15 research outputs found
The GRAVITY fringe tracker: correlation between optical path residuals and atmospheric parameters
After the first year of observations with the GRAVITY fringe tracker, we
compute correlations between the optical path residuals and atmospheric and
astronomical parameters. The median residuals of the optical path residuals are
180 nm on the ATs and 270 nm on the UTs. The residuals are uncorrelated with
the target magnitudes for Kmag below 5.5 on ATs (9 on UTs). The correlation
with the coherence time is however extremely clear, with a drop-off in fringe
tracking performance below 3 ms.Comment: submitted to SPIE Astronomical Telescopes & Instrumentation 201
Polarimetry and Astrometry of NIR Flares as Event Horizon Scale, Dynamical Probes for the Mass of Sgr A*
We present new astrometric and polarimetric observations of flares from Sgr
A* obtained with GRAVITY, the near-infrared interferometer at ESO's Very Large
Telescope Interferometer (VLTI), bringing the total sample of well-covered
astrometric flares to four and polarimetric ones to six, where we have for two
flares good coverage in both domains. All astrometric flares show clockwise
motion in the plane of the sky with a period of around an hour, and the
polarization vector rotates by one full loop in the same time. Given the
apparent similarities of the flares, we present a common fit, taking into
account the absence of strong Doppler boosting peaks in the light curves and
the EHT-measured geometry. Our results are consistent with and significantly
strengthen our model from 2018: We find that a) the combination of polarization
period and measured flare radius of around nine gravitational radii (, innermost stable circular orbit) is consistent with
Keplerian orbital motion of hot spots in the innermost accretion zone. The mass
inside the flares' radius is consistent with the measured from stellar orbits at several thousand . This
finding and the diameter of the millimeter shadow of Sgr A* thus support a
single black hole model. Further, b) the magnetic field configuration is
predominantly poloidal (vertical), and the flares' orbital plane has a moderate
inclination with respect to the plane of the sky, as shown by the non-detection
of Doppler-boosting and the fact that we observe one polarization loop per
astrometric loop. Moreover, c) both the position angle on sky and the required
magnetic field strength suggest that the accretion flow is fueled and
controlled by the winds of the massive, young stars of the clockwise stellar
disk 1-5 arcsec from Sgr A*, in agreement with recent simulations.Comment: 10 pages, 12 figures. Submitted to A&
Direct discovery of the inner exoplanet in the HD206893 system. Evidence for deuterium burning in a planetary-mass companion
Long term precise radial velocity (RV) monitoring of the nearby star
HD206893, as well as anomalies in the system proper motion, have suggested the
presence of an additional, inner companion in the system. Here we describe the
results of a multi-epoch search for the companion responsible for this RV drift
and proper motion anomaly using the VLTI/GRAVITY instrument. Utilizing
information from ongoing precision RV measurements with the HARPS spectrograph,
as well as Gaia host star astrometry, we report a high significance detection
of the companion HD206893c over three epochs, with clear evidence for Keplerian
orbital motion. Our astrometry with 50-100 arcsec precision afforded
by GRAVITY allows us to derive a dynamical mass of 12.7 M and an orbital separation of 3.53 au for HD206893c. Our
fits to the orbits of both companions in the system utilize both Gaia
astrometry and RVs to also provide a precise dynamical estimate of the
previously uncertain mass of the B component, and therefore derive an age of
Myr. We find that theoretical atmospheric/evolutionary models
incorporating deuterium burning for HD206893c, parameterized by cloudy
atmospheres provide a good simultaneous fit to the luminosity of both HD206893B
and c. In addition to utilizing long-term RV information, this effort is an
early example of a direct imaging discovery of a bona fide exoplanet that was
guided in part with Gaia astrometry. Utilizing Gaia astrometry is expected to
be one of the primary techniques going forward to identify and characterize
additional directly imaged planets. Lastly, this discovery is another example
of the power of optical interferometry to directly detect and characterize
extrasolar planets where they form at ice-line orbital separations of 2-4\,au.Comment: Accepted to A&
The mass of ÎČ Pictoris c from ÎČ Pictoris b orbital motion
Aims. We aim to demonstrate that the presence and mass of an exoplanet can now be effectively derived from the astrometry of another exoplanet. Methods. We combined previous astrometry of ÎČ Pictoris b with a new set of observations from the GRAVITY interferometer. The orbital motion of ÎČ Pictoris b is fit using Markov chain Monte Carlo simulations in Jacobi coordinates. The inner planet, ÎČ Pictoris c, was also reobserved at a separation of 96 mas, confirming the previous orbital estimations. Results. From the astrometry of planet b only, we can (i) detect the presence of ÎČ Pictoris c and (ii) constrain its mass to 10.04-3.10+4.53 MJup. If one adds the astrometry of ÎČ Pictoris c, the mass is narrowed down to 9.15-1.06+1.08 MJup. The inclusion of radial velocity measurements does not affect the orbital parameters significantly, but it does slightly decrease the mass estimate to 8.89-0.75+0.75 MJup. With a semimajor axis of 2.68 ± 0.02 au, a period of 1221 ± 15 days, and an eccentricity of 0.32 ± 0.02, the orbital parameters of ÎČ Pictoris c are now constrained as precisely as those of ÎČ Pictoris b. The orbital configuration is compatible with a high-order mean-motion resonance (7:1). The impact of the resonance on the planets' dynamics would then be negligible with respect to the secular perturbations, which might have played an important role in the eccentricity excitation of the outer planet. © 2021 S. Lacour et al
Constraining the nature of the PDS 70 protoplanets with VLTI/GRAVITY
We present K-band interferometric observations of the PDS 70 protoplanets
along with their host star using VLTI/GRAVITY. We obtained K-band spectra and
100 as precision astrometry of both PDS 70 b and c in two epochs, as well
as spatially resolving the hot inner disk around the star. Rejecting unstable
orbits, we found a nonzero eccentricity for PDS 70 b of , a
near-circular orbit for PDS 70 c, and an orbital configuration that is
consistent with the planets migrating into a 2:1 mean motion resonance.
Enforcing dynamical stability, we obtained a 95% upper limit on the mass of PDS
70 b of 10 , while the mass of PDS 70 c was unconstrained. The
GRAVITY K-band spectra rules out pure blackbody models for the photospheres of
both planets. Instead, the models with the most support from the data are
planetary atmospheres that are dusty, but the nature of the dust is unclear.
Any circumplanetary dust around these planets is not well constrained by the
planets' 1-5 m spectral energy distributions (SEDs) and requires longer
wavelength data to probe with SED analysis. However with VLTI/GRAVITY, we made
the first observations of a circumplanetary environment with sub-au spatial
resolution, placing an upper limit of 0.3~au on the size of a bright disk
around PDS 70 b
The power of coordinate transformations in dynamical interpretations of Galactic structure
Modeling the orbital motion of Sgr A*'s near-infrared flares
Infrared observations of Sgr A* probe the region close to the event horizon of the black hole at the Galactic center. These observations can constrain the properties of low-luminosity accretion as well as that of the black hole itself. The GRAVITY instrument at the ESO VLTI has recently detected continuous circular relativistic motion during infrared flares which has been interpreted as orbital motion near the event horizon. Here we analyze the astrometric data from these flares, taking into account the effects of out-of-plane motion and orbital shear of material near the event horizon of the black hole. We have developed a new code to predict astrometric motion and flux variability from compact emission regions following particle orbits. Our code combines semi-analytic calculations of timelike geodesics that allow for out-of-plane or elliptical motions with ray tracing of photon trajectories to compute time-dependent images and light curves. We apply our code to the three flares observed with GRAVITY in 2018. We show that all flares are consistent with a hotspot orbiting at R similar to 9 gravitational radii with an inclination of i similar to 140 degrees. The emitting region must be compact and less than similar to 5 gravitational radii in diameter. We place a further limit on the out-of-plane motion during the flare
The flux distribution of Sgr A*
The Galactic center black hole Sagittarius A* is a variable near-infrared (NIR) source that exhibits bright flux excursions called flares. When flux from Sgr A* is detected, the light curve has been shown to exhibit red noise characteristics and the distribution of flux densities is non-linear, non-Gaussian, and skewed to higher flux densities. However, the low-flux density turnover of the flux distribution is below the sensitivity of current single-aperture telescopes. For this reason, the median NIR flux has only been inferred indirectly from model fitting, but it has not been directly measured. In order to explore the lowest flux ranges, to measure the median flux density, and to test if the previously proposed flux distributions fit the data, we use the unprecedented resolution of the GRAVITY instrument at the VLTI. We obtain light curves using interferometric model fitting and coherent flux measurements. Our light curves are unconfused, overcoming the confusion limit of previous photometric studies. We analyze the light curves using standard statistical methods and obtain the flux distribution. We find that the flux distribution of Sgr A* turns over at a median flux density of (1.1 +/- 0.3) mJy. We measure the percentiles of the flux distribution and use them to constrain the NIR K-band spectral energy distribution. Furthermore, we find that the flux distribution is intrinsically right-skewed to higher flux density in log space. Flux densities below 0.1 mJy are hardly ever observed. In consequence, a single powerlaw or lognormal distribution does not suffice to describe the observed flux distribution in its entirety. However, if one takes into account a power law component at high flux densities, a lognormal distribution can describe the lower end of the observed flux distribution. We confirm the rms-flux relation for Sgr A* and find it to be linear for all flux densities in our observation. We conclude that Sgr A* has two states: the bulk of the emission is generated in a lognormal process with a well-defined median flux density and this quiescent emission is supplemented by sporadic flares that create the observed power law extension of the flux distribution
The GRAVITY Young Stellar Object survey. VII. The inner dusty disks of T Tauri stars
Accepted for publication in A&A; 20 pages, 15 figures, 9 tablesInternational audienceThese protoplanetary disks in T Tauri stars play a central role in star and planet formation. We spatially resolve at sub-au scales the innermost regions of a sample of T Tauri's disks to better understand their morphology and composition. We extended our homogeneous data set of 27 Herbig stars and collected near-IR K-band observations of 17 T Tauri stars, spanning effective temperatures and luminosities in the ranges of ~4000-6000 K and ~0.4-10 Lsun. We focus on the continuum emission and develop semi-physical geometrical models to fit the interferometric data and search for trends between the properties of the disk and the central star. The best-fit models of the disk's inner rim correspond to wide rings. We extend the Radius-luminosity relation toward the smallest luminosities (0.4-10 Lsun) and find the R~L^(1/2) trend is no longer valid, since the K-band sizes measured with GRAVITY are larger than the predicted sizes from sublimation radius computation. No clear correlation between the K-band half-flux radius and the mass accretion rate is seen. Having magnetic truncation radii in agreement with the K-band GRAVITY sizes would require magnetic fields as strong as a few kG, which should have been detected, suggesting that accretion is not the main process governing the location of the half-flux radius of the inner dusty disk. Our measurements agree with models that take into account the scattered light. The N-to-K band size ratio may be a proxy for disentangling disks with silicate features in emission from disks with weak and/or in absorption silicate features. When comparing inclinations and PA of the inner disks to those of the outer disks (ALMA) in nine objects of our sample, we detect misalignments for four objects