109 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
Infrared wavefront sensing for adaptive optics assisted Galactic Center observations with the VLT interferometer and GRAVITY: operation and results
This article describes the operation of the near-infrared wavefront sensing
based Adaptive Optics (AO) system CIAO. The Coud\'e Infrared Adaptive Optics
(CIAO) system is a central auxiliary component of the Very Large Telescope
(VLT) interferometer (VLTI). It enables in particular the observations of the
Galactic Center (GC) using the GRAVITY instrument. GRAVITY is a highly
specialized beam combiner, a device that coherently combines the light of the
four 8-m telescopes and finally records interferometric measurements in the
K-band on 6 baselines simultaneously. CIAO compensates for phase disturbances
caused by atmospheric turbulence, which all four 8 m Unit Telescopes (UT)
experience during observation. Each of the four CIAO units generates an almost
diffraction-limited image quality at its UT, which ensures that maximum flux of
the observed stellar object enters the fibers of the GRAVITY beam combiner. We
present CIAO performance data obtained in the first 3 years of operation as a
function of weather conditions. We describe how CIAO is configured and used for
observations with GRAVITY. In addition, we focus on the outstanding features of
the near-infrared sensitive Saphira detector, which is used for the first time
on Paranal, and show how it works as a wavefront sensor detector.Comment: 12 pages, 8 figures, accepted for publication in Instruments (open
access journal from mdpi
First direct detection of an exoplanet by optical interferometry; Astrometry and K-band spectroscopy of HR8799 e
To date, infrared interferometry at best achieved contrast ratios of a few
times on bright targets. GRAVITY, with its dual-field mode, is now
capable of high contrast observations, enabling the direct observation of
exoplanets. We demonstrate the technique on HR8799, a young planetary system
composed of four known giant exoplanets. We used the GRAVITY fringe tracker to
lock the fringes on the central star, and integrated off-axis on the HR8799e
planet situated at 390 mas from the star. Data reduction included
post-processing to remove the flux leaking from the central star and to extract
the coherent flux of the planet. The inferred K band spectrum of the planet has
a spectral resolution of 500. We also derive the astrometric position of the
planet relative to the star with a precision on the order of 100as. The
GRAVITY astrometric measurement disfavors perfectly coplanar stable orbital
solutions. A small adjustment of a few degrees to the orbital inclination of HR
8799 e can resolve the tension, implying that the orbits are close to, but not
strictly coplanar. The spectrum, with a signal-to-noise ratio of
per spectral channel, is compatible with a late-type L brown dwarf. Using
Exo-REM synthetic spectra, we derive a temperature of \,K and a
surface gravity of cm/s. This corresponds to a radius
of and a mass of , which is an independent confirmation of mass estimates from evolutionary
models. Our results demonstrate the power of interferometry for the direct
detection and spectroscopic study of exoplanets at close angular separations
from their stars.Comment: published in A&
The GRAVITY young stellar object survey XIII. Tracing the time-variable asymmetric disk structure in the inner AU of the Herbig star HD98922
Temporal variability in the photometric and spectroscopic properties of
protoplanetary disks is common in YSO. However, evidence pointing toward
changes in their morphology over short timescales has only been found for a few
sources, mainly due to a lack of high cadence observations at mas resolution.
We combine GRAVITY multi-epoch observations of HD98922 at mas resolution with
PIONIER archival data covering a total time span of 11 years. We interpret the
interferometric visibilities and spectral energy distribution with geometrical
models and through radiative transfer techniques. We investigated
high-spectral-resolution quantities to obtain information on the properties of
the HI BrG-line-emitting region. The observations are best fitted by a model of
a crescent-like asymmetric dust feature located at 1 au and accounting for 70%
of the NIR emission. The feature has an almost constant magnitude and orbits
the central star with a possible sub-Keplerian period of 12 months, although a
9 month period is another, albeit less probable, solution. The radiative
transfer models show that the emission originates from a small amount of
carbon-rich (25%) silicates, or quantum-heated particles located in a
low-density region. Among different possible scenarios, we favor hydrodynamical
instabilities in the inner disk that can create a large vortex. The high
spectral resolution differential phases in the BrG-line show that the hot-gas
component is offset from the star and in some cases is located between the star
and the crescent feature. The scale of the emission does not favor
magnetospheric accretion as a driving mechanism. The scenario of an asymmetric
disk wind or a massive accreting substellar or planetary companion is
discussed. With this unique observational data set for HD98922, we reveal
morphological variability in the innermost 2 au of its disk region.Comment: 45 pages, 20 figures, accepted by and to be published in Astronomy &
Astrophysics (A&A
Simulations of the Milky Way's central molecular zone -- II. Star formation
The Milky Way's central molecular zone (CMZ) has emerged in recent years as a
unique laboratory for the study of star formation. Here we use the simulations
presented in Tress et al. 2020 to investigate star formation in the CMZ. These
simulations resolve the structure of the interstellar medium at sub-parsec
resolution while also including the large-scale flow in which the CMZ is
embedded. Our main findings are as follows. (1) While most of the star
formation happens in the CMZ ring at , a significant
amount also occurs closer to SgrA* at . (2) Most of
the star formation in the CMZ happens downstream of the apocentres, consistent
with the "pearls-on-a-string" scenario, and in contrast to the notion that an
absolute evolutionary timeline of star formation is triggered by pericentre
passage. (3) Within the timescale of our simulations ( Myr), the
depletion time of the CMZ is constant within a factor of . This suggests
that variations in the star formation rate are primarily driven by variations
in the mass of the CMZ, caused for example by AGN feedback or
externally-induced changes in the bar-driven inflow rate, and not by variations
in the depletion time. (4) We study the trajectories of newly born stars in our
simulations. We find several examples that have age and 3D velocity compatible
with those of the Arches and Quintuplet clusters. Our simulations suggest that
these prominent clusters originated near the collision sites where the
bar-driven inflow accretes onto the CMZ, at symmetrical locations with respect
to the Galactic centre, and that they have already decoupled from the gas in
which they were born.Comment: Accepted for publication in MNRAS. Movies of the simulations can be
found at: https://www.youtube.com/channel/UCwnzfO-xLxzRDz9XsexfPo
Using the motion of S2 to constrain vector clouds around SgrA*
The dark compact object at the centre of the Milky Way is well established to
be a supermassive black hole with mass , but the nature of its environment is still under debate. In this
work, we used astrometric and spectroscopic measurements of the motion of the
star S2, one of the closest stars to the massive black hole, to determine an
upper limit on an extended mass composed of a massive vector field around
Sagittarius A*. For a vector with effective mass , our Markov Chain Monte Carlo analysis shows
no evidence for such a cloud, placing an upper bound at confidence level. We show that dynamical
friction exerted by the medium on S2 motion plays no role in the analysis
performed in this and previous works, and can be neglected thus.Comment: 9 pages, 5 figures, accepted to MNRA
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&
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