3,647 research outputs found
The effect of deposition forming by metal inert gas welding and rolling on the properties of steels
Imperial Users onl
A Self-Occulting Accretion Disk in the SW Sex Star DW UMa
We present the ultraviolet spectrum of the SW Sex star and nova-like variable
DW UMa in an optical low state, as observed with the Space Telescope Imaging
Spectrograph on board the Hubble Space Telescope (HST). The data are well
described by a synthetic white dwarf (WD) spectrum with T_eff = 46,000 +/- 1000
K, log g = 7.60 +/- 0.15, v*sin(i) = 370 +/- 100 km/s and Z/Z_solar = 0.47 +/-
0.15. For this combination of T_eff and log g, WD models predict M_WD = 0.48
+/- 0.06 M_solar and R_WD = (1.27 +/- 0.18) * 10^9 cm. Combining the radius
estimate with the normalization of the spectral fit, we obtain a distance
estimate of d = 830 +/-150 pc.
During our observations, DW UMa was approximately 3 magnitudes fainter in V
than in the high state. A comparison of our low-state HST spectrum to a
high-state spectrum obtained with the International Ultraviolet Explorer shows
that the former is much bluer and has a higher continuum level shortward of
1450 A. Since DW UMa is an eclipsing system, this suggests that an optically
thick accretion disk rim blocks our view of the WD primary in the high state.
If self-occulting accretion disks are common among the SW Sex stars, we can
account for (i) the preference for high-inclination systems within the class
and (ii) their V-shaped continuum eclipses. Moreover, even though the emission
lines produced by a self-obscured disk are generally still double-peaked, they
are weaker and narrower than those produced by an unobscured disk. This may
allow a secondary line emission mechanism to dominate and produce the
single-peaked, optical lines that are a distinguishing characteristic of the SW
Sex stars.Comment: 9 pages, including 2 figures; accepted for publication in
Astrophysical Journal Letters; New version matches version in press (footnote
added to discussion section; figures now use color
On the use of electron-multiplying CCDs for astronomical spectroscopy
Conventional CCD detectors have two major disadvantages: they are slow to
read out and they suffer from read noise. These problems combine to make
high-speed spectroscopy of faint targets the most demanding of astronomical
observations. It is possible to overcome these weaknesses by using
electron-multiplying CCDs (EMCCDs). EMCCDs are conventional frame-transfer
CCDs, but with an extended serial register containing high-voltage electrodes.
An avalanche of secondary electrons is produced as the photon-generated
electrons are clocked through this register, resulting in signal amplification
that renders the read noise negligible. Using a combination of laboratory
measurements with the QUCAM2 EMCCD camera and Monte Carlo modelling, we show
that it is possible to significantly increase the signal-to-noise ratio of an
observation by using an EMCCD, but only if it is optimised and utilised
correctly. We also show that even greater gains are possible through the use of
photon counting. We present a recipe for astronomers to follow when setting up
a typical EMCCD observation which ensures that maximum signal-to-noise ratio is
obtained. We also discuss the benefits that EMCCDs would bring if used with the
next generation of extremely large telescopes. Although we mainly consider the
spectroscopic use of EMCCDs, our conclusions are equally applicable to imaging.Comment: 18 figures, 3 tables, 18 page
Information Aggregation in Exponential Family Markets
We consider the design of prediction market mechanisms known as automated
market makers. We show that we can design these mechanisms via the mold of
\emph{exponential family distributions}, a popular and well-studied probability
distribution template used in statistics. We give a full development of this
relationship and explore a range of benefits. We draw connections between the
information aggregation of market prices and the belief aggregation of learning
agents that rely on exponential family distributions. We develop a very natural
analysis of the market behavior as well as the price equilibrium under the
assumption that the traders exhibit risk aversion according to exponential
utility. We also consider similar aspects under alternative models, such as
when traders are budget constrained
An observational test of common-envelope evolution
By analysing and modelling the change in the abundance ratio of
C/C and O/O on the surface of the lower mass star
of a binary during the common-envelope (CE) phase of evolution, we propose a
simple observational test of the CE scenario. The test is based on the infrared
measurement of either the C/C or O/O ratio of red
dwarfs in post-common envelope binaries (PCEB's). In certain cases
(main-sequence red dwarf secondaries in PCEB's without planetary nebulae), as
well as determining whether or not accretion has occurred during the CE phase,
we can determine the amount of mass accreted during the CE phase and hence the
initial mass of the red dwarf component prior to the CE phase. In the other
cases considered (low-mass red dwarfs in PCEB's and red dwarf's in PCEB's with
planetary nebulae) we can only say whether or not accretion has occurred during
the CE phase.Comment: uuencoded compressed postscript. The preprint are also available at
URL http://www.ast.cam.ac.uk/preprint/PrePrint.htm
LS Peg: A Low-Inclination SW Sextantis-Type Cataclysmic Binary with High-Velocity Balmer Emission Line Wings
We present time-resolved spectroscopy and photometry of the bright
cataclysmic variable LS Peg (= S193). The Balmer lines exhibit broad,
asymmetric wings Doppler-shifted by about 2000 km/s at the edges, while the HeI
lines show phase-dependent absorption features strikingly similar to SW
Sextantis stars, as well as emission through most of the phase. The CIII/NIII
emission blend does not show any phase dependence. From velocities of Halpha
emission lines, we determine an orbital period of 0.174774 +/- 0.000003 d (=
4.1946 h), which agrees with Szkody's (1995) value of approximately 4.2 hours.
No stable photometric signal was found at the orbital period. A non-coherent
quasi-periodic photometric signal was seen at a period of 20.7 +/- 0.3 min.
The high-velocity Balmer wings most probably arise from a stream re-impact
point close to the white dwarf. We present simulated spectra based on a
kinematic model similar to the modified disk-overflow scenario of Hellier &
Robinson (1994). The models reproduce the broad line wings, though some other
details are unexplained.
Using an estimate of dynamical phase based on the model, we show that the
phasing of the emission- and absorption-line variations is consistent with that
in (eclipsing) SW Sex stars. We therefore identify LS Peg as a low-inclination
SW Sex star.
Our model suggests i = 30 deg, and the observed absence of any photometric
signal at the orbital frequency establishes i < 60 deg. This constraint puts a
severe strain on interpretations of the SW Sex phenomenon which rely on disk
structures lying slightly out of the orbital plane.Comment: 29 pages, 13 figures, to be published in PASP Feb. 199
A Risk Comparison of Ordinary Least Squares vs Ridge Regression
We compare the risk of ridge regression to a simple variant of ordinary least
squares, in which one simply projects the data onto a finite dimensional
subspace (as specified by a Principal Component Analysis) and then performs an
ordinary (un-regularized) least squares regression in this subspace. This note
shows that the risk of this ordinary least squares method is within a constant
factor (namely 4) of the risk of ridge regression.Comment: Appearing in JMLR 14, June 201
Complex Multimodality Central Airway Management of Aspergillus Pseudomembranous Tracheobronchitis
A 69-year-old woman developed central airway obstruction due to invasive Aspergillus infection resulting in pseudomembranous tracheobronchitis (PTB). Several challenges were encountered in the airway management of this patient including her having relatively smaller airways which were more prone to obstruction by pseudomembranes and made airway interventions difficult. The patient had clinical deterioration in spite of antifungal therapy and bronchoscopic debridement. The multimodality airway techniques included the use of smaller biliary balloons for dilatation, using a hybrid stent to slowly dilate and maintain patency of right main stem and finally the insertion of right secondary carina peripheral Y stent, which resulted in clinical improvement and allowed time for antifungal therapy to take effect. To the best of our knowledge, the use of a peripheral Y stent has never been described in the setting of PTB
Infrared spectroscopy of cataclysmic variables: III. Dwarf novae below the period gap and novalike variables
We present K-band spectra of the short-period dwarf novae YZ Cnc, LY Hya, BK
Lyn, T Leo, SW UMa and WZ Sge, the novalike variables DW UMa, V1315 Aql, RW
Tri, VY Scl, UU Aqr and GP Com, and a series of field dwarf stars with spectral
types ranging from K2-M6.
The spectra of the dwarf novae are dominated by emission lines of HI and HeI.
The large velocity and equivalent widths of these lines, in conjunction with
the fact that the lines are double-peaked in the highest inclination systems,
indicate an accretion disc origin. In the case of YZ Cnc and T Leo, for which
we obtained time-resolved data covering a complete orbital cycle, the emission
lines show modulations in their equivalent widths which are most probably
associated with the bright spot (the region where the gas stream collides with
the accretion disc). There are no clear detections of the secondary star in any
of the dwarf novae below the period gap, yielding upper limits of 10-30% for
the contribution of the secondary star to the observed K-band flux. In
conjunction with the K-band magnitudes of the dwarf novae, we use the derived
secondary star contributions to calculate lower limits to the distances to
these systems.
The spectra of the novalike variables are dominated by broad, single-peaked
emission lines of HI and HeI - even the eclipsing systems we observed do not
show the double-peaked profiles predicted by standard accretion disc theory.
With the exception of RW Tri, which exhibits NaI, CaI and 12CO absorption
features consistent with a M0V secondary contributing 65% of the observed
K-band flux, we find no evidence for the secondary star in any of the novalike
variables. The implications of this result are discussed.Comment: 13 pages, 5 figures, to appear in MNRA
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