34 research outputs found
Propagation characteristics of 20/30 GHz links with a 40 deg masking angle
An effective means of reducing Ka-band propagation loss is the use of high elevation angle paths, i.e., a large masking angle, between earth stations and the space platform. Experimental data have shown that the signal loss associated with most atmospheric effects is inversely proportional to sin(theta), where theta denotes the path elevation angle. A large masking angle and a generous link margin are the primary tools used in the Teledesic Corporation network to minimize atmospheric-related signal outages. This report documents the results of a study sponsored by Teledesic Corporation to characterize the effect of radiowave propagation on Teledesic's links. The recent Olympus campaign in Europe and the U.S. has provided new information that is not included. Therefore, CCIR recommendations and NASA Propagation Handbook models constitute the base of this study, and, when applicable, data from other sources have been used to improve the predictions. Furthermore, attention has been given to data from the Olympus campaign. The effects investigated during this study include gas, rain, fog, sand, and cloud attenuation; diversity gain; scintillation; and depolarization
Is there really a debris disc around ?
The presence of a debris disc around the Gyr-old solar-type star
was suggested by the infrared
excess detection. Follow-up observations with /PACS revealed
a double-lobed feature, that displayed asymmetries both in brightness and
position. Therefore, the disc was thought to be edge-on and significantly
eccentric. Here we present ALMA/ACA observations in Band 6 and 7 which
unambiguously reveal that these lobes show no common proper motion with
. In these observations, no flux has been detected
around that exceeds the levels. We
conclude that surface brightness upper limits of a debris disc around
are at 1.3 mm, and
at 870 microns. Our results overall demonstrate
the capability of the ALMA/ACA to follow-up observations of
debris discs and clarify the effects of background confusion.Comment: 6 pages, 2 figures, 2 table
Fomalhaut b could be massive and sculpting the narrow, eccentric debris disc, if in mean-motion resonance with it
The star Fomalhaut hosts a narrow, eccentric debris disc, plus a highly
eccentric companion Fomalhaut b. It is often argued that Fomalhaut b cannot
have significant mass, otherwise it would quickly perturb the disc. We show
that material in internal mean-motion resonances with a massive, coplanar
Fomalhaut b would actually be long-term stable, and occupy orbits similar to
the observed debris. Furthermore, millimetre dust released in collisions
between resonant bodies could reproduce the width, shape and orientation of the
observed disc. We first re-examine the possible orbits of Fomalhaut b, assuming
that it moves under gravity alone. If Fomalhaut b orbits close to the disc
midplane then its orbit crosses the disc, and the two are apsidally aligned.
This alignment may hint at an ongoing dynamical interaction. Using the
observationally allowed orbits, we then model the interaction between a massive
Fomalhaut b and debris. Whilst most debris is unstable in such an extreme
configuration, we identify several resonant populations that remain stable for
the stellar lifetime, despite crossing the orbit of Fomalhaut b. This debris
occupies low-eccentricity orbits similar to the observed debris ring. These
resonant bodies would have a clumpy distribution, but dust released in
collisions between them would form a narrow, relatively smooth ring similar to
observations. We show that if Fomalhaut b has a mass between those of Earth and
Jupiter then, far from removing the observed debris, it could actually be
sculpting it through resonant interactions.Comment: 24 pages, 11 figures, accepted for publication in MNRA
A gap in the planetesimal disc around HD 107146 and asymmetric warm dust emission revealed by ALMA
While detecting low mass exoplanets at tens of au is beyond current
instrumentation, debris discs provide a unique opportunity to study the outer
regions of planetary systems. Here we report new ALMA observations of the
80-200 Myr old Solar analogue HD 107146 that reveal the radial structure of its
exo-Kuiper belt at wavelengths of 1.1 and 0.86 mm. We find that the
planetesimal disc is broad, extending from 40 to 140 au, and it is
characterised by a circular gap extending from 60 to 100 au in which the
continuum emission drops by about 50%. We also report the non-detection of the
CO J=3-2 emission line, confirming that there is not enough gas to affect the
dust distribution. To date, HD 107146 is the only gas-poor system showing
multiple rings in the distribution of millimeter sized particles. These rings
suggest a similar distribution of the planetesimals producing small dust grains
that could be explained invoking the presence of one or more perturbing
planets. Because the disk appears axisymmetric, such planets should be on
circular orbits. By comparing N-body simulations with the observed visibilities
we find that to explain the radial extent and depth of the gap, it would be
required the presence of multiple low mass planets or a single planet that
migrated through the disc. Interior to HD 107146's exo-Kuiper belt we find
extended emission with a peak at ~20 au and consistent with the inner warm belt
that was previously predicted based on 22m excess as in many other
systems. This warm belt is the first to be imaged, although unexpectedly
suggesting that it is asymmetric. This could be due to a large belt
eccentricity or due to clumpy structure produced by resonant trapping with an
additional inner planet
The clumpy structure of Eridani's debris disc revisited by ALMA
Eridani is the closest star to our Sun known to host a debris
disc. Prior observations in the (sub-)millimetre regime have potentially
detected clumpy structure in the disc and attributed this to interactions with
an (as yet) undetected planet. However, the prior observations were unable to
distinguish between structure in the disc and background confusion. Here we
present the first ALMA image of the entire disc, which has a resolution of
1.6"1.2". We clearly detect the star, the main belt and two point
sources. The resolution and sensitivity of this data allow us to clearly
distinguish background galaxies (that show up as point sources) from the disc
emission. We show that the two point sources are consistent with background
galaxies. After taking account of these, we find that resolved residuals are
still present in the main belt, including two clumps with a
significance -- one to the east of the star and the other to the northwest. We
perform -body simulations to demonstrate that a migrating planet can form
structures similar to those observed by trapping planetesimals in resonances.
We find that the observed features can be reproduced by a migrating planet
trapping planetesimals in the 2:1 mean motion resonance and the symmetry of the
most prominent clumps means that the planet should have a position angle of
either or . Observations over multiple epochs
are necessary to test whether the observed features rotate around the star.Comment: 16 pages, 10 figures, accepted for publication in MNRA
An interferometric study of the Fomalhaut inner debris disk. III. Detailed models of the exozodiacal disk and its origin
Can Eccentric Debris Disks Be Long-lived? A First Numerical Investigation and Application to Zeta(exp 2) Reticuli
Context. Imaging of debris disks has found evidence for both eccentric and offset disks. One hypothesis is that they provide evidence for massive perturbers, for example, planets or binary companions, which sculpt the observed structures. One such disk was recently observed in the far-IR by the Herschel Space Observatory around Zeta2 Reticuli. In contrast with previously reported systems, the disk is significantly eccentric, and the system is several Gyr old. Aims. We aim to investigate the long-term evolution of eccentric structures in debris disks caused by a perturber on an eccentric orbit around the star. We hypothesise that the observed eccentric disk around Zeta2 Reticuli might be evidence of such a scenario. If so, we are able to constrain the mass and orbit of a potential perturber, either a giant planet or a binary companion. Methods. Analytical techniques were used to predict the effects of a perturber on a debris disk. Numerical N-body simulations were used to verify these results and further investigate the observable structures that may be produced by eccentric perturbers. The long-term evolution of the disk geometry was examined, with particular application to the Zeta2 Reticuli system. In addition, synthetic images of the disk were produced for direct comparison with Herschel observations. Results. We show that an eccentric companion can produce both the observed offsets and eccentric disks. These effects are not immediate, and we characterise the timescale required for the disk to develop to an eccentric state (and any spirals to vanish). For Zeta2 Reticuli, we derive limits on the mass and orbit of the companion required to produce the observations. Synthetic images show that the pattern observed around Zeta2 Reticuli can be produced by an eccentric disk seen close to edge-on, and allow us to bring additional constraints on the disk parameters of our model (disk flux and extent). Conclusions. We conclude that eccentric planets or stellar companions can induce long-lived eccentric structures in debris disks. Observations of such eccentric structures thus provide potential evidence of the presence of such a companion in a planetary system. We considered the specific example of Zeta2 Reticuli, whose observed eccentric disk can be explained by a distant companion (at tens of AU) on an eccentric orbit (ep greater than approx. 0.3)
An inner warp discovered in the disk around HD 110058 using VLT/SPHERE and HST/STIS
An edge-on debris disk was detected in 2015 around the young, nearby A0V star
HD 110058. The disk showed features resembling those seen in the disk of beta
Pictoris that could indicate the presence of a perturbing planetary-mass
companion in the system. We investigated new and archival scattered light
images of the disk in order to characterise its morphology and spectrum. In
particular, we analysed the disk's warp to constrain the properties of possible
planetary perturbers. Our work uses data from two VLT/SPHERE observations and
archival data from HST/STIS. We measured the morphology of the disk by
analysing vertical profiles along the length of the disk to extract the
centroid spine position and vertical height. We extracted the surface
brightness and reflectance spectrum of the disk. We detect the disk between 20
au (with SPHERE) and 150 au (with STIS), at a position angle of
159.60.6. Analysis of the spine shows an asymmetry between
the two sides of the disk, with a 3.40.9 warp between ~20 au
and 60 au. The disk is marginally vertically resolved in scattered light, with
a vertical aspect ratio of 9.30.7% at 45 au. The extracted reflectance
spectrum is featureless, flat between 0.95 micron and 1.1 micron, and red from
1.1 micron to 1.65 micron. The outer parts of the disk are also asymmetric with
a tilt between the two sides compatible with a disk made of forward-scattering
particles and seen not perfectly edge-on, suggesting an inclination of
<84. The presence of an undetected planetary-mass companion on an
inclined orbit with respect to the disk could explain the warp. The
misalignment of the inner parts of the disk with respect to the outer disk
suggests a warp that has not yet propagated to the outer parts of the disk,
favouring the scenario of an inner perturber as the origin of the warp.Comment: 17 pages, 15 figures, 3 tables; accepted for publication in A&
Insights into the planetary dynamics of HD 206893 with ALMA
Radial substructure in the form of rings and gaps has been shown to be ubiquitous among protoplanetary discs. This could be the case in exo-Kuiper belts as well, and evidence for this is emerging. In this paper, we present ALMA observations of the debris/planetesimal disc surrounding HD 206893, a system that also hosts two massive companions at 2 and 11 au. Our observations reveal a disc extending from 30 to 180 au, split by a 27 au wide gap centred at 74 au, and no dust surrounding the reddened brown dwarf (BD) at 11 au. The gap width suggests the presence of a 0.9MJup planet at 74 au, which would be the third companion in this system. Using previous astrometry of the BD, combined with our derived disc orientation as a prior, we were able to better constrain its orbit finding it is likely eccentric ( 0.14+0.05â0.04 ). For the innermost companion, we used radial velocity, proper motion anomaly, and stability considerations to show its mass and semimajor axis are likely in the ranges 4-100MJup and 1.4-4.5 au. These three companions will interact on secular time-scales and perturb the orbits of planetesimals, stirring the disc and potentially truncating it to its current extent via secular resonances. Finally, the presence of a gap in this system adds to the growing evidence that gaps could be common in wide exo-Kuiper belts. Out of six wide debris discs observed with ALMA with enough resolution, four to five show radial substructure in the form of gaps
Chasing extreme planetary architectures: I- HD196885Ab, a super-Jupiter dancing with two stars?
Planet(s) in binaries are unique architectures for testing predictions of
planetary formation and evolution theories in very hostile environments. We
used the IRDIS dual-band imager of SPHERE at VLT, and the speckle
interferometric camera HRCAM of SOAR, to acquire high-angular resolution images
of HD 196885 AB between 2015 and 2020. Radial velocity observations have been
extended over almost 40 yr extending the radial velocity measurements HD 196885
A and resolving both the binary companion and the inner giant planet HD 196885
Ab. Finally, we took advantage of the exquisite astrometric precision of the
dual-field mode of VLTI/GRAVITY (down to 30 {\mu}as) to monitor the relative
position of HD 196885 A and B to search for the 3.6 yr astrometric wobble of
the circumprimary planet Ab imprinted on the binary separation. Our
observations enable to accurately constrain the orbital properties of the
binary HD 196885 AB, seen on an inclined and retrograde orbit (iAB = 120.43
deg) with a semi-major axis of 19.78 au, and an eccentricity of 0.417. The
GRAVITY measurements confirm for the first time the nature of the inner planet
HD 196885 Ab by rejecting all families of pole-on solutions in the stellar or
brown dwarf masses. The most favored island of solutions is associated with a
Jupiter-like planet (MAb = 3.39 MJup), with moderate eccentricity (eAaAb =
0.44), and inclination close to 143.04 deg. This results points toward a
significant mutual inclination (Phi = 24.36 deg) between the orbital planes
(relative to the star) of the binary companion B and the planet Ab. Our
dynamical simulations indicate that the system is dynamically stable over time.
Eccentricity and mutual inclination variations could be expected for moderate
von Zipele Kozai Lidov cycles that may affect the inner planet.Comment: 12 pages, 6 figures, accepted in A&