2,956 research outputs found
On the evidence for brown-dwarf secondary stars in cataclysmic variables
We present the K-band spectrum of the cataclysmic variable LL And, obtained
using NIRSPEC on Keck-II. The spectrum shows no evidence for the absorption
features observed by Howell & Ciardi (2001), which these authors used to claim
a detection of a brown-dwarf secondary star in LL And. In light of our new
data, we review the evidence for brown-dwarf secondary stars in this and other
cataclysmic variables.Comment: 6 pages, to appear in Monthly Notices, accepte
To achieve and to conform: Motivational values predict social comparison orientation
As personal motivational values have been shown to associate with personality traits, we explore whether they
also predict social comparison orientation (SCO), the proclivity to compare with other people in order to inform
self-evaluation judgements. Participants completed the Portrait Values Questionnaire (Schwartz et al., 2001) to
measure personal values and INCOM (Gibbons & Buunk, 1999) to assess SCO. Bayesian analyses revealed that
the personal values of achievement and conformity strongly predicted SCO. The association between SCO and
three other personal values—power, universalism, and benevolence—were mediated by achievement. The
findings suggest that the tendency to compare to others is determined by very two different motivational values,
gaining personal success through demonstrating competence according to social standards and exercising behaviours and attitudes that adhere to social norm
Random Projections For Large-Scale Regression
Fitting linear regression models can be computationally very expensive in
large-scale data analysis tasks if the sample size and the number of variables
are very large. Random projections are extensively used as a dimension
reduction tool in machine learning and statistics. We discuss the applications
of random projections in linear regression problems, developed to decrease
computational costs, and give an overview of the theoretical guarantees of the
generalization error. It can be shown that the combination of random
projections with least squares regression leads to similar recovery as ridge
regression and principal component regression. We also discuss possible
improvements when averaging over multiple random projections, an approach that
lends itself easily to parallel implementation.Comment: 13 pages, 3 Figure
First Lunar Occultation Results from the 2.4 m Thai National Telescope equipped with ULTRASPEC
The recently inaugurated 2.4\,m Thai National Telescope (TNT) is equipped,
among other instruments, with the ULTRASPEC low-noise, frame-transfer EMCCD
camera. At the end of its first official observing season, we report on the use
of this facility to record high time resolution imaging using small detector
subarrays with sampling as fast as several \,Hz. In particular, we have
recorded lunar occultations of several stars which represent the first
contribution to this area of research made from South-East Asia with a
telescope of this class. Among the results, we discuss an accurate measurement
of ~Cnc, which has been reported previously as a suspected close
binary. Attempts to resolve this star by several authors have so far met with a
lack of unambiguous confirmation. With our observation we are able to place
stringent limits on the projected angular separation (<0\farcs003) and
brightness () of a putative companion. We also present a
measurement of the binary {HR~7072}, which extends considerably the time
coverage available for its yet undetermined orbit. We discuss our precise
determination of the flux ratio and projected separation in the context of
other available data. We conclude by providing an estimate of the performance
of ULTRASPEC at TNT for lunar occultation work. This facility can help to
extend the lunar occultation technique in a geographical area where no
comparable resources were available until now.Comment: Accepted for publication in Astronomical Journa
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
Estimating the masses of extra-solar planets
All extra-solar planet masses that have been derived spectroscopically are
lower limits since the inclination of the orbit to our line-of-sight is unknown
except for transiting systems. It is, however, possible to determine the
inclination angle, i, between the rotation axis of a star and an observer's
line-of-sight from measurements of the projected equatorial velocity (v sin i),
the stellar rotation period (P_rot) and the stellar radius (R_star). This
allows the removal of the sin i dependency of spectroscopically derived
extra-solar planet masses under the assumption that the planetary orbits lie
perpendicular to the stellar rotation axis. We have carried out an extensive
literature search and present a catalogue of v sin i, P_rot, and R_star
estimates for exoplanet host stars. In addition, we have used Hipparcos
parallaxes and the Barnes-Evans relationship to further supplement the R_star
estimates obtained from the literature. Using this catalogue, we have obtained
sin i estimates using a Markov-chain Monte Carlo analysis. This allows proper
1-sigma two-tailed confidence limits to be placed on the derived sin i's along
with the transit probability for each planet to be determined. While a small
proportion of systems yield sin i's significantly greater than 1, most likely
due to poor P_rot estimations, the large majority are acceptable. We are
further encouraged by the cases where we have data on transiting systems, as
the technique indicates inclinations of ~90 degrees and high transit
probabilities. In total, we estimate the true masses of 133 extra-solar
planets. Of these, only 6 have revised masses that place them above the 13
Jupiter mass deuterium burning limit. Our work reveals a population of
high-mass planets with low eccentricities and we speculate that these may
represent the signature of different planetary formation mechanisms at work.Comment: 40 pages, 6 tables, 2 figures. Accepted for publication in the
Monthly Notices of the Royal Astronomical Society after editing of Tables 1 &
6 for electronic publication. Html abstract shortened for astro-ph submissio
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Aggressive digital papillary adenocarcinoma and sentinel node biopsy: A case report and literature review.
Aggressive digital papillary adenocarcinomas (ADPA) are malignant tumours of the sweat gland. Due to the high tendency to recur and metastasise, wide local resection of the lesion is recommended. However, there are a limited number of cases reporting the use of sentinel node biopsy in the management of ADPA, thus its effectiveness remains unclear. We present a case of ADPA of the right middle finger treated with digital amputation and sentinel node biopsy, and review the current literature focusing on the usefulness of sentinel node biopsy
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