2,220 research outputs found
The PRIMA fringe sensor unit
The Fringe Sensor Unit (FSU) is the central element of the Phase Referenced
Imaging and Micro-arcsecond Astrometry (PRIMA) dual-feed facility and provides
fringe sensing for all observation modes, comprising off-axis fringe tracking,
phase referenced imaging, and high-accuracy narrow-angle astrometry. It is
installed at the Very Large Telescope Interferometer (VLTI) and successfully
servoed the fringe tracking loop during the initial commissioning phase. Unique
among interferometric beam combiners, the FSU uses spatial phase modulation in
bulk optics to retrieve real-time estimates of fringe phase after spatial
filtering. A R=20 spectrometer across the K-band makes the retrieval of the
group delay signal possible. The FSU was integrated and aligned at the VLTI in
summer 2008. It yields phase and group delay measurements at sampling rates up
to 2 kHz, which are used to drive the fringe tracking control loop. During the
first commissioning runs, the FSU was used to track the fringes of stars with
K-band magnitudes as faint as m_K=9.0, using two VLTI Auxiliary Telescopes (AT)
and baselines of up to 96 m. Fringe tracking using two Very Large Telescope
(VLT) Unit Telescopes (UT) was demonstrated. During initial commissioning and
combining stellar light with two ATs, the FSU showed its ability to improve the
VLTI sensitivity in K-band by more than one magnitude towards fainter objects,
which is of fundamental importance to achieve the scientific objectives of
PRIMA.Comment: 19 pages, 23 figures. minor changes and language editing. this
version equals the published articl
Massive binaries in the vicinity of Sgr A*
A long-term spectroscopic and photometric survey of the most luminous and
massive stars in the vicinity of the super-massive black hole Sgr A* revealed
two new binaries; a long-period Ofpe/WN9 binary, GCIRS 16NE, with a modest
eccentricity of 0.3 and a period of 224 days and an eclipsing Wolf-Rayet binary
with a period of 2.3 days. Together with the already identified binary GCIRS
16SW, there are now three confirmed OB/WR binaries in the inner 0.2\,pc of the
Galactic Center. Using radial velocity change upper limits, we were able to
constrain the spectroscopic binary fraction in the Galactic Center to at a confidence level of 95%, a massive binary
fraction similar to that observed in dense clusters. The fraction of eclipsing
binaries with photometric amplitudes is , which is consistent with local OB star clusters ().
Overall the Galactic Center binary fraction seems to be close to the binary
fraction in comparable young clusters.Comment: 5 figures, submitted to Ap
Mid-infrared interferometry with K band fringe-tracking I. The VLTI MIDI+FSU experiment
Context: A turbulent atmosphere causes atmospheric piston variations leading
to rapid changes in the optical path difference of an interferometer, which
causes correlated flux losses. This leads to decreased sensitivity and accuracy
in the correlated flux measurement. Aims: To stabilize the N band
interferometric signal in MIDI (MID-infrared Interferometric instrument), we
use an external fringe tracker working in K band, the so-called FSU-A (fringe
sensor unit) of the PRIMA (Phase-Referenced Imaging and Micro-arcsecond
Astrometry) facility at VLTI. We present measurements obtained using the newly
commissioned and publicly offered MIDI+FSU-A mode. A first characterization of
the fringe-tracking performance and resulting gains in the N band are
presented. In addition, we demonstrate the possibility of using the FSU-A to
measure visibilities in the K band. Methods: We analyzed FSU-A fringe track
data of 43 individual observations covering different baselines and object K
band magnitudes with respect to the fringe-tracking performance. The N band
group delay and phase delay values could be predicted by computing the relative
change in the differential water vapor column density from FSU-A data.
Visibility measurements in the K band were carried out using a scanning mode of
the FSU-A. Results: Using the FSU-A K band group delay and phase delay
measurements, we were able to predict the corresponding N band values with high
accuracy with residuals of less than 1 micrometer. This allows the coherent
integration of the MIDI fringes of faint or resolved N band targets,
respectively. With that method we could decrease the detection limit of
correlated fluxes of MIDI down to 0.5 Jy (vs. 5 Jy without FSU-A) and 0.05 Jy
(vs. 0.2 Jy without FSU-A) using the ATs and UTs, respectively. The K band
visibilities could be measured with a precision down to ~2%.Comment: 11 pages, 13 figures, Accepted for publication in A&
VLT/SINFONI time-resolved spectroscopy of the central, luminous, H-rich WN stars of R136
Using the Very Large Telescope's Spectrograph for INtegral Field Observation
in the Near-Infrared (VLT/SINFONI), we have obtained repeated AO-assisted, NIR
spectroscopy of the six central luminous, Wolf-Rayet (WR) stars in the core of
the very young (~1 Myr), massive and dense cluster R136, in the Large
Magellanic Cloud (LMC). We also de-archived available images that were obtained
with the Hubble Space Telescope's Space Telescope Imaging Spectrograph
(HST/STIS), and extracted high-quality, differential photometry of our target
stars to check for any variability related to binary motion.
Previous studies, relying on spatially unresolved, integrated, optical
spectroscopy, had reported that one of these stars was likely to be a 4.377-day
binary. Our study set out to identify the culprit and any other short-period
system among our targets. However, none displays significant photometric
variability, and only one star, BAT99-112 (R136c), located on the outer fringe
of R136, displays a marginal variability in its radial velocities; we
tentatively report an 8.2-day period. The binary status of BAT99-112 is
supported by the fact that it is one of the brightest X-ray sources among all
known WR stars in the LMC, consistent with it being a colliding-wind system.
Follow-up observations have been proposed to confirm the orbital period of this
potentially very massive system.Comment: 9 pages, 6 figures; accepted for publication in MNRA
The nuclear star cluster of the Milky Way
The nuclear star cluster of the Milky Way is a unique target in the Universe.
Contrary to extragalactic nuclear star clusters, using current technology it
can be resolved into tens of thousands of individual stars. This allows us to
study in detail its spatial and velocity structure as well as the different
stellar populations that make up the cluster. Moreover, the Milky Way is one of
the very few cases where we have firm evidence for the co-existence of a
nuclear star cluster with a central supermassive black hole, Sagittarius A*.
The number density of stars in the Galactic center nuclear star cluster can be
well described, at distances pc from Sagittarius A*, by a power-law
of the form with an index of .
In the central parsec the index of the power-law becomes much flatter and
decreases to . We present proper motions for more than 6000
stars within 1 pc in projection from the central black hole. The cluster
appears isotropic at projected distances pc from Sagittarius A*.
Outside of 0.5 pc and out to 1.0 pc the velocity dispersion appears to stay
constant. A robust result of our Jeans modeling of the data is the required
presence of of extended (stellar) mass in the
central parsec of the Galaxy.Comment: To appear in the proceedings of "The Universe under the Microscope -
Astrophysics at High Angular Resolution", Journal of Physics:Conference
Series (IOP; http://www.iop.org/EJ/conf) This version has been slightly
modified (e.g. double-log plot in right hand panel of Figure 5
The Post-Pericenter Evolution of the Galactic Center Source G2
In early 2014 the fast-moving near-infrared source G2 reached its closest
approach to the supermassive black hole Sgr A* in the Galactic Center. We
report on the evolution of the ionized gaseous component and the dusty
component of G2 immediately after this event, revealed by new observations
obtained in 2015 and 2016 with the SINFONI integral field spectrograph and the
NACO imager at the ESO VLT. The spatially resolved dynamics of the Br
line emission can be accounted for by the ballistic motion and tidal shearing
of a test-particle cloud that has followed a highly eccentric Keplerian orbit
around the black hole for the last 12 years. The non-detection of a drag force
or any strong hydrodynamic interaction with the hot gas in the inner accretion
zone limits the ambient density to less than a few 10 cm at the
distance of closest approach (1500 ), assuming G2 is a spherical cloud
moving through a stationary and homogeneous atmosphere. The dust continuum
emission is unresolved in L'-band, but stays consistent with the location of
the Br emission. The total luminosity of the Br and L' emission
has remained constant to within the measurement uncertainty. The nature and
origin of G2 are likely related to that of the precursor source G1, since their
orbital evolution is similar, though not identical. Both object are also likely
related to a trailing tail structure, which is continuously connected to G2
over a large range in position and radial velocity.Comment: 17 pages, 12 figures; accepted for publication in Ap
The supermassive black hole in NGC4486a detected with SINFONI at the VLT
The near-infrared integral field spectrograph SINFONI at the ESO VLT opens a
new window for the study of central supermassive black holes. With a near-IR
spatial resolution similar to HST optical and the ability to penetrate dust it
provides the possibility to explore the low-mass end of the M-sigma relation
(sigma<120km/s) where so far very few black hole masses were measured with
stellar dynamics. With SINFONI we observed the central region of the
low-luminosity elliptical galaxy NGC4486a at a spatial resolution of ~0.1arcsec
in the K band. The stellar kinematics was measured with a maximum penalised
likelihood method considering the region around the CO absorption band heads.
We determined a black hole mass of M_BH=1.25^{+0.75}_{-0.79} x 10^7 M_sun (90%
C.L.) using the Schwarzschild orbit superposition method including the full
2-dimensional spatial information. This mass agrees with the predictions of the
M-sigma relation, strengthening its validity at the lower sigma end.Comment: 7 pages, 7 figures. Accepted by MNRA
The Quintuplet Cluster I. A K-band spectral catalog of stellar sources
Three very massive clusters are known to reside in the Galactic Center
region, the Arches cluster, the Quintuplet cluster and the Central parsec
cluster. We obtained spectroscopic observations of the Quintuplet cluster with
the Integral Field Spectrograph SINFONI-SPIFFI at the ESO-VLT. The spectral
range comprises the near-IR K-band from 1.94 to 2.45 micrometer. The 3D data
cubes of the individual fields were flux-calibrated and combined to one
contiguous cube, from which the spectra of all detectable point sources were
extracted. We present a catalog of 160 stellar sources in the inner part of the
Quintuplet cluster.Comment: 14 pages, 9 figures, 3 tables, accepted by A&
Making SPIFFI SPIFFIER: Upgrade of the SPIFFI instrument for use in ERIS and performance analysis from re-commissioning
SPIFFI is an AO-fed integral field spectrograph operating as part of SINFONI
on the VLT, which will be upgraded and reused as SPIFFIER in the new VLT
instrument ERIS. In January 2016, we used new technology developments to
perform an early upgrade to optical subsystems in the SPIFFI instrument so
ongoing scientific programs can make use of enhanced performance before ERIS
arrives in 2020. We report on the upgraded components and the performance of
SPIFFI after the upgrade, including gains in throughput and spatial and
spectral resolution. We show results from re-commissioning, highlighting the
potential for scientific programs to use the capabilities of the upgraded
SPIFFI. Finally, we discuss the additional upgrades for SPIFFIER which will be
implemented before it is integrated into ERIS.Comment: 20 pages, 12 figures. Proceedings from SPIE Astronomical Telescopes
and Instrumentation 201
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