3,367 research outputs found
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
A near-IR variability study of the Galactic black hole: a red noise source with no detected periodicity
We present the results of near-infrared (2 and 3 microns) monitoring of Sgr
A*-IR with 1 min time sampling using the natural and laser guide star adaptive
optics (LGS AO) system at the Keck II telescope. Sgr A*-IR was observed
continuously for up to three hours on each of seven nights, between 2005 July
and 2007 August. Sgr A*-IR is detected at all times and is continuously
variable, with a median observed 2 micron flux density of 0.192 mJy,
corresponding to 16.3 magnitude at K'. These observations allow us to
investigate Nyquist sampled periods ranging from about 2 minutes to an hour.
Using Monte Carlo simulations, we find that the variability of Sgr A* in this
data set is consistent with models based on correlated noise with power spectra
having frequency dependent power law slopes between 2.0 to 3.0, consistent with
those reported for AGN light curves. Of particular interest are periods of ~20
min, corresponding to a quasi-periodic signal claimed based upon previous
near-infrared observations and interpreted as the orbit of a 'hot spot' at or
near the last stable orbit of a spinning black hole. We find no significant
periodicity at any time scale probed in these new observations for periodic
signals. This study is sensitive to periodic signals with amplitudes greater
than 20% of the maximum amplitude of the underlying red noise component for
light curves with duration greater than ~2 hours at a 98% confidence limit.Comment: 37 pages, 2 tables, 17 figures, accepted by Ap
High angular resolution integral-field spectroscopy of the Galaxy's nuclear cluster: a missing stellar cusp?
We report on the structure of the nuclear star cluster in the innermost 0.16
pc of the Galaxy as measured by the number density profile of late-type giants.
Using laser guide star adaptive optics in conjunction with the integral field
spectrograph, OSIRIS, at the Keck II telescope, we are able to differentiate
between the older, late-type ( 1 Gyr) stars, which are presumed to be
dynamically relaxed, and the unrelaxed young ( 6 Myr) population. This
distinction is crucial for testing models of stellar cusp formation in the
vicinity of a black hole, as the models assume that the cusp stars are in
dynamical equilibrium in the black hole potential. Based on the late-type stars
alone, the surface stellar number density profile, , is flat, with . Monte Carlo simulations of
the possible de-projected volume density profile, n(r) ,
show that is less than 1.0 at the 99.73 % confidence level. These
results are consistent with the nuclear star cluster having no cusp, with a
core profile that is significantly flatter than predicted by most cusp
formation theories, and even allows for the presence of a central hole in the
stellar distribution. Of the possible dynamical interactions that can lead to
the depletion of the red giants observable in this survey -- stellar
collisions, mass segregation from stellar remnants, or a recent merger event --
mass segregation is the only one that can be ruled out as the dominant
depletion mechanism. The lack of a stellar cusp around a supermassive black
hole would have important implications for black hole growth models and
inferences on the presence of a black hole based upon stellar distributions.Comment: 35 pages, 5 tables, 12 figures, accepted by Ap
Adaptive Optics Observations of the Galactic Center Young Stars
Adaptive Optics observations have dramatically improved the quality and
versatility of high angular resolution measurements of the center of our
Galaxy. In this paper, we quantify the quality of our Adaptive Optics
observations and report on the astrometric precision for the young stellar
population that appears to reside in a stellar disk structure in the central
parsec. We show that with our improved astrometry and a 16 year baseline,
including 10 years of speckle and 6 years of laser guide star AO imaging, we
reliably detect accelerations in the plane of the sky as small as 70
microarcsec/yr/yr (~2.5 km/s/yr) and out to a projected radius from the
supermassive black hole of 1.5" (~0.06 pc). With an increase in sensitivity to
accelerations by a factor of ~6 over our previous efforts, we are able to
directly probe the kinematic structure of the young stellar disk, which appears
to have an inner radius of 0.8". We find that candidate disk members are on
eccentric orbits, with a mean eccentricity of = 0.30 +/- 0.07. Such
eccentricities cannot be explained by the relaxation of a circular disk with a
normal initial mass function, which suggests the existence of a top-heavy IMF
or formation in an initially eccentric disk.Comment: 7 pages, 4 figures, SPIE Astronomical Telescopes and Instrumentation
201
Stellar Dynamics at the Galactic Center with an Extremely Large Telescope
We discuss experiments achievable via monitoring of stellar dynamics near the
massive black hole at the Galactic center with a next generation, extremely
large telescope (ELT). Given the likely observational capabilities of an ELT
and current knowledge of the stellar environment at the Galactic center, we
synthesize plausible samples of stellar orbits around the black hole. We use
the Markov Chain Monte Carlo method to evaluate the constraints that orbital
monitoring places on the matter content near the black hole. Results are
expressed as functions of the number N of stars with detectable orbital motions
and the astrometric precision dtheta and spectroscopic precision dv at which
stellar proper motions and radial velocities are monitored. For N = 100, dtheta
= 0.5 mas, and dv = 10 km/s -- a conservative estimate of the capabilities of a
30 meter telescope -- the extended matter distribution enclosed by the orbits
will produce measurable deviations from Keplerian motion if >1000 Msun is
enclosed within 0.01 pc. The black hole mass and distance to the Galactic
center will be measured to better than ~0.1%. Lowest-order relativistic
effects, such as the prograde precession, will be detectable if dtheta < 0.5
mas. Higher-order effects, including frame dragging due to black hole spin,
requires dtheta < 0.05 mas, or the favorable discovery of a compact, highly
eccentric orbit. Finally, we calculate the rate at which monitored stars
undergo detectable nearby encounters with background stars. Such encounters
probe the mass function of stellar remnants that accumulate near the black
hole. We find that ~30 encounters will be detected over a 10 yr baseline for
dtheta = 0.5 mas.Comment: 14 pages, 5 figures; discussion no longer aperture-specific (TMT ->
ELT), matches ApJ versio
Testing for periodicities in near-IR light curves of Sgr A
We present the results of near-infrared (2 ÎŒm) monitoring of Sgr A*-IR with 1 minute time sampling using laser guide star adaptive optics (LGS AO) system at the Keck II
telescope. Sgr A*-IR was observed continuously for up to three hours on each of seven nights, between 2006 May and 2007 August. Sgr A*-IR is detected at all times and is continuously variable. These observations allow us to investigate Nyquist sampled periods ranging from
about 2 minutes to an hour. Of particular interest are periods of ~20 min, which corresponds to a quasi-periodic (QPO) signal claimed based upon previous near-infrared observations and interpreted as the orbit of a âhot spotâ at or near the last stable orbit of a spinning black hole.
We investigate these claims by comparing periodograms of the light curves with models for red noise and find no significant deviations that would indicate QPO activity at any time scale probed in the study. We find that the variability of Sgr A* is consistent with a model based on
correlated noise with a power spectrum having a frequency dependence of ~ f^(2.5), consistent with that observed in AGNs. Furthermore, the periodograms show power down to the minimum sampling time of 2 min, well below the period of the last stable orbit of a maximally spinning black hole, indicating that the Sgr A*-IR light curves observed in this study is unlikely to be from the Keplerian motion of a single âhot spotâ of orbiting plasma
High Proper Motion Stars in the Vicinity of Sgr A*: Evidence for a Supermassive Black Hole at the Center of Our Galaxy
Over a two year period (1995-1997), we have conducted a diffraction-limited
imaging study at 2.2 microns of the inner 6"x6" of the Galaxy's central stellar
cluster using the Keck 10-m telescope. The K band images obtained reveal a
large population of faint stars. We use an unbiased approach for identifying
and selecting stars to be included in this proper motion study, which results
in a sample of 90 stars with brightness ranging from K=9-17 and velocities as
large as 1,400+-100 km/sec. Compared to earlier work (Eckart et al. 1997;
Genzel et al. 1997), the source confusion is reduced by a factor of 9, the
number of stars with proper motion measurement in the central 25 arcsec^2 of
our galaxy is doubled, and the accuracy of the velocity measurements in the
central 1 arcsec^2 is improved by a factor of 4. The peaks of both the stellar
surface density and the velocity dispersion are consistent with the position of
the unusual radio source and blackhole candidate, Sgr A*, suggesting that Sgr
A* is coincident (+-0."1) with the dynamical center of the Galaxy. As a
function of distance from Sgr A*, the velocity dispersion displays a falloff
well fit by Keplerian motion about a central dark mass of 2.6(+-0.2)x10^6 Mo
confined to a volume of at most 10^-6 pc^3, consistent with earlier results.
Although uncertainties in the measurements mathematically allow for the matter
to be distributed over this volume as a cluster, no realistic cluster is
physically tenable. Thus, independent of the presence of Sgr A*, the large
inferred central density of at least 10^12 Mo/pc^3, which exceeds the
volume-averaged mass densities found at the center of any other galaxy, leads
us to the conclusion that our Galaxy harbors a massive central black hole.Comment: 19 pages, 8 figures, accepted for publications in the Astrophysical
Journa
Orbits and Masses in the T Tauri System
We investigate the binary star T Tauri South, presenting the orbital
parameters of the two components and their individual masses. We combined
astrometric positions from the literature with previously unpublished VLT
observations. Model fits yield the orbital elements of T Tau Sa and Sb. We use
T Tau N as an astrometric reference to derive an estimate for the mass ratio of
Sa and Sb. Although most of the orbital parameters are not well constrained, it
is unlikely that T Tau Sb is on a highly elliptical orbit or escaping from the
system. The total mass of T Tau S is rather well constrained to 3.0 +0.15/-0.24
M_sun. The mass ratio Sb:Sa is about 0.4, corresponding to individual masses of
M_Sa = 2.1+/-0.2 M_sun and M_Sb = 0.8+/-0.1 M_sun. This confirms that the
infrared companion in the T Tauri system is a pair of young stars obscured by
circumstellar material.Comment: 10 pages, 11 figures, accepted by Astronomy and Astrophysic
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