1,920 research outputs found
An Extreme-AO Search for Giant Planets around a White Dwarf --VLT/SPHERE performance on a faint target GD 50
CONTEXT. Little is known about the planetary systems around single white
dwarfs although there is strong evidence that they do exist.
AIMS. We performed a pilot study with the extreme-AO system on the
Spectro-Polarimetric High-contrast Exoplanet REsearch (SPHERE) on the Very
Large Telescopes (VLT) to look for giant planets around a young white dwarf, GD
50.
METHODS. We were awarded science verification time on the new ESO instrument
SPHERE. Observations were made with the InfraRed Dual-band Imager and
Spectrograph in classical imaging mode in H band.
RESULTS. Despite the faintness of the target (14.2 mag in R band), the AO
loop was closed and a strehl of 37\% was reached in H band. No objects were
detected around GD 50. We achieved a 5-sigma contrast of 6.2, 8.0 and 8.25 mags
at 0{\farcs}2, 0{\farcs}4 and 0{\farcs}6 and beyond, respectively. We exclude
any substellar objects more massive than 4.0 M at 6.2 AU, 2.9
M at 12.4 AU and 2.8 M at 18.6 AU and beyond. This
rivals the previous upper limit set by Spitzer. We further show that SPHERE is
the most promising instrument available to search for close-in substellar
objects around nearby white dwarfs.Comment: A&A letters, accepte
Collisional modelling of the debris disc around HIP 17439
We present an analysis of the debris disc around the nearby K2 V star HIP
17439. In the context of the Herschel DUNES key programme the disc was observed
and spatially resolved in the far-IR with the Herschel PACS and SPIRE
instruments. In a first model, Ertel et al. (2014) assumed the size and radial
distribution of the circumstellar dust to be independent power laws. There, by
exploring a very broad range of possible model parameters several scenarios
capable of explaining the observations were suggested. In this paper, we
perform a follow-up in-depth collisional modelling of these scenarios trying to
further distinguish between them. In our models we consider collisions, direct
radiation pressure, and drag forces, i.e. the actual physical processes
operating in debris discs. We find that all scenarios discussed in Ertel et al.
are physically sensible and can reproduce the observed SED along with the PACS
surface brightness profiles reasonably well. In one model, the dust is produced
beyond 120au in a narrow planetesimal belt and is transported inwards by
Poynting-Robertson and stellar wind drag. A good agreement with the observed
radial profiles would require stellar winds by about an order of magnitude
stronger than the solar value, which is not supported, although not ruled out,
by observations. Another model consists of two spatially separated planetesimal
belts, a warm inner and a cold outer one. This scenario would probably imply
the presence of planets clearing the gap between the two components. Finally,
we show qualitatively that the observations can be explained by assuming the
dust is produced in a single, but broad planetesimal disc with a surface
density of solids rising outwards, as expected for an extended disc that
experiences a natural inside-out collisional depletion. Prospects of
discriminating between the competing scenarios by future observations are
discussed.Comment: Astronomy and Astrophysics (accepted for publication). 11 pages, 8
figure
RB loss contributes to aggressive tumor phenotypes in MYC-driven triple negative breast cancer
Triple negative breast cancer (TNBC) is characterized by multiple genetic events occurring in concert to drive pathogenic features of the disease. Here we interrogated the coordinate impact of p53, RB, and MYC in a genetic model of TNBC, in parallel with the analysis of clinical specimens. Primary mouse mammary epithelial cells (mMEC) with defined genetic features were used to delineate the combined action of RB and/or p53 in the genesis of TNBC. In this context, the deletion of either RB or p53 alone and in combination increased the proliferation of mMEC; however, the cells did not have the capacity to invade in matrigel. Gene expression profiling revealed that loss of each tumor suppressor has effects related to proliferation, but RB loss in particular leads to alterations in gene expression associated with the epithelial-to-mesenchymal transition. The overexpression of MYC in combination with p53 loss or combined RB/p53 loss drove rapid cell growth. While the effects of MYC overexpression had a dominant impact on gene expression, loss of RB further enhanced the deregulation of a gene expression signature associated with invasion. Specific RB loss lead to enhanced invasion in boyden chambers assays and gave rise to tumors with minimal epithelial characteristics relative to RB-proficient models. Therapeutic screening revealed that RB-deficient cells were particularly resistant to agents targeting PI3K and MEK pathway. Consistent with the aggressive behavior of the preclinical models of MYC overexpression and RB loss, human TNBC tumors that express high levels of MYC and are devoid of RB have a particularly poor outcome. Together these results underscore the potency of tumor suppressor pathways in specifying the biology of breast cancer. Further, they demonstrate that MYC overexpression in concert with RB can promote a particularly aggressive form of TNB
The influence of dust grain porosity on the analysis of debris disc observations
Debris discs are often modelled assuming compact dust grains, but more and more evidence for the presence of porous grains is found. We aim at quantifying the systematic errors introduced when modelling debris discs composed of porous dust with a disc model assuming spherical, compact grains. We calculate the optical dust properties derived via the fast, but simple effective medium theory. The theoretical lower boundary of the size distribution – the so-called ‘blowout size’ – is compared in the cases of compact and porous grains. Finally, we simulate observations of hypothetical debris discs with different porosities and feed them into a fitting procedure using only compact grains. The deviations of the results for compact grains from the original model based on porous grains are analysed. We find that the blowout size increases with increasing grain porosity up to a factor of 2. An analytical approximation function for the blowout size as a function of porosity and stellar luminosity is derived. The analysis of the geometrical disc set-up, when constrained by radial profiles, is barely affected by the porosity. However, the determined minimum grain size and the slope of the grain size distribution derived using compact grains are significantly overestimated. Thus, the unexpectedly high ratio of minimum grain size to blowout size found by previous studies using compact grains can be partially described by dust grain porosity, although the effect is not strong enough to completely explain the trend
Collisional modelling of the AU Microscopii debris disc
The spatially resolved AU Mic debris disc is among the most famous and
best-studied debris discs. We aim at a comprehensive understanding of the dust
production and the dynamics of the disc objects with in depth collisional
modelling including stellar radiative and corpuscular forces. Our models are
compared to a suite of observational data for thermal and scattered light
emission, ranging from the ALMA radial surface brightness profile at 1.3mm to
polarisation measurements in the visible. Most of the data can be reproduced
with a planetesimal belt having an outer edge at around 40au and subsequent
inward transport of dust by stellar winds. A low dynamical excitation of the
planetesimals with eccentricities up to 0.03 is preferred. The radial width of
the planetesimal belt cannot be constrained tightly. Belts that are 5au and
17au wide, as well as a broad 44au-wide belt are consistent with observations.
All models show surface density profiles increasing with distance from the star
as inferred from observations. The best model is achieved by assuming a stellar
mass loss rate that exceeds the solar one by a factor of 50. While the SED and
the shape of the ALMA profile are well reproduced, the models deviate from the
scattered light data more strongly. The observations show a bluer disc colour
and a lower degree of polarisation for projected distances <40au than predicted
by the models. The problem may be mitigated by irregularly-shaped dust grains
which have scattering properties different from the Mie spheres used. From
tests with a handful of selected dust materials, we derive a preference for
mixtures of silicate, carbon, and ice of moderate porosity. We address the
origin of the unresolved central excess emission detected by ALMA and show that
it cannot stem from an additional inner belt alone. Instead, it should derive,
at least partly, from the chromosphere of the central star.Comment: Astronomy and Astrophysics (accepted for publication), 18 pages, 11
figure
Interpreting the extended emission around three nearby debris disc host stars
Cool debris discs are a relic of the planetesimal formation process around
their host star, analogous to the solar system's Edgeworth-Kuiper belt. As
such, they can be used as a proxy to probe the origin and formation of
planetary systems like our own. The Herschel Open Time Key Programmes "DUst
around NEarby Stars" (DUNES) and "Disc Emission via a Bias-free Reconnaissance
in the Infrared/Submillimetre" (DEBRIS) observed many nearby, sun-like stars at
far-infrared wavelengths seeking to detect and characterize the emission from
their circumstellar dust. Excess emission attributable to the presence of dust
was identified from around 20% of stars. Herschel's high angular
resolution ( 7" FWHM at 100 m) provided the capacity for resolving
debris belts around nearby stars with radial extents comparable to the solar
system (50 to 100 au). As part of the DUNES and DEBRIS surveys, we obtained
observations of three debris disc stars, HIP 22263 (HD 30495), HIP 62207 (HD
110897), and HIP 72848 (HD 131511), at far-infrared wavelengths with the
Herschel PACS instrument. Combining these new images and photometry with
ancilliary data from the literature, we undertook simultaneous multi-wavelength
modelling of the discs' radial profiles and spectral energy distributions using
three different methodologies: single annulus, modified black body, and a
radiative transfer code. We present the first far-infrared spatially resolved
images of these discs and new single-component debris disc models. We
characterize the capacity of the models to reproduce the disc parameters based
on marginally resolved emission through analysis of two sets of simulated
systems (based on the HIP 22263 and HIP 62207 data) with the noise levels
typical of the Herschel images. We find that the input parameter values are
recovered well at noise levels attained in the observations presented here.Comment: 13 pages, 5 figures, 5 tables, accepted for publication in A&
First L band detection of hot exozodiacal dust with VLTI/MATISSE
For the first time, we observed the emission of hot exozodiacal dust in L band. We used the new instrument MATISSE at the Very Large Telescope Interferometer to detect the hot dust around κ Tuc with a significance of 3σ to 6σ at wavelengths between 3.37 and 3.85μm and a dust-to-star flux ratio of 5 to 7 per cent. We modelled the spectral energy distribution based on the new L band data alone and in combination with H band data published previously. In all cases we find 0.58μm grains of amorphous carbon to fit the κ Tuc observations the best, however, also nanometre or micrometre grains and other carbons or silicates reproduce the observations well. Since the H band data revealed a temporal variability, while our Lband data were taken at a different epoch, we combine them in different ways. Depending on the approach, the best fits are obtained for a narrow dust ring at a stellar distance in the 0.1–029 au range and thus with a temperature between 940 and 1430K. Within the 1σ uncertainty dust location and temperature are confined to 0.032−1.18au and 600−2000K
J004457+4123 (Sharov 21): not a remarkable nova in M31 but a background quasar with a spectacular UV flare
We announce the discovery of a quasar behind the disk of M31, which was
previously classified as a remarkable nova in our neighbour galaxy. The paper
is primarily aimed at the outburst of J004457+4123 (Sharov 21), with the first
part focussed on the optical spectroscopy and the improvement in the
photometric database. Both the optical spectrum and the broad band spectral
energy distribution of Sharov 21 are shown to be very similar to that of
normal, radio-quiet type 1 quasars. We present photometric data covering more
than a century and resulting in a long-term light curve that is densely sampled
over the past five decades. The variability of the quasar is characterized by a
ground state with typical fluctuation amplitudes of ~0.2 mag around B~20.5,
superimposed by a singular flare of ~2 yr duration (observer frame) with the
maximum at 1992.81 where the UV flux has increased by a factor of ~20. The
total energy in the flare is at least three orders of magnitudes higher than
the radiated energy of the most luminous supernovae, provided that it comes
from an intrinsic process and the energy is radiated isotropically. The profile
of the flare light curve appears to be in agreement with the standard
predictions for a stellar tidal disruption event where a ~10 M_sun giant star
was shredded in the tidal field of a ~2...5 10^8 M_sun black hole. The short
fallback time derived from the light curve requires an ultra-close encounter
where the pericentre of the stellar orbit is deep within the tidal disruption
radius. Gravitational microlensing provides an alternative explanation, though
the probability of such a high amplification event is very low.Comment: Accepted for publication in Astronomy and Astrophysics, 14 pages, 11
figure
Observing planet-disk interaction in debris disks
Context. Structures in debris disks induced by planetdisk interaction are
promising to provide valuable constraints on the existence and properties of
embedded planets. Aims. We investigate the observability of structures in
debris disks induced by planet-disk interaction. Methods. The observability of
debris disks with the Atacama Large Millimeter/submillimeter Array (ALMA) is
studied on the basis of a simple analytical disk model. Furthermore, N-body
simulations are used to model the spatial dust distribution in debris disks
under the influence of planet-disk interaction. Images at optical scattered
light to millimeter thermal re-emission are computed. Available information
about the expected capabilities of ALMA and the James Webb Space Telescope
(JWST) are used to investigate the observability of characteristic disk
structures through spatially resolved imaging. Results. Planet-disk interaction
can result in prominent structures. This provides the opportunity of detecting
and characterizing extrasolar planets in a range of masses and radial distances
from the star that is not accessible to other techniques. Facilities that will
be available in the near future are shown to provide the capabilities to
spatially resolve and characterize structures in debris disks. Limitations are
revealed and suggestions for possible instrument setups and observing
strategies are given. In particular, ALMA is limited by its sensitivity to
surface brightness, which requires a trade-off between sensitivity and spatial
resolution. Space-based midinfrared observations will be able to detect and
spatially resolve regions in debris disks even at a distance of several tens of
AU from the star, where the emission from debris disks in this wavelength range
is expected to be low. [Abridged]Comment: 16 pages, 10 figures, accepted by A&
Planet Signatures in Collisionally Active Debris Discs: scattered light images
Planet perturbations are often invoked as a potential explanation for many
spatial structures that have been imaged in debris discs. So far this issue has
been mostly investigated with collisionless N-body numerical models. We
numerically investigate how the coupled effect of collisions and radiation
pressure can affect the formation and survival of radial and azimutal
structures in a disc perturbed by a planet. We consider two set-ups: a planet
embedded within an extended disc and a planet exterior to an inner debris ring.
We use the DyCoSS code of Thebault(2012) and derive synthetic images of the
system in scattered light. The planet's mass and orbit, as well as the disc's
collisional activity are explored as free parameters.
We find that collisions always significantly damp planet-induced structures.
For the case of an embedded planet, the planet's signature, mostly a density
gap around its radial position, should remain detectable in head-on images if
M_planet > M_Saturn. If the system is seen edge-on, however, inferring the
presence of the planet is much more difficult, although some planet-induced
signatures might be observable under favourable conditions.
For the inner-ring/external-planet case, planetary perturbations cannot
prevent collision-produced small fragments from populating the regions beyond
the ring: The radial luminosity profile exterior to the ring is close to the
one it should have in the absence of the planet. However, a Jovian planet on a
circular orbit leaves precessing azimutal structures that can be used to
indirectly infer its presence. For a planet on an eccentric orbit, the ring is
elliptic and the pericentre glow effect is visible despite of collisions and
radiation pressure, but detecting such features in real discs is not an
unambiguous indicator of the presence of an outer planet.Comment: Accepted for Publication in A&A (NOTE: Abridged abstract and
(very)LowRes Figures. Better version, with High Res figures and full abstract
can be found at http://lesia.obspm.fr/perso/philippe-thebault/planpapph.pdf
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