7,779 research outputs found
Improved astrometry for the Bohannan & Epps catalogue
Aims: Accurate astrometry is required to reliably cross-match 20th-century
catalogues against 21st-century surveys. The present work aims to provide such
astrometry for the 625 entries of the Bohannan & Epps (BE74) catalogue of
H emission-line stars. Methods: BE74 targets have been individually
identified in digital images and, in most cases, unambiguously matched to
entries in the UCAC4 astrometric catalogue. Results: Sub-arcsecond astrometry
is now available for almost all BE74 stars. Several identification errors in
the literature illustrate the perils of relying solely on positional
coincidences using poorer-quality astrometry.Comment: 3 pages, 1 figure. Accepted in A&
Emission-line stars in the LMC: the Armagh survey, and a metacatalogue
[Aims] Accurate astrometry is required to reliably cross-match 20th-century
photographic catalogues against 21st-century digital surveys. The present work
provides modern-era identifications and astrometry for the 801 emission-line
objects "of stellar appearance" in the Armagh survey (the largest of its nature
to date). [Methods] Targets have been individually identified in digital images
using the Armagh Atlas and, in most cases, unambiguously matched to entries in
the UCAC astrometric catalogues. [Results] Astrometry with sub-arcsecond
precision is now available for all the major photographic spectroscopic surveys
of the LMC. The results are used to compile an annotated metacatalogue of 1675
individual, spectroscopically identified candidate H-alpha-emission stars,
including detailed cross-matching between catalogues, and resolving many
(though not all) identification ambiguities in individual primary sources
A reappraisal of parameters for the putative planet PTFO 8-8695b and its potentially precessing parent star
Published photometry of fading events in the PTFO 8-8695 system is modelled
using improved treatments of stellar geometry, surface intensities, and,
particularly, gravity darkening, with a view to testing the planetary-transit
hypothesis. Variability in the morphology of fading events can be reproduced by
adopting convective-envelope gravity darkening, but near-critical stellar
rotation is required. This leads to inconsistencies with spectroscopic
observations; the model also predicts substantial photometric variability
associated with stellar precession, contrary to observations. Furthermore, the
empirical ratio of orbital to rotational angular momenta is at odds with
physically plausible values. An exoplanet transiting a precessing,
gravity-darkened star may not be the correct explanation of periodic fading
events in this system
Rapid rotators revisited: absolute dimensions of KOI-13
We analyse Kepler light-curves of the exoplanet KOI-13b transiting its
moderately rapidly rotating (gravity-darkened) parent star. A physical model,
with minimal ad hoc free parameters, reproduces the time-averaged light-curve
at the ca. 10 parts per million level. We demonstrate that this Roche-model
solution allows the absolute dimensions of the system to be determined from the
star's projected equatorial rotation speed, v(e)sin(i), without any additional
assumptions; we find a planetary radius 1.33+/-0.05 R(Jup), stellar polar
radius 1.55+/-0.06 R(sun), combined mass M(*) + M(P) (\simeq M*) = 1.47 +/-
0.17 M(sun), and distance d \simeq 370+/-25 pc, where the errors are dominated
by uncertainties in relative flux contribution of the visual-binary companion
KOI-13B. The implied stellar rotation period is within ca. 5% of the
non-orbital, 25.43-hr signal found in the Kepler photometry. We show that the
model accurately reproduces independent tomographic observations, and yields an
offset between orbital and stellar-rotation angular-momentum vectors of
60.25+/-0.05 degrees.Comment: Accepted in MNRA
Characteristics and classification of A-type supergiants in the Small Magellanic Cloud
We address the relationship between spectral type and physical properties for
A-type supergiants in the SMC. We first construct a self-consistent
classification scheme for A supergiants, employing the calcium K to H epsilon
line ratio as a temperature-sequence discriminant. Following the precepts of
the `MK process', the same morphological criteria are applied to Galactic and
SMC spectra with the understanding there may not be a correspondence in
physical properties between spectral counterparts in different environments. We
then discuss the temperature scale, concluding that A supergiants in the SMC
are systematically cooler than their Galactic counterparts at the same spectral
type, by up to ~10%. Considering the relative line strengths of H gamma and the
CH G-band we extend our study to F and early G-type supergiants, for which
similar effects are found. We note the implications for analyses of
extragalactic luminous supergiants, for the flux-weighted gravity-luminosity
relationship and for population synthesis studies in unresolved stellar
systems.Comment: 14 pages, 14 figures, accepted by MNRAS; minor section removed prior
to final publicatio
A recalibration of IUE NEWSIPS low dispersion data
While the low dispersion IUE NEWSIPS data products represent a significant
improvement over original IUE SIPS data, they still contain serious systematic
effects which compromise their utility for certain applications. We show that
NEWSIPS low resolution data are internally consistent to only 10-15% at best,
with the majority of the problem due to time dependent systematic effects. In
addition, the NEWSIPS flux calibration is shown to be inconsistent by nearly
10%.
We examine the origin of these problems and proceed to formulate and apply
algorithms to correct them to ~ 3% level -- a factor of 5 improvement in
accuracy. Because of the temporal systematics, transforming the corrected data
to the IUE flux calibration becomes ambiguous. Therefore, we elect to transform
the corrected data onto the HST FOS system. This system is far more
self-consistent, and transforming the IUE data to it places data from both
telescopes on a single system.
Finally, we argue that much of the remaining 3% systematic effects in the
corrected data is traceable to problems with the NEWSIPS intensity
transformation function (ITF). The accuracy could probably be doubled by
rederiving the ITF.Comment: Submitted to ApJ Supplement, 35 pages, 13 figures, LaTeX - AASTEX
aas2pp4.st
Discourse Analysis: varieties and methods
This paper presents and analyses six key approaches to discourse analysis, including political discourse theory, rhetorical political analysis, the discourse historical approach in critical discourse analysis, interpretive policy analysis, discursive psychology and Q methodology. It highlights differences and similarities between the approaches along three distinctive dimensions, namely, ontology, focus and purpose. Our analysis reveals the difficulty of arriving at a fundamental matrix of dimensions which would satisfactorily allow one to organize all approaches in a coherent theoretical framework. However, it does not preclude various theoretical articulations between the different approaches, provided one takes a problem-driven approach to social science as one?s starting-point
High-precision stellar limb-darkening in exoplanetary transits
Characterization of the atmospheres of transiting exoplanets relies on
accurate measurements of the extent of the optically thick area of the planet
at multiple wavelengths with a precision 100 parts per million (ppm).
Next-generation instruments onboard the James Webb Space Telescope (JWST) are
expected to achieve 10 ppm precision for several tens of targets. A
similar precision can be obtained in modelling only if other astrophysical
effects, including the stellar limb-darkening, are accounted for properly. In
this paper, we explore the limits on precision due to the mathematical formulas
currently adopted to approximate the stellar limb-darkening, and to the use of
limb-darkening coefficients obtained either from stellar-atmosphere models or
empirically. We propose a new limb-darkening law with two coefficients,
`power-2', which outperforms other two-coefficient laws adopted in the
literature in most cases, and particularly for cool stars. Empirical
limb-darkening based on two-coefficient formulas can be significantly biased,
even if the light-curve residuals are nearly photon-noise limited. We
demonstrate an optimal strategy to fitting for the four-coefficients
limb-darkening in the visible, using prior information on the exoplanet orbital
parameters to break some of the degeneracies that otherwise would prevent the
convergence of the fit. Infrared observations taken with the James Webb Space
Telescope (JWST) will provide accurate measurements of the exoplanet orbital
parameters with unprecedented precision, which can be used as priors to improve
the stellar limb-darkening characterization, and therefore the inferred
exoplanet parameters, from observations in the visible, such as those taken
with Kepler/K2, JWST, other past and future instruments
- …