First Resolution of Microlensed Images Dong, Subo; Mérand, A.; Delplancke-Ströbele, F. ...
The Astrophysical journal,
01/2019, Volume:
871, Issue:
1
Journal Article
Peer reviewed
Open access
Abstract
We employ Very Large Telescope Interferometer GRAVITY to resolve, for the first time, the two images generated by a gravitational microlens. The measurements of the image separation
mas, and ...hence the Einstein radius
θ
E
= 1.87 ± 0.03 mas, are precise. This demonstrates the robustness of the method, provided that the source is bright enough for GRAVITY (
K
≲ 10.5) and the image separation is of order of or larger than the fringe spacing. When
θ
E
is combined with a measurement of the “microlens parallax”
, the two will together yield the lens mass and lens–source relative parallax and proper motion. Because the source parallax and proper motion are well measured by
Gaia
, this means that the lens characteristics will be fully determined, whether or not it proves to be luminous. This method can be a powerful probe of dark, isolated objects, which are otherwise quite difficult to identify, much less characterize. Our measurement contradicts Einstein’s prediction that “the luminous circle i.e., microlensed image cannot be distinguished” from a star.
Context.
Interferometric observables are strongly correlated, yet it is common practice to ignore these correlations in the data analysis process.
Aims.
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.
Methods.
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.
Results.
When accounting for the correlations, the faint source detection limits improve by a factor of up to ∼2 at angular separations > 20 mas. For commonly used detection criteria based on
χ
2
statistics, this mostly results in claimed detections being more reliable.
Conclusions.
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.
ABSTRACT
The fate of a massive star during the latest stages of its evolution is highly dependent on its mass-loss history and geometry, with the yellow hypergiants (YHGs) being key objects. We ...present near-IR interferometric observations of the famous YHG IRC+10420 and blue spectra taken between 1994 and 2019. Our 2.2-μm GRAVITY/VLTI observations attain a spatial resolution of ∼5 stellar radii and spatially resolve the hot emission in the K-band tracing the gas via Na i doublet emission and the Br γ emission. Our geometric modelling reveals a compact neutral zone (Na i) which is slightly larger than the continuum but within an extended Br γ emitting region. Our study confirms an hour-glass geometry of the wind, but we find no signature of a companion at 7–800 au separations at the contrast limit of our observations (3.7 mag at 3σ) to explain this geometry. We report an evolution of the ejecta over 7 yr, and constrain the opening angle of the hour-glass to be <10°. Lastly, we present the first blue optical spectra of IRC+10420 since 1994. The multi-epoch data indicate that the spectral type, and thus temperature, of the object has essentially remained constant during the intervening years. Therefore, the observed increase in temperature of 2000 K in less than two decades prior to 1994 is now halted. This suggests that this YHG has ‘hit’ the White Wall in the HR-diagram preventing it from evolving blue-wards, and will likely undergo a major mass-loss event in the near future.
Context. The precise determinations of stellar mass at ≲1% provide important constraints on stellar evolution models. Accurate parallax measurements can also serve as independent benchmarks for the ...next Gaia data release. Aims. We measured the masses and distance of binary systems with a precision level better than 1% using a fully geometrical and empirical method. Methods. We obtained the first interferometric observations for the eclipsing systems AI Phe, AL Dor, KW Hya, NN Del, ψ Cen and V4090 Sgr with the VLTI/PIONIER combiner, which we combined with radial velocity measurements to derive their three-dimensional orbit, masses, and distance. Results. We determined very precise stellar masses for all systems, ranging in precision from 0.04% to 3.3%. We combined these measurements with the stellar effective temperature and linear radius to fit stellar isochrones models and determined the age of the systems. We also derived the distance to the systems with a precision level of 0.4%. Conclusions. The comparison of theoretical models with stellar parameters shows that stellar models are still deficient in simultaneously fitting the stellar parameters (Teff, R and M) with this level of precision on individual masses. This stresses the importance of precisely measuring the stellar parameters to better calibrate stellar evolution models. The precision of our model-independent orbital parallaxes varies from 24 μas as to 70 μas and the parallaxes provide a unique opportunity to verify whether the future Gaia measurements have systematic errors.
Context.
Transition disks are protoplanetary disks with dust-depleted cavities, possibly indicating substantial clearing of their dust content by a massive companion. For several known transition ...disks, dark regions interpreted as shadows have been observed in scattered light imaging and are hypothesized to originate from misalignments between distinct regions of the disk.
Aims.
We aim to investigate the presence of misalignments in transition disks. We study the inner disk (<1 au) geometries of a sample of 20 well-known transition disks with Very Large Telescope Interferometer (VLTI) GRAVITY observations and use complementary
12
CO and
13
CO molecular line archival data from the Atacama Large Millimeter/submillimeter Array (ALMA) to derive the orientation of the outer disk regions (>10 au).
Methods.
We fit simple parametric models to the visibilities and closure phases of the GRAVITY data to derive the inclination and position angle of the inner disks. The outer disk geometries were derived from Keplerian fits to the ALMA velocity maps and compared to the inner disk constraints. We also predicted the locations of expected shadows for significantly misaligned systems.
Results.
Our analysis reveals six disks to exhibit significant misalignments between their inner and outer disk structures. The predicted shadow positions agree well with the scattered light images of HD 100453 and HD 142527, and we find supporting evidence for a shadow in the south of the disk around CQ Tau. In the other three targets for which we infer significantly misaligned disks, V1247 Ori, V1366 Ori, and RY Lup, we do not see any evident sign of shadows in the scattered light images. The scattered light shadows observed in DoAr 44, HD 135344 B, and HD 139614 are consistent with our observations, yet the underlying morphology is likely too complex to be described properly by our models and the accuracy achieved by our observations.
Conclusions.
The combination of near infrared and submillimeter interferometric observations allows us to assess the geometries of the innermost disk regions and those of the outer disk. Whereas we can derive precise constraints on the potential shadow positions for well-resolved inner disks around Herbig Ae/Be stars, the large statistical uncertainties for the marginally resolved inner disks around the T Tauri stars of our sample make it difficult to extract conclusive constraints for the presence of shadows in these systems.
We report the lens mass and distance measurements of the nearby microlensing event TCP J05074264+2447555 (Kojima-1). We measure the microlens parallax vector using Spitzer and ground-based light ...curves with constraints on the direction of lens-source relative proper motion derived from Very Large Telescope Interferometer (VLTI) GRAVITY observations. Combining this determination with the angular Einstein radius measured by VLTI-GRAVITY observations, we find that the lens is a star with mass at a distance DL = 429 21 pc. We find that the blended light basically all comes from the lens. The lens-source proper motion is , so with currently available adaptive-optics instruments, the lens and source can be resolved in 2021. This is the first microlensing event whose lens mass is unambiguously measured by interferometry + satellite-parallax observations, which opens a new window for mass measurements of isolated objects such as stellar-mass black holes.
Observations of 48 red-clump stars were obtained in the H band with the PIONIER instrument installed at the Very Large Telescope Interferometer. Limb-darkened angular diameters were measured by ...fitting radial intensity profile I(r) to square visibility measurements. Half the angular diameters determined have formal errors better than 1.2%, while the overall accuracy is better than 2.7%. Average stellar atmospheric parameters (effective temperatures, metallicities and surface gravities) were determined from new spectroscopic observations and literature data and combined with precise Gaia parallaxes to derive a set of fundamental stellar properties. These intrinsic parameters were then fitted to existing isochrone models to infer masses and ages of the stars. The added value from interferometry imposes a better and independent constraint on the R −Teff plane. Our derived values are consistent with previous works, although there is a strong scatter in age between various models. This shows that atmospheric parameters, mainly metallicities and surface gravities, still suffer from a non-accurate determination, limiting constraints on input physics and parameters of stellar evolution models.
Context.
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.
Aims.
With the high-precision parallaxes delivered by the early third
Gaia
data release (EDR3), 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.
Methods.
We applied an implementation of the parallax-of-pulsation method through the algorithm called spectro-photo-interferometry of pulsating stars (SPIPS), which combines all types of available data for a variable star (multiband and multicolor photometry, radial velocity, effective temperature, and interferometry measurements) in a global modeling of its pulsation.
Results.
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.
Conclusions.
Using the best set of data and the most precise distances for Milky Way Cepheids, we derive new calibrations of the period-luminosity and period-radius relations:
M
K
S
= −5.529
±0.015
− 3.141
±0.050
(log
P
− 0.9) and log
R
= 1.763
±0.003
+ 0.653
±0.012
(log
P
− 0.9). After investigating the dependences of the projection 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 such as radial velocity, temperature, or metallicity. Statistically, the
p
-factor has an average value of
p
= 1.26 ± 0.07, but with an unsatisfactory agreement (
σ
= 0.15). 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.
Context.
The infrared (IR) excess of classical Cepheids is seldom studied and poorly understood despite observational evidence and the potential for its contribution to induce systematics on the ...period-luminosity (PL) relation used in the calibration of the extragalactic distance scale.
Aims.
This study aims to understand the physical origin of the IR excess found in the spectral energy distribution (SED) of 5 Cepheids: RS Pup (
P
= 41.46d),
ζ
Gem (
P
= 10.15d),
η
Aql (
P
= 7.18d), V Cen (
P
= 5.49d) and SU Cyg (
P
= 3.85d).
Methods.
A time series of atmospheric models along the pulsation cycle were fitted to a compilation of data, including optical and near-IR photometry,
Spitzer
spectra (secured at a specific phase), interferometric angular diameters, effective temperature estimates, and radial velocity measurements.
Herschel
images in two bands were also analyzed qualitatively. In this fitting process, based on the SPIPS algorithm, a residual was found in the SED, whatever the pulsation phase, and for wavelengths larger than about 1.2
μ
m, which corresponds to the so-determined infrared excess of Cepheids. This IR excess was then corrected from interstellar medium absorption in order to infer the presence (or absence) of dust shells and was, ultimately, used in order to fit a model for a shell of ionized gas.
Results.
For all Cepheids, we find a continuum IR excess increasing up to approximately −0.1 magnitudes at 30
μ
m, which cannot be explained by a hot or cold dust model of CircumStellar Environment (CSE). However, a weak but significant dust emission at 9.7
μ
m is found for
ζ
Gem,
η
Aql and RS Pup, while clear interstellar clouds are seen in the
Herschel
images for V Cen and RS Pup. We show, for the first time, that the IR excess of Cepheids can be explained by free–free emission from a thin shell of ionized gas, with a thickness of ≃15% of the star radius, a mass of 10
−9
−10
−7
M
⊙
and a temperature ranging between 3500 and 4500 K.
Conclusions.
The presence of a thin shell of ionized gas around Cepheids must be tested with interferometers operating in the visible or mid-IR, or using radio telescopes. The impact of such CSEs of ionized gas on the PL relation of Cepheids also calls for further investigation.
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