773 research outputs found
Flaring and self-shadowed disks around Herbig Ae stars: simulations for 10 micron interferometers
We present simulations of the interferometric visibilities of Herbig Ae star
disks. We investigate whether interferometric measurements in the 10 micrometer
atmospheric window are sensitive to the presence of an increased scale height
at the inner disk edge, predicted by recent models. Furthermore, we investigate
whether such measurements can discriminate between disks with a ``flaring''
geometry and disks with a ``flat'' geometry. We show that both these questions
can be addressed, using measurements at a small number of appropriately chosen
baselines. The classification of Herbig Ae stars in two groups, based on the
appearance of the spectral energy distribution (SED), has been attributed to a
difference in disk geometry. Sources with a group I SED would have a flaring
outer disk geometry, whereas the disk of group II sources is proposed to be
flat (or ``self-shadowed''). We show that this hypothesis can be tested using
long-baseline interferometric measurements in the micrometer atmospheric
window.Comment: 10 pages, 7 figures, accepted for publiction in Astronomy and
Astrophysic
Governance for quality management in smallholder-based tropical food chains
Abstract The paper provides a framework that focuses on the linkages between several key dimensions of supply chain organization and performance of perishable tropical food products. The focus is on the relationship between governance regime and quality management. However, two other but related variables are taken into account because they impact on the relationship between governance and quality management. These variables are channel choice and value added distribution in the supply chain. Governance regime is reflecting how to enhance coordination and trust amongst supply chain partners and how to reduce transaction costs. Quality management is dealing with how to manage food technology processes such that required quality levels can be improved and variability in quality of natural products can be exploited. Governance regimes in relation to quality management practices are discussed to the extent that supply chain partners are able, or are enabled, to invest in required quality improve-ments. Reduction of transaction costs, creation of trust-based networks and proper trade-offs between direct and future gains may offer substantial contributions to effective quality management and enforcement. This framework has been applied to nine case studies on smallholder-based food supply chains originating from developing countries (Ruben et al., 2007). Three of these case studies are discussed in this paper to illustrate what challenges can be derived from the case studies. The selected case studies concern fish originating from Kenya, mango originating from Costa Rica and vegetables produced in China
Dissecting Massive YSOs with Mid-Infrared Interferometry
The very inner structure of massive YSOs is difficult to trace. With
conventional observational methods we often identify structures still several
hundreds of AU in size. But we also need information about the innermost
regions where the actual mass transfer onto the forming high-mass star occurs.
An innovative way to probe these scales is to utilise mid-infrared
interferometry. Here, we present first results of our MIDI GTO programme at the
VLTI. We observed 10 well-known massive YSOs down to scales of 20 mas. We
clearly resolve these objects which results in low visibilities and sizes in
the order of 30 - 50 mas. Thus, with MIDI we can for the first time quantify
the extent of the thermal emission from the warm circumstellar dust and thus
calibrate existing concepts regarding the compactness of such emission in the
pre-UCHII region phase. Special emphasis will be given to the BN-type object
M8E-IR where our modelling is most advanced and where there is indirect
evidence for a strongly bloated central star.Comment: 8 pages, 6 figures, proceedings contribution for the conference
"Massive Star Formation: Observations confront Theory", held in September
2007 in Heidelberg, Germany; to appear in ASP Conf. Ser. 387, H. Beuther et
al. (eds.
Young stars in Epsilon Cha and their disks: disk evolution in sparse associations
(abridge) The nearby young stellar association Epsilon Cha association has an
estimated age of 3-5 Myr, making it an ideal laboratory to study the disk
dissipation process and provide empirical constraints on the timescale of
planet formation. We combine the available literature data with our Spitzer IRS
spectroscopy and VLT/VISIR imaging data. The very low mass stars USNO-B120144.7
and 2MASS J12005517 show globally depleted spectral energy distributions
pointing at strong dust settling. 2MASS J12014343 may have a disk with a very
specific inclination where the central star is effectively screened by the cold
outer parts of a flared disk but the 10 micron radiation of the warm inner disk
can still reach us. We find the disks in sparse stellar associations are
dissipated more slowly than those in denser (cluster) environments. We detect
C_{2}H_{2} rovibrational band around 13.7 micron on the IRS spectrum of
USNO-B120144.7. We find strong signatures of grain growth and crystallization
in all Epsilon Cha members with 10 micron features detected in their IRS
spectra. We combine the dust properties derived in the Epsilon Cha sample with
those found using identical or similar methods in the MBM 12, Coronet cluster,
Eta Cha associations, and in the cores to disks (c2d) legacy program. We find
that disks around low-mass young stars show a negative radial gradient in the
mass-averaged grain size and mass fraction of crystalline silicates. A positive
correlation exists between the mass-averaged grain sizes of amorphous silicates
and the accretion rates if the latter is above ~10^{-9} Msun/yr, possibly
indicating that those disks are sufficiently turbulent to prevent grains of
several microns in size to sink into the disk interior.Comment: 17 pages, 18 figures, 6 tables, language revised; accepted to A&
petitRADTRANS: a Python radiative transfer package for exoplanet characterization and retrieval
We present the easy-to-use, publicly available, Python package petitRADTRANS,
built for the spectral characterization of exoplanet atmospheres. The code is
fast, accurate, and versatile; it can calculate both transmission and emission
spectra within a few seconds at low resolution ( = 1000;
correlated-k method) and high resolution (;
line-by-line method), using only a few lines of input instruction. The somewhat
slower correlated-k method is used at low resolution because it is more
accurate than methods such as opacity sampling. Clouds can be included and
treated using wavelength-dependent power law opacities, or by using optical
constants of real condensates, specifying either the cloud particle size, or
the atmospheric mixing and particle settling strength. Opacities of amorphous
or crystalline, spherical or irregularly-shaped cloud particles are available.
The line opacity database spans temperatures between 80 and 3000 K, allowing to
model fluxes of objects such as terrestrial planets, super-Earths, Neptunes, or
hot Jupiters, if their atmospheres are hydrogen-dominated. Higher temperature
points and species will be added in the future, allowing to also model the
class of ultra hot-Jupiters, with equilibrium temperatures K. Radiative transfer results were tested by cross-verifying the low- and
high-resolution implementation of petitRADTRANS, and benchmarked with the
petitCODE, which itself is also benchmarked to the ATMO and Exo-REM codes. We
successfully carried out test retrievals of synthetic JWST emission and
transmission spectra (for the hot Jupiter TrES-4b, which has a of
1800 K). The code is publicly available at
http://gitlab.com/mauricemolli/petitRADTRANS, and its documentation can be
found at https://petitradtrans.readthedocs.io.Comment: 17 pages, 7 figures, published in A&
Refined physical properties and g',r',i',z',J,H,K transmission spectrum of WASP-23b from the ground
Multi-band observations of planetary transits using the telescope defocus
technique may yield high-quality light curves suitable for refining the
physical properties of exoplanets even with small or medium size telescopes.
Such observations can be used to construct a broad-band transmission spectrum
of transiting planets and search for the presence of strong absorbers. We have
thoroughly characterised the orbital ephemeris and physical properties of the
transiting planet and host star in the WASP-23b system, constructed a
broad-band transmission spectrum of WASP-23b and performed a comparative
analysis with theoretical models of hot Jupiters. We observed a complete
transit of WASP-23b in seven bands simultaneously, using the GROND instrument
on the MPG/ESO 2.2m telescope at La Silla Observatory and telescope
defocussing. The optical data were taken in the Sloan g',r',i' and z' bands.
The resulting light curves are of high quality, with a root-mean-square scatter
of the residual as low as 330ppm in the z'-band, with a cadence of 90s.
Near-infrared data were obtained in the JHK bands. We performed MCMC analysis
of our photometry plus existing radial velocity data to refine measurements of
the ephemeris and physical properties of the WASP-23. We constructed a
broad-band transmission spectrum of WASP-23b and compared it with a theoretical
transmission spectrum of a Hot Jupiter. We measured the central transit time
with a precision about 8s. From this and earlier observations we obtain an
orbital period of P=2.9444300+/-0.0000011d. Our analysis also yielded a larger
radius and mass for the planet (Rp=1.067+0.045-0.038 RJup and,
Mp=0.917+0.040-0.039MJup). The transmission spectrum is marginally flat, given
the limited precision of the measurements for the planet radius and poor
spectral resolution of the data.Comment: 8 pages, 5 figures, accepted for publication in Astronomy &
Astrophysic
The structure of disks around intermediate-mass young stars from mid-infrared interferometry. Evidence for a population of group II disks with gaps
The disks around Herbig Ae/Be stars are commonly divided into group I and
group II based on their far-infrared spectral energy distribution, and the
common interpretation for that is flared and flat disks. Recent observations
suggest that many flaring disks have gaps, whereas flat disks are thought to be
gapless. The different groups of objects can be expected to have different
structural signatures in high-angular-resolution data. Over the past 10 years,
the MIDI instrument on the Very Large Telescope Interferometer has collected
observations of several tens of protoplanetary disks. We model the large set of
observations with simple geometric models. A population of radiative-transfer
models is synthesized for interpreting the mid-infrared signatures. Objects
with similar luminosities show very different disk sizes in the mid-infrared.
Restricting to the young objects of intermediate mass, we confirm that most
group I disks are in agreement with being transitional. We find that several
group II objects have mid-infrared sizes and colors overlapping with sources
classified as group I, transition disks. This suggests that these sources have
gaps, which has been demonstrated for a subset of them. This may point to an
intermediate population between gapless and transition disks. Flat disks with
gaps are most likely descendants of flat disks without gaps. Gaps, potentially
related to the formation of massive bodies, may therefore even develop in disks
in a far stage of grain growth and settling. The evolutionary implications of
this new population could be twofold. Either gapped flat disks form a separate
population of evolved disks, or some of them may further evolve into flaring
disks with large gaps. The latter transformation may be governed by the
interaction with a massive planet, carving a large gap and dynamically exciting
the grain population in the disk.Comment: 24 pages, 11 figures, A&A in pres
Evidence for grain growth in T Tauri disks
In this article we present the results from mid-infrared spectroscopy of a
sample of 14 T Tauri stars with silicate emission. The qualitative analysis of
the spectra reveals a correlation between the strength of the silicate feature
and its shape similar to the one which was found recently for the more massive
Herbig Ae/Be stars by van Boekel et al. (2003). The comparison with theoretical
spectra of amorphous olivine with different grain sizes suggests that this
correlation is indicating grain growth in the disks of T Tauri stars. Similar
mechanisms of grain processing appear to be effective in both groups of young
stars.Comment: 4 pages A&A lette
Grain growth in newly discovered young eruptive stars
FU Orionis-type stars are young stellar objects showing large outbursts due
to highly enhanced accretion from the circumstellar disk onto the protostar.
FUor-type outbursts happen in a wide variety of sources from the very embedded
ones to those with almost no sign of extended emission beyond the disk. The
subsequent eruptions might gradually clear up the obscuring envelope material
and drive the protostar on its way to become a disk-only T Tauri star. We used
VLT/VISIR to obtain the first spectra that cover the 8-13 m mid-infrared
wavelength range in low-resolution of five recently discovered FUors. Four
objects from our sample show the 10 m silicate feature in emission. We
study the shape and strength of the silicate feature in these objects and find
that they mostly contain large amorphous grains, suggesting that large grains
are typically not settled to the midplane in FUor disks. This is a general
characteristic of FUors, as opposed to regular T Tauri-type stars whose disks
display anything from pristine small grains to significant grain growth. We
classify our targets by determining whether the silicate feature is in emission
or in absorption, and confront them with the evolutionary scenarios on the
dispersal of the envelopes around young stars. In our sample, all Class II
objects exhibit silicate emission, while for Class I objects, the appearance of
the feature in emission or absorption depends on the viewing angle with respect
to the outflow cavity. This highlights the importance of geometric effects when
interpreting the silicate feature.Comment: 7 pages, 1 table, 3 figures, accepted for publication in the
Astrophysical Journal Letter
Scattered light images of spiral arms in marginally gravitationally unstable discs with an embedded planet
Scattered light images of transition discs in the near-infrared often show
non-axisymmetric structures in the form of wide-open spiral arms in addition to
their characteristic low-opacity inner gap region. We study self-gravitating
discs and investigate the influence of gravitational instability on the shape
and contrast of spiral arms induced by planet-disc interactions.
Two-dimensional non-isothermal hydrodynamical simulations including viscous
heating and a cooling prescription are combined with three-dimensional dust
continuum radiative transfer models for direct comparison to observations. We
find that the resulting contrast between the spirals and the surrounding disc
in scattered light is by far higher for pressure scale height variations, i.e.
thermal perturbations, than for pure surface density variations. Self-gravity
effects suppress any vortex modes and tend to reduce the opening angle of
planet-induced spirals, making them more tightly wound. If the disc is only
marginally gravitationally stable with a Toomre parameter around unity, an
embedded massive planet (planet-to-star mass ratio of ) can trigger
gravitational instability in the outer disc. The spirals created by this
instability and the density waves launched by the planet can overlap resulting
in large-scale, more open spiral arms in the outer disc. The contrast of these
spirals is well above the detection limit of current telescopes.Comment: Accepted for publication in MNRAS; 13 pages, 8 figure
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