Near-infrared Interferometric Observations Research Articles

Three-dimensional Orbit of AC Her Determined: Binary-induced Truncation Cannot Explain the Large Cavity in This Post-AGB Transition Disk

Some evolved binaries, namely post–asymptotic giant branch (AGB) binaries, are surrounded by stable and massive circumbinary disks similar to protoplanetary disks found around young stars. Around 10% of these disks are transition disks: they have a large inner cavity in the dust. Previous interferometric measurements and modeling have ruled out these cavities being formed by dust sublimation and suggested that they are due to massive circumbinary planets that trap dust in the disk and produce the observed depletion of refractory elements on the surfaces of the post-AGB stars. In this study, we test an alternative scenario in which the large cavities could be due to dynamical truncation from the inner binary. We performed near-infrared interferometric observations with the CHARA Array on the archetype of such a transition disk around a post-AGB binary: AC Her. We detect the companion at ten epochs over 4 yr and determine the three-dimensional orbit using these astrometric measurements in combination with a radial velocity time series. This is the first astrometric orbit constructed for a post-AGB binary system. We derive the best-fit orbit with a semimajor axis of 2.01 ± 0.01 mas (2.83 ± 0.08 au), inclination (142.9 ± 1.1)°, and longitude of the ascending node (155.1 ± 1.8)°. We find that the theoretical dynamical truncation and dust sublimation radii are at least ∼3× smaller than the observed inner disk radius (∼21.5 mas or 30 au). This strengthens the hypothesis that the origin of the cavity is due to the presence of a circumbinary planet.

Binarity and beyond in A stars II. Disentangling the four stars in the vicinity of the triple HIP 87813 within the quintuple system HJ2814

ABSTRACT A-type stars are the progenitors of about half of the white dwarfs (WDs) that currently exist. The observational mappings of both multiplicities are far from complete and their connection is not known. We are in the process of obtaining tight constraints on a sample of 108 southern and nearby A-stars that are part of the VAST sample by conducting near-infrared (NIR) interferometric observations to the stars among them which have large Gaia-Hipparcos accelerations. In this paper, we combine spectroscopy, adaptive optics imaging, NIR interferometry, and Gaia-Hipparcos astrometry in order to disentangle the stars in the complicated HIP 87813 = HJ2814A system. We show that (i) a previously discovered faint star separated by 2 arcsec is actually a background source; (ii) the Gaia-Hipparcos acceleration is caused by a newly discovered M ∼ 0.77 M⊙ star that was missed in previous AO images and we solve for its a = 27 au (P ∼ 70 yrs) astrometric orbit; (iii) by combining archival spectra with interferometric observations, we show that the A star has a very close M ∼ 0.85 M⊙ companion on a a = 0.16 au (P = 13.4 d) orbit. The mutual inclination in the hierarchical triple is constrained to $\phi _{\mathrm{rel}}=46\pm 15^{^{\circ }}$ but Kozai–Lidov eccentricity oscillations in the inner binary are currently very small and damped by general relativistic precession. The system HJ2814 is one of only about fifteen known 5+ multiples with an A-star primary, and will result in a system of at least two WDs within about a Hubble time.

A detailed analysis of the Gl 486 planetary system

Context.The Gl 486 system consists of a very nearby, relatively bright, weakly active M3.5 V star at just 8 pc with a warm transiting rocky planet of about 1.3R⊕and 3.0M⊕. It is ideal for both transmission and emission spectroscopy and for testing interior models of telluric planets.Aims.To prepare for future studies, we aim to thoroughly characterise the planetary system with new accurate and precise data collected with state-of-the-art photometers from space and spectrometers and interferometers from the ground.Methods.We collected light curves of seven new transits observed with the CHEOPS space mission and new radial velocities obtained with MAROON-X at the 8.1 m Gemini North telescope and CARMENES at the 3.5 m Calar Alto telescope, together with previously published spectroscopic and photometric data from the two spectrographs and TESS. We also performed near-infrared interferometric observations with the CHARA Array and new photometric monitoring with a suite of smaller telescopes (AstroLAB, LCOGT, OSN, TJO). This extraordinary and rich data set was the input for our comprehensive analysis.Results.From interferometry, we measure a limb-darkened disc angular size of the star Gl 486 atθLDD= 0.390 ± 0.018 mas. Together with a correctedGaiaEDR3 parallax, we obtain a stellar radiusR*= 0.339 ± 0.015R⊕. We also measure a stellar rotation period atProt= 49.9 ± 5.5 days, an upper limit to its XUV (5-920 A) flux informed by newHubble/STIS data, and, for the first time, a variety of element abundances (Fe, Mg, Si, V, Sr, Zr, Rb) and C/O ratio. Moreover, we imposed restrictive constraints on the presence of additional components, either stellar or sub-stellar, in the system. With the input stellar parameters and the radial-velocity and transit data, we determine the radius and mass of the planet Gl 486 b atRp= 1.343−0.062+0.063R⊕andMp= 3.00−0.12+0.13M⊕, with relative uncertainties of the planet radius and mass of 4.7% and 4.2%, respectively. From the planet parameters and the stellar element abundances, we infer the most probable models of planet internal structure and composition, which are consistent with a relatively small metallic core with respect to the Earth, a deep silicate mantle, and a thin volatile upper layer. With all these ingredients, we outline prospects for Gl 486 b atmospheric studies, especially with forthcomingJames WebbSpace Telescope (Webb) observations.

The GRAVITY young stellar object survey

Context. Protoplanetary disks drive some of the formation process (e.g., accretion, gas dissipation, formation of structures) of stars and planets. Understanding such physical processes is one of the most significant astrophysical questions. HD 163296 is an interesting young stellar object for which infrared and sub-millimeter observations have shown a prominent circumstellar disk with gaps plausibly created by forming planets. Aims. This study aims to characterize the morphology of the inner disk in HD 163296 with multi-epoch, near-infrared interferometric observations performed with GRAVITY at the Very Large Telescope Interferometer. Our goal is to depict the K-band (λ0 ~ 2.2 μm) structure of the inner rim with milliarcsecond (sub-au) angular resolution. Our data is complemented with archival Precision Integrated-Optics Near-infrared Imaging ExpeRiment (H-band; λ0 ~ 1.65 μm) data of the source. Methods. We performed a gradient descent parametric model fitting to recover the sub-au morphology of our source. Results. Our analysis shows the existence of an asymmetry in the disk surrounding the central star of HD 163296. We confirm variability of the disk structure in the inner ~2 mas (0.2 au). While variability of the inner disk structure in this source has been suggested by previous interferometric studies, this is the first time that it is confirmed in the H- and K-bands by using a complete analysis of the closure phases and squared visibilities over several epochs. Because of the separation from the star, position changes, and the persistence of this asymmetric structure on timescales of several years, we argue that it is probably a dusty feature (e.g., a vortex or dust clouds) made by a mixing of silicate and carbon dust and/or refractory grains, inhomogeneously distributed above the mid-plane of the disk.

Determining Subparsec Supermassive Black Hole Binary Orbits with Infrared Interferometry

Abstract Radial-velocity monitoring has revealed the presence of moving broad emission lines in some quasars, potentially indicating the presence of a subparsec binary system. Phase-referenced, near-infrared interferometric observations could map out the binary orbit by measuring the photocenter difference between a broad emission line and the hot dust continuum. We show that astrometric data over several years may be able to detect proper motions and accelerations, confirming the presence of a binary and constraining system parameters. The brightness, redshifts, and astrometric sizes of current candidates are well matched to the capabilities of the upgraded Very Large Telescope Interferometer/GRAVITY+ instrument, and we identify a first sample of 10 possible candidates. The astrometric signature depends on the morphology and evolution of hot dust emission in supermassive black hole binary systems. Measurements of the photocenter offset may reveal binary motion whether the hot dust emission region is fixed to the inner edge of the circumbinary disk, or moves in response to the changing irradiation pattern from an accreting secondary black hole.

The Inner Disk of RY Tau: Evidence of Stellar Occultation by the Disk Atmosphere at the Sublimation Rim from K-band Continuum Interferometry

Abstract We present models of the inner region of the circumstellar disk of RY Tau that aim to explain our near-infrared (K-band: 2.1 μm) interferometric observations, while remaining consistent with the optical to near-infrared portions of the spectral energy distribution. Our submilliarcsecond-resolution CHARA Array observations are supplemented with shorter baseline, archival data from PTI, KI, and VLTI/GRAVITY and modeled using an axisymmetric Monte Carlo radiative transfer code. The K-band visibilities are well fit by models incorporating a central star illuminating a disk with an inner edge shaped by dust sublimation at 0.210 ± 0.005 au, assuming a viewing geometry adopted from millimeter interferometry (65° inclined with a disk major axis position angle of 23°). This sublimation radius is consistent with that expected of silicate grains with a maximum size of 0.36–0.40 μm contributing to the opacity, and is an order of magnitude further from the star than the theoretical magnetospheric truncation radius. The visibilities on the longest baselines probed by CHARA indicate that we lack a clear line of sight to the stellar photosphere. Instead, our analysis shows that the central star is occulted by the disk surface layers close to the sublimation rim. While we do not see direct evidence of temporal variability in our multiepoch CHARA observations, we suggest the aperiodic photometric variability of RY Tau is likely related temporal and/or azimuthal variations in the structure of the disk surface layers.

A family portrait of disk inner rims around Herbig Ae/Be stars

Context. The innermost astronomical unit (au) in protoplanetary disks is a key region for stellar and planet formation, as exoplanet searches have shown a large occurrence of close-in planets that are located within the first au around their host star. Aims. We aim to reveal the morphology of the disk inner rim using near-infrared interferometric observations with milli-arcsecond resolution provided by near-infrared multitelescope interferometry. Methods. We provide model-independent reconstructed images of 15 objects selected from the Herbig AeBe survey carried out with PIONIER at the Very Large Telescope Interferometer, using the semi-parametric approach for image reconstruction of chromatic objects. We propose a set of methods to reconstruct and analyze the images in a consistent way. Results. We find that 40% of the systems (6/15) are centrosymmetric at the angular resolution of the observations. For the rest of the objects, we find evidence for asymmetric emission due to moderate-to-strong inclination of a disk-like structure for ~30% of the objects (5/15) and noncentrosymmetric morphology due to a nonaxisymmetric and possibly variable environment (4/15, ~27%). Among the systems with a disk-like structure, 20% (3/15) show a resolved dust-free cavity. Finally, we do not detect extended emission beyond the inner rim. Conclusions. The image reconstruction process is a powerful tool to reveal complex disk inner rim morphologies, which is complementary to the fit of geometrical models. At the angular resolution reached by near-infrared interferometric observations, most of the images are compatible with a centrally peaked emission (no cavity). For the most resolved targets, image reconstruction reveals morphologies that cannot be reproduced by generic parametric models (e.g., perturbed inner rims or complex brightness distributions). Moreover, the nonaxisymmetric disks show that the spatial resolution probed by optical interferometers makes the observations of the near-infrared emission (inside a few au) sensitive to temporal evolution with a time-scale down to a few weeks. The evidence of nonaxisymmetric emission that cannot be explained by simple inclination and radiative transfer effects requires alternative explanations, such as a warping of the inner disks. Interferometric observations can therefore be used to follow the evolution of the asymmetry of those disks at an au or sub-au scale.

Variable Warm Dust around the Herbig Ae Star HD 169142: Birth of a Ring?*

Abstract The Herbig Ae star HD 169142 is known to have a gaseous disk with a large inner hole, and also a photometrically variable inner dust component in the sub-astronomical-unit region. Following up on our previous analysis, we further studied the temporal evolution of inner dust around HD 169142, which may provide information on the evolution from late-stage protoplanetary disks to debris disks. We used near-infrared interferometric observations obtained with the Very Large Telescope Interferometer/PIONIER to constrain the dust distribution at three epochs spanning six years. We also studied the photometric variability of HD 169142 using our optical–infrared observations and archival data. Our results indicate that a dust ring at ∼0.3 au formed some time between 2013 and 2018, and then faded (but did not completely disappear) by 2019. The short-term variability resembles that observed in extreme debris disks, and is likely related to short-lived dust of secondary origin, though variable shadowing from the inner ring could be an alternative interpretation. If confirmed, this is the first direct detection of secondary dust production inside a protoplanetary disk.

First NIR interferometrically resolved high-order Brackett and forbidden Fe lines of a B[e] star: V921 Sco

ABSTRACT We present near-infrared interferometric AMBER observations of the B[e] binary V921 Sco at low (R ∼ 30) and medium spectral resolution (R∼ 1500) in the K and H bands. Low spectral resolution AMBER data were used to estimate the position of the companion V921 Sco B and confirmed a clockwise movement on sky with respect to the primary of 33° between 2008 and 2012. Our observations resolve for the first time higher order Brackett lines (Br6–Br12). The modelling of the different line transitions revealed a decrease in the size of the line-emitting regions from Br3 to Br12. We are able to reproduce this decrease with a simple radiative transfer model of an equatorial disc in local thermodynamic equilibrium. In addition to the Brackett series, we also resolve permitted and forbidden Fe line emission. Our modelling shows that these lines originate from ∼2 au from the star, corresponding roughly to the measured dust sublimation region. This might indicate that the forbidden line emission arises from shock excitation at the base of a disc wind.

A study of dust properties in the inner sub-au region of the Herbig Ae star HD 169142 with VLTI/PIONIER

Context. An essential step to understanding protoplanetary evolution is the study of disks that contain gaps or inner holes. The pre-transitional disk around the Herbig star HD 169142 exhibits multi-gap disk structure, differentiated gas and dust distribution, planet candidates, and near-infrared fading in the past decades, which make it a valuable target for a case study of disk evolution. Aims. Using near-infrared interferometric observations with VLTI/PIONIER, we aim to study the dust properties in the inner sub-au region of the disk in the years 2011−2013, when the object is already in its near-infrared faint state. Methods. We first performed simple geometric modeling to characterize the size and shape of the NIR-emitting region. We then performed Monte-Carlo radiative transfer simulations on grids of models and compared the model predictions with the interferometric and photometric observations. Results. We find that the observations are consistent with optically thin gray dust lying at Rin ~ 0.07 au, passively heated to T ~ 1500 K. Models with sub-micron optically thin dust are excluded because such dust will be heated to much higher temperatures at similar distance. The observations can also be reproduced with a model consisting of optically thick dust at Rin ~ 0.06 au, but this model is plausible only if refractory dust species enduring ~ 2400 K exist in the inner disk.

The wind and the magnetospheric accretion onto the T Tauri star S Coronae Australis at sub-au resolution

Aims. To investigate the inner regions of protoplanetary discs, we performed near-infrared interferometric observations of the classical T Tauri binary system S CrA. Methods. We present the first VLTI-GRAVITY high spectral resolution (R ~ 4000) observations of a classical T Tauri binary, S CrA (composed of S CrA N and S CrA S and separated by ~1.̋4), combining the four 8m telescopes in dual-field mode. Results. Our observations in the near-infrared K-band continuum reveal a disc around each binary component, with similar half-flux radii of about 0.1 au at d ~ 130 pc, inclinations (i = 28 ± 3° and i = 22 ± 6°), and position angles (PA = 0°± 6° and PA = –2°± 12°), suggesting that they formed from the fragmentation of a common disc. The S CrA N spectrum shows bright He i and Brγ line emission exhibiting inverse P Cygni profiles, typically associated with infalling gas. The continuum-compensated Brγ line visibilities of S CrA N show the presence of a compact Brγ emitting region whose radius is about ~0.06 au, which is twice as big as the truncation radius. This component is mostly tracing a wind. Moreover, a slight radius change between the blue- and red-shifted Brγ line components is marginally detected. Conclusions. The presence of an inverse P Cygni profile in the He i and Brγ lines, along with the tentative detection of a slightly larger size of the blue-shifted Brγ line component, hint at the simultaneous presence of a wind and magnetospheric accretion in S CrA N.

A disk asymmetry in motion around the B[e] star MWC158

MWC158 is a star with the B[e] 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. 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. 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. 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. 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.

Warm dust resolved in the cold disk around T Chamaeleontis with VLTI/AMBER

The transition between massive Class II circumstellar disks and Class III debris disks, with dust residuals, has not yet been clearly understood. Disks are expected to dissipate with time, and dust clearing in the inner regions can be the consequence of several mechanisms. Planetary formation is one of them that will possibly open a gap inside the disk. According to recent models based on photometric observations, T Cha is expected to present a large gap within its disk, meaning that an inner dusty disk is supposed to have survived close to the star. We investigate this scenario with new near-infrared interferometric observations. We observed T Cha in the H and K bands using the AMBER instrument at VLTI and used the MCFOST radiative transfer code to model the SED of T Cha and the interferometric observations simultaneously and to test the scenario of an inner dusty structure. We also used a toy model of a binary to check that a companion close to the star can reproduce our observations. The scenario of a close (few mas) companion cannot satisfactorily reproduce the visibilities and SED, while a disk model with a large gap and an inner ring producing the bulk of the emission (in H and K-bands) close to 0.1 AU is able to account for all the observations. With this study, the presence of an optically thick inner dusty disk close to the star and dominating the H and K- bands emission is confirmed. According to our model, the large gap extends up to ~ 7.5 AU. This points toward a companion (located at several AU) gap-opening scenario to explain the morphology of T Cha.

A hot compact dust disk around a massive young stellar object

Circumstellar disks are an essential ingredient of the formation of low-mass stars. It is unclear, however, whether the accretion-disk paradigm can also account for the formation of stars more massive than about 10 solar masses, in which strong radiation pressure might halt mass infall. Massive stars may form by stellar merging, although more recent theoretical investigations suggest that the radiative-pressure limit may be overcome by considering more complex, non-spherical infall geometries. Clear observational evidence, such as the detection of compact dusty disks around massive young stellar objects, is needed to identify unambiguously the formation mode of the most massive stars. Here we report near-infrared interferometric observations that spatially resolve the astronomical-unit-scale distribution of hot material around a high-mass ( approximately 20 solar masses) young stellar object. The image shows an elongated structure with a size of approximately 13 x 19 astronomical units, consistent with a disk seen at an inclination angle of approximately 45 degrees . Using geometric and detailed physical models, we found a radial temperature gradient in the disk, with a dust-free region less than 9.5 astronomical units from the star, qualitatively and quantitatively similar to the disks observed in low-mass star formation. Perpendicular to the disk plane we observed a molecular outflow and two bow shocks, indicating that a bipolar outflow emanates from the inner regions of the system.

INTERFEROMETRIC EVIDENCE FOR RESOLVED WARM DUST IN THE DQ TAU SYSTEM

We report on near-infrared (IR) interferometric observations of the double-lined pre-main sequence binary system DQ Tau. We model these data with a visual orbit for DQ Tau supported by the spectroscopic orbit and analysis of Mathieu et al. Further, DQ Tau exhibits significant near-IR excess; modeling our data requires inclusion of near-IR light from an source. Remarkably, the excess source is resolved in our data, similar in scale to the binary itself (~0.2 AU at apastron), rather than the larger circumbinary disk (~0.4 AU radius). Our observations support the Mathieu et al. and Carr et al. inference of significant warm material near the DQ Tau binary.

Perspectives in Radiative Transfer and Interferometry: Protoplanetary Disks

Near-infrared and mid-infrared broad band interferometric observations of young stellar objects trace the inner part of the warm dusty circumstellar environment (smaller than a few AUs) and can be used to constrain its physical properties, in particular to characterize the size, location and composition of the emitting region. Because the spatial frequency coverage of current instruments remain sparse, modeling, including detailed radiative transfer (RT), is necessary to get a precise estimate of these properties.

Water vapour and hydrogen in the terrestrial-planet-forming region of a protoplanetary disk

Planetary systems (ours included) formed in disks of dust and gas around young stars. Disks are an integral part of the star and planet formation process, and knowledge of the distribution and temperature of inner-disk material is crucial for understanding terrestrial planet formation, giant planet migration, and accretion onto the central star. Although the inner regions of protoplanetary disks in nearby star-forming regions subtend only a few nano-radians, near-infrared interferometry has recently enabled the spatial resolution of these terrestrial zones. Most observations have probed only dust, which typically dominates the near-infrared emission. Here I report spectrally dispersed near-infrared interferometric observations that probe the gas (which dominates the mass and dynamics of the inner disk), in addition to the dust, within one astronomical unit (1 au, the Sun-Earth distance) of the young star MWC 480. I resolve gas, including water vapour and atomic hydrogen, interior to the edge of the dust disk; this contrasts with results of previous spectrally dispersed interferometry observations. Interactions of this accreting gas with migrating planets may lead to short-period exoplanets like those detected around main-sequence stars. The observed water vapour is probably produced by the sublimation of migrating icy bodies, and provides a potential reservoir of water for terrestrial planets.

Large dust grains in the inner region of circumstellar disks

Context. Simple geometrical ring models account well for near-infrared interferometric observations of dusty disks surrounding pre-main sequence stars of intermediate mass. Such models demonstrate that the dust distribution in these disks has an inner hole and puffed-up inner edge consistent with theoretical expectations. Aims. In this paper, we reanalyze the available interferometric observations of six intermediate mass pre-main sequence stars (CQ Tau, VV Ser, MWC 480, MWC 758, V1295 Aql and AB Aur) in the framework of a more detailed physical model of the inner region of the dusty disk. Our aim is to verify whether the model will allow us to constrain the disk and dust properties. Methods. Observed visibilities from the literature are compared with theoretical visibilities from our model. With the assumption that silicates are the most refractory dust species, our model computes self-consistently the shape and emission of the inner edge of the dusty disk (and hence its visibilities for given interferometer configurations). The only free parameters in our model are the inner disk orientation and the size of the dust grains. Results. In all objects with the exception of AB Aur, our self-consistent models reproduce both the interferometric results and the near-infrared spectral energy distribution. In four cases, grains larger than ∼1.2 μm, and possibly much larger are either required by or consistent with the observations. The inclination of the inner disk is found to be always larger than ∼30°, and in at least two objects much larger.

A Gas + Dust Model of the Peculiar Envelope of HD 62623 Based on Interferometric Observations

We present an improved model for the circumstellar environment of the peculiar star HD 62623 taking into account the gas around the central star and the dust present in the outer parts of the envelope. We compare our results with photometric observations and near-infrared interferometric observations obtained with aperture masking interferometry performed at the Keck I telescope from the literature. This modeling has two goals: to provide a framework for understanding the partially resolved object shown by the visibility curves, and to allow investigation of the envelope of HD 62623 by discrimination between proposed models. We show that a better fit for both the photometric and interferometric data is obtained by adding the gas in the inner part of the envelope. The present model allows us to confirm a smaller value for the inner dust radius, i.e., Rin = 15-20 R*. The dust temperature we obtain at this boundary is Tin = 1500 K. Our simulation seems to indicate that, even with a nearly spherical model, a sufficient density (ρ ~ 10-18 g cm-3) to form dust at the inner radius is obtained. Despite our multiple tentatives, we were not able to reproduce the visibility curve at 1.65 μm showing an unresolved object. At 1.24, 2.26, and 2.08 μm, the agreement between our best model and the visibility data is not good enough to draw a very strong conclusion, but it may indicate that the circumstellar environment of HD 62623 is based on different grain sizes and optically thick dust.