336 research outputs found
Flexible Commissioning: A prevention and access focused approach in Yorkshire and the Humber
No abstract available
The chromospherically--active binary CF Tuc revisited
New high-resolution spectra, of the chromospherically active binary system CF
Tuc, taken at the Mt. John University Observatory in 2007, were analyzed using
two methods: cross-correlation and Fourier--based disentangling. As a result,
new radial velocity curves of both components were obtained. The resulting
orbital elements of CF Tuc are: = AU,
= AU, =
, and = . The cooler
component of the system shows H and CaII H & K emissions. Our
spectroscopic data and recent light curves were solved simultaneously
using the Wilson-Devinney code. A dark spot on the surface of the cooler
component was assumed to explain large asymmetries observed in the light
curves. The following absolute parameters of the components were determined:
= , = ,
= , = ,
= and = . The
orbital period of the system was studied using the O-C analysis. The O-C
diagram could be interpreted in terms of either two abrupt changes or a
quasi-sinusoidal form superimposed on a downward parabola. These variations are
discussed by reference to the combined effect of mass transfer and mass loss,
the Applegate mechanism and also a light-time effect due to the existence of a
massive third body (possibly a black hole) in the system. The distance to CF
Tuc was calculated to be pc from the dynamic parallax, neglecting
interstellar absorption, in agreement with the Hipparcos value.Comment: 33 pages, 10 figures, accepted for publication by MNRA
G-band Spectral Synthesis in Solar Magnetic Concentrations
Narrow band imaging in the G-band is commonly used to trace the small
magnetic field concentrations of the Sun, although the mechanism that makes
them bright has remained unclear. We carry out LTE syntheses of the G-band in
an assorted set of semi-empirical model magnetic concentrations. The syntheses
include all CH lines as well as the main atomic lines within the band-pass. The
model atmospheres produce bright G-band spectra having many properties in
common with the observed G-band bright points. In particular, the contrast
referred to the quiet Sun is about twice the contrast in continuum wavelengths.
The agreement with observations does not depend on the specificities of the
model atmosphere, rather it holds from single fluxtubes to MIcro-Structured
Magnetic Atmospheres. However, the agreement requires that the real G-band
bright points are not spatially resolved, even in the best observations. Since
the predicted G-band intensities exceed by far the observed values, we foresee
a notable increase of contrast of the G-band images upon improvement of the
angular resolution. According to the LTE modeling, the G-band spectrum emerges
from the deep photosphere that produces the continuum. Our syntheses also
predict solar magnetic concentrations showing up in continuum images but not in
the G-band . Finally, we have examined the importance of the CH
photo-dissociation in setting the amount of G-band absorption. It turns out to
play a minor role.Comment: To appear in ApJ, 554 n2 Jun 20, 33 pages and 9 figure
Study of variable stars in the MOA data base: long-period red variables in the Large Magellanic Cloud
One hundred and forty six long-period red variable stars in the Large
Magellanic Cloud (LMC) from the three year MOA project database were analysed.
A careful periodic analysis was performed on these stars and a catalogue of
their magnitudes, colours, periods and amplitudes is presented. We convert our
blue and red magnitudes to band values using 19 oxygen-rich stars. A group
of red short-period stars separated from the Mira sequence has been found on a
(log P, K) diagram. They are located at the short period side of the Mira
sequence consistent with the work of Wood and Sebo (1996). There are two
interpretations for such stars; a difference in pulsation mode or a difference
in chemical composition. We investigated the properties of these stars together
with their colour, amplitude and periodicity. We conclude that they have small
amplitudes and less regular variability. They are likely to be higher mode
pulsators. A large scatter has been also found on the long period side of the
(log P, K) diagram. This is possibly a systematic spread given that the blue
band of our photometric system covers both standard B and V bands and affects
carbon-rich stars.Comment: 19 pages, 19 figures, accepted for publication in MNRA
The Mt John University Observatory Search For Earth-mass Planets In The Habitable Zone Of Alpha Centauri
The "holy grail" in planet hunting is the detection of an Earth-analog: a
planet with similar mass as the Earth and an orbit inside the habitable zone.
If we can find such an Earth-analog around one of the stars in the immediate
solar neighborhood, we could potentially even study it in such great detail to
address the question of its potential habitability. Several groups have focused
their planet detection efforts on the nearest stars. Our team is currently
performing an intensive observing campaign on the alpha Centauri system using
the Hercules spectrograph at the 1-m McLellan telescope at Mt John University
Observatory (MJUO) in New Zealand. The goal of our project is to obtain such a
large number of radial velocity measurements with sufficiently high temporal
sampling to become sensitive to signals of Earth-mass planets in the habitable
zones of the two stars in this binary system. Over the past years, we have
collected more than 45,000 spectra for both stars combined. These data are
currently processed by an advanced version of our radial velocity reduction
pipeline, which eliminates the effect of spectral cross-contamination. Here we
present simulations of the expected detection sensitivity to low-mass planets
in the habitable zone by the Hercules program for various noise levels. We also
discuss our expected sensitivity to the purported Earth-mass planet in an
3.24-d orbit announced by Dumusque et al.~(2012).Comment: 16 pages, 7 figures, accepted for publication in the International
Journal of Astrobiolog
The origins of specificity in the microcin-processing protease TldD/E
TldD and TldE proteins are involved in the biosynthesis of microcin B17 (MccB17), an Escherichia coli thiazole/oxazole-modified peptide toxin targeting DNA gyrase. Using a combination of biochemical and crystallographic methods we show that E. coli TldD and TldE interact to form a heterodimeric metalloprotease. TldD/E cleaves the N-terminal leader sequence from the modified MccB17 precursor peptide, to yield mature antibiotic, while it has no effect on the unmodified peptide. Both proteins are essential for the activity; however, only the TldD subunit forms a novel metal-containing active site within the hollow core of the heterodimer. Peptide substrates are bound in a sequence-independent manner through beta sheet interactions with TldD and are likely cleaved via a thermolysin-type mechanism. We suggest that TldD/E acts as a "molecularpencil sharpener'': unfoldedpoly-peptides are fed through a narrow channel into the active site and processively truncated through the cleavage of short peptides from the N-terminal end
MOA 2003-BLG-37: A Bulge Jerk-Parallax Microlens Degeneracy
We analyze the Galactic bulge microlensing event MOA-2003-BLG-37. Although
the Einstein timescale is relatively short, t_e=43 days, the lightcurve
displays deviations consistent with parallax effects due to the Earth's
accelerated motion. We show that the chi^2 surface has four distinct local
minima that are induced by the ``jerk-parallax'' degeneracy, with pairs of
solutions having projected Einstein radii, \tilde r_e = 1.76 AU and 1.28 AU,
respectively. This is the second event displaying such a degeneracy and the
first toward the Galactic bulge. For both events, the jerk-parallax formalism
accurately describes the offsets between the different solutions, giving hope
that when extra solutions exist in future events, they can easily be found.
However, the morphologies of the chi^2 surfaces for the two events are quite
different, implying that much remains to be understood about this degeneracy.Comment: 19 pages, 3 figures, 1 table, ApJ, in press, 1 July 200
Structure and Evolution of Nearby Stars with Planets. I. Short-Period Systems
Using the Yale stellar evolution code, we have calculated theoretical models
for nearby stars with planetary-mass companions in short-period nearly circular
orbits: 51 Pegasi, Tau Bootis, Upsilon Andromedae, Rho Cancri, and Rho Coronae
Borealis. We present tables listing key stellar parameters such as mass,
radius, age, and size of the convective envelope as a function of the
observable parameters (luminosity, effective temperature, and metallicity), as
well as the unknown helium fraction. For each star we construct best models
based on recently published spectroscopic data and the present understanding of
galactic chemical evolution. We discuss our results in the context of planet
formation theory, and, in particular, tidal dissipation effects and stellar
metallicity enhancements.Comment: 48 pages including 13 tables and 5 figures, to appear in Ap
On Planetary Companions to the MACHO-98-BLG-35 Microlens Star
We present observations of microlensing event MACHO-98-BLG-35 which reached a
peak magnification factor of almost 80. These observations by the Microlensing
Planet Search (MPS) and the MOA Collaborations place strong constraints on the
possible planetary system of the lens star and show intriguing evidence for a
low mass planet with a mass fraction . A giant planet with is excluded from 95%
of the region between 0.4 and 2.5 from the lens star, where is the
Einstein ring radius of the lens. This exclusion region is more extensive than
the generic "lensing zone" which is . For smaller mass planets,
we can exclude 57% of the "lensing zone" for and 14% of
the lensing zone for . The mass fraction corresponds to an Earth mass planet for a lensing star of mass \sim
0.3 \msun. A number of similar events will provide statistically significant
constraints on the prevalence of Earth mass planets. In order to put our limits
in more familiar terms, we have compared our results to those expected for a
Solar System clone averaging over possible lens system distances and
orientations. We find that such a system is ruled out at the 90% confidence
level. A copy of the Solar System with Jupiter replaced by a second Saturn mass
planet can be ruled out at 70% confidence. Our low mass planetary signal (few
Earth masses to Neptune mass) is significant at the confidence
level. If this planetary interpretation is correct, the MACHO-98-BLG-35 lens
system constitutes the first detection of a low mass planet orbiting an
ordinary star without gas giant planets.Comment: ApJ, April 1, 2000; 27 pages including 8 color postscript figure
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