786 research outputs found
Influence of roughness on ZDDP tribofilm formation in boundary lubricated fretting
Influence of initial surface topography on tribofilm formation in ZDDP lubricated contact was analysed. A small displacement fretting tests with sinusoidal motion were carried out in classical sphere/plane configuration. A range of surfaces with different initial roughness were prepared by milling and grinding processes. Tests were carried out using variable displacement method where amplitude of imposed displacement was gradually increased after every 1000 cycles from 2 to 30 ”m. The surfaces after tribological tests were measured by interferometric profiler. Main findings confirm that initial roughness has a significant influence on antiwear tribofilm formation in boundary lubricated contact. Tribofilm form faster and require less energy to activate in case of rough surface obtained by milling process than in case of smooth grinded surface. However, in contact lubricated by ZDDP additive a significant transfer of material occurred from plane to sphere specimen
Elemental boron doping behavior in silicon molecular beam epitaxy
Boron-doped Si epilayers were grown by molecular beam epitaxy (MBE) using an elemental boron source, at levels up to 2Ă1020 cmâ3, to elucidate profile control and electrical activation over the growth temperature range 450â900 °C. Precipitation and surface segregation effects were observed at doping levels of 2Ă1020 cmâ3 for growth temperatures above 600 °C. At growth temperatures below 600 °C, excellent profile control was achieved with complete electrical activation at concentrations of 2Ă1020 cmâ3, corresponding to the optimal MBE growth conditions for a range of Si/SixGe1âx heterostructures
OGLE-2002-BLG-360: from a gravitational microlensing candidate to an overlooked red transient
OGLE-2002-BLG-360 was discovered as a microlensing candidate by the OGLE-III
project. The subsequent light curve however clearly showed that the brightening
of the object could not have resulted from the gravitational microlensing
phenomenon. We aim to explain the nature of OGLE-2002-BLG-360 and its eruption
observed in 2002--2006. The observational data primarily come from the archives
of the OGLE project, which monitored the object in 2001--2009. The archives of
the MACHO and MOA projects also provided us with additional data obtained in
1995--99 and 2000--2005, respectively. These data allowed us to analyse the
light curve of the object during its eruption, as well as the potential
variability of its progenitor. In the archives of several infrared surveys,
namely 2MASS, MSX, Spitzer, AKARI, WISE, and VVV, we found measurements of the
object, which allowed us to study the spectral energy distribution (SED) of the
object. We constructed a simple model of a star surrounded by a dusty envelope,
which was used to interpret the observed SED. Our analysis of the data clearly
shows that OGLE-2002-BLG-360 was most probably a red transient, i.e. an object
similar in nature to V838 Mon, whose eruption was observed in 2002. The SED in
all phases, i.e. progenitor, eruption, and remnant, was dominated by infrared
emission, which we interpret as evidence of dust formation in an intense mass
outflow. Since 2009 the object has been completely embedded in dust. We suggest
that the progenitor of OGLE-2002-BLG-360 was a binary, which had entered the
common-envelope phase a long time (at least decades) before the observed
eruption, and that the eruption resulted from the final merger of the binary
components. We point out similarities between OGLE-2002-BLG-360 and CK Vul,
whose eruption was observed in 1670--72, and this strengthens the hypothesis
that CK Vul was also a red transient.Comment: accepted in A&
Growth studies on Si0.8Ge0.2 channel two-dimensional hole gases
We report a study of the influences of MBE conditions on the low-temperature mobilities of Si/Si0.8Ge0.2 2DHG structures. A significant dependence of 2DHG mobility on growth temperature is observed with the maximum mobility of 3640 cm2 Vâ1 sâ1 at 5.4 K being achieved at the relatively high-growth temperature of 640 °C. This dependence is associated with a reduction in interface charge density. Studies on lower mobility samples show that Cu contamination can be reduced both by growth interruptions and by modifications to the Ge source; this reduction produces improvements in the low-temperature mobilities. We suggest that interface charge deriving from residual metal contamination is currently limiting the 4-K mobility
Warm Breeze from the starboard bow: a new population of neutral helium in the heliosphere
We investigate the signals from neutral He atoms observed from Earth orbit in
2010 by IBEX. The full He signal observed during the 2010 observation season
can be explained as a superposition of pristine neutral interstellar He gas and
an additional population of neutral He that we call the Warm Breeze. The Warm
Breeze is approximately two-fold slower and 2.5 times warmer than the primary
interstellar He population, and its density in front of the heliosphere is ~7%
that of the neutral interstellar helium. The inflow direction of the Warm
Breeze differs by ~19deg from the inflow direction of interstellar gas. The
Warm Breeze seems a long-term feature of the heliospheric environment. It has
not been detected earlier because it is strongly ionized inside the
heliosphere, which brings it below the threshold of detection via pickup ion
and heliospheric backscatter glow observations, as well as by the direct
sampling of GAS/Ulysses. Possible sources for the Warm Breeze include (1) the
secondary population of interstellar helium, created via charge exchange and
perhaps elastic scattering of neutral interstellar He atoms on interstellar He+
ions in the outer heliosheath, or (2) a gust of interstellar He originating
from a hypothetic wave train in the Local Interstellar Cloud. A secondary
population is expected from models, but the characteristics of the Warm Breeze
do not fully conform to modeling results. If, nevertheless, this is the
explanation, IBEX-Lo observations of the Warm Breeze provide key insights into
the physical state of plasma in the outer heliosheath. If the second hypothesis
is true, the source is likely to be located within a few thousand of AU from
the Sun, which is the propagation range of possible gusts of interstellar
neutral helium with the Warm Breeze characteristics against dissipation via
elastic scattering in the Local Cloud.Comment: submitted to ApJ
Interstellar neutral helium in the heliosphere from IBEX observations. III. Mach number of the flow, velocity vector, and temperature from the first six years of measurements
We analyzed observations of interstellar neutral helium (ISN~He) obtained
from the Interstellar Boundary Explorer (IBEX) satellite during its first six
years of operation. We used a refined version of the ISN~He simulation model,
presented in the companion paper by Sokol_et al. 2015, and a sophisticated data
correlation and uncertainty system and parameter fitting method, described in
the companion paper by Swaczyna et al 2015. We analyzed the entire data set
together and the yearly subsets, and found the temperature and velocity vector
of ISN~He in front of the heliosphere. As seen in the previous studies, the
allowable parameters are highly correlated and form a four-dimensional tube in
the parameter space. The inflow longitudes obtained from the yearly data
subsets show a spread of ~6 degree, with the other parameters varying
accordingly along the parameter tube, and the minimum chi-square value is
larger than expected. We found, however, that the Mach number of the ISN~He
flow shows very little scatter and is thus very tightly constrained. It is in
excellent agreement with the original analysis of ISN~He observations from IBEX
and recent reanalyses of observations from Ulysses. We identify a possible
inaccuracy in the Warm Breeze parameters as the likely cause of the scatter in
the ISN~He parameters obtained from the yearly subsets, and we suppose that
another component may exist in the signal, or a process that is not accounted
for in the current physical model of ISN~He in front of the heliosphere. From
our analysis, the inflow velocity vector, temperature, and Mach number of the
flow are equal to lambda_ISNHe = 255.8 +/- 0.5 degree, beta_ISNHe = 5.16 +/-
0.10 degree, T_ISNHe = 7440 +/- 260 K, v_ISNHe = 25.8 +/- 0.4$ km/s, and
M_ISNHe = 5.079 +/- 0.028, with uncertainties strongly correlated along the
parameter tube.Comment: Updated reference
Microlensing optical depth toward the Galactic Bulge using bright sources from OGLE-II
We present a measurement of the microlensing optical depth toward the
Galactic Bulge based on 4 years of the OGLE-II survey using Red Clump Giant
(RCG). Using 32 events we find tau=2.55_{-0.46}^{+0.57}* 10^{-6} at
(l,b)=(1.16, -2.75). Taking into account the measured gradient along the
Galactic latitude b, tau = [ (4.48+/- 2.37) + (0.78+/- 0.84)* b]* 10^{-6}, this
value is consistent with previous measurements using RCG sources and recent
theoretical predictions. We determine the microlensing parameters and select
events using a model light curve with the flux blending. We find that ~38% of
the OGLE-II events which appear to have RCG sources are actually due to much
fainter stars blended with a bright companion. We show explicitly that model
fits without blending result in similar tau estimates through partial
cancellation of contributions from higher detection efficiency, underestimated
time-scales and larger number of selected events. This approach, however, leads
to biased time-scale distributions and event rates. Consequently, microlensing
studies should carefully consider source confusion effects even for bright
stars.Comment: 49 pages and 18 figures, ApJ in press, the value changed due to the
systematic correctio
The distance to the Galactic Centre based on Population-II Cepheids and RR Lyrae stars
Context: The distance to the Galactic Centre (GC) is of importance for the
distance scale in the Universe. The value derived by Eisenhauer et al. (2005)
of 7.62 +- 0.32 kpc based on the orbit of one star around the central black
hole is shorter than most other distance estimates based on a variety of
different methods. Aim: To establish an independent distance to the GC with
high accuracy. To this end Population-II Cepheids are used that have been
discovered in the OGLE-II and III surveys. Method: Thirty-nine Pop-II Cepheids
have been monitored on 4 nights spanning 14 days. Light curves have been fitted
using the known periods from the OGLE data to determine the mean K-band
magnitude. It so happens that 37 RR Lyrae stars are in the field-of-views and
mean K-band magnitudes are derived for this sample as well. Results: The
period-luminosity relation of Pop-II Cepheids in the K-band is determined, and
the derived slope of -2.24 +- 0.14 is consistent with the value derived by
Matsunaga et al. (2006). Fixing the slope to their more accurate value results
in a zero point, and implies a distance modulus to the GC of 14.51 +- 0.12,
with an additional systematic uncertainty of 0.07 mag. Similarly, from the RR
Lyrae K-band PL-relation we derive a value of 14.48 +- 0.17 (random) +- 0.07
(syst.). The two independent determinations are averaged to find 14.50 +- 0.10
(random) +- 0.07 (syst.), or 7.94 +- 0.37 +- 0.26 kpc.Comment: A&A accepte
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