3,603 research outputs found
Spectroscopic observations of the eclipsing Polar MN Hya (RX J0929--24)
We present low--medium resolution optical spectroscopy of the eclipsing AM
Her system MN Hya (RX J0929--24). We determine the magnetic field strength at
the primary accretion region of the white dwarf to be 42MG from the spacing of
cyclotron features visible during ~0.4--0.7. From spectra taken during the
eclipse we find that the secondary has a M3--4 spectral type. Combined with the
eclipse photometry of Sekiguchi, Nakada & Bassett and an estimate of the
interstellar extinction we find a distance of ~300--700pc. We find unusual line
variations at phase ~0.9: Halpha is seen in absorption and emission. This is at
the same point in the orbital phase that a prominent absorption dip is seen in
soft X-rays.Comment: accepted by MNRAS, 7 pages, 5 fig
LRG-BEASTS III: Ground-based transmission spectrum of the gas giant orbiting the cool dwarf WASP-80
We have performed ground-based transmission spectroscopy of the hot Jupiter
orbiting the cool dwarf WASP-80 using the ACAM instrument on the William
Herschel Telescope (WHT) as part of the LRG-BEASTS programme. This is the third
paper of a ground-based transmission spectroscopy survey of hot Jupiters using
low-resolution grism spectrographs. We observed two transits of the planet and
have constructed transmission spectra spanning a wavelength range of
4640-8840A. Our transmission spectrum is inconsistent with a previously claimed
detection of potassium in WASP-80b's atmosphere, and is instead most consistent
with a haze. We also do not see evidence for sodium absorption at a resolution
of 100A.Comment: 11 pages, 9 figures. Accepted for publication in MNRA
Rayleigh scattering in the transmission spectrum of HAT-P-18b
We have performed ground-based transmission spectroscopy of the hot Jupiter
HAT-P-18b using the ACAM instrument on the William Herschel Telescope (WHT).
Differential spectroscopy over an entire night was carried out at a resolution
of using a nearby comparison star. We detect a bluewards slope
extending across our optical transmission spectrum which runs from 4750 to
9250\AA. The slope is consistent with Rayleigh scattering at the equilibrium
temperature of the planet (852K). We do not detect enhanced sodium absorption,
which indicates that a high-altitude haze is masking the feature and giving
rise to the Rayleigh slope. This is only the second discovery of a Rayleigh
scattering slope in a hot Jupiter atmosphere from the ground, and our study
illustrates how ground-based observations can provide transmission spectra with
precision comparable to the Hubble Space Telescope.Comment: 11 pages, 9 figures, accepted for publication in MNRA
Physical properties and radius variations in the HAT-P-5 planetary system from simultaneous four-colour photometry
The radii of giant planets, as measured from transit observations, may vary
with wavelength due to Rayleigh scattering or variations in opacity. Such an
effect is predicted to be large enough to detect using ground-based
observations at multiple wavelengths. We present defocussed photometry of a
transit in the HAT-P-5 system, obtained simultaneously through Stromgren u,
Gunn g and r, and Johnson I filters. Two more transit events were observed
through a Gunn r filter. We detect a substantially larger planetary radius in
u, but the effect is greater than predicted using theoretical model atmospheres
of gaseous planets. This phenomenon is most likely to be due to systematic
errors present in the u-band photometry, stemming from variations in the
transparency of Earth's atmosphere at these short wavelengths. We use our data
to calculate an improved orbital ephemeris and to refine the measured physical
properties of the system. The planet HAT-P-5b has a mass of 1.06 +/- 0.11 +/-
0.01 Mjup and a radius of 1.252 +/- 0.042 +/- 0.008 Rjup (statistical and
systematic errors respectively), making it slightly larger than expected
according to standard models of coreless gas-giant planets. Its equilibrium
temperature of 1517 +/- 29 K is within 60K of that of the extensively-studied
planet HD 209458b.Comment: Version 2 corrects the accidental omission of one author in the arXiv
metadata. Accepted for publication in MNRAS. 9 pages, 4 figures, 7 tables.
The properties of HAT-P-5 have been added to the Transiting Extrasolar Planet
Catalogue at http://www.astro.keele.ac.uk/~jkt/tepcat
K2 Variable Catalogue II: Machine Learning Classification of Variable Stars and Eclipsing Binaries in K2 Fields 0-4
We are entering an era of unprecedented quantities of data from current and
planned survey telescopes. To maximise the potential of such surveys, automated
data analysis techniques are required. Here we implement a new methodology for
variable star classification, through the combination of Kohonen Self
Organising Maps (SOM, an unsupervised machine learning algorithm) and the more
common Random Forest (RF) supervised machine learning technique. We apply this
method to data from the K2 mission fields 0-4, finding 154 ab-type RR Lyraes
(10 newly discovered), 377 Delta Scuti pulsators, 133 Gamma Doradus pulsators,
183 detached eclipsing binaries, 290 semi-detached or contact eclipsing
binaries and 9399 other periodic (mostly spot-modulated) sources, once class
significance cuts are taken into account. We present lightcurve features for
all K2 stellar targets, including their three strongest detected frequencies,
which can be used to study stellar rotation periods where the observed
variability arises from spot modulation. The resulting catalogue of variable
stars, classes, and associated data features are made available online. We
publish our SOM code in Python as part of the open source PyMVPA package, which
in combination with already available RF modules can be easily used to recreate
the method.Comment: Accepted for publication in MNRAS, 16 pages, 13 figures. Updated with
proof corrections. Full catalogue tables available at
https://www2.warwick.ac.uk/fac/sci/physics/research/astro/people/armstrong/
or at the CD
Magnetism, X-rays, and Accretion Rates in WD 1145+017 and other Polluted White Dwarf Systems
This paper reports circular spectropolarimetry and X-ray observations of
several polluted white dwarfs including WD 1145+017, with the aim to constrain
the behavior of disk material and instantaneous accretion rates in these
evolved planetary systems. Two stars with previously observed Zeeman splitting,
WD 0322-019 and WD 2105-820, are detected above 5 sigma and > 1 kG, while
WD 1145+017, WD 1929+011, and WD 2326+049 yield (null) detections below this
minimum level of confidence. For these latter three stars, high-resolution
spectra and atmospheric modeling are used to obtain limits on magnetic field
strengths via the absence of Zeeman splitting, finding B* < 20 kG based on data
with resolving power R near 40 000. An analytical framework is presented for
bulk Earth composition material falling onto the magnetic polar regions of
white dwarfs, where X-rays and cyclotron radiation may contribute to accretion
luminosity. This analysis is applied to X-ray data for WD 1145+017, WD
1729+371, and WD 2326+049, and the upper bound count rates are modeled with
spectra for a range of plasma kT = 1 - 10 keV in both the magnetic and
non-magnetic accretion regimes. The results for all three stars are consistent
with a typical dusty white dwarf in a steady-state at 1e8 - 1e9 g/s. In
particular, the non-magnetic limits for WD 1145+017 are found to be well below
previous estimates of up to 1e12 g/s, and likely below 1e10 g/s, thus
suggesting the star-disk system may be average in its evolutionary state, and
only special in viewing geometry.Comment: 14 pages, 7 figures, 3 tables; accepted to MNRA
Reconnaissance of the TRAPPIST-1 exoplanet system in the Lyman- line
The TRAPPIST-1 system offers the opportunity to characterize terrestrial,
potentially habitable planets orbiting a nearby ultracool dwarf star. We
performed a four-orbit reconnaissance with the Space Telescope Imaging
Spectrograph onboard the Hubble Space Telescope to study the stellar emission
at Lyman-, to assess the presence of hydrogen exospheres around the two
inner planets, and to determine their UV irradiation. We detect the
Lyman- line of TRAPPIST-1, making it the coldest exoplanet host star
for which this line has been measured. We reconstruct the intrinsic line
profile, showing that it lacks broad wings and is much fainter than expected
from the stellar X-ray emission. TRAPPIST-1 has a similar X-ray emission as
Proxima Cen but a much lower Ly- emission. This suggests that
TRAPPIST-1 chromosphere is only moderately active compared to its transition
region and corona. We estimated the atmospheric mass loss rates for all
planets, and found that despite a moderate extreme UV emission the total XUV
irradiation could be strong enough to strip the atmospheres of the inner
planets in a few billions years. We detect marginal flux decreases at the times
of TRAPPIST-1b and c transits, which might originate from stellar activity, but
could also hint at the presence of extended hydrogen exospheres. Understanding
the origin of these Lyman- variations will be crucial in assessing the
atmospheric stability and potential habitability of the TRAPPIST-1 planets.Comment: Published in A&A as a Letter to the Edito
Umklapp scattering from spin fluctuations in Copper-Oxides
The -dependent electronic momentum relaxation rate due to Umklapp
scattering from antiferromagnetic spin fluctuations is studied within a
renormalized mean-field approach to an extended model appropriate to
YBaCuO and other cuprates. Transport coefficients are
calculated in a relaxation time approximation. We compare these results with
those obtained with the phenomenological assumption that all scattering
processes dissipate momentum. We show that the latter, which violates momentum
conservation, leads to quite different magnitudes and temperature dependences
of resistivities and Hall coefficients.Comment: replaced by LaTeX file (due to problems with PostScript
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