378 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
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
A detailed X-ray investigation of zeta Puppis IV. Further characterization of the variability
Previously, the X-ray emission of zeta Puppis was found to be variable with
light curves harbouring "trends" with a typical timescale longer than the
exposure length. The origin of these changes was proposed to be linked to
large-scale structures in the wind, but further characterization of the
variability at high energies was needed. Since then, a number of new X-ray
observations have become available. Furthermore, a cyclic behaviour with a
1.78d period was identified in long optical photometric runs, which is thought
to be associated with the launching mechanism of large-scale wind structures.
We analysed these new X-ray data, revisited the old data, and compared X-ray
with optical data, including when simultaneous. We found that the behaviour in
X-rays cannot be explained in terms of a perfect clock because the amplitude
and shape of its variations change with time. For example, zeta Puppis was much
more strongly variable between 2007 and 2011 than before and after this
interval. Comparing the X-ray spectra of the star at maximum and minimum
brightness yields no compelling difference beyond the overall flux change: the
temperatures, absorptions, and line shapes seem to remain constant, well within
errors. The only common feature between X-ray datasets is that the variation
amplitudes appear maximum in the medium (0.6-1.2keV) energy band. Finally, no
clear and coherent correlation can be found between simultaneous X-ray and
optical data. Only a subgroup of observations may be combined coherently with
the optical period of 1.78d, although the simultaneous optical behaviour is
unknown. The currently available data do not reveal any obvious, permanent, and
direct correlation between X-ray and optical variations. The origin of the
X-ray variability therefore still needs to be ascertained, highlighting the
need for long-term monitoring in multiwavelengths, i.e. X-ray, UV, and optical.Comment: accepted for publication by A&
Time-series photometry of the O4 I(n)fp star zeta Puppis
We report a time-series analysis of the O4 I(n)fp star zeta Pup, based on
optical photometry obtained with the SMEI instrument on the Coriolis satellite,
2003--2006. A single astrophysical signal is found, with P = (1.780938 \pm
0.000093) d and a mean semi-amplitude of (6.9 \pm 0.3) mmag. There is no
evidence for persistent coherent signals with semi-amplitudes in excess of ca.
2~mmag on any of the timescales previously reported in the literature. In
particular, there is no evidence for a signature of the proposed rotation
period, ca. 5.1~days; zeta Pup is therefore probably not an oblique magnetic
rotator. The 1.8-day signal varies in amplitude by a factor ca. 2 on timescales
of 10--100d (and probably by more on longer timescales), and exhibits modest
excursions in phase, but there is no evidence for systematic changes in period
over the 1000-d span of our observations. Rotational modulation and
stellar-wind variability appear to be unlikely candidates for the underlying
mechanism; we suggest that the physical origin of the signal may be pulsation
associated with low-l oscillatory convection modes.Comment: MNRAS, in pres
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