397 research outputs found
The power of monitoring stellar orbits
The center of the Milky Way hosts a massive black hole. The observational
evidence for its existence is overwhelming. The compact radio source Sgr A* has
been associated with a black hole since its discovery. In the last decade,
high-resolution, near-infrared measurements of individual stellar orbits in the
innermost region of the Galactic Center have shown that at the position of Sgr
A* a highly concentrated mass of 4 x 10^6 M_sun is located. Assuming that
general relativity is correct, the conclusion that Sgr A* is a massive black
hole is inevitable. Without doubt this is the most important application of
stellar orbits in the Galactic Center. Here, we discuss the possibilities going
beyond the mass measurement offered by monitoring these orbits. They are an
extremely useful tool for many scientific questions, such as a geometric
distance estimate to the Galactic Center or the puzzle, how these stars reached
their current orbits. Future improvements in the instrumentation will open up
the route to testing relativistic effects in the gravitational potential of the
black hole, allowing to take full advantage of this unique laboratory for
celestial mechanics.Comment: Proceedings of the Galactic Center Workshop 2009, Shangha
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
Hydrodynamical simulations of a compact source scenario for G2
The origin of the dense gas cloud G2 discovered in the Galactic Center
(Gillessen et al. 2012) is still a debated puzzle. G2 might be a diffuse cloud
or the result of an outflow from an invisible star embedded in it. We present
here detailed simulations of the evolution of winds on G2's orbit. We find that
the hydrodynamic interaction with the hot atmosphere present in the Galactic
Center and the extreme gravitational field of the supermassive black hole must
be taken in account when modeling such a source scenario. We find that the
hydrodynamic interaction with the hot atmosphere present in the Galactic Center
and the extreme gravitational field of the supermassive black hole must be
taken in account when modeling such a source scenario. We also find that in
this scenario most of the Br\gamma\ luminosity is expected to come from the
highly filamentary densest shocked wind material. G2's observational properties
can be used to constrain the properties of the outflow and our best model has a
mass outflow rate of Mdot,w=8.8 x 10^{-8} Msun/yr and a wind velocity of vw =
50 km/s. These values are compatible with those of a young TTauri star wind, as
already suggested by Scoville & Burkert (2013).Comment: 4 pages, 3 figures; Proceeding of the IAU 303: "The GC: Feeding and
Feedback in a Normal Galactic Nucleus" / September 30 - October 4, 2013,
Santa Fe, New Mexico (USA
3D AMR hydrosimulations of a compact source scenario for the Galactic Centre cloud G2
The nature of the gaseous and dusty cloud G2 in the Galactic Centre is still
under debate. We present three-dimensional hydrodynamical adaptive mesh
refinement (AMR) simulations of G2, modeled as an outflow from a "compact
source" moving on the observed orbit. The construction of mock
position-velocity (PV) diagrams enables a direct comparison with observations
and allow us to conclude that the observational properties of the gaseous
component of G2 could be matched by a massive () and slow ()
outflow, as observed for T Tauri stars. In order for this to be true, only the
material at larger () distances from the source must be
actually emitting, otherwise G2 would appear too compact compared to the
observed PV diagrams. On the other hand, the presence of a central dusty source
might be able to explain the compactness of G2's dust component. In the present
scenario, 5-10 years after pericentre the compact source should decouple from
the previously ejected material, due to the hydrodynamic interaction of the
latter with the surrounding hot and dense atmosphere. In this case, a new
outflow should form, ahead of the previous one, which would be the smoking gun
evidence for an outflow scenario.Comment: resubmitted to MNRAS after referee report, 16 pages, 11 figure
The interferometric baselines and GRAVITY astrometric error budget
GRAVITY is a new generation beam combination instrument for the VLTI. Its
goal is to achieve microarsecond astrometric accuracy between objects separated
by a few arcsec. This accuracy on astrometric measurements is the most
important challenge of the instrument, and careful error budget have been
paramount during the technical design of the instrument. In this poster, we
will focus on baselines induced errors, which is part of a larger error budget.Comment: SPIE Meeting 2014 -- Montrea
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
Reaching micro-arcsecond astrometry with long baseline optical interferometry; application to the GRAVITY instrument
A basic principle of long baseline interferometry is that an optical path
difference (OPD) directly translates into an astrometric measurement. In the
simplest case, the OPD is equal to the scalar product between the vector
linking the two telescopes and the normalized vector pointing toward the star.
However, a too simple interpretation of this scalar product leads to seemingly
conflicting results, called here "the baseline paradox". For micro-arcsecond
accuracy astrometry, we have to model in full the metrology measurement. It
involves a complex system subject to many optical effects: from pure baseline
errors to static, quasi-static and high order optical aberrations. The goal of
this paper is to present the strategy used by the "General Relativity Analysis
via VLT InTerferometrY" instrument (GRAVITY) to minimize the biases introduced
by these defects. It is possible to give an analytical formula on how the
baselines and tip-tilt errors affect the astrometric measurement. This formula
depends on the limit-points of three type of baselines: the wide-angle
baseline, the narrow-angle baseline, and the imaging baseline. We also,
numerically, include non-common path higher-order aberrations, whose amplitude
were measured during technical time at the Very Large Telescope Interferometer.
We end by simulating the influence of high-order common-path aberrations due to
atmospheric residuals calculated from a Monte-Carlo simulation tool for
Adaptive optics systems. The result of this work is an error budget of the
biases caused by the multiple optical imperfections, including optical
dispersion. We show that the beam stabilization through both focal and pupil
tracking is crucial to the GRAVITY system. Assuming the instrument pupil is
stabilized at a 4 cm level on M1, and a field tracking below 0.2, we
show that GRAVITY will be able to reach its objective of 10as accuracy.Comment: 14 pages. Accepted by A&
The orbit of the star S2 around SgrA* from VLT and Keck data
Two recent papers (Ghez et al. 2008, Gillessen et al. 2009) have estimated
the mass of and the distance to the massive black hole in the center of the
Milky Way using stellar orbits. The two astrometric data sets are independent
and yielded consistent results, even though the measured positions do not match
when simply overplotting the two sets. In this letter we show that the two sets
can be brought to excellent agreement with each other when allowing for a small
offset in the definition of the reference frame of the two data sets. The
required offsets in the coordinates and velocities of the origin of the
reference frames are consistent with the uncertainties given in Ghez et al.
(2008). The so combined data set allows for a moderate improvement of the
statistical errors of mass of and distance to Sgr A*, but the overall
accuracies of these numbers are dominated by systematic errors and the
long-term calibration of the reference frame. We obtain R0 = 8.28 +- 0.15(stat)
+- 0.29(sys) kpc and M(MBH) = 4.30 +- 0.20(stat) +- 0.30(sys) x 10^6 Msun as
best estimates from a multi-star fit.Comment: submitted to ApJ
GCIRS 7, a pulsating M1 supergiant at the Galactic centre. Physical properties and age
The stellar population in the central parsec of the Galaxy is dominated by an
old (several Gyr) population, but young, massive stars dominate the luminosity
function. We have studied the most luminous of these stars, GCIRS 7, in order
to constrain the age of the recent star formation event in the Galactic Centre
and to characterise it as an interferometric reference for observations of the
Galactic Centre with the instrument GRAVITY, which will equip the Very Large
Telescope Interferometer in the near future. We present the first H-band
interferometric observations of GCIRS 7, obtained using the PIONIER visitor
instrument on the VLTI using the four 8.2-m unit telescopes. In addition, we
present unpublished K-band VLTI/AMBER data, build JHKL light-curves based on
data spanning 4 decades, and measured the star's effective temperature using
SINFONI spectroscopy. GCIRS 7 is marginally resolved at H-band (in 2013:
uniform-disk diameter=1.076+/-0.093mas, R=960+/-92Rsun at 8.33+/-0.35kpc). We
detect a significant circumstellar contribution at K-band. The star and its
environment are variable in brightness and in size. The photospheric H-band
variations are well modelled with two periods: P0~470+/-10 days (amplitude
~0.64mag) and long secondary period LSP~2700-2850 days (~1.1mag). As measured
from CO equivalent width, =3600+/-195K. The size, periods, luminosity
(=-8.44+/-0.22) and effective temperature are consistent with an M1
supergiant with an initial mass of 22.5+/-2.5Msun and an age of 6.5-10Myr
(depending on rotation). This age is in remarkable agreement with most
estimates for the recent star formation event in the central parsec. Caution
should be taken when using this star as an interferometric reference as it is
variable in size, is surrounded by a variable circumstellar environment and
large convection cells may form on its photosphere.Comment: Accepted for publication in A&A. 10 pages, 12 figure
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