3,369 research outputs found
3D stellar kinematics at the Galactic center: measuring the nuclear star cluster spatial density profile, black hole mass, and distance
We present 3D kinematic observations of stars within the central 0.5 pc of
the Milky Way nuclear star cluster using adaptive optics imaging and
spectroscopy from the Keck telescopes. Recent observations have shown that the
cluster has a shallower surface density profile than expected for a dynamically
relaxed cusp, leading to important implications for its formation and
evolution. However, the true three dimensional profile of the cluster is
unknown due to the difficulty in de-projecting the stellar number counts. Here,
we use spherical Jeans modeling of individual proper motions and radial
velocities to constrain for the first time, the de-projected spatial density
profile, cluster velocity anisotropy, black hole mass (), and
distance to the Galactic center () simultaneously. We find that the inner
stellar density profile of the late-type stars, to
have a power law slope , much more shallow than
the frequently assumed Bahcall Wolf slope of . The measured
slope will significantly affect dynamical predictions involving the cluster,
such as the dynamical friction time scale. The cluster core must be larger than
0.5 pc, which disfavors some scenarios for its origin. Our measurement of
and
kpc is consistent with that derived from stellar
orbits within 1 of Sgr A*. When combined with the orbit of
S0-2, the uncertainty on is reduced by 30% ( kpc).
We suggest that the MW NSC can be used in the future in combination with
stellar orbits to significantly improve constraints on .Comment: 7 pages, 3 figures, 2 tables, ApJL accepte
Constraining scalar fields with stellar kinematics and collisional dark matter
The existence and detection of scalar fields could provide solutions to
long-standing puzzles about the nature of dark matter, the dark compact objects
at the centre of most galaxies, and other phenomena. Yet, self-interacting
scalar fields are very poorly constrained by astronomical observations, leading
to great uncertainties in estimates of the mass and the
self-interacting coupling constant of these fields. To counter this,
we have systematically employed available astronomical observations to develop
new constraints, considerably restricting this parameter space. In particular,
by exploiting precise observations of stellar dynamics at the centre of our
Galaxy and assuming that these dynamics can be explained by a single boson
star, we determine an upper limit for the boson star compactness and impose
significant limits on the values of the properties of possible scalar fields.
Requiring the scalar field particle to follow a collisional dark matter model
further narrows these constraints. Most importantly, we find that if a scalar
dark matter particle does exist, then it cannot account for both the
dark-matter halos and the existence of dark compact objects in galactic nucleiComment: 23 pages, 8 figures; accepted for publication by JCAP after minor
change
First VLTI infrared spectro-interferometry on GCIRS 7 - Characterizing the prime reference source for Galactic center observations at highest angular resolution
Investigating the environment of the massive black hole SgrA* at the center
of the Galaxy requires the highest angular resolution available to avoid source
confusion and to study the physical properties of the individual objects.
GCIRS7 has been used as wavefront and astrometric reference. Our studies
investigate, for the first time, its properties at 2&10um using VLTI/AMBER and
MIDI. We aim at analyzing the suitability of IRS7 as an IF-phase-reference for
the upcoming generation of dual-field facilities at optical interferometers. We
observed with (R~30) and 50m (proj.) baseline, resulting in 9 and 45mas
resolution for NIR and MIR, resp. The first K-band fringe detection of a GC
star suggests that IRS7 could be marginally resolved at 2um, which would imply
that the photosphere of the supergiant is enshrouded by a molecular and dusty
envelope. At 10um, IRS7 is strongly resolved with a visibility of approximately
0.2. The MIR is dominated by moderately warm (200 K), extended dust, mostly
distributed outside of a radius of about 120 AU (15 mas) around the star. A
deep 9.8-silicate absorption in excess of the usual extinction law with respect
to the NIR extinction has been found. This confirms recent findings of a
relatively enhanced, interstellar 9.8-silicate absorption with respect to the
NIR extinction towards another star in the central arcsec, suggesting an
unusual dust composition in that region. Our VLTI observations show that
interferometric NIR phase-referencing experiments with mas resolution using
IRS7 as phase-reference appear to be feasible, but more such studies are
required to definitely characterize the close environment around this star. We
demonstrate that interferometry is required to resolve the innermost
environment of stars at the Galactic center.Comment: 6 pages, 2 figures, accepted for publication in A&
Source-intrinsic near-infrared properties of Sgr A*: Total intensity measurements
We present a comprehensive data description for Ks-band measurements of Sgr
A*. We characterize the statistical properties of the variability of Sgr A* in
the near-infrared, which we find to be consistent with a single-state process
forming a power-law distribution of the flux density. We discover a linear
rms-flux relation for the flux-density range up to 12 mJy on a timescale of 24
minutes. This and the power-law flux density distribution implies a
phenomenological, formally non-linear statistical variability model with which
we can simulate the observed variability and extrapolate its behavior to higher
flux levels and longer timescales. We present reasons why data with our cadence
cannot be used to decide on the question whether the power spectral density of
the underlying random process shows more structure at timescales between 25 min
and 100 min compared to what is expected from a red noise random process.Comment: Accepted to ApJS, August 27, 201
An Improved Distance and Mass Estimate for Sgr A* from a Multistar Orbit Analysis
We present new, more precise measurements of the mass and distance of our
Galaxy's central supermassive black hole, Sgr A*. These results stem from a new
analysis that more than doubles the time baseline for astrometry of faint stars
orbiting Sgr A*, combining two decades of speckle imaging and adaptive optics
data. Specifically, we improve our analysis of the speckle images by using
information about a star's orbit from the deep adaptive optics data (2005 -
2013) to inform the search for the star in the speckle years (1995 - 2005).
When this new analysis technique is combined with the first complete
re-reduction of Keck Galactic Center speckle images using speckle holography,
we are able to track the short-period star S0-38 (K-band magnitude = 17,
orbital period = 19 years) through the speckle years. We use the kinematic
measurements from speckle holography and adaptive optics to estimate the orbits
of S0-38 and S0-2 and thereby improve our constraints of the mass ()
and distance () of Sgr A*: and kpc. The
uncertainties in and as determined by the combined orbital fit
of S0-2 and S0-38 are improved by a factor of 2 and 2.5, respectively, compared
to an orbital fit of S0-2 alone and a factor of 2.5 compared to previous
results from stellar orbits. This analysis also limits the extended dark mass
within 0.01 pc to less than at 99.7% confidence, a
factor of 3 lower compared to prior work.Comment: 56 pages, 14 figures, accepted to Ap
Stellar and circumstellar properties of visual binaries in the Orion Nebula Cluster
Our general understanding of multiple star and planet formation is primarily
based on observations of young multiple systems in low density regions like
Tau-Aur and Oph. Since many, if not most, of the stars are born in clusters,
observational constraints from young binaries in those environments are
fundamental for understanding both the formation of multiple systems and
planets in multiple systems throughout the Galaxy. We build upon the largest
survey for young binaries in the Orion Nebula Cluster (ONC) which is based on
Hubble Space Telescope observations to derive both stellar and circumstellar
properties of newborn binary systems in this cluster environment. We present
Adaptive Optics spatially-resolved JHKL'-band photometry and K-band
R\,5000 spectra for a sample of 8 ONC binary systems from this database.
We characterize the stellar properties of binary components and obtain a census
of protoplanetary disks through K-L' color excess. For a combined sample of ONC
binaries including 7 additional systems with NIR spectroscopy from the
literature, we derive mass ratio and relative age distributions. We compare the
stellar and circumstellar properties of binaries in ONC with those in Tau-Aur
and Oph from samples of binaries with stellar properties derived for each
component from spectra and/or visual photometry and with a disk census obtained
through K-L color excess. The mass ratio distribution of ONC binaries is found
to be indistinguishable from that of Tau-Aur and, to some extent, to that of
Oph in the separation range 85-560\,AU and for primary mass in the range 0.15
to 0.8\,M_{\sun}.A trend toward a lower mass ratio with larger separation is
suggested in ONC binaries which is not seen in Tau-Aur binaries.The components
of ONC binaries are found to be significantly more coeval than the overall ONC
population and as coeval as components of binaries in Tau-Aur and Oph[...]Comment: Accepted for publication in Astronomy & Astrophysic
The Keplerian orbit of G2
We give an update of the observations and analysis of G2 - the gaseous red
emission-line object that is on a very eccentric orbit around the Galaxy's
central black hole and predicted to come within 2400 Rs in early 2014. During
2013, the laser guide star adaptive optics systems on the W. M. Keck I and II
telescopes were used to obtain three epochs of spectroscopy and imaging at the
highest spatial resolution currently possible in the near-IR. The updated
orbital solution derived from radial velocities in addition to Br-Gamma line
astrometry is consistent with our earlier estimates. Strikingly, even ~6 months
before pericenter passage there is no perceptible deviation from a Keplerian
orbit. We furthermore show that a proposed "tail" of G2 is likely not
associated with it but is rather an independent gas structure. We also show
that G2 does not seem to be unique, since several red emission-line objects can
be found in the central arcsecond. Taken together, it seems more likely that G2
is ultimately stellar in nature, although there is clearly gas associated with
it.Comment: Proceedings of IAU Symposium #303, "The Galactic Center: Feeding and
Feedback in a Normal Galactic Nucleus"; 2013 September 30 - October 4, Santa
Fe New Mexico (USA
Spitzer/IRAC Observations of the Variability of Sgr A* and the Object G2 at 4.5 microns
We present the first detection from the Spitzer Space Telescope of 4.5 micron
variability from Sgr A*, the emitting source associated with the Milky Way's
central black hole. The >23 hour continuous light curve was obtained with the
IRAC instrument in 2013 December. The result characterizes the variability of
Sgr A* prior to the closest approach of the G2 object, a putative infalling gas
cloud that orbits close to Sgr A*. The high stellar density at the location of
Sgr A* produces a background of ~250 mJy at 4.5 microns in each pixel with a
large pixel-to-pixel gradient, but the light curve for the highly variable Sgr
A* source was successfully measured by modeling and removing the variations due
to pointing wobble. The observed flux densities range from the noise level of
~0.7 mJy rms in a 6.4-s measurement to ~10 mJy. Emission was seen above the
noise level ~34% of the time. The light curve characteristics, including the
flux density distribution and structure function, are consistent with those
previously derived at shorter infrared wavelengths. We see no evidence in the
light curve for activity attributable to the G2 interaction at the observing
epoch, ~100 days before the expected G2 periapsis passage. The IRAC light curve
is more than a factor of two longer than any previous infrared observation,
improving constraints on the timescale of the break in the power spectral
distribution of Sgr A* flux densities. The data favor the longer of the two
previously published values for the timescale.Comment: 13 pages, 10 figures, 2 tables, accepted for publication in the Ap
The instrumental polarization of the Nasmyth focus polarimetric differential imager NAOS/CONICA (NACO) at the VLT - Implications for time-resolved polarimetric measurements of Sgr A*
We report on the results of calibrating and simulating the instrumental
polarization properties of the ESO VLT adaptive optics camera system
NAOS/CONICA (NACO) in the Ks-band. We use the Stokes/Mueller formalism for
metallic reflections to describe the instrumental polarization. The model is
compared to standard-star observations and time-resolved observations of bright
sources in the Galactic center. We find the instrumental polarization to be
highly dependent on the pointing position of the telescope and about 4% at
maximum. We report a polarization angle offset of 13.28{\deg} due to a position
angle offset of the half-wave plate that affects the calibration of NACO data
taken before autumn 2009. With the new model of the instrumental polarization
of NACO it is possible to measure the polarization with an accuracy of 1% in
polarization degree. The uncertainty of the polarization angle is < 5{\deg} for
polarization degrees > 4%. For highly sampled polarimetric time series we find
that the improved understanding of the polarization properties gives results
that are fully consistent with the previously used method to derive the
polarization. The small difference between the derived and the previously
employed polarization calibration is well within the statistical uncertainties
of the measurements, and for Sgr A* they do not affect the results from our
relativistic modeling of the accretion process.Comment: 16 pages, 15 figures, 5 tables, accepted by A&A on 2010 October 1
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