283 research outputs found
Resolving the dusty circumstellar environment of the A[e] supergiant HD 62623 with the VLTI/MIDI
B[e] stars are hot stars surrounded by circumstellar gas and dust responsible
for the presence of emission lines and IR-excess in their spectra. How dust can
be formed in this highly illuminated and diluted environment remains an open
issue. HD 62623 is one of the very few A-type supergiants showing the B[e]
phenomenon. We obtained nine calibrated visibility measurements using the
VLTI/MIDI instrument in SCI-PHOT mode and PRISM spectral dispersion mode with
projected baselines ranging from 13 to 71 m and with various position angles.
We used geometrical models and physical modeling with a radiative transfer code
to analyze these data. The dusty circumstellar environment of HD 62623 is
partially resolved by the VLTI/MIDI even with the shortest baselines. The
environment is flattened and can be separated into two components: a compact
one whose extension grows from 17 mas at 8 microns to 30 mas at 9.6 microns and
stays almost constant up to 13 microns, and a more extended one that is
over-resolved even with the shortest baselines. Using the radiative transfer
code MC3D, we managed to model HD 62623's circumstellar environment as a dusty
disk with an inner radius of 3.85+-0.6 AU, an inclination angle of 60+-10 deg,
and a mass of 2x10^-7Mo. It is the first time that the dusty disk inner rim of
a supergiant star exhibiting the B[e] phenomenon is significantly constrained.
The inner gaseous envelope likely contributes up to 20% to the total N band
flux and acts like a reprocessing disk. Finally, the hypothesis of a stellar
wind deceleration by the companion's gravitational effects remains the most
probable case since the bi-stability mechanism does not seem to be efficient
for this star.Comment: 13 pages, 11 figures. A&A accepted pape
MoSâ‚‚ tribofilm distribution from low viscosity lubricants and its effect on friction
The current study analyses the friction performance of low viscosity fully-formulated oils containing the Molybdenum Dialkyl Dithiocarbamate (MoDTC) friction modifier at different concentrations. The MoDTC friction modifier is known to produce MoS2 sheets in the tribocontact providing a low coefficient of friction under boundary lubrication conditions. However, there is a little knowledge around the quantitative relationship between the concentration of MoDTC in the oil and MoS2 amount and distribution in the contact. The study uses Raman spectroscopy mapping capability to characterise the tribofilm formed from different chemistry lubricants and under different tribological conditions as defined by the lambda ratio. After qualitative and quantitative chemical surface characterisation a discussion is presented to highlight some important aspects to relate the formed MoS2 sheets, their spatial distribution in tribofilms and the subsequent tribological performance
The origin of dust polarization in the Orion Bar
The linear polarization of thermal dust emission provides a powerful tool to
probe interstellar and circumstellar magnetic fields, because aspherical grains
tend to align themselves with magnetic field lines. While the Radiative
Alignment Torque (RAT) mechanism provides a theoretical framework to this
phenomenon, some aspects of this alignment mechanism still need to be
quantitatively tested. One such aspect is the possibility that the reference
alignment direction changes from the magnetic field ("B-RAT") to the radiation
field k-vector ("k-RAT") in areas of strong radiation fields. We investigate
this transition toward the Orion Bar PDR, using multi-wavelength SOFIA HAWC+
dust polarization observations. The polarization angle maps show that the
radiation field direction is on average not the preferred grain alignment axis.
We constrain the grain sizes for which the transition from B-RAT to k-RAT occur
in the Orion Bar (grains > 0.1 {\mu}m toward the most irradiated locations),
and explore the radiatively driven rotational disruption that may take place in
the high-radiation environment of the Bar for large grains. While the grains
susceptible to rotational disruption should be in supra-thermal rotation and
aligned with the magnetic field, k-RAT aligned grains would rotate at thermal
velocities. We find that the grain size at which the alignment shifts from
B-RAT to k-RAT corresponds to grains too large to survive the rotational
disruption. Therefore, we expect a large fraction of grains to be aligned at
supra-thermal rotation with the magnetic field, and potentially be subject to
rotational disruption depending on their tensile strength
Pre-maximum spectro-imaging of the Mira star T Lep with AMBER/VLTI
Diffuse envelopes around Mira variables are among the most important sources
influencing the chemical evolution of galaxies. However they represent an
observational challenge because of their complex spectral features and their
rapid temporal variability. We constrained the exact brightness distribution of
the Mira star TLep with a model-independent analysis. We obtained single-epoch
interferometric observations with a dataset continuous in the spectral domain
(1.5-2.4mum) and in the spatial domain (baselines ranging from 11 to 96m). We
performed a model independent image reconstruction for each spectral bin using
the MIRA software. We completed the analysis by modeling the data with a simple
star+layer model inspired from the images. Reconstructed images confirm the
general picture of a central star partially obscured by the surrounding
molecular shell of changing opacity. At 1.7mum, the shell becomes optically
thin, with corresponding emission appearing as a ring circling the star. This
is the first direct evidence of the spherical morphology of the molecular
shell. Model fitting confirmed a spherical layer of constant size and changing
opacity over the wavelengths. Rough modeling points to a continuum opacity
within the shell, in addition to the CO and H2O features. Accordingly, it
appeared impossible to model the data by a photosphere alone in any of the
spectral bins.Comment: Accepted in A&
Milli-arcsecond astrophysics with VSI, the VLTI spectro-imager in the ELT era
Nowadays, compact sources like surfaces of nearby stars, circumstellar
environments of stars from early stages to the most evolved ones and
surroundings of active galactic nuclei can be investigated at milli-arcsecond
scales only with the VLT in its interferometric mode. We propose a
spectro-imager, named VSI (VLTI spectro-imager), which is capable to probe
these sources both over spatial and spectral scales in the near-infrared
domain. This instrument will provide information complementary to what is
obtained at the same time with ALMA at different wavelengths and the extreme
large telescopes.Comment: 8 pages. To be published in the proceedings of the ESO workshop
"Science with the VLT in the ELT Era", held in Garching (Germany) on 8-12
October 2007, A. Moorwood edito
The Stellar Composition of the Star Formation Region CMa R1 -- III. A new outburst of the Be star component in Z CMa
We report on a recent event in which, after more than a decade of slowly
fading, the visual brightness of the massive young binary Z CMa suddenly
started to rise by about 1 magnitude in December 1999, followed by a rapid
decline to its previous brightness over the next six months. This behaviour is
similar to that exhibited by this system around its eruption in February 1987.
A comparison of the intrinsic luminosities of the system with recent
evolutionary calculations shows that Z CMa may consist of a 16 M_sun B0 IIIe
primary star and a ~ 3 M_sun FUOr secondary with a common age of ~ 3 x 10^5 yr.
We also compare new high-resolution spectra obtained in Jan. and Feb. 2000,
during the recent rise in brightness, with archive data from 1991 and 1996. The
spectra are rich in emission lines, which originate from the envelope of the
early B-type primary star. The strength of these emission lines increased
strongly with the brightness of Z CMa. We interpret the collected spectral data
in terms of an accretion disc with atmosphere around the Herbig B0e component
of Z CMa, which has expanded during the outbursts of 1987 and 2000. A high
resolution profile of the 6300 A [O I] emission line, obtained by us in March
2002 shows an increase in flux and a prominent blue shoulder to the feature
extending to ~ -700 km/s, which was much fainter in the pre-outburst spectra.
We propose that this change in profile is a result of a strong change in the
collimation of a jet, as a result of the outburst at the start of this century.Comment: 22 pages, 12 figures, accepted for publication in MNRA
Imaging Young Stellar Objects with VLTi/PIONIER
OHP 2013: International Colloquium at Haute-Provence Observatory, France, 23-27 September 2013 - Improving the performances of current optical interferometers and future designsThis is the final version of the paper. Available from the publisher via the URL in this record.Optical interferometry imaging is designed to help us to reveal complex astronomical sources without a prior model. Among these complex objects are the young stars and their environments, which have a typical morphology with a point-like source, surrounded by circumstellar material with unknown morphology. To image them, we have developed a numerical method that removes completely the stellar point source and reconstructs the rest of the image, using the differences in the spectral behavior between the star and its circumstellar material. We aim to reveal the first Astronomical
Units of these objects where many physical phenomena could interplay: the dust sublimation causing a puffed-up inner rim, a dusty halo, a dusty wind or an inner gaseous component. To investigate more deeply these regions, we carried out the first Large Program survey of HAeBe stars with two main goals: statistics on the geometry of these objects at the first astronomical unit scale and imaging their very close environment. The images reveal the environment, which is not polluted by the star and allows us to derivethe best fit for the flux ratio and the spectral slope. We present the first images from this survey and the application of the imaging method on other astronomical objects.This work is supported by the French ANR POLCA project (Processing of pOLychromatic interferometriC data for Astrophysics, ANR-10-BLAN-0511
Imaging the heart of astrophysical objects with optical long-baseline interferometry
The number of publications of aperture-synthesis images based on optical
long-baseline interferometry measurements has recently increased due to easier
access to visible and infrared interferometers. The interferometry technique
has now reached a technical maturity level that opens new avenues for numerous
astrophysical topics requiring milli-arcsecond model-independent imaging. In
writing this paper our motivation was twofold: 1) review and publicize
emblematic excerpts of the impressive corpus accumulated in the field of
optical interferometry image reconstruction; 2) discuss future prospects for
this technique by selecting four representative astrophysical science cases in
order to review the potential benefits of using optical long baseline
interferometers. For this second goal we have simulated interferometric data
from those selected astrophysical environments and used state-of-the-art codes
to provide the reconstructed images that are reachable with current or
soon-to-be facilities. The image reconstruction process was "blind" in the
sense that reconstructors had no knowledge of the input brightness
distributions. We discuss the impact of optical interferometry in those four
astrophysical fields. We show that image reconstruction software successfully
provides accurate morphological information on a variety of astrophysical
topics and review the current strengths and weaknesses of such reconstructions.
We investigate how to improve image reconstruction and the quality of the image
possibly by upgrading the current facilities. We finally argue that optical
interferometers and their corresponding instrumentation, existing or to come,
with 6 to 10 telescopes, should be well suited to provide images of complex
sceneries.Comment: Acccepted to Astronomy and Astrophysics Revie
Experimental Assessment of the Water Quality Influence on the Phosphorus Uptake of an Invasive Aquatic Plant: Biological Responses throughout Its Phenological Stage
International audienceUnderstanding how an invasive plant can colonize a large range of environments is still a great challenge in freshwater ecology. For the first time, we assessed the relative importance of four factors on the phosphorus uptake and growth of an invasive macrophyte Elodea nuttallii (Planch.) St. John. This study provided data on its phenotypic plasticity, which is frequently suggested as an important mechanism but remains poorly investigated. The phosphorus uptake of two Elodea nuttallii subpopulations was experimentally studied under contrasting environmental conditions. Plants were sampled in the Rhine floodplain and in the Northern Vosges mountains, and then maintained in aquaria in hard (Rhine) or soft (Vosges) water. Under these conditions, we tested the influence of two trophic states (eutrophic state, 100 mu g.l(-1) P-PO43- and hypertrophic state, 300 mu g.l(-1) P-PO43-) on the P metabolism of plant subpopulations collected at three seasons (winter, spring and summer). Elodea nuttallii was able to absorb high levels of phosphorus through its shoots and enhance its phosphorus uptake, continually, after an increase of the resource availability (hypertrophic > eutrophic). The lowest efficiency in nutrient use was observed in winter, whereas the highest was recorded in spring, what revealed thus a storage strategy which can be beneficial to new shoots. This experiment provided evidence that generally, the water trophic state is the main factor governing P uptake, and the mineral status (softwater > hardwater) of the stream water is the second main factor. The phenological stage appeared to be a confounding factor to P level in water. Nonetheless, phenology played a role in P turnover in the plant. Finally, phenotypic plasticity allows both subpopulations to adapt to a changing environment
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