481 research outputs found
Micro-abrasion resistance of thermochemically treated steels in aqueous solutions: Mechanisms, maps, materials selection
The area of micro-abrasion is an interesting and relatively recent area in tribo-testing methodologies, where small particles of less than 10 μm are employed between interacting surfaces. It is topical for a number of reasons; its direct relation to the mechanisms of the wear process in bio-tribological applications, ease in conducting tests and the good repeatability of the test results. It has widespread applications in conditions used in the space and offshore industries to bio-engineering for artificial joints and implants. There have been many recent studies on the micro-abrasion performance of materials, ranging from work basic metals to nano-structured coatings. However, no significant work is reported on the micro-abrasion resistance of thermochemically treated steels. Hence, this paper looks at the performance of two thermochemically treated steels, Tenifer bath nitride stainless steel (T-SS) and vanadized carbon steel (V-CS) in such conditions with reference to the stainless steel (SS) by varying the applied load and sliding distance. The results indicated that T-SS demonstrates exceptionally poor resistance to micro-abrasion. It was observed that the heat treatment process and properties of the hardened layer (hardness and thickness) are extremely important in determining the micro-abrasion resistance of such steels. Finally, the results were used to develop micro-abrasion mechanism and wastage maps, which can be used to optimize the surface treated materials for micro-abrasion resistance
-prime and -primary -ideals on -schemes
Let be a flat finite-type group scheme over a scheme , and a
noetherian -scheme on which -acts. We define and study -prime and
-primary -ideals on and study their basic properties. In particular,
we prove the existence of minimal -primary decomposition and the
well-definedness of -associated -primes. We also prove a generalization
of Matijevic-Roberts type theorem. In particular, we prove Matijevic-Roberts
type theorem on graded rings for -regular and -rational properties.Comment: 54pages, added Example 6.16 and the reference [8]. The final versio
Single-lined Spectroscopic Binary Star Candidates in the RAVE Survey
Repeated spectroscopic observations of stars in the Radial Velocity
Experiment (RAVE) database are used to identify and examine single-lined binary
(SB1) candidates. The RAVE latest internal database (VDR3) includes radial
velocities, atmospheric and other parameters for approximately quarter million
of different stars with little less than 300,000 observations. In the sample of
~20,000 stars observed more than once, 1333 stars with variable radial
velocities were identified. Most of them are believed to be SB1 candidates. The
fraction of SB1 candidates among stars with several observations is between 10%
and 15% which is the lower limit for binarity among RAVE stars. Due to the
distribution of time spans between the re-observation that is biased towards
relatively short timescales (days to weeks), the periods of the identified SB1
candidates are most likely in the same range. Because of the RAVE's narrow
magnitude range most of the dwarf candidates belong to the thin Galactic disk
while the giants are part of the thick disk with distances extending to up to a
few kpc. The comparison of the list of SB1 candidates to the VSX catalog of
variable stars yielded several pulsating variables among the giant population
with the radial velocity variations of up to few tens of km/s. There are 26
matches between the catalog of spectroscopic binary orbits (SB9) and the whole
RAVE sample for which the given periastron time and the time of RAVE
observation were close enough to yield a reliable comparison. RAVE measurements
of radial velocities of known spectroscopic binaries are consistent with their
published radial velocity curves.Comment: 10 pages, 7 figures, accepted for publication in A
Exploring the Morphology of RAVE Stellar Spectra
The RAdial Velocity Experiment (RAVE) is a medium resolution R~7500
spectroscopic survey of the Milky Way which already obtained over half a
million stellar spectra. They present a randomly selected magnitude-limited
sample, so it is important to use a reliable and automated classification
scheme which identifies normal single stars and discovers different types of
peculiar stars. To this end we present a morphological classification of
350,000 RAVE survey stellar spectra using locally linear embedding, a
dimensionality reduction method which enables representing the complex spectral
morphology in a low dimensional projected space while still preserving the
properties of the local neighborhoods of spectra. We find that the majority of
all spectra in the database ~90-95% belong to normal single stars, but there is
also a significant population of several types of peculiars. Among them the
most populated groups are those of various types of spectroscopic binary and
chromospherically active stars. Both of them include several thousands of
spectra. Particularly the latter group offers significant further investigation
opportunities since activity of stars is a known proxy of stellar ages.
Applying the same classification procedure to the sample of normal single stars
alone shows that the shape of the projected manifold in two dimensional space
correlates with stellar temperature, surface gravity and metallicity.Comment: 28 pages, 11 figures, accepted for publication in ApJ
4MOST Consortium Survey 3: Milky Way Disc and Bulge Low-Resolution Survey (4MIDABLE-LR)
The mechanisms of the formation and evolution of the Milky Way are encoded in
the orbits, chemistry and ages of its stars. With the 4MOST MIlky way Disk And
BuLgE Low-Resolution Survey (4MIDABLE-LR) we aim to study kinematic and
chemical substructures in the Milky Way disc and bulge region with samples of
unprecedented size out to larger distances and greater precision than
conceivable with Gaia alone or any other ongoing or planned survey. Gaia gives
us the unique opportunity for target selection based almost entirely on
parallax and magnitude range, hence increasing the efficiency in sampling
larger Milky Way volumes with well-defined and effective selection functions.
Our main goal is to provide a detailed chrono-chemo-kinematical extended map of
our Galaxy and the largest Gaia follow-up down to magnitudes (Vega).
The complex nature of the disc components (for example, large target densities
and highly structured extinction distribution in the Milky Way bulge and disc
area), prompted us to develop a survey strategy with five main sub-surveys that
are tailored to answer the still open questions about the assembly and
evolution of our Galaxy, while taking full advantage of the Gaia data.Comment: Part of the 4MOST issue of The Messenger, published in preparation of
4MOST Community Workshop, see http://www.eso.org/sci/meetings/2019/4MOST.htm
The RAVE survey: the Galactic escape speed and the mass of the Milky Way
We construct new estimates on the Galactic escape speed at various
Galactocentric radii using the latest data release of the Radial Velocity
Experiment (RAVE DR4). Compared to previous studies we have a database larger
by a factor of 10 as well as reliable distance estimates for almost all stars.
Our analysis is based on the statistical analysis of a rigorously selected
sample of 90 high-velocity halo stars from RAVE and a previously published data
set. We calibrate and extensively test our method using a suite of cosmological
simulations of the formation of Milky Way-sized galaxies. Our best estimate of
the local Galactic escape speed, which we define as the minimum speed required
to reach three virial radii , is km/s (90%
confidence) with an additional 5% systematic uncertainty, where is
the Galactocentric radius encompassing a mean over-density of 340 times the
critical density for closure in the Universe. From the escape speed we further
derive estimates of the mass of the Galaxy using a simple mass model with two
options for the mass profile of the dark matter halo: an unaltered and an
adiabatically contracted Navarro, Frenk & White (NFW) sphere. If we fix the
local circular velocity the latter profile yields a significantly higher mass
than the un-contracted halo, but if we instead use the statistics on halo
concentration parameters in large cosmological simulations as a constraint we
find very similar masses for both models. Our best estimate for , the
mass interior to (dark matter and baryons), is M (corresponding to M). This estimate is in good agreement with recently published
independent mass estimates based on the kinematics of more distant halo stars
and the satellite galaxy Leo I.Comment: 16 pages, 15 figures; accepted for publication in Astronomy &
Astrophysic
Synthesis of CdS and CdSe nanocrystallites using a novel single-molecule precursors approach
The synthesis of CdS and CdSe nanocrystallites using the thermolysis of several dithioor
diselenocarbamato complexes of cadmium in trioctylphosphine oxide (TOPO) is reported.
The nanodispersed materials obtained show quantum size effects in their optical spectra
and exhibit near band-edge luminescence. The influence of experimental parameters on
the properties of the nanocrystallites is discussed. HRTEM images of these materials show
well-defined, crystalline nanosized particles. Standard size fractionation procedures can
be performed in order to narrow the size dispersion of the samples. The TOPO-capped CdS
and CdSe nanocrystallites and simple organic bridging ligands, such as 2,2¢-bipyrimidine,
are used as the starting materials for the preparation of novel nanocomposites. The optical
properties shown by these new nanocomposites are compared with those of the starting
nanodispersed materials
The VMC Survey -- XXXIV. Morphology of Stellar Populations in the Magellanic Clouds
The Magellanic Clouds are nearby dwarf irregular galaxies whose morphologies
show different properties when traced by different stellar populations, making
them an important laboratory for studying galaxy morphologies. We study the
morphology of the Magellanic Clouds using data from the VISTA survey of the
Magellanic Clouds system (VMC). We used about and million sources
across an area of deg and deg towards the Large and
Small Magellanic Cloud (LMC and SMC), respectively. We estimated median ages of
stellar populations occupying different regions of the near-infrared
() colour-magnitude diagram. Morphological maps
were produced and detailed features in the central regions were characterised
for the first time with bins corresponding to a spatial resolution of
kpc (LMC) and kpc (SMC). In the LMC, we find that main sequence stars
show coherent structures that grow with age and trace the multiple spiral arms
of the galaxy, star forming regions become dimmer as we progress in age, while
supergiant stars are centrally concentrated. Intermediate-age stars, despite
tracing a regular and symmetrical morphology, show central clumps and hints of
spiral arms. In the SMC, young main sequence stars depict a broken bar.
Intermediate-age populations show signatures of elongation towards the
Magellanic Bridge that can be attributed to the LMC-SMC interaction
Myr ago. They also show irregular central features suggesting that the inner
SMC has also been influenced by tidal interactions.Comment: Accepted for publication in MNRAS, 20 pages, 12 figures and 2 table
Yule-Simpson's paradox in Galactic Archaeology
Simpson’s paradox, or Yule–Simpson effect, arises when a trend appears in different subsets of data but disappears or reverses when these subsets are combined. We describe here seven cases of this phenomenon for chemo-kinematical relations believed to constrain the Milky Way disc formation and evolution. We show that interpreting trends in relations, such as the radial and vertical chemical abundance gradients, the age–metallicity relation, and the metallicity–rotational velocity relation (MVR), can lead to conflicting conclusions about the Galaxy past if analyses marginalize over stellar age and/or birth radius. It is demonstrated that the MVR in RAVE giants is consistent with being always strongly negative, when narrow bins of [Mg/Fe] are considered. This is directly related to the negative radial metallicity gradients of stars grouped by common age (mono-age populations) due to the inside-out disc formation. The effect of the asymmetric drift can then give rise to a positive MVR trend in high-[α/Fe] stars, with a slope dependent on a given survey’s selection function and observational uncertainties. We also study the variation of lithium abundance, A(Li), with [Fe/H] of AMBRE:HARPS dwarfs. A strong reversal in the positive A(Li)–[Fe/H] trend of the total sample is found for mono-age populations, flattening for younger groups of stars. Dissecting by birth radius shows strengthening in the positive A(Li)–[Fe/H] trend, shifting to higher [Fe/H] with decreasing birth radius; these observational results suggest new constraints on chemical evolution models. This work highlights the necessity for precise age estimates for large stellar samples covering wide spatial regions
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