Context.
The low wind effect (LWE) occurs at the aperture of 8-meter class telescopes when the spiders holding the secondary mirror get significantly cooler than the air. The effect creates phase ...discontinuities in the incoming wavefront at the location of the spiders. Under the LWE, the wavefront residuals after correction of the adaptive optics (AO) are dominated by low-order aberrations, pistons, and tip-tilts, contained in the pupil quadrants separated by the spiders. Those aberrations, called petal modes, degrade the AO performances during the best atmospheric turbulence conditions. Ultimately, the LWE is an obstacle for high-contrast exoplanet observations at a small angular separation from the host star.
Aims.
We aim to understand why extreme AO with a Shack-Hartmann (SH) wavefront sensor fails to correct for the petal tip and tilt modes, while these modes imprint a measurable signal in the SH slopes. We explore if the petal tip and tilt content of the LWE can be controlled and mitigated without an additional wavefront sensor.
Methods.
We simulated the sensitivity of a single subaperture of a SH wavefront sensor in the presence of a phase discontinuity across this subaperture. We explored the effect of the most important parameters: the amplitude of the discontinuity, the spider thickness, and the field of view. We then performed end-to-end simulations to reproduce and explain the behavior of extreme AO systems based on a SH in the presence of the LWE. We then evaluated the efficiency of a new mitigation strategy by running simulations, including atmosphere and realistic LWE phase perturbations.
Results.
For realistic parameters (i.e. a spider thickness at 25% of a SH subaperture, and a field of view of 3.5
λ/d
), we find that the sensitivity of the SH to a phase discontinuity is dramatically reduced, or even reversed. Under the LWE, a nonzero curl path is created in the measured slopes, which transforms into vortex-structures in the residuals when the loop is closed. While these vortexes are easily seen in the residual wavefront and slopes, they cannot be controlled by the system. We used this understanding to propose a strategy for controlling the petal tip and tilt modes of the LWE by using the measurements from the SH, but excluding the faulty subapertures.
Conclusions.
The proposed mitigation strategy may be of use in all extreme AO systems based on SH for which the LWE is an issue, such as SPHERE and GRAVITY+.
A close encounter of the massive kind Apellaniz, J Maiz; Sana, H; Barba, R H ...
Monthly Notices of the Royal Astronomical Society,
01/2017, Volume:
464, Issue:
3
Journal Article
Peer reviewed
Open access
We have used (i) Hubble Space Telescope Advanced Camera for Surveys imaging and Space Telescope Imaging Spectrograph spectroscopy, (ii) ground-based Precision Integrated-Optics Near-infrared Imaging ...ExpeRiment/Very Large Telescope long-baseline interferometry, and (iii) ground-based spectroscopy from different instruments to study the orbit of the extreme multiple system HD 93...129 Aa,Ab, which is composed of (at least) two very massive stars in a long-period orbit with e > 0.92, which will pass through periastron in 2017/2018. In several ways, the system is an ... Car precursor. Around the time of periastron passage, the two very strong winds will collide and generate an outburst of non-thermal hard X-ray emission without precedent in an O+O binary since astronomers have been able to observe above Earth's atmosphere. A coordinated multiwavelength monitoring in the next two years will enable a breakthrough understanding of the wind interactions in such extreme close encounters. Furthermore, we have found evidence that HD 93 129 Aa may be a binary system itself. In that case, we could witness a three-body interaction which may yield a runaway star or a stellar collision close to or shortly after the periastron passage. Either of those outcomes would be unprecedented, as they are predicted to be low-frequency events in the Milky Way. (ProQuest: ... denotes formulae/symbols omitted.)
Stellar orbits at the Galactic Center provide a very clean probe of the gravitational potential of the supermassive black hole. They can be studied with unique precision, beyond the confusion limit ...of a single telescope, with the near-infrared interferometer GRAVITY. Imaging is essential to search the field for faint, unknown stars on short orbits which potentially could constrain the black hole spin. Furthermore, it provides the starting point for astrometric fitting to derive highly accurate stellar positions. Here, we present G
R
, a new imaging tool specifically designed for Galactic Center observations with GRAVITY. The algorithm is based on a Bayesian interpretation of the imaging problem, formulated in the framework of information field theory and building upon existing works in radio-interferometric imaging. Its application to GRAVITY observations from 2021 yields the deepest images to date of the Galactic Center on scales of a few milliarcseconds. The images reveal the complicated source structure within the central 100 mas around Sgr A*, where we detected the stars S29 and S55 and confirm S62 on its trajectory, slowly approaching Sgr A*. Furthermore, we were able to detect S38, S42, S60, and S63 in a series of exposures for which we offset the fiber from Sgr A*. We provide an update on the orbits of all aforementioned stars. In addition to these known sources, the images also reveal a faint star moving to the west at a high angular velocity. We cannot find any coincidence with any known source and, thus, we refer to the new star as S300. From the flux ratio with S29, we estimate its
K
-band magnitude as
m
K
(S300) ≃ 19.0 − 19.3. Images obtained with CLEAN confirm the detection. To assess the sensitivity of our images, we note that fiber damping reduces the apparent magnitude of S300 and the effect increases throughout the year as the star moves away from the field center. Furthermore, we performed a series of source injection tests. Under favorable circumstances, sources well below a magnitude of 20 can be recovered, while 19.7 is considered the more universal limit for a good data set.
Context.
Past observations of O-type stars in the Galaxy have shown that almost all massive stars are part of a binary or higher-order multiple system. Given the wide range of separations at which ...these companions are found, several observational techniques have been adopted to characterize them. Despite the recent advancements in interferometric and adaptive optics observations, contrasts greater than 4 in the
H
band have never been reached between 100 and 1000 mas.
Aims.
Using new adaptive optics (AO) assisted coronagraphic observations, we aim to study the multiplicity properties of a sample of 18 dwarf (or sub-giant) O stars in the galactic field and in OB associations to probe the existence of stellar companions in the angular separation range from 0.″15 to 6″ down to very low mass ratios.
Methods.
We used VLT/SPHERE to observe simultaneously with the IRDIS and IFS sub-systems 18 O-type stars within 6 kpc and ages between 1 and 5 Myr. The IFS
YJH
band observations have allowed us to probe the presence of sub-solar companions in a 1.7″ × 1.7″ field-of-view down to magnitude limits of Δ
H
= 10 at 0.″4. In the wider 12″ × 12″ IRDIS field-of-view, we reached contrasts of Δ
K
= 12 at 1″, enabling us to look for even fainter companions at larger angular separations and to probe the source density of the surrounding portion of the sky.
Results.
This paper presents five newly discovered intermediate (< 1″) separation companions, three of which are smaller than 0.2
M
⊙
. If confirmed by future analyses of proper motions, these new detections represent the lowest-mass companions ever found around O-type stars. Additionally, 29 other sources are found in the IRDIS field-of-view with spurious association probabilities smaller than 5%. Assuming that all sources detected within 1″ are physically bound companions, the observed (uncorrected for bias) fraction of companions for O-type stars between 150 and 900 mas is 0.39 ± 0.15, whereas it increases to 1.6 ± 0.3 in the separation range from 0.″9 to 6″.
Conclusions.
These findings clearly support the notion that massive stars form almost exclusively in multiple systems, serving as proof of concept that supports the application of larger AO-assisted coronagraphic surveys as a crucial step in placing constraints on the multiplicity properties of massive star companions in regions of the parameter space that have previously gone unexplored. These results also demonstrate that the companion mass function is populated down to the lowest stellar masses.
Context. HD 150136 is a triple hierarchical system and a non-thermal radio emitter. It is formed by an O3−3.5 V + O5.5−6 V close binary and a more distant O6.5−7 V tertiary. So far, only the inner ...orbital properties have been reliably constrained. Aims. To quantitatively understand the non-thermal emission process, accurate knowledge of the physical and orbital properties of the object is crucial. Here, we aim to investigate the orbital properties of the wide system and to constrain the inclinations of the inner and outer binaries, and with these the absolute masses of the system components. Methods. We used the PIONIER combiner at the Very Large Telescope Interferometer to obtain the very first interferometric measurements of HD 150136. We combined the interferometric observations with new and existing high-resolution spectroscopic data to derive the orbital solution of the outer companion in the three-dimensional space. Results. The wide system is clearly resolved by PIONIER, with a projected separation on the plane of the sky of about 9 milli-arcsec. The best-fit orbital period, eccentricity, and inclination are 8.2 yr, 0.73, and 108°. We constrain the masses of the three stars of the system to 63 ± 10, 40 ± 6, and 33 ± 12 M⊙ for the O3−3.5 V, O5.5−6 V, and O6.5−7 V components. Conclusions. The dynamical masses agree within errors with the evolutionary masses of the components. Future interferometric and spectroscopic monitoring of HD 150136 should allow one to reduce the uncertainties to a few per cent only and to accurately constrain the distance to the system. This makes HD 150136 an ideal system to quantitatively test evolutionary models of high-mass stars as well as the physics of non-thermal processes occurring in O-type systems.
We report the detection of continuous positional and polarization changes of the compact source SgrA* in high states (“flares”) of its variable near-infrared emission with the near-infrared ...GRAVITY-Very Large Telescope Interferometer (VLTI) beam-combining instrument. In three prominent bright flares, the position centroids exhibit clockwise looped motion on the sky, on scales of typically 150 μas over a few tens of minutes, corresponding to about 30% the speed of light. At the same time, the flares exhibit continuous rotation of the polarization angle, with about the same 45(±15) min period as that of the centroid motions. Modelling with relativistic ray tracing shows that these findings are all consistent with a near face-on, circular orbit of a compact polarized “hot spot” of infrared synchrotron emission at approximately six to ten times the gravitational radius of a black hole of 4 million solar masses. This corresponds to the region just outside the innermost, stable, prograde circular orbit (ISCO) of a Schwarzschild–Kerr black hole, or near the retrograde ISCO of a highly spun-up Kerr hole. The polarization signature is consistent with orbital motion in a strong poloidal magnetic field.
Context. MWC158 is a star with the Be phenomenon that shows strong spectrophotometric variability (in lines and in UV and visible continuum) attributed to phases of shell ejection. The evolutionary ...stage of this star was never clearly determined. Previous interferometric, spectropolarimetric and spectro-interferometric studies suggest a disk morphology for its environment. Aims. We investigate the origin of the variability within the inner astronomical unit of the central star using near-infrared interferometric observations with PIONIER at the VLTI over a two-year period. Methods. We performed an image reconstruction of the circumstellar environment using the SPARCO method. We discovered that the morphology of the circumstellar environment could vary on timescales of weeks or days. We carried out a parametric fit of the data with a model consisting of a star, a disk and a bright spot that represents a brighter emission in the disk. Results. We detect strong morphological changes in the first astronomical unit around the star, that happen on a timescale of few months. We cannot account for such variability well with a binary model. Our parametric model fits the data well and allows us to extract the location of the asymmetry for different epochs. Conclusions. For the first time, we detect a morphological variability in the environment of MWC158. This variability is reproduced by a model of a disk and a bright spot. The locations of the bright spot suggest that it is located in the disk, but its precise motion is not determined. The origin of the asymmetry in the disk is complex and may be related to asymmetric shell ejections.
Stars orbiting the compact radio source Sgr A* in the Galactic Center serve as precision probes of the gravitational field around the closest massive black hole. In addition to adaptive ...optics-assisted astrometry (with NACO/VLT) and spectroscopy (with SINFONI/VLT, NIRC2/Keck and GNIRS/Gemini) over three decades, we have obtained 30–100 μas astrometry since 2017 with the four-telescope interferometric beam combiner GRAVITY/VLTI, capable of reaching a sensitivity of
m
K
= 20 when combining data from one night. We present the simultaneous detection of several stars within the diffraction limit of a single telescope, illustrating the power of interferometry in the field. The new data for the stars S2, S29, S38, and S55 yield significant accelerations between March and July 2021, as these stars pass the pericenters of their orbits between 2018 and 2023. This allows for a high-precision determination of the gravitational potential around Sgr A*. Our data are in excellent agreement with general relativity orbits around a single central point mass,
M
•
= 4.30 × 10
6
M
⊙
, with a precision of about ±0.25%. We improve the significance of our detection of the Schwarzschild precession in the S2 orbit to 7
σ
. Assuming plausible density profiles, the extended mass component inside the S2 apocenter (≈0.23″ or 2.4 × 10
4
R
S
) must be ≲3000
M
⊙
(1
σ
), or ≲0.1% of
M
•
. Adding the enclosed mass determinations from 13 stars orbiting Sgr A* at larger radii, the innermost radius at which the excess mass beyond Sgr A* is tentatively seen is
r
≈ 2.5″ ≥ 10× the apocenter of S2. This is in full harmony with the stellar mass distribution (including stellar-mass black holes) obtained from the spatially resolved luminosity function.
The mass-loss mechanism of cool massive evolved stars is poorly understood. The proximity of Betelgeuse makes it an appealing target to study its atmosphere, map the shape of its envelope, and follow ...the structure of its wind from the photosphere out to the interstellar medium. A link is suspected between the powerful convective motions in Betelgeuse and its mass loss. We aim to constrain the spatial structure and temporal evolution of the convective pattern on the photosphere and to search for evidence of this link. We report new interferometric observations in the infrared H-band using the VLTI/PIONIER instrument. We monitored the photosphere of Betelgeuse between 2012 January and 2014 November to look for evolutions that may trigger the outflow. We bring new evidence for the presence of a convective pattern in the photosphere of red supergiants. We propose that the large observed feature is modifying the signature of the convective pattern at the surface of the star in a way that simulations cannot reproduce.
Context.
Asymptotic giant branch (AGB) stars are cool luminous evolved stars that are well observable across the Galaxy and populating
Gaia
data. They have complex stellar surface dynamics, which ...amplifies the uncertainties on stellar parameters and distances.
Aims.
On the AGB star CL Lac, it has been shown that the convection-related variability accounts for a substantial part of the
Gaia
DR2 parallax error. We observed this star with the MIRC-X beam combiner installed at the CHARA interferometer to detect the presence of stellar surface inhomogeneities.
Methods.
We performed the reconstruction of aperture synthesis images from the interferometric observations at different wavelengths. Then, we used 3D radiative hydrodynamics (RHD) simulations of stellar convection with CO5BOLD and the post-processing radiative transfer code O
PTIM
3D to compute intensity maps in the spectral channels of MIRC-X observations. Then, we determined the stellar radius using the average 3D intensity profile and, finally, compared the 3D synthetic maps to the reconstructed ones focusing on matching the intensity contrast, the morphology of stellar surface structures, and the photocentre position at two different spectral channels, 1.52 and 1.70
μ
m, simultaneously.
Results.
We measured the apparent diameter of CL Lac at two wavelengths (3.299 ± 0.005 mas and 3.053 ± 0.006 mas at 1.52 and 1.70
μ
m, respectively) and recovered the radius (
R
= 307 ± 41 and
R
= 284 ± 38
R
⊙
) using a
Gaia
parallax. In addition to this, the reconstructed images are characterised by the presence of a brighter area that largely affects the position of the photocentre. The comparison with 3D simulation shows good agreement with the observations both in terms of contrast and surface structure morphology, meaning that our model is adequate for explaining the observed inhomogenities.
Conclusions.
This work confirms the presence of convection-related surface structures on an AGB star of
Gaia
DR2. Our result will help us to take a step forward in exploiting
Gaia
measurement uncertainties to extract the fundamental properties of AGB stars using appropriate RHD simulations.
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