36 research outputs found
Probing the origin of UX Ori-type variability in the YSO binary CO Ori with VLTI/GRAVITY
The primary star in the young stellar object (YSO) binary CO Ori displays UX
Ori-type variability: irregular, high amplitude optical and near-infrared
photometric fluctuations where flux minima coincide with polarization maxima.
This is attributed to changes in local opacity. In CO Ori A, these variations
exhibit a 12.4 yr cycle. Here, we investigate the physical origin of the
fluctuating opacity and its periodicity using interferometric observations of
CO Ori obtained using VLTI/GRAVITY. Continuum K-band circum-primary and
circum-secondary emission are marginally spatially resolved for the first time
while Br emission is detected in the spectrum of the secondary. We
estimate a spectral type range for CO Ori B of K2-K5 assuming visual
extinction, and a distance of 430 pc. From geometric modelling
of the continuum visibilities, the circum-primary emission is consistent with a
central point source plus a Gaussian component with a full-width-half-maximum
of 2.310.04 milliarcseconds (mas), inclined at 30.22.2 and
with a major axis position angle of 406. This inclination is
lower than that reported for the discs of other UX Ori-type stars, providing a
first indication that the UX Ori phenomena may arise through fluctuations in
circumstellar material exterior to a disc, e.g. in a dusty outflow. An
additional wide, symmetric Gaussian component is required to fit the
visibilities of CO Ori B, signifying a contribution from scattered light.
Finally, closure phases of CO Ori A were used to investigate whether the 12.4
yr periodicity is associated with an undetected third component, as has been
previously suggested. We rule out any additional companions contributing more
than 3.6% to the K-band flux within ~7.3-20 mas of CO Ori A.Comment: 7 pages, 4 figures, accepted for publication in MNRA
Imagerie de l'environnement protoplanétaire des étoiles jeunes par interférométrie optique
An effective way to understand the formation of planets is the study of protoplanetary disks. The first images of these disks were obtained in the infrared and the millimeter in the 80s. These images unveiled for the first time the morphology of the infrared excess seen in the spectral energy distributions of young stellar objects. Since then, significant progress has been made and, in addition to the direct detection of planets, we are able to distinguish the disruption they could cause in these disks. The inner region of these disks, where the majority of planets are found, is complex as being the scene of many phenomena still poorly constrained (dust sublimation, winds, accretion). For the closest young stars, observing these regions amounts to achieve an angular resolution of the order of a milliarcsecond, unattainable with monolithic telescopes. The optical interferometry can reach such a small angle. This technique consists in combining the light of two or more telescopes to make it interfere. These interferences can be used to constrain the morphology of the observed object by using models. But to understand the phenomena involved in the inner parts of young stellar objects, it is necessary to have an independent image. Image reconstruction is possible with the recent advent of interferometers with 4 or more telescopes. The first images were able to be rebuilt. However, the central star does not allow easy access to the environment morphology. The goal of my thesis was to bypass this difficulty by developing a method of image reconstruction which is adapted to the protoplanetary environment of young stars. It consists in separating the central star of the image to reconstruct its environment while taking into account the temperature difference between the two. With this method and the VLTI interferometric instruments, I reconstructed the images of the first astronomical unit of a dozen of Herbig stars and revealed their morphologies. I was able to apply a novel geometric analysis to characterize them. Finally, I have analyzed in more detail a particular star, MWC158, which I imaged the variability that could be interpreted as a matter ejection. My thesis demonstrates the importance of the inclusion of chromatic aspects in image reconstruction and adaptation of this method to the specific characteristics of young stars.Une manière efficace de contraindre la formation des planètes est l'étude des disques protoplanétaires. Les premières images de ces disques ont été obtenues dans les années 80 en infrarouge et en millimétrique. Ces images dévoilaient pour la première fois la morphologie de l'excès infrarouge vu dans les distributions spectrales d'énergies des étoiles jeunes. Depuis, de nets progrès ont été faits et, outre la détection directe de planètes, nous sommes capables de distinguer les perturbations que celles-ci pourraient engendrer dans ces disques. La région interne de ces disques, où la majorité des planètes sont détectées, est complexe car étant le théâtre de nombreux phénomènes encore mal contraints (sublimation de la poussière, vents, accrétion). Pour les étoiles jeunes les plus proches, observer ces régions revient à atteindre une résolution angulaire de l'ordre de la milliseconde d'arc, inatteignable avec un télescope monolithique. L'interférométrie optique permet de satisfaire cette contrainte. Cette technique consiste à combiner la lumière de deux télescopes ou plus afin de la faire interférer. Ces interférences permettent de contraindre la morphologie de l'objet observé à l'aide de modèles. Mais afin de comprendre les phénomènes en jeu il est nécessaire d'avoir une image indépendante de ces modèles. La reconstruction d'images est possible avec l'avènement récent d'interféromètres à 4 télescopes ou plus. Les premières images ont ainsi pu être reconstruites. Cependant, l'étoile centrale ne permet pas d'accéder facilement à l'image de l'environnement. Ma thèse a donc consisté à outrepasser cette difficulté en développant une méthode de reconstruction d'image adaptée à l'environnement protoplanétaire des étoiles jeunes. Elle consiste à séparer l'étoile centrale de l'image afin de reconstruire son environnement tout en prenant en compte la différence de température entre ces deux éléments. Grâce à cette méthode et aux instruments interférométriques du VLTI, j'ai pu reconstruire les images des premières unités astronomiques d'une douzaine d'étoiles de Herbig et de révéler leurs morphologies. J'ai ainsi pu appliquer une analyse géométrique originale afin de les caractériser. Enfin, j'ai analysé plus en détail un étoile particulière, MWC158, dont j'ai imagé la variabilité qui pourrait être interprétée comme une éjection de matière. Ma thèse démontre l'importance de la prise en compte des aspects chromatiques dans la reconstruction d'image ainsi que de l'adaptation de cette méthode à la spécificité des étoiles jeunes
Gas dynamics in the inner few AU around the Herbig B[e] star MWC297: Indications of a disk wind from kinematic modeling and velocity-resolved interferometric imaging
We present near-infrared AMBER (R = 12, 000) and CRIRES (R = 100, 000)
observations of the Herbig B[e] star MWC297 in the hydrogen Br-gamma-line.
Using the VLTI unit telescopes, we obtained a uv-coverage suitable for aperture
synthesis imaging. We interpret our velocity-resolved images as well as the
derived two-dimensional photocenter displacement vectors, and fit kinematic
models to our visibility and phase data in order to constrain the gas velocity
field on sub-AU scales. The measured continuum visibilities constrain the
orientation of the near-infrared-emitting dust disk, where we determine that
the disk major axis is oriented along a position angle of 99.6 +/- 4.8 degrees.
The near-infrared continuum emission is 3.6 times more compact than the
expected dust-sublimation radius, possibly indicating the presence of highly
refractory dust grains or optically thick gas emission in the inner disk. Our
velocity-resolved channel maps and moment maps reveal the motion of the
Br-gamma-emitting gas in six velocity channels, marking the first time that
kinematic effects in the sub-AU inner regions of a protoplanetary disk could be
directly imaged. We find a rotation-dominated velocity field, where the blue-
and red-shifted emissions are displaced along a position angle of 24 +/- 3
degrees and the approaching part of the disk is offset west of the star. The
visibility drop in the line as well as the strong non-zero phase signals can be
modeled reasonably well assuming a Keplerian velocity field, although this
model is not able to explain the 3 sigma difference that we measure between the
position angle of the line photocenters and the position angle of the dust
disk. We find that the fit can be improved by adding an outflowing component to
the velocity field, as inspired by a magneto-centrifugal disk-wind scenario.Comment: 15 pages, 13 Figure
Intricate visibility effects from resolved emission of young stellar objects: the case of MWC158 observed with the VLTI
In the course of our VLTI young stellar object PIONIER imaging program, we
have identified a strong visibility chromatic dependency that appeared in
certain sources. This effect, rising value of visibilities with decreasing
wavelengths over one base, is also present in previous published and archival
AMBER data. For Herbig AeBe stars, the H band is generally located at the
transition between the star and the disk predominance in flux for Herbig AeBe
stars. We believe that this phenomenon is responsible for the visibility rise
effect. We present a method to correct the visibilities from this effect in
order to allow "gray" image reconstruction software, like Mira, to be used. In
parallel we probe the interest of carrying an image reconstruction in each
spectral channel and then combine them to obtain the final broadband one. As an
illustration we apply these imaging methods to MWC158, a (possibly Herbig) B[e]
star intensively observed with PIONIER. Finally, we compare our result with a
parametric model fitted onto the data.Comment: 14 pages, 7 figure
The Red Rectangle: a thin disk with big grains
The Red Rectangle is a nebula surrounding the post-AGB star HD 44179. It is
the prototype of a particular class of nebulae associated with post-AGB
binaries characterised by the presence of stable circumbinary disks in
(quasi-)Keplerian rotation. Here we present the results of new high-resolution
(20-50 mas) ALMA observations of continuum and line emissions at 0.9 mm. The
continuum maps are analysed through a simple model of dust emission, which can
reproduce the observational data. We find that most dust emission in the Red
Rectangle is concentrated in the central regions of the rotating disk and that
the settlement of dust grains onto the equatorial plane is very significant,
particularly in comparison with the much larger scale height displayed by the
gas distribution. The diameter of the dust-emitting region is about 250 au,
with a total width of about 50 au. This region coincides with the warm PDR
where certain molecules (like HCN), CI, and CII are presumably formed, as well
as probably PAHs. From the spectral index, we confirm the presence in the disk
of large grains, with a typical radius of about 0.150 mm, which supports the
long-lived hypothesis for this structure. We also confirm the existence of a
compact ionised wind at the centre of the nebula, probably emerging from the
accretion disk around the companion, for which we derive an extent of about 10
au and a total flux of 8 mJy. We also briefly present the results on molecular
lines of 12CO, 13CO, and other less abundant species.Comment: 6 pages, 4 figures, 1 table, Proceedings IAU Symposium No. 384
"Planetary Nebulae: a Universal Toolbox in the Era of Precision Astrophysics
The structure of jets launched from post-AGB binary systems
We focus on post-asymptotic giant branch (post-AGB) binaries and study the
interaction between the different components of these complex systems. These
components comprise the post-AGB primary, a main sequence secondary, a
circumbinary disk, as well as a fast bipolar outflow (jet) launched by the
companion. We obtained well-sampled time series of high resolution optical
spectra over the last decade and these spectra provide the basis of our study.
The jet is detected in absorption, at superior conjunction, when the line of
sight towards the primary goes through the bipolar cone. Our spectral time
series scan the jets during orbital motion. Our spatio-kinematic model is
constrained by these dynamical spectra. We complement this with a
radiative-transfer model in which the Balmer series are used to derive total
mass-loss rates in the jets. The jets are found to be wide and display an
angle-dependent density structure with a dense and slower outer region near the
jet cone and a fast inner part along the jet symmetry axes. The deprojected
outflow velocities confirm that the companions are main sequence companions.
The total mass-loss rates are large (10^{-8} and 10^{-5}\,solar mass per year),
from which we can infer that the mass-accretion rates onto the companion star
must be high as well. The circumbinary disk is likely the main source for the
accretion disk around the companion. All systems with full disks that start
near the sublimation radius show jets, whereas for systems with evolved
transition disks, this lowers to a detection rate of 50%. Objects without an
infrared excess do not show jets. We conclude that jet creation in post-AGB
binaries is a mainstream process. The interaction between the circumbinary
disks and the central binary provide the needed accretion flow, but the
presence of a circumbinary disk does not seem to be the only prerequisite to
launch a jet.Comment: 17 pages, 12 figures, accepted by Astronomy and Astrophysic
The shadow knows: using shadows to investigate the structure of the pretransitional disk of HD 100453
We present GPI polarized intensity imagery of HD 100453 in Y-, J-, and K1
bands which reveals an inner gap ( au), an outer disk ( au) with
two prominent spiral arms, and two azimuthally-localized dark features also
present in SPHERE total intensity images (Wagner 2015). SED fitting further
suggests the radial gap extends to au. The narrow, wedge-like shape of the
dark features appears similar to predictions of shadows cast by a inner disk
which is misaligned with respect to the outer disk. Using the Monte Carlo
radiative transfer code HOCHUNCK3D (Whitney 2013), we construct a model of the
disk which allows us to determine its physical properties in more detail. From
the angular separation of the features we measure the difference in inclination
between the disks 45, and their major axes, PA = 140 east
of north for the outer disk and 100for the inner disk. We find an
outer disk inclination of from face-on in broad agreement
with the Wagner 2015 measurement of 34. SPHERE data in J- and H-bands
indicate a reddish disk which points to HD 100453 evolving into a young debris
disk
Dusty disk winds at the sublimation rim of the highly inclined, low mass YSO SU Aurigae
T Tauri stars are low-mass young stars whose disks provide the setting for
planet formation. Despite this, their structure is poorly understood. We
present new infrared interferometric observations of the SU Aurigae
circumstellar environment that offer 3 x higher resolution and better baseline
position angle coverage over previous observations. We investigate the
characteristics of circumstellar material around SU Aur, constrain the disk
geometry, composition and inner dust rim structure. The CHARA array offers
opportunities for long baseline observations, with baselines up to 331 m. Using
the CLIMB 3-telescope combiner in the K-band allows us to measure visibilities
as well as closure phase. We undertook image reconstruction for
model-independent analysis, and geometric modeling. Additionally, the fitting
of radiative transfer models constrains the physical parameters of the disk.
For the first time, a dusty disk wind is introduced to the radiative transfer
code TORUS to model protoplanetary disks. Our implementation is motivated by
theoretical dusty disk winds, where magnetic field lines drive dust above the
disk plane close to the sublimation zone. Image reconstruction reveals an
inclined disk with slight asymmetry along its minor-axis, likely due to
inclination effects obscuring the inner disk rim through absorption of incident
star light on the near-side and thermal re-emission/scattering of the far-side.
Geometric modelling of a skewed ring finds the inner rim at 0.17+/-0.02 au with
an inclination of 50.9+/-1.0 degrees and minor axis position angle 60.8+/-1.2
degrees. Radiative transfer modelling shows a flared disk with an inner radius
at 0.18 au which implies a grain size of 0.4 um and a scale height of 15.0 au
at 100 au. Among the tested radiative transfer models, only the dusty disk wind
successfully accounts for the K-band excess by introducing dust above the
mid-plane.Comment: Accepted for publication in Astronomy \& Astrophysic