowly. Both brachiopods and phoronids evolve slowly by comparison with other invertebrates. A number of palaeontologically dated times of earliest appearance are used to make upper and lower estimates of the global rate of brachiopod SSU rDNA evolution, and these estimates are used to infer the likely divergence times of other nodes in the gene tree. There is reasonable agreement between most inferred molecular and palaeontological ages. The estimated rates of SSU rDNA sequence evolution suggest that the last common ancestor of brachiopods, chitons and other protostome invertebrates (Lophotrochozoa and Ecdysozoa) lived deep in Precambrian time. Results of this first DNA-based, taxonomically representative analysis of brachiopod phylogeny are in broad agreement with current morphology-based classification and systematics and are largely consistent with the hypothesis that brachiopod shell ontogeny and morphology are a good guide to phylogeny
Cicrumnuclear Supernova Remnants and HII Regions in NGC 253
Archival VLA data has been used to produce arcsecond-resolution 6- and 20-cm
images of the region surrounding the nuclear 200-pc (~15") starburst in NGC
253. Twenty-two discrete sources stronger than 0.4 mJy have been detected
within ~2 kpc (~3') of the galaxy nucleus; almost all these sources must be
associated with the galaxy. None of the radio sources coincides with a detected
X-ray binary, so they appear to be due to supernova remnants and H II regions.
The region outside the central starburst has a derived radio supernova rate of
<~0.1/yr, and may account for at least 20% of the recent star formation in NGC
253. Most of the newly identified sources have steep, nonthermal radio spectra,
but several relatively strong thermal sources also exist, containing the
equivalent of tens of O5 stars. These stars are spread over tens of parsecs,
and are embedded in regions having average ionized gas densities of
20-200/cm^3, much lower than in the most active nuclear star-forming regions in
NGC 253 or in the super star clusters seen in other galaxies. The strongest
region of thermal emission coincides with a highly reddened area seen at
near-infrared wavelengths, possibly containing optically obscured H II regions.Comment: 17 pages, 3 postscript figures, AASTeX format, in press for
Astronomical Journal, July 200
Elemental Abundances of Nearby Galaxies through High Signal-to-Noise XMM-Newton Observations of ULXs
(abridged) In this paper, we examined XMM Newton EPIC spectra of 14
ultra-luminous X-ray sources (ULXs)in addition to the XMM RGS spectra of two
sources (Holmberg II X-1 and Holmberg IX X-1). We determined oxygen and iron
abundances of the host galaxy's interstellar medium (ISM) using K-shell (O) and
L-shell (Fe) X-ray photo-ionization edges towards these ULXs. We found that the
oxygen abundances closely matched recent solar abundances for all of our
sources, implying that ULXs live in similar local environments despite the wide
range of galaxy host properties. Also, we compare the X-ray hydrogen column
densities (n_H) for 8 ULX sources with column densities obtained from radio H I
observations. The X-ray model n_H values are in good agreement with the H I n_H
values, implying that the hydrogen absorption towards the ULXs is not local to
the source (with the exception of the source M81 XMM1). In order to obtain the
column density and abundance values, we fit the X-ray spectra of the ULXs with
a combined power law and one of several accretion disk models. We tested the
abundances obtained from the XSPEC models bbody, diskbb, grad, and diskpn along
with a power law, finding that the abundances were independent of the thermal
model used. We comment on the physical implications of these different model
fits. We also note that very deep observations allow a breaking of the
degeneracy noted by Stobbart et al. (2006) favoring a high mass solution for
the absorbed grad + power law model.Comment: 18 pages, accepted to Ap
XMM-Newton Archival Study of the ULX Population in Nearby Galaxies
We present the results of an archival XMM-Newton study of the bright X-ray
point sources (L_X > 10^38 erg/s) in 32 nearby galaxies. From our list of
approximately 100 point sources, we attempt to determine if there is a
low-state counterpart to the Ultraluminous X-ray (ULX) population, searching
for a soft-hard state dichotomy similar to that known for Galactic X-ray
binaries and testing the specific predictions of the IMBH hypothesis. To this
end, we searched for "low-state" objects, which we defined as objects within
our sample which had a spectrum well fit by a simple absorbed power law, and
"high-state" objects, which we defined as objects better fit by a combined
blackbody and a power law. Assuming that ``low-state'' objects accrete at
approximately 10% of the Eddington luminosity (Done & Gierlinski 2003) and that
"high-state" objects accrete near the Eddington luminosity we further divided
our sample of sources into low and high state ULX sources. We classify 16
sources as low-state ULXs and 26 objects as high-state ULXs. As in Galactic
black hole systems, the spectral indices, Gamma, of the low-state objects, as
well as the luminosities, tend to be lower than those of the high-state
objects. The observed range of blackbody temperatures for the high state is
0.1-1 keV, with the most luminous systems tending toward the lowest
temperatures. We therefore divide our high-state ULXs into candidate IMBHs
(with blackbody temperatures of approximately 0.1 keV) and candidate stellar
mass BHs (with blackbody temperatures of approximately 1.0 keV). A subset of
the candidate stellar mass BHs have spectra that are well-fit by a
Comptonization model, a property similar of Galactic BHs radiating in the
"very-high" state near the Eddington limit.Comment: 54 pages, submitted to ApJ (March 2005), accepted (May 2006); changes
to organization of pape
Radio Continuum Jet in NGC 7479
The barred galaxy NGC 7479 hosts a remarkable jet-like radio continuum
feature: bright, 12-kpc long in projection, and hosting an aligned magnetic
field. The degree of polarization is 6%-8% along the jet, and remarkably
constant, which is consistent with helical field models. The radio brightness
of the jet suggests strong interaction with the ISM and hence a location near
the disk plane. We observed NGC 7479 at four wavelengths with the VLA and
Effelsberg radio telescopes. The equipartition strength is 35-40 micro-G for
the total and >10 micro-G for the ordered magnetic field in the jet. The jet
acts as a bright, polarized background. Faraday rotation between 3.5 and 6 cm
and depolarization between 6 and 22 cm can be explained by magneto-ionic gas in
front of the jet, with thermal electron densities of ~0.06 cm**(-3) in the bar
and ~0.03 cm**(-3) outside the bar. The regular magnetic field along the bar
points toward the nucleus on both sides. The regular field in the disk reveals
multiple reversals, probably consisting of field loops stretched by a shearing
gas flow in the bar. The projection of the jet bending in the sky plane is in
the sense opposite to that of the underlying stellar and gaseous spiral
structure. The bending in 3-D is most easily explained as a precessing jet,
with an age less than 10**6 years. Our observations are consistent with very
recent triggering, possibly by a minor merger. NGC 7479 provides a unique
opportunity to study interaction-triggered 15-kpc scale radio jets within a
spiral galaxy.Comment: 18 pages, 21 figures, accepted for publication in the Astrophysical
Journa
A Search for Extraplanar Dust in Nearby Edge-On Spirals
We present high resolution BV images of 12 edge-on spiral galaxies observed
with the WIYN 3.5-m telescope. These images were obtained to search for
extraplanar (|z| > 0.4 kpc) absorbing dust structures similar to those
previously found in NGC 891 (Howk & Savage 1997). Our imaged galaxies include a
sample of seven massive L_*-like spiral galaxies within D<25 Mpc that have
inclinations i > 87 deg from the plane of the sky. We find that five of these
seven systems show extraplanar dust, visible as highly-structured absorbing
clouds against the background stellar light of the galaxies. The more prominent
structures are estimated to have associated gas masses >10^5 M_sun; the implied
potential energies are > 10^(52) ergs. All of the galaxies in our sample that
show detectable halpha emission at large z also show extraplanar dust
structures. None of those galaxies for which extraplanar halpha searches were
negative show evidence for extensive high-z dust. The existence of extraplanar
dust is a common property of massive spiral galaxies. We discuss several
mechanisms for shaping the observed dust features, emphasizing the possibility
that these dusty clouds represent the dense phase of a multiphase medium at
high-z in spiral galaxies. The correlation between high-z dust and extraplanar
Halpha emission may simply suggest that both trace the high-z interstellar
medium in its various forms (or phases), the existence of which may ultimately
be driven by vigorous star formation in the underlying disk. (Abstract
abridged)Comment: 26 pages; 15 jpeg figures. To appear in The Astronomical Journal, May
1999. Gzipped tar files of high-resolution figures in postscript and jpeg
formats are available at
http://www.astro.wisc.edu/~howk/Papers/papers.html#surve
Exceeding the threshold value for Trioza apicalis Förster 1848 in carrot fields did not cause damage as revealed during monitoring in Germany from 2017–2020
The carrot psyllid Trioza apicalis Förster 1848 is a carrot pest in Europe that can cause serious damages in case of massive occurrence. Damages up to a total loss of yield have been reported from Scandinavian countries but also from Switzerland. The action threshold to control the pest with chemical pesticides is 0.2 T. apicalis per day and trap caught by sticky traps. We investigated the number of T. apicalis with sticky traps on carrot fields of the study regions Lüneburg/Uelzen and Hameln/Bad Pyrmont in Germany, during the period 2017–2020. The number of T. apicalis caught was generally very low in both study regions. On several fields in successive weeks almost no individuals were found in the study region Hameln/Bad Pyrmont. In Lüneburg/Uelzen was at least one field each year where the number of carrot psyllid was clearly higher than in all other fields and exceeded the threshold level. Surprisingly on carrot fields in close proximity to carrot fields from the previous year, the T. apicalis numbers were only slightly increased. Nonetheless, no loss of yield was reported for any of the fields in the four years of the study, although the generally defined threshold has been exceeded on many of the investigated carrot fields.Bundesanstalt für Landwirtschaft und Ernährung
http://dx.doi.org/10.13039/501100010771Bundesanstalt für Landwirtschaft und Ernährung (DE)Julius Kühn-Institut (JKI), Bundesforschungsinstitut für Kulturpflanzen (4250)Peer Reviewe
Exceeding the threshold value for Trioza apicalis Förster 1848 in carrot fields did not cause damage as revealed during monitoring in Germany from 2017–2020
The carrot psyllid Trioza apicalis Förster 1848 is a carrot pest in Europe that can cause serious damages in case of massive occurrence. Damages up to a total loss of yield have been reported from Scandinavian countries but also from Switzerland. The action threshold to control the pest with chemical pesticides is 0.2 T. apicalis per day and trap caught by sticky traps. We investigated the number of T. apicalis with sticky traps on carrot fields of the study regions Lüneburg/Uelzen and Hameln/Bad Pyrmont in Germany, during the period 2017–2020. The number of T. apicalis caught was generally very low in both study regions. On several fields in successive weeks almost no individuals were found in the study region Hameln/Bad Pyrmont. In Lüneburg/Uelzen was at least one field each year where the number of carrot psyllid was clearly higher than in all other fields and exceeded the threshold level. Surprisingly on carrot fields in close proximity to carrot fields from the previous year, the T. apicalis numbers were only slightly increased. Nonetheless, no loss of yield was reported for any of the fields in the four years of the study, although the generally defined threshold has been exceeded on many of the investigated carrot fields
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