117 research outputs found
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 two. 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, are 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.Comment: accepted by MNRA
Inner mean-motion resonances with eccentric planets: A possible origin for exozodiacal dust clouds
High levels of dust have been detected in the immediate vicinity of many
stars, both young and old. A promising scenario to explain the presence of this
short-lived dust is that these analogues to the Zodiacal cloud (or exozodis)
are refilled in situ through cometary activity and sublimation. As the
reservoir of comets is not expected to be replenished, the presence of these
exozodis in old systems has yet to be adequately explained.
It was recently suggested that mean-motion resonances (MMR) with exterior
planets on moderately eccentric () orbits could
scatter planetesimals on to cometary orbits with delays of the order of several
100 Myr. Theoretically, this mechanism is also expected to sustain continuous
production of active comets once it has started, potentially over
Gyr-timescales.
We aim here to investigate the ability of this mechanism to generate
scattering on to cometary orbits compatible with the production of an exozodi
on long timescales. We combine analytical predictions and complementary
numerical N-body simulations to study its characteristics.
We show, using order of magnitude estimates, that via this mechanism, low
mass discs comparable to the Kuiper Belt could sustain comet scattering at
rates compatible with the presence of the exozodis which are detected around
Solar-type stars, and on Gyr timescales. We also find that the levels of dust
detected around Vega could be sustained via our proposed mechanism if an
eccentric Jupiter-like planet were present exterior to the system's cold debris
disc.Comment: 15 pages, 12 figures; Accepted for publication in MNRA
Modellierung der raeumlichen Struktur von Debris-Scheiben
The subject of the present thesis is the modeling of the spatial structure of debris disks from high-quality data, the dynamical modeling of planet-disk interaction in debris disks and the simulation of the detectability of the found structures with present and future instruments.Gegenstand der vorliegenden Dissertation ist die Modellierung der raeumlichen Struktur von Debris-Scheiben anhand qualitativ hochwertiger Daten, die dynamische Modellierung von Planet-Scheibe-Wechselwirkung in Debris-Scheiben, sowie die Simulation der Beobachtbarkeit der modellierten Strukturen mit aktuellen und zukuenftigen Instrumenten
Multi-wavelength modeling of the spatially resolved debris disk of HD 107146
(abridged) We aim to constrain the location, composition, and dynamical state
of planetesimal populations and dust around the young, sun-like (G2V) star HD
107146}. We consider coronagraphic observations obtained with the Advanced
Camera for Surveys (HST/ACS) onboard the HST in broad V and broad I filters, a
resolved 1.3mm map obtained with the Combined Array for Research in
Millimeter-Wave Astronomy (CARMA), Spitzer/IRS low resolution spectra, and the
spectral energy distribution (SED) of the object at wavelengths ranging from
3.5micron to 3.1mm. We complement these data with new coronagraphic high
resolution observations of the debris disk using the Near Infrared Camera and
Multi-Object Spectrometer (HST/NICMOS) aboard the HST in the F110W filter. The
SED and images of the disk in scattered light as well as in thermal reemission
are combined in our modeling using a parameterized model for the disk density
distribution and optical properties of the dust. A detailed analytical model of
the debris disk around HD 107146 is presented that allows us to reproduce the
almost entire set of spatially resolved and unresolved multi-wavelength
observations. Considering the variety of complementary observational data, we
are able to break the degeneracies produced by modeling SED data alone. We find
the disk to be an extended ring with a peak surface density at 131AU.
Furthermore, we find evidence for an additional, inner disk probably composed
of small grains released at the inner edge of the outer disk and moving inwards
due to Poynting-Robertson drag. A birth ring scenario (i.e., a more or less
broad ring of planetesimals creating the dust disk trough collisions) is found
to be the most likely explanation of the ringlike shape of the disk.Comment: 15 pages, 9 figures, accepted for publication in A&
Evidence of a discontinuous disk structure around the Herbig Ae star HD 139 614
A new class of pre-main sequence objects has been recently identified as
pre-transitional disks. They present near-infrared excess coupled to a flux
deficit at about 10 microns and a rising mid-infrared and far-infrared
spectrum. These features suggest a disk structure with inner and outer dust
components, separated by a dust-depleted region (or gap). We here report on the
first interferometric observations of the disk around the Herbig Ae star HD
139614. Its infrared spectrum suggests a flared disk, and presents
pre-transitional features,namely a substantial near-infrared excess accompanied
by a dip around 6 microns and a rising mid-infrared part. In this framework, we
performed a study of the spectral energy distribution (SED) and the
mid-infrared VLTI/MIDI interferometric data to constrain thespatial structure
of the inner dust disk region and assess its possibly multi-component
structure. We based our work on a temperature-gradient disk model that includes
dust opacity. While we could not reproduce the SED and interferometric
visibilities with a one-component disk, a better agreement was obtained with a
two-component disk model composed of an optically thin inner disk extending
from 0.22 to 2.3 au, a gap, and an outer temperature-gradient disk starting at
5.6 au. Therefore, our modeling favors an extended and optically thin inner
dust component and in principle rules out the possibility that the
near-infrared excess originates only from a spatially confined region.
Moreover, the outer disk is characterized by a very steep temperature profile
and a temperature higher than 300 K at its inner edge. This suggests the
existence of a warm component corresponding to a scenario where the inner edge
of the outer disk is directly illuminated by the central star. This is an
expected consequence of the presence of a gap, thus indicative of a
pre-transitional structure.Comment: 14 pages, 6 figure
Searching for faint companions with VLTI/PIONIER. II. 92 main sequence stars from the Exozodi survey
The Exozodi survey aims to determine the occurrence rate of bright
exozodiacal discs around nearby main sequence stars using infrared
interferometry. Although the Exozodi survey targets have been carefully
selected to avoid the presence of binary stars, the results of this survey can
still be biased by the presence of unidentified stellar companions. Using the
PIONIER data set collected within the Exozodi survey, we aim to search for the
signature of point-like companions around the Exozodi target stars. We use both
the closure phases and squared visibilities collected by PIONIER to search for
companions within the ~100 mas interferometric field of view. The presence of a
companion is assessed by computing the goodness of fit to the data for a series
of binary models with various separations and contrasts. Five stellar
companions are resolved for the first time around five A-type stars: HD 4150,
HD 16555, HD 29388, HD 202730, and HD 224392 (although the companion to HD
16555 was independently resolved by speckle interferometry while we were
carrying out the survey). In the most likely case of main sequence companions,
their spectral types range from A5V to K4V. Three of these stars were already
suspected to be binaries from Hipparcos astrometric measurements, although no
information was available on the companions themselves so far. In addition to
debiasing the statistics of the Exozodi survey, these results can also be used
to revise the fraction of visual binaries among A-type stars, suggesting that
an extra ~13% A-type stars are visual binaries in addition to the ones detected
in previous direct imaging surveys. We estimate that about half the population
of nearby A-type stars could be resolved as visual binaries using a combination
of state-of-the-art interferometry and single-aperture imaging, and we suggest
that a significant fraction of these binaries remains undetected to date.Comment: Accepted for publication in A&
An independent determination of Fomalhaut b's orbit and the dynamical effects on the outer dust belt
The nearby star Fomalhaut harbours a cold, moderately eccentric dust belt
with a sharp inner edge near 133 au. A low-mass, common proper motion companion
(Fom b), was discovered near the inner edge and was identified as a planet
candidate that could account for the belt morphology. However, the most recent
orbit determination based on four epochs of astrometry over eight years reveals
a highly eccentric orbit that appears to cross the belt in the sky plane
projection. We perform here a full orbital determination based on the available
astrometric data to independently validate the orbit estimates previously
presented. Adopting our values for the orbital elements and their associated
uncertainties, we then study the dynamical interaction between the planet and
the dust ring, to check whether the proposed disk sculpting scenario by Fom b
is plausible. We used a dedicated MCMC code to derive the statistical
distributions of the orbital elements of Fom b. Then we used symplectic N-body
integration to investigate the dynamics of the dust belt, as perturbed by a
single planet. Different attempts were made assuming different masses for Fom
b. We also performed a semi-analytical study to explain our results. Our
results are in good agreement with others regarding the orbit of Fom b. We find
that the orbit is highly eccentric, is close to apsidally aligned with the
belt, and has a moderate mutual inclination relative to the belt plane of. If
coplanar, this orbit crosses the disk. Our dynamical study then reveals that
the observed planet could sculpt a transient belt configuration with a similar
eccentricity to what is observed, but it would not be simultaneously apsidally
aligned with the planet. This transient configuration only occurs a short time
after the planet is placed on such an orbit (assuming an initially circular
disk), a time that is inversely proportional to the planet's mass, and that is
in any case much less than the 440 Myr age of the star. We constrain how long
the observed dust belt could have survived with Fom b on its current orbit, as
a function of its possible mass. This analysis leads us to conclude that Fom b
is likely to have low mass, that it is unlikely to be responsible for the
sculpting of the belt, and that it supports the hypothesis of a more massive,
less eccentric planet companion Fom c.Comment: 17 pages, 15 figures, accepted for publication in Astronomy \&
Astrophysic
Unveiling new stellar companions from the PIONIER exozodi survey
The main goal of the EXOZODI survey is to detect and characterize
circumstellar dust and to propose the first statistical study of exozodiacal
disks in the near-infrared using telescopes in both hemispheres. For this
purpose, Ertel et al. have conducted in 2012 a survey of nearby main sequence
stars with VLTI/PIONIER to search for the presence of circumstellar dust. This
survey, carried out during 12 nights, comprises about 100 stars. For each star,
we obtained typically three OBs and we searched for circumstellar emission
based on the measurement of squared visibilities at short baselines. A drop in
the measured visibilities with respect to the expected photospheric visibility
indicates the presence of resolved emission around the target star. It is
however generally not possible to conclude on the morphology of the detected
emission based solely on the squared visibilities. Here, we focus on closure
phases to search for faint companions around the whole sample. Indeed, to
derive robust statistics on the occurrence rate of bright exozodiacal disks, we
need to discriminate between companions and disks. For this reason, the main
goal of this paper is to discriminate between circumstellar disks (which show
no closure phase provided that they are point-symmetric) and faint companions
(point-like sources, creating non-zero closure phases). We also aim to reveal
new companions that do not necessarily produce a significant signature in the
squared visibilities, as the signature of the companion may show up more
prominently in the closure phases. In this process, we reveal four new stellar
companions with contrasts ranging from 2% to 95% (i.e., up to equal flux
binaries). We also tentatively detect faint companions around one other target
that will require follow-up observations to be confirmed or infirmed. We
discuss the implications of these discoveries on the results of the exozodi
survey.Comment: To appear in SPIE proceedings vol. 914
HD139614: the interferometric case for a group-Ib pre-transitional young disk
The Herbig Ae star HD 139614 is a group-Ib object, which featureless SED
indicates disk flaring and a possible pre-transitional evolutionary stage. We
present mid- and near-IR interferometric results collected with MIDI, AMBER and
PIONIER with the aim of constraining the spatial structure of the 0.1-10 AU
disk region and assess its possible multi-component structure. A two-component
disk model composed of an optically thin 2-AU wide inner disk and an outer
temperature-gradient disk starting at 5.6 AU reproduces well the observations.
This is an additional argument to the idea that group-I HAeBe inner disks could
be already in the disk-clearing transient stage. HD 139614 will become a prime
target for mid-IR interferometric imaging with the second-generation instrument
MATISSE of the VLTI.Comment: SPIE Astronomical Telescopes and Instrumentation conference, June
2014, 11 pages, 7 Figure
The TRENDS High-Contrast Imaging Survey. VII. Discovery of a Nearby Sirius-like White Dwarf System (HD 169889)
Monitoring the long-term radial velocity (RV) and acceleration of nearby
stars has proven an effective method for directly detecting binary and
substellar companions. Some fraction of nearby RV trend systems are expected to
be comprised of compact objects that likewise induce a systemic Doppler signal.
In this paper, we report the discovery of a white dwarf companion found to
orbit the nearby ( mas) G9 V star HD 169889.
High-contrast imaging observations using NIRC2 at Keck and LMIRCam at the LBT
uncover the (, ) companion
at an angular separation of 0.8'' (28 au). Thirteen years of precise Doppler
observations reveal a steep linear acceleration in RV time series and place a
dynamical constraint on the companion mass of . This "Sirius-like" system adds to the census of white dwarf
companions suspected to be missing in the solar neighborhood.Comment: Accepted to Ap
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