1,832 research outputs found
Orbits and origins of the young stars in the central parsec of the galaxy
We present new proper motions from the 10 m Keck telescopes for a puzzling population of massive, young stars located within a parsec of the supermassive black hole at the Galactic Center. Our proper motion measurements have uncertainties of only 0.07 mas yr^(â1) (3 km s^(â1) ), which is âł7 times better than previous proper motion measurements for these stars, and enables us to measure accelerations as low as 0.2 mas yr^(â2) (7 km s^(â1) yr^(â1) ). These measurements, along with stellar line-of-sight velocities from the literature, constrain the true orbit of each individual star and allow us to directly test the hypothesis that the massive stars reside in two stellar disks as has been previously proposed. Analysis of the stellar orbits reveals only one disk of young stars using a method that is capable of detecting disks containing at least 7 stars. The detected disk contains 50% (38 of 73) of the young stars, is inclined by ~115° from the plane of the sky, and is oriented at a position angle of âŒ100° East of North. The on-disk and off-disk populations have similar K-band luminosity functions and radial distributions that decrease at larger radii as â r^(â2). The disk has an out-of-the-disk velocity dispersion of 28±6 km s^(â1) , which corresponds to a half-opening angle of 7°±2° , and several candidate disk members have eccentricities greater than 0.2. Our findings suggest that the young stars may have formed in situ but in a more complex geometry than a simple thin circular disk
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
Galactic Center Youth: Orbits and Origins of the Young Stars in the Central Parsec
We present new proper motions for the massive, young stars at the Galactic Center, based on 10 years of diffraction limited data from the Keck telescopes. Our proper motion measurements now have uncertainties of only 1-2 km/s and allow us to explore the origin of the young stars that reside within the sphere of inflience of the supermassive black hole whose strong tidal forces make this region inhospitable for star formation. Their presence, however, may be explained either by in situ star formation in an accretion disk or as the remnants of a massive stellar cluster which spiraled in via dynamical friction. Earlier stellar velocity vectors were used to postulate that all the young stars resided in two counter-rotating stellar disks, which is consistent with both of the above formation scenarios. Our precise proper motions allow us, for the frst time, to determine the orbital parameters of each individual star and thereby to test the hypothesis that the massive stars reside in two stellar disks. Of the 26 young stars in this study that were previously proposed to lie on the inner, clockwise disk, we find that nearly all exhibit orbital constraints consistent with such a disk. On the other hand, of the 7 stars in this study previously proposed to lie in the outer, less well-defhed counter-clockwise disk, 6 exhibit inclinations that are inconsistent with such a disk, bringing into question the existence of the outer disk. Furthermore, for stars in the inner disk that have eccentricity constraints, we find several that have lower limits to the eccentricity of more than 0.4, implying highly eccentric orbits. This stands in contrast to simple accretion disk formation scenarios which typically predict predominantly circular orbits
A Constant Spectral Index for Sagittarius A* During Infrared/X-ray Intensity Variations
We report the first time-series of broadband infrared (IR) color measurements
of Sgr A*, the variable emission source associated with the supermassive black
hole at the Galactic Center. Using the laser and natural guide star AO systems
on the Keck II telescope, we imaged Sgr A* in multiple near-infrared broadband
filters with a typical cycle time of ~3 min during 4 observing runs
(2005-2006), two of which were simultaneous with Chandra X-ray measurements. In
spite of the large range of dereddened flux densities for Sgr A* (2-30 mJy),
all of our near-IR measurements are consistent with a constant spectral index
of alpha = -0.6+-0.2. Furthermore, this value is consistent with the spectral
indices observed at X-ray wavelengths during nearly all outbursts; which is
consistent with the synchrotron self-Compton model for the production of the
X-ray emission. During the coordinated observations, one IR outburst occurs <36
min after a possibly associated X-ray outburst, while several similar IR
outbursts show no elevated X-ray emission. A variable X-ray to IR ratio and
constant infrared spectral index challenge the notion that the IR and X-ray
emission are connected to the same electrons. We, therefore, posit that the
population of electrons responsible for both the IR and X-ray emission are
generated by an acceleration mechanism that leaves the bulk of the electron
energy distribution responsible for the IR emission unchanged, but has a
variable high-energy cutoff. Occasionally a tail of electrons >1 GeV is
generated, and it is this high-energy tail that gives rise to the X-ray
outbursts. One possible explanation for this type of variation is from the
turbulence induced by a magnetorotational instability, in which the outer scale
length of the turbulence varies and changes the high-energy cutoff.Comment: 11 pages, 7 figures (color), Accepted for publication in ApJ.
Resolution (Fig 1&2) downgraded for astro-ph. For full resolution, see
http://casa.colorado.edu/~hornstei/sgracolor.pd
The Accelerations of Stars Orbiting the Milky Way's Central Black Hole
Recent measurements, of the velocities of stars near the center of the Milky
Way have provided the strongest evidence for the presence of a supermassive
black hole in a galaxy, but the observational uncertainties poorly constrain
many of the properties of the black hole. Determining the accelerations of
stars in their orbits around the center provides much more precise information
about the position and mass of the black hole. Here we report measurements of
the accelerations for three stars located ~0.005 pc from the central radio
source Sgr A*; these accelerations are comparable to those experienced by the
Earth as it orbits the Sun. These data increase the inferred minimum mass
density in the central region of the Galaxy by an order of magnitude relative
to previous results and localized the dark mass to within 0.05 +- 0.04 arcsec
of the nominal position of Sgr A*. In addition, the orbital period of one of
the observed stars could be as short as 15 years, allowing us the opportunity
in the near future to observe an entire period.Comment: To appear in September 21 2000 issue of Natur
The Shortest Known Period Star Orbiting our Galaxy's Supermassive Black Hole
Stars with short orbital periods at the center of our galaxy offer a powerful
and unique probe of a supermassive black hole. Over the past 17 years, the W.
M. Keck Observatory has been used to image the Galactic center at the highest
angular resolution possible today. By adding to this data set and advancing
methodologies, we have detected S0-102, a star orbiting our galaxy's
supermassive black hole with a period of just 11.5 years. S0-102 doubles the
number of stars with full phase coverage and periods less than 20 years. It
thereby provides the opportunity with future measurements to resolve
degeneracies in the parameters describing the central gravitational potential
and to test Einstein's theory of General Relativity in an unexplored regime.Comment: Science, in press (published Oct 5, 2012). See Science Online for the
Supplementary Material, or here:
http://www.astro.ucla.edu/~ghezgroup/gc/research/S02_S0102_orbits.htm
The proper motion of the Arches cluster with Keck Laser-Guide Star Adaptive Optics
We present the first measurement of the proper motion of the young, compact
Arches cluster near the Galactic center from near-infrared adaptive optics (AO)
data taken with the recently commissioned laser-guide star (LGS) at the Keck
10-m telescope. The excellent astrometric accuracy achieved with LGS-AO
provides the basis for a detailed comparison with VLT/NAOS-CONICA data taken
4.3 years earlier. Over the 4.3 year baseline, a spatial displacement of the
Arches cluster with respect to the field population is measured to be 24.0 +/-
2.2 mas, corresponding to a proper motion of 5.6 +/- 0.5 mas/yr or 212 +/- 29
km/s at a distance of 8 kpc. In combination with the known line-of-sight
velocity of the cluster, we derive a 3D space motion of 232 +/- 30 km/s of the
Arches relative to the field. The large proper motion of the Arches cannot be
explained with any of the closed orbital families observed in gas clouds in the
bar potential of the inner Galaxy, but would be consistent with the Arches
being on a transitional trajectory from x1 to x2 orbits. We investigate a
cloud-cloud collision as the possible origin for the Arches cluster. The
integration of the cluster orbit in the potential of the inner Galaxy suggests
that the cluster passes within 10 pc of the supermassive black hole only if its
true GC distance is very close to its projected distance. A contribution of
young stars from the Arches cluster to the young stellar population in the
inner few parsecs of the GC thus appears increasingly unlikely. The measurement
of the 3D velocity and orbital analysis provides the first observational
evidence that Arches-like clusters do not spiral into the GC. This confirms
that no progenitor clusters to the nuclear cluster are observed at the present
epoch.Comment: 22 pdflatex pages including 12 figures, reviewed version accepted by
Ap
Measuring Distance and Properties of the Milky Way's Central Supermassive Black Hole with Stellar Orbits
We report new precision measurements of the properties of our Galaxy's
supermassive black hole. Based on astrometric (1995-2007) and radial velocity
(2000-2007) measurements from the W. M. Keck 10-meter telescopes, a fully
unconstrained Keplerian orbit for the short period star S0-2 provides values
for Ro of 8.0+-0.6 kpc, M_bh of 4.1+-0.6x10^6 Mo, and the black hole's radial
velocity, which is consistent with zero with 30 km/s uncertainty. If the black
hole is assumed to be at rest with respect to the Galaxy, we can further
constrain the fit and obtain Ro = 8.4+-0.4 kpc and M_bh = 4.5+-0.4x10^6 Mo.
More complex models constrain the extended dark mass distribution to be less
than 3-4x10^5 Mo within 0.01 pc, ~100x higher than predictions from stellar and
stellar remnant models. For all models, we identify transient astrometric
shifts from source confusion and the assumptions regarding the black hole's
radial motion as previously unrecognized limitations on orbital accuracy and
the usefulness of fainter stars. Future astrometric and RV observations will
remedy these effects. Our estimates of Ro and the Galaxy's local rotation
speed, which it is derived from combining Ro with the apparent proper motion of
Sgr A*, (theta0 = 229+-18 km/s), are compatible with measurements made using
other methods. The increased black hole mass found in this study, compared to
that determined using projected mass estimators, implies a longer period for
the innermost stable orbit, longer resonant relaxation timescales for stars in
the vicinity of the black hole and a better agreement with the M_bh-sigma
relation.Comment: ApJ, accepted (26 pages, 16 figures, 7 tables
Stellar Populations in the Central 0.5 pc of the Galaxy. II. The Initial Mass Function
The supermassive black hole at the center of the Milky Way plays host to a massive, young cluster that may have formed in one of the most inhospitable environments in the Galaxy. We present new measurements of the global properties of this cluster, including the initial mass function (IMF), age, and cluster mass. These results are based on Keck laser-guide-star adaptive optics observations used to identify the young stars and measure their K p-band luminosity function as presented in Do et al. A Bayesian inference methodology is developed to simultaneously fit the global properties of the cluster utilizing the observations and extensive simulations of synthetic star clusters. We find that the slope of the mass function for this cluster is α = 1.7 ± 0.2, which is steeper than previously reported, but still flatter than the traditional Salpeter slope of 2.35. The age of the cluster is between 2.5 and 5.8 Myr with 95% confidence, which is a younger age than typically adopted but consistent within the uncertainties of past measurements. The exact age of the cluster is difficult to determine since our results show two distinct age solutions (3.9 Myr and 2.8 Myr) due to model degeneracies in the relative number of Wolf-Rayet and OB stars. The total cluster mass is between 14,000 and 37,000 M_â above 1 M_â and it is necessary to include multiple star systems in order to fit the observed luminosity function and the number of observed Wolf-Rayet stars. The new IMF slope measurement is now consistent with X-ray observations indicating a factor of 10 fewer X-ray emitting pre-main-sequence stars than expected when compared with a Salpeter IMF. The young cluster at the Galactic center is one of the few definitive examples of an IMF that deviates significantly from the near-universal IMFs found in the solar neighborhood
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