918 research outputs found
Detection limits for close eclipsing and transiting sub-stellar and planetary companions to white dwarfs in the WASP survey
We used photometric data from the WASP (Wide-Angle Search for Planets) survey
to explore the possibility of detecting eclipses and transit signals of brown
dwarfs, gas giants and terrestrial companions in close orbit around white
dwarfs. We performed extensive Monte Carlo simulations and we found that for
Gaussian random noise WASP is sensitive to companions as small as the Moon
orbiting a 12 white dwarf. For fainter stars WASP is sensitive to
increasingly larger bodies. Our sensitivity drops in the presence of co-variant
noise structure in the data, nevertheless Earth-size bodies remain readily
detectable in relatively low S/N data. We searched for eclipses and transit
signals in a sample of 194 white dwarfs in the WASP archive however, no
evidence for companions was found. We used our results to place tentative upper
limits to the frequency of such systems. While we can only place weak limits on
the likely frequency of Earth-sized or smaller companions; brown dwarfs and gas
giants (radius R) with periods 0.2 days must certainly be
rare (). More stringent constraints requires significantly larger white
dwarf samples, higher observing cadence and continuous coverage. The short
duration of eclipses and transits of white dwarfs compared to the cadence of
WASP observations appears to be one of the main factors limiting the detection
rate in a survey optimised for planetary transits of main sequence stars.Comment: 8 pages, 3 figure
Phase-resolved far-ultraviolet HST spectroscopy of the peculiar magnetic white dwarf RE J0317-853
We present phase resolved FUV HST FOS spectra of the rapidly rotating, highly
magnetic white dwarf RE J0317-853. Using these data, we construct a new model
for the magnetic field morphology across the stellar surface. From an expansion
into spherical harmonics, we find the range of magnetic field strengths present
is 180-800MG. For the first time we could identify an absorption feature
present at certain phases at 1160A as a ``forbidden'' 1s_0 -> 2s_0 component,
due to the combined presence of an electric and magnetic field.Comment: 15 pages including 4 figures. Accepted for publication in ApJ Letter
Origin of electron cyclotron maser-induced radio emissions at ultra-cool dwarfs: magnetosphere-ionosphere coupling currents
A number of ultra-cool dwarfs emit circularly polarised radio waves generated
by the electron cyclotron maser instability. In the solar system such radio is
emitted from regions of strong auroral magnetic field-aligned currents. We thus
apply ideas developed for Jupiter's magnetosphere, being a well-studied
rotationally-dominated analogue in our solar system, to the case of
fast-rotating UCDs. We explain the properties of the radio emission from UCDs
by showing that it would arise from the electric currents resulting from an
angular velocity shear in the fast-rotating magnetic field and plasma, i.e. by
an extremely powerful analogue of the process which causes Jupiter's auroras.
Such a velocity gradient indicates that these bodies interact significantly
with their space environment, resulting in intense auroral emissions. These
results strongly suggest that auroras occur on bodies outside our solar system.Comment: Accepted for publication in the Astrophysical Journa
NGTS-4b: A sub-Neptune transiting in the desert
We report the discovery of NGTS-4b, a sub-Neptune-sized planet transiting a 13th magnitude K-dwarf in a 1.34 d orbit. NGTS-4b has a mass M = 20.6 ± 3.0 M⊕ and radius R = 3.18 ± 0.26 R⊕, which places it well within the so-called ‘Neptunian Desert’. The mean density of the planet (3.45 ± 0.95 g cm−3) is consistent with a composition of 100  per cent H2O or a rocky core with a volatile envelope. NGTS-4b is likely to suffer significant mass loss due to relatively strong EUV/X-ray irradiation. Its survival in the Neptunian desert may be due to an unusually high-core mass, or it may have avoided the most intense X-ray irradiation by migrating after the initial activity of its host star had subsided. With a transit depth of 0.13 ± 0.02 per cent, NGTS-4b represents the shallowest transiting system ever discovered from the ground, and is the smallest planet discovered in a wide-field ground-based photometric survey
Dynamical constraints on some orbital and physical properties of the WD0137-349 A/B binary system
In this paper I deal with the WD0137-349 binary system consisting of a white
dwarf (WD) and a brown dwarf (BD) in a close circular orbit of about 116 min.
I, first, constrain the admissible range of values for the inclination i by
noting that, from looking for deviations from the third Kepler law, the
quadrupole mass moment Q would assume unlikely large values, incompatible with
zero at more than 1-sigma level for i 43 deg. Then, by
conservatively assuming that the most likely values for i are those that
prevent such an anomalous behavior of Q, i.e. those for which the third Kepler
law is an adequate modeling of the orbital period, I obtain i=39 +/- 2 deg.
Such a result is incompatible with the value i=35 deg quoted in literature by
more than 2 sigma. Conversely, it is shown that the white dwarf's mass range
obtained from spectroscopic measurements is compatible with my experimental
range, but not for i=35 deg. As a consequence, my estimate of yields an
orbital separation of a=(0.59 +/- 0.05)R_Sun and an equilibrium temperature of
BD of T_eq=(2087 +/- 154)K which differ by 10% and 4%, respectively, from the
corresponding values for i=35 deg.Comment: LaTex2e, 11 pages, 3 figures, no tables. It refers to gr-qc/0611126
and better clarify the result obtained there. Accepted by Astrophysics and
Space Scienc
WD0837+185:the formation and evolution of an extreme mass ratio white dwarf-brown dwarf binary in Praesepe
There is a striking and unexplained dearth of brown dwarf companions in close
orbits (< 3AU) around stars more massive than the Sun, in stark contrast to the
frequency of stellar and planetary companions. Although rare and relatively
short-lived, these systems leave detectable evolutionary end points in the form
of white dwarf - brown dwarf binaries and these remnants can offer unique
insights into the births and deaths of their parent systems. We present the
discovery of a close (orbital separation ~ 0.006 AU) substellar companion to a
massive white dwarf member of the Praesepe star cluster. Using the cluster age
and the mass of the white dwarf we constrain the mass of the white dwarf
progenitor star to lie in the range 3.5 - 3.7 Msun (B9). The high mass of the
white dwarf means the substellar companion must have been engulfed by the B
star's envelope while it was on the late asymptotic giant branch (AGB). Hence,
the initial separation of the system was ~2 AU, with common envelope evolution
reducing the separation to its current value. The initial and final orbital
separations allow us to constrain the combination of the common envelope
efficiency (alpha) and binding energy parameters (lambda) for the AGB star to
alpha lambda ~3. We examine the various formation scenarios and conclude that
the substellar object was most likely to have been captured by the white dwarf
progenitor early in the life of the cluster, rather than forming in situ.Comment: Accepted for publication in ApJ
Hubble Space Telescope observations of the NUV transit of WASP-12b
We present new observations of four closely-spaced NUV transits of the hot
Jupiter-like exoplanet WASP-12b using HST/COS, significantly increasing the
phase resolution of the observed NUV light curve relative to previous
observations, while minimising the temporal variation of the system. We observe
significant excess NUV absorption during the transit, with mean normalised
in-transit fluxes of , i.e. 2-5
deeper than the optical transit level of for a uniform stellar
disk (the exact confidence level depending on the normalisation method used).
We further observe an asymmetric transit shape, such that the post-conjunction
fluxes are overall 2-3 higher than pre-conjunction values, and
characterised by rapid variations in count rate between the pre-conjunction and
out of transit levels. We do not find evidence for an early ingress to the NUV
transit as suggested by earlier HST observations. However, we show that the NUV
count rate observed prior to the optical transit is highly variable, but
overall 2.2-3.0 below the post-transit values and comparable
in depth to the optical transit, possibly forming a variable region of NUV
absorption from at least phase 0.83, limited by the data coverage.Comment: Accepted into the Astrophysical Journa
Metabolomic profiling of submaximal exercise at a standardised relative intensity in healthy adults
Ten physically active subjects underwent two cycling exercise trials. In the first, aerobic capacity (VO2max) was determined and the second was a 45 min submaximal exercise test. Urine samples were collected separately the day before (day 1) , the day of (day 2), and the day after (day 3) the submaximal exercise test (12 samples per subject). Metabolomic profiling of the samples was carried out using hydrophilic interaction chromatography (HILIC) coupled to an Orbitrap Exactive mass spectrometer. Data were extracted, database searched and then subjected to principle components (PCA) and orthogonal partial least squares (OPLSDA) modelling. The best results were obtained from pre-treating the data by normalising the metabolites to their mean output on days 1 and 2 of the trial. This allowed PCA to separate the day 2 first void samples (D2S1) from the day 2 post-exercise samples (D2S3) PCA also separated the equivalent samples obtained on day 1 (D1S1 and D1S3). OPLSDA modelling separated both the D2S1 and D2S3 samples and D1S1 and D1S3 samples. The metabolites affected by the exercise samples included a range of purine metabolites and several acyl carnitines. Some metabolites were subject to diurnal variation these included bile acids and several amino acids, the variation of these metabolites was similar on day 1 and day 2 despite the exercise intervention on day 2. Using OPLS modelling it proved possible to identify a single abundant urinary metabolite provisionally identified as oxo-aminohexanoic acid (OHA) as being strongly correlated with VO2max when the levels in the D2S3 samples were considered
A LOFAR mini-survey for low-frequency radio emission from the nearest brown dwarfs
We have conducted a mini-survey for low-frequency radio emission from some of the closest brown dwarfs to the Sun with rapid rotation rates: SIMP J013656.5 +093347, WISEPC 150649.97+702736.0, and WISEPA J174124.26+255319.5.We have placed robust 3s upper limits on the flux density in the 111 – 169 MHz frequency range for these targets: WISE 1506: < 0:72 mJy; WISE 1741: < 0:87 mJy; SIMP 0136: < 0:66 mJy. At 8 hours of integration per target to achieve these limits, we find that systematic and detailed study of this class of object at LOFAR frequencies will require a substantial dedication of resources
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