60 research outputs found
Constraining the dark cusp in the Galactic Center by long-period binaries
Massive black holes (MBHs) in galactic nuclei are believed to be surrounded
by a high density stellar cluster, whose mass is mostly in hard-to-detect faint
stars and compact remnants. Such dark cusps dominate the dynamics near the MBH:
a dark cusp in the Galactic center (GC) of the Milky Way would strongly affect
orbital tests of General Relativity there; on cosmic scales, dark cusps set the
rates of gravitational wave emission events from compact remnants that spiral
into MBHs, and they modify the rates of tidal disruption events, to list only
some implications. A recently discovered long-period massive young binary (P_12
<~ 1 yr, M_12 ~ O(100 M_sun), T_12 ~ 6x10^6 yr), only ~0.1 pc from the Galactic
MBH (Pfuhl et al 2013), sets a lower bound on the 2-body relaxation timescale
there, min t_rlx ~ (P_12/M_12)^(2/3)T_12 ~ 10^7 yr, and correspondingly, an
upper bound on the stellar number density, max n ~ few x 10^8/
1/pc^3, based on the binary's survival against evaporation by the dark cusp.
However, a conservative dynamical estimate, the drain limit, implies t_rlx >
O(10^8) yr. Such massive binaries are thus too short-lived and tightly bound to
constrain a dense relaxed dark cusp. We explore here in detail the use of
longer-period, less massive and longer-lived binaries (P_12 ~ few yr, M_12 ~
2-4 M_sun, T_12 ~ 10^8-10^10 yr), presently just below the detection threshold,
for probing the dark cusp, and develop the framework for translating their
future detections among the giants in the GC into dynamical constraints.Comment: 13 pp. Submitted to Ap
The GRAVITY interferometer and the Milky Wayâs nuclear star cluster
This thesis is divided into two parts: an instrumentation part and an astrophysical part.
The instrumentation part describes the development and implementation of the fiber coupler
and guiding subsystems of the 2nd generation VLTI instrument GRAVITY. The astrophysical
part describes the derivation of the star formation history of the Milky Wayâs
nuclear star cluster based on imaging and spectroscopic data obtained at the Very Large
Telescope.
The future VLTI instrument GRAVITY will deliver micro-arcsecond astrometry, using the
interferometric combination of four telescopes. The instrument is a joint project of several
European institutes lead by the Max Planck Institut fšur extraterrestrische Physik.
The instrumental part of this thesis describes the fiber coupler unit and the guiding system.
They serve for beam stabilization and light injection in GRAVITY. In order to deliver
micro-arcsecond astrometry, GRAVITY requires an unprecedented stability of the VLTI
optical train. We therefore developed a dedicated guiding system, correcting the longitudinal
and lateral pupil wanderas well as the image jitter in VLTI tunnel. The actuators for
the correction are provided by four fiber coupler units located in the GRAVITY cryostat.
Each fiber coupler picks the light of one telescope and stabilizes the beam. Furthermore
each unit provides field de-rotation, polarization adjustment as well as atmospheric piston
correction. A novel roof-prism design offers the possibility of on-axis as well as off-axis
fringe tracking. Finally the stabilized beam is injected with minimized losses into singlemode
fibers via parabolic mirrors. We present lab results of the first guiding- as well as
the first fiber coupler prototype, in particular the closed loop performance and the optical
quality. Based on the lab results we derive the on-sky performance of the systems and the
implications concerning the sensitivity of GRAVITY.
The astrophysical part of this thesis presents imaging and integral field spectroscopy data
for 450 cool giant stars within 1 pc from Sgr A*. We use the prominent CO bandheads
to derive effective temperatures of individual giants. Additionally we present the deepest
spectroscopic observation of the Galactic Center so far, probing the number of B9/A0 main
sequence stars (2.2 â 2.8M) in two deep fields. From spectro-photometry we construct
a Hertzsprung-Russell diagram of the red giant population and fit the observed diagram
with model populations to derive the star formation history of the nuclear cluster. We
find that (1) the average nuclear star-formation rate dropped from an initial maximum
10Gyrs ago to a deep minimum 1-2Gyrs ago and increased again during the last few
hundred Myrs, and (2) that roughly 80% of the stellar mass formed more than 5Gyrs ago; (3) mass estimates within R 1 pc from Sgr A* favor a dominant star formation
mode with a normal Chabrier/Kroupa initial mass function for the majority of the past
star formation in the Galactic Center. The bulk stellar mass seems to have formed under
conditions significantly different from the observed young stellar disks, perhaps because at
the time of the formation of the nuclear cluster the massive black hole and its sphere of
influence was much smaller than today
On the origin of the B-stars in the Galactic center
We present a new directly-observable statistic which uses sky position and
proper motion of stars near the Galactic center massive black hole to identify
populations with high orbital eccentricities. It is most useful for stars with
large orbital periods for which dynamical accelerations are difficult to
determine. We apply this statistic to a data set of B-stars with projected
radii 0."1 < p < 25" (~0.004 - 1 pc) from the massive black hole in the
Galactic center. We compare the results with those from N-body simulations to
distinguish between scenarios for their formation. We find that the scenarios
favored by the data correlate strongly with particular K-magnitude intervals,
corresponding to different zero-age main-sequence (MS) masses and lifetimes.
Stars with 14 < mK < 15 (15 - 20 solar masses, t_{MS} = 8-13 Myr) match well to
a disk formation origin, while those with mK > 15 (13
Myr), if isotropically distributed, form a population that is more eccentric
than thermal, which suggests a Hills binary-disruption origin.Comment: Updated paper. 21 pages, 28 figures, 6 tables, ApJ accepte
Pericenter passage of the gas cloud G2 in the Galactic Center
We have further followed the evolution of the orbital and physical properties
of G2, the object currently falling toward the massive black hole in the
Galactic Center on a near-radial orbit. New, very sensitive data were taken in
April 2013 with NACO and SINFONI at the ESO VLT . The 'head' of G2 continues to
be stretched ever further along the orbit in position-velocity space. A
fraction of its emission appears to be already emerging on the blue-shifted
side of the orbit, past pericenter approach. Ionized gas in the head is now
stretched over more than 15,000 Schwarzschild radii RS around the pericenter of
the orbit, at ~ 2000 RS ~ 20 light hours from the black hole. The pericenter
passage of G2 will be a process stretching over a period of at least one year.
The Brackett-{\gamma} luminosity of the head has been constant over the past 9
years, to within +- 25%, as have the line ratios Brackett-{\gamma} /
Paschen-{\alpha} and Brackett-{\gamma} / Helium-I. We do not see any
significant evidence for deviations of G2's dynamical evolution, due to
hydrodynamical interactions with the hot gas around the black hole, from a
ballistic orbit of an initially compact cloud with moderate velocity
dispersion. The constant luminosity and the increasingly stretched appearance
of the head of G2 in the position-velocity plane, without a central peak, is
not consistent with several proposed models with continuous gas release from an
initially bound zone around a faint star on the same orbit as G2.Comment: 10 figures, submitted to Ap
Infrared interferometry to spatially and spectrally resolve jets in X-ray binaries
Infrared interferometry is a new frontier for precision ground based
observing, with new instrumentation achieving milliarcsecond (mas) spatial
resolutions for faint sources, along with astrometry on the order of 10
microarcseconds. This technique has already led to breakthroughs in the
observations of the supermassive black hole at the Galactic centre and its
orbiting stars, AGN, and exo-planets, and can be employed for studying X-ray
binaries (XRBs), microquasars in particular. Beyond constraining the orbital
parameters of the system using the centroid wobble and spatially resolving jet
discrete ejections on mas scales, we also propose a novel method to discern
between the various components contributing to the infrared bands: accretion
disk, jets and companion star. We demonstrate that the GRAVITY instrument on
the Very Large Telescope Interferometer (VLTI) should be able to detect a
centroid shift in a number of sources, opening a new avenue of exploration for
the myriad of transients expected to be discovered in the coming decade of
radio all-sky surveys. We also present the first proof-of-concept GRAVITY
observation of a low-mass X-ray binary transient, MAXI J1820+070, to search for
extended jets on mas scales. We place the tightest constraints yet via direct
imaging on the size of the infrared emitting region of the compact jet in a
hard state XRB.Comment: 12 Pages, 3 figures, accepted for publication in MNRA
The metrology system of the VLTI instrument GRAVITY
The VLTI instrument GRAVITY combines the beams from four telescopes and
provides phase-referenced imaging as well as precision-astrometry of order 10
microarcseconds by observing two celestial objects in dual-field mode. Their
angular separation can be determined from their differential OPD (dOPD) when
the internal dOPDs in the interferometer are known. Here, we present the
general overview of the novel metrology system which performs these
measurements. The metrology consists of a three-beam laser system and a
homodyne detection scheme for three-beam interference using phase-shifting
interferometry in combination with lock-in amplifiers. Via this approach the
metrology system measures dOPDs on a nanometer-level.Comment: 9 pages, 5 figure
A Detection of Sgr A* in the far infrared
We report the first detection of the Galactic Centre massive black hole,
Sgr~A*, in the far infrared. Our measurements were obtained with PACS on board
the \emph{Herschel} satellite at and .
While the warm dust in the Galactic Centre is too bright to allow for a direct
detection of Sgr~A*, we measure a significant and simultaneous variation of its
flux of and during one observation. The significance level of
the band variability is and the corresponding
band variability is significant at . We find
no example of an equally significant false positive detection. Conservatively
assuming a variability of in the FIR, we can provide upper limits to the
flux. Comparing the latter with theoretical models we find that 1D RIAF models
have difficulties explaining the observed faintness. However, the upper limits
are consistent with modern ALMA and VLA observations. Our upper limits provide
further evidence for a spectral peak at and
constrain the number density of electrons in the accretion
disk and or outflow.Comment: accepted for publication in AP
The GRAVITY instrument software / High-level software
GRAVITY is the four-beam, near- infrared, AO-assisted, fringe tracking,
astrometric and imaging instrument for the Very Large Telescope Interferometer
(VLTI). It is requiring the development of one of the most complex instrument
software systems ever built for an ESO instrument. Apart from its many
interfaces and interdependencies, one of the most challenging aspects is the
overall performance and stability of this complex system. The three infrared
detectors and the fast reflective memory network (RMN) recorder contribute a
total data rate of up to 20 MiB/s accumulating to a maximum of 250 GiB of data
per night. The detectors, the two instrument Local Control Units (LCUs) as well
as the five LCUs running applications under TAC (Tools for Advanced Control)
architecture, are interconnected with fast Ethernet, RMN fibers and dedicated
fiber connections as well as signals for the time synchronization. Here we give
a simplified overview of all subsystems of GRAVITY and their interfaces and
discuss two examples of high-level applications during observations: the
acquisition procedure and the gathering and merging of data to the final FITS
file.Comment: 8 pages, 7 figures, published in Proc. SPIE 9146, Optical and
Infrared Interferometry IV, 91462
- âŠ