arXiv subjects

Antoine Mérand

Publications and source records attributed to Antoine Mérand.

At least 37 records · Page 2Linked to original sources

arXiv2023

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 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 the cavity being formed by dust sublimation and suggested that the cavity is due to a massive circumbinary planet that traps the dust in the disk and produces the observed depletion of refractory elements on the surface of the post-AGB star. In this study, we test alternative scenario in which the large cavity 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 years and determine the 3-dimensional orbit using these astrometric measurements in combination with the 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 semi-major axis $2.01 \pm 0.01$ mas ($2.83\pm0.08$ au), inclination $(142.9 \pm 1.1)^\circ$ and longitude of the ascending node $(155.1 \pm 1.8)^\circ$. We find that the theoretical dynamical truncation and dust sublimation radius are at least $\sim3\times$ smaller than the observed inner disk radius ($\sim21.5$ mas or 30 au). This strengthens the hypothesis that the origin of such a cavity is due to the presence of a circumbinary planet.

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arXiv2023

Imaging the inner astronomical unit of Herbig Be star HD 190073

Inner regions of protoplanetary disks host many complex physical processes such as star-disk interactions, magnetic fields, planet formation, and the migration of new planets. To directly study this region requires milli-arcsecond angular resolution, beyond the diffraction limit of the world's largest optical telescopes and even too small for the mm-wave interferometer ALMA. However, we can use infrared interferometers to image the inner astronomical unit. Here, we present new results from the CHARA and VLTI arrays for the young and luminous Herbig Be star HD 190073. We detect a sub-AU cavity surrounded by a ring-like structure that we interpret as the dust destruction front. We model the shape with 6 radial profiles, 3 symmetric and 3 asymmetric, and present a model-free image reconstruction. All the models are consistent with a near face-on disk with inclination $\lesssim 20^\circ$, and we measure an average ring radius of 1.4 $\pm 0.2$ mas (1.14 AU). Around $48\%$ of the total flux comes from the disk with ~$15\%$ of that emission appearing to emerge from inside the inner rim. The cause of emission is still unclear, perhaps due to different dust grain compositions or gas emission. The skewed models and the imaging point to an off-center star, possibly due to binarity. Our image shows a sub-AU structure, which seems to move between the two epochs inconsistently with Keplerian motion and we discuss possible explanations for this apparent change.

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arXiv2023

Multiplicity of northern bright O-type stars with optical long baseline interferometry

The study of the multiplicity of massive stars gives hints on their formation processes and their evolutionary paths, which are still not fully understood. Large separation binaries (>50 milliseconds of arc, mas) can be probed by adaptive-optics-assisted direct imaging and sparse aperture masking, while close binaries can be resolved by photometry and spectroscopy. However, optical long baseline interferometry is mandatory to establish the multiplicity of Galactic massive stars at the separation gap between 1 and 50 mas. In this paper, we aim to demonstrate the capability of the new interferometric instrument MIRC-X, located at the CHARA Array, to study the multiplicity of O-type stars and therefore probe the full range of separation for more than 120 massive stars (H<7.5 mag). We initiated a pilot survey of bright O-type stars (H<6.5mag) observable with MIRC-X. We observed 29 O-type stars, including two systems in average atmospheric conditions around a magnitude of H=7.5 mag. We systematically reduced the obtained data with the public reduction pipeline of the instrument. We analyzed the reduced data using the dedicated python software CANDID to detect companions. Out of these 29 systems, we resolved 19 companions in 17 different systems with angular separations between ~0.5 and 50 mas. This results in a multiplicity fraction fm=17/29=0.59+/-0.09, and an average number of companions fc=19/29=0.66+/-0.13. Those results are in agreement with the results of the SMASH+ survey in the Southern Hemisphere. Thirteen of these companions have been resolved for the first time, including the companion responsible for the nonthermal emission in Cyg OB2-5 A and the confirmation of the candidate companion of HD 47129 suggested by SMASH+. A large survey on more than 120 northern O-type stars (H<7.5) is possible with MIRC-X and will be fruitful.

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arXiv2023

The GRAVITY+ Project: Towards All-sky, Faint-Science, High-Contrast Near-Infrared Interferometry at the VLTI

The GRAVITY instrument has been revolutionary for near-infrared interferometry by pushing sensitivity and precision to previously unknown limits. With the upgrade of GRAVITY and the Very Large Telescope Interferometer (VLTI) in GRAVITY+, these limits will be pushed even further, with vastly improved sky coverage, as well as faint-science and high-contrast capabilities. This upgrade includes the implementation of wide-field off-axis fringe-tracking, new adaptive optics systems on all Unit Telescopes, and laser guide stars in an upgraded facility. GRAVITY+ will open up the sky to the measurement of black hole masses across cosmic time in hundreds of active galactic nuclei, use the faint stars in the Galactic centre to probe General Relativity, and enable the characterisation of dozens of young exoplanets to study their formation, bearing the promise of another scientific revolution to come at the VLTI.

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arXiv2022

The Near Infrared Imager and Slitless Spectrograph for JWST -- V. Kernel Phase Imaging and Data Analysis

Kernel phase imaging (KPI) enables the direct detection of substellar companions and circumstellar dust close to and below the classical (Rayleigh) diffraction limit. We present a kernel phase analysis of JWST NIRISS full pupil images taken during the instrument commissioning and compare the performance to closely related NIRISS aperture masking interferometry (AMI) observations. For this purpose, we develop and make publicly available the custom "Kpi3Pipeline" enabling the extraction of kernel phase observables from JWST images. The extracted observables are saved into a new and versatile kernel phase FITS file (KPFITS) data exchange format. Furthermore, we present our new and publicly available "fouriever" toolkit which can be used to search for companions and derive detection limits from KPI, AMI, and long-baseline interferometry observations while accounting for correlated uncertainties in the model fitting process. Among the four KPI targets that were observed during NIRISS instrument commissioning, we discover a low-contrast (~1:5) close-in (~1 $λ/D$) companion candidate around CPD-66~562 and a new high-contrast (~1:170) detection separated by ~1.5 $λ/D$ from 2MASS~J062802.01-663738.0. The 5-$σ$ companion detection limits around the other two targets reach ~6.5 mag at ~200 mas and ~7 mag at ~400 mas. Comparing these limits to those obtained from the NIRISS AMI commissioning observations, we find that KPI and AMI perform similar in the same amount of observing time. Due to its 5.6 times higher throughput if compared to AMI, KPI is beneficial for observing faint targets and superior to AMI at separations >325 mas. At very small separations (<100 mas) and between ~250-325 mas, AMI slightly outperforms KPI which suffers from increased photon noise from the core and the first Airy ring of the point-spread function.

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arXiv2022

Flexible Spectro Interferometric modelling of OIFITS data with PMOIRED

Despite image reconstruction becoming more widespread when interpreting OIFITS Data, model fitting in u,v space often remains the best way to interpret data, either because of the sparsity of the data, or because a quantitative measurement needs to be done. PMOIRED, is a flexible Python library to visualize, manipulate and model OIFITS data using simple geometric models. The strength of PMOIRED resides in its capability to combine linearly various simple components to create complex scenes, while linking, constraining, and adding priors to fitted parameters. The code also enables grid search to find global minima, as well as data resampling to better evaluate uncertainties. In addition to analytical functions, arbitrary radial profiles, azimuthal variations or sparse wavelet modelling of spectra are implemented.

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arXiv2022

Characterising the orbit and circumstellar environment of the high-mass binary MWC 166 A

Context: Stellar evolution models are highly dependent on accurate mass estimates, especially for high-mass stars in the early stages of evolution. The most direct method for obtaining model-independent masses is derivation from the orbit of close binaries. Aims: To derive the first astrometric+RV orbit solution for the single-lined spectroscopic binary MWC 166 A, based on CHARA and VLTI near-infrared interferometry over multiple epochs and ~100 archival radial velocity measurements, and to derive fundamental stellar parameters from this orbit. We also sought to model circumstellar activity in the system from K-band spectral lines. Methods: We geometrically modelled the dust continuum to derive astrometry at 13 epochs and constrain individual stellar parameters. We used the continuum models as a base to examine differential phases, visibilities and closure phases over the Br-$γ$ and He-I emission lines. Results: Our orbit solution suggests a period of $367.7\pm0.1$ d, twice as long as found with previous RV orbit fits, subsequently constraining the component masses to $M_1=12.2\pm2.2 M_\odot$ and $M_2=4.9\pm0.5 M_\odot$. The line-emitting gas was found to be localised around the primary and is spatially resolved on scales of ~11 stellar radii, with the spatial displacement between the line wings consistent with a rotating disc. Conclusions: The large radius and stable orientation of the line emission are inconsistent with magnetospheric or boundary-layer accretion, but indicate an ionised inner gas disk around MWC 166 Aa. We observe line variability that could be explained either with generic line variability in a Herbig star disc or V/R variations in a decretion disc. We also constrained the age of the system to ~$(7\pm2)\times10^5$ yr, consistent with the system being comprised of a main-sequence primary and a secondary still contracting towards the main sequence.

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arXiv2021

Inspecting the Cepheid parallax of pulsation using Gaia EDR3 parallaxes. Projection factor and period-luminosity and period-radius relations

As primary anchors of the distance scale, Cepheid stars play a crucial role in our understanding of the distance scale of the Universe because of their period-luminosity relation. Determining precise and consistent parameters (radius, temperature, color excess, and projection factor) of Cepheid pulsating stars is therefore very important. With the high-precision parallaxes delivered by the early third Gaia data release, we aim to derive various parameters of Cepheid stars in order to calibrate the period-luminosity and period-radius relations and to investigate the relation of period to p-factor. We applied an implementation of the parallax-of-pulsation method through the algorithm called Spectro-Photo-Interferometry of Pulsating Stars, which combines all types of available data for a variable star in a global modeling of its pulsation. We present the SPIPS modeling of a sample of 63 Galactic Cepheids. Adopting Gaia EDR3 parallaxes as an input associated with the best available dataset, we derive consistent values of parameters for these stars such as the radius, multiband apparent magnitudes, effective temperatures, color excesses, period changes, Fourier parameters, and the projection factor. We then derive new calibrations of the period-luminosity and period-radius relations. After investigating the dependences of the p-factor on the parameters of the stars, we find a high dispersion of its values and no evidence of its correlation with the period or with any other parameters. Statistically, the p-factor has an average value of p=1.26$\pm$0.07, but with an unsatisfactory agreement. In absence of any clear correlation between the p-factor and other quantities, the best agreement is obtained under the assumption that the p-factor can take any value in a band with a width of 0.15. This result highlights the need for a further examination of the physics behind the p-factor.

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arXiv2021

X-rays in Cepheids: XMM-Newton Observations of $η$ Aql

X-ray bursts have recently been discovered in the Cepheids $δ$ Cep and $β$ Dor modulated by the pulsation cycle. We have obtained an observation of the Cepheid $η$ Aql with the XMM-Newton satellite at the phase of maximum radius, the phase at which there is a burst of X-rays in $δ$ Cep. No X-rays were seen from the Cepheid $η$ Aql at this phase, and the implications for Cepheid upper atmospheres are discussed. We have also used the combination of X-ray sources and Gaia and 2MASS data to search for a possible grouping around the young intermediate mass Cepheid. No indication of such a group was found.

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arXiv2021

Maintaining scientific discourse during a global pandemic: ESO's first e-conference #H02020

From 22 to 26 June 2020, we hosted ESO's first live e-conference, #H02020, from within ESO headquarters in Garching, Germany. Every day, between 200 and 320 researchers around the globe tuned in to discuss the nature and implications of the discord between precise determinations of the Universe's expansion rate, H0. Originally planned as an in-person meeting, we moved to the virtual domain to maintain strong scientific discourse despite the SARS-CoV-2 (COVID-19) pandemic. Here, we describe our conference setup, participants feedback gathered before and after the meeting, and lessons learned from this unexpected exercise. As e-conferencing will become increasingly common in the future, we provide our perspective on how e-conferences can make scientific exchange more effective and inclusive, in addition to climate friendly.

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arXiv2021

Inspecting the Cepheid distance ladder: The Hubble Space Telescope distance to the SNIa host galaxy NGC 5584

The current tension between the direct and the early Universe measurements of the Hubble Constant, $H_0$, requires detailed scrutiny of all the data and methods used in the studies on both sides of the debate. The Cepheids in the type Ia supernova (SNIa) host galaxy NGC 5584 played a key role in the local measurement of $H_0$. The SH0ES project used the observations of this galaxy to derive a relation between Cepheids' periods and ratios of their amplitudes in different optical bands of the Hubble Space Telescope (HST), and used these relations to analyse the light curves of the Cepheids in around half of the current sample of local SNIa host galaxies. In this work, we present an independent detailed analysis of the Cepheids in NGC 5584. We employ different tools for our photometric analysis and a completely different method for our light curve analysis, and we do not find a systematic difference between our period and mean magnitude measurements compared to those reported by SH0ES. By adopting a period-luminosity relation calibrated by the Cepheids in the Milky Way, we measure a distance modulus $μ=31.810\pm0.047$ (mag) which is in agreement with $μ=31.786\pm0.046$ (mag) measured by SH0ES. In addition, the relations we find between periods and amplitude ratios of the Cepheids in NGC 5584 are significantly tighter than those of SH0ES and their potential impact on the direct $H_0$ measurement will be investigated in future studies.

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arXiv2020

Increasing the achievable contrast of infrared interferometry with an error correlation model

Interferometric observables are strongly correlated, yet it is common practice to ignore these correlations in the data analysis process. We develop an empirical model for the correlations present in Very Large Telescope Interferometer GRAVITY data and show that properly accounting for them yields fainter detection limits and increases the reliability of potential detections. We extracted the correlations of the (squared) visibility amplitudes and the closure phases directly from intermediate products of the GRAVITY data reduction pipeline and fitted our empirical models to them. Then, we performed model fitting and companion injection and recovery tests with both simulated and real GRAVITY data, which are affected by correlated noise, and compared the results when ignoring the correlations and when properly accounting for them with our empirical models. When accounting for the correlations, the faint source detection limits improve by a factor of up to $\sim 2$ at angular separations $> 20~\rm{mas}$. For commonly used detection criteria based on $χ^2$ statistics, this mostly results in claimed detections being more reliable. Ignoring the correlations present in interferometric data is a dangerous assumption which might lead to a large number of false detections. The commonly used detection criteria (e.g. in the model fitting pipeline CANDID) are only reliable when properly accounting for the correlations; furthermore, instrument teams should work on providing full covariance matrices instead of statistically independent error bars as part of the official data reduction pipelines.

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arXiv2020

The Milky Way Cepheid Leavitt law based on Gaia DR2 parallaxes of companion stars and host open cluster populations

Classical Cepheids provide the foundation for the empirical extragalactic distance ladder. Milky Way Cepheids are the only stars in this class accessible to trigonometric parallax measurements. However, the parallaxes of Cepheids from the second Gaia data release (GDR2) are affected by systematics because of the absence of chromaticity correction, and occasionally by saturation. As a proxy for the parallaxes of 36 Galactic Cepheids, we adopt either the GDR2 parallaxes of their spatially resolved companions or the GDR2 parallax of their host open cluster. This novel approach allows us to bypass the systematics on the GDR2 Cepheids parallaxes that is induced by saturation and variability. We adopt a GDR2 parallax zero-point (ZP) of -0.046 mas with an uncertainty of 0.015 mas that covers most of the recent estimates. We present new Galactic calibrations of the Leavitt law in the V, J, H, K_S , and Wesenheit W_H bands. We compare our results with previous calibrations based on non-Gaia measurements and compute a revised value for the Hubble constant anchored to Milky Way Cepheids. From an initial Hubble constant of 76.18 +/- 2.37 km/s/Mpc based on parallax measurements without Gaia, we derive a revised value by adopting companion and average cluster parallaxes in place of direct Cepheid parallaxes, and we find H_0 = 72.8 +/- 1.9 (statistical + systematics) +/- 1.9 (ZP) km/s/Mpc when all Cepheids are considered and H0 = 73.0 +/- 1.9 (statistical + systematics) +/- 1.9 (ZP) km/s/Mpc for fundamental mode pulsators only.

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arXiv2020

The Leavitt law of Milky Way Cepheids from Gaia DR2 static companion parallaxes

Classical Cepheids (CCs) are at the heart of the empirical extragalactic distance ladder. Milky Way CCs are the only stars of this class accessible to trigonometric parallax measurements. Until recently, the most accurate trigonometric parallaxes of Milky Way CCs were the HST/FGS measurements collected by Benedict et al. (2002, 2007) and HST/WFC3 measurements by Riess et al. (2018). Unfortunately, the second Gaia data release (GDR2) has not yet delivered reliable parallaxes for Galactic CCs, failing to replace the HST as the foundation of the Galactic calibrations of the Leavitt law. We aim at calibrating independently the Leavitt law of Milky Way CCs based on the GDR2 catalog of trigonometric parallaxes. As a proxy for the parallaxes of a sample of 23 Galactic CCs, we adopt the GDR2 parallaxes of their spatially resolved companions. As the latter are unsaturated, photometrically stable stars, this novel approach allows us to bypass the GDR2 bias on the parallax of the CCs that is induced by saturation and variability. We present new Galactic calibrations of the Leavitt law in the J, H, K, V, Wesenheit WH and Wesenheit WVK bands based on the GDR2 parallaxes of the CC companions. We show that the adopted value of the zero point of the GDR2 parallaxes, within a reasonable range, has a limited impact on our Leavitt law calibration.

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arXiv2019

Diagnosing 0.1-10 au scale morphology of the FU\,Ori disk using ALMA and VLTI/GRAVITY

We report new Atacama Large Millimeter/submillimeter Array Band 3 (86-100 GHz; $\sim$80 mas angular resolution) and Band 4 (146-160 GHz; $\sim$50 mas angular resolution) observations of the dust continuum emission towards the archetypal and ongoing accretion burst young stellar object FU Ori, which simultaneously covered its companion, FU Ori S. In addition, we present near-infrared (2-2.45 $μ$m) observations of FU Ori taken with the General Relativity Analysis via VLT InTerferometrY (GRAVITY; $\sim$1 mas angular resolution) instrument on the Very Large Telescope Interferometer (VLTI). We find that the emission in both FU Ori and FU Ori S at (sub)millimeter and near infrared bands is dominated by structures inward of $\sim$10 au radii. We detected closure phases close to zero from FU Ori with VLTI/GRAVITY, which indicate the source is approximately centrally symmetric and therefore is likely viewed nearly face-on. Our simple model to fit the GRAVITY data shows that the inner 0.4 au radii of the FU Ori disk has a triangular spectral shape at 2-2.45 $μ$m, which is consistent with the H$_{2}$O and CO absorption features in a $\dot{M}\sim$10$^{-4}$ $M_{\odot}\,yr^{-1}$, viscously heated accretion disk. At larger ($\sim$0.4-10 au) radii, our analysis shows that viscous heating may also explain the observed (sub)millimeter and centimeter spectral energy distribution when we assume a constant, $\sim$10$^{-4}$ $M_{\odot}\,yr^{-1}$ mass inflow rate in this region. This explains how the inner 0.4 au disk is replenished with mass at a modest rate, such that it neither depletes nor accumulates significant masses over its short dynamic timescale. Finally, we tentatively detect evidence of vertical dust settling in the inner 10 au of the FU Ori disk, but confirmation requires more complete spectral sampling in the centimeter bands.

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arXiv2019

Consistent radial velocities of classical Cepheids from the cross-correlation technique

Accurate radial velocities ($v_{\rm rad}$) of Cepheids are mandatory within the context of distance measurements via the Baade-Wesselink technique. The most common $v_{\rm rad}$ derivation method consists in cross-correlating the observed spectrum with a binary template and measuring a velocity on the resulting profile. Yet for Cepheids, the spectral lines selected within the template as well as the way of fitting the cross-correlation function (CCF) have a significant impact on the measured $v_{\rm rad}$. We detail the steps to compute consistent Cepheid CCFs and $v_{\rm rad}$, and we characterise the impact of Cepheid spectral properties and $v_{\rm rad}$ computation method on the resulting line profiles. We collected more than 3900 high-resolution spectra from seven different spectrographs of 64 classical Cepheids. These spectra were standardised through a single process on pre-defined wavelength ranges. We built six correlation templates selecting un-blended lines of different depths from a synthetic Cepheid spectrum, on three different wavelength ranges from 390 to 800 nm. Each spectrum was cross-correlated with these templates to build the corresponding CCFs. We derived a set of line profile observables as well as three different $v_{\rm rad}$ measurements from each CCF. This study confirms that both the template wavelength range, its mean line depth and width, and the $v_{\rm rad}$ computation method significantly impact the $v_{\rm rad}$. Deriving more robust Cepheid $v_{\rm rad}$ time series require to minimise the asymmetry of the line profile and its impact on the $v_{\rm rad}$. Centroid $v_{\rm rad}$, that exhibit slightly smaller amplitudes but significantly smaller scatter than Gaussian or biGaussian $v_{\rm rad}$, should thus be favoured. Stronger lines are also less asymmetric and lead to more robust $v_{\rm rad}$ than weaker lines.

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arXiv2018

Multiple Star Systems in the Orion Nebula

This work presents an interferometric study of the massive-binary fraction in the Orion Trapezium Cluster with the recently comissioned GRAVITY instrument. We observe a total of 16 stars of mainly OB spectral type. We find three previously unknown companions for $θ^1$ Ori B, $θ^2$ Ori B, and $θ^2$ Ori C. We determine a separation for the previously suspected companion of NU Ori. We confirm four companions for $θ^1$ Ori A, $θ^1$ Ori C, $θ^1$ Ori D, and $θ^2$ Ori A, all with substantially improved astrometry and photometric mass estimates. We refine the orbit of the eccentric high-mass binary $θ^1$ Ori C and we are able to derive a new orbit for $θ^1$ Ori D. We find a system mass of 21.7 $M_{\odot}$ and a period of $53$ days. Together with other previously detected companions seen in spectroscopy or direct imaging, eleven of the 16 high-mass stars are multiple systems. We obtain a total number of 22 companions with separations up to 600 AU. The companion fraction of the early B and O stars in our sample is about 2, significantly higher than in earlier studies of mostly OB associations. The separation distribution hints towards a bimodality. Such a bimodality has been previously found in A stars, but rarely in OB binaries, which up to this point have been assumed to be mostly compact with a tail of wider companions. We also do not find a substantial population of equal-mass binaries. The observed distribution of mass ratios declines steeply with mass, and like the direct star counts, indicates that our companions follow a standard power law initial mass function. Again, this is in contrast to earlier findings of flat mass ratio distributions in OB associations. We exclude collision as a dominant formation mechanism but find no clear preference for core accretion or competitive accretion.

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arXiv2018

Optical interferometry and Gaia parallaxes for a robust calibration of the Cepheid distance scale

We present the modeling tool we developed to incorporate multi-technique observations of Cepheids in a single pulsation model: the Spectro-Photo-Interferometry of Pulsating Stars (SPIPS). The combination of angular diameters from optical interferometry, radial velocities and photometry with the coming Gaia DR2 parallaxes of nearby Galactic Cepheids will soon enable us to calibrate the projection factor of the classical Parallax-of-Pulsation method. This will extend its applicability to Cepheids too distant for accurate Gaia parallax measurements, and allow us to precisely calibrate the Leavitt law's zero point. As an example application, we present the SPIPS model of the long-period Cepheid RS Pup that provides a measurement of its projection factor, using the independent distance estimated from its light echoes.

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