351 research outputs found
The orbital motion of the Arches cluster — clues on cluster formation near the galactic center
The Arches cluster is one of the most massive, young clusters in the Milky Way. Located inside the central molecular zone in the inner 200 pc of the Galactic center, it formed in one of the most extreme star-forming environments in the present-day Galaxy. Its young age of only 2.5 Myr allows us to observe the cluster despite the strong tidal shear forces in the inner Galaxy. The orbit of the cluster determines its dynamical evolution, tidal stripping, and hence its fate. We have measured the proper motion of the Arches cluster relative to the ambient field from Keck/NIRC2 LGS-AO and VLT/NAOS-CONICA NGS-AO observations taken 4.3 years earlier. When combined with the radial velocity, we derive a 3D space motion of 232 ± 30 km/s for the Arches. This motion is exceptionally large when compared to molecular cloud orbits in the GC, and places stringent constraints on the formation scenarios for starburst clusters in dense, nuclear environments
Young Binary Stars and Associated Disks
The typical product of the star formation process is a binary star. Binaries
have provided the first dynamical measures of the masses of pre-main-sequence
(PMS) stars, providing support for the calibrations of PMS evolutionary tracks.
Surprisingly, in some star-forming regions PMS binary frequencies are higher
than among main-sequence solar-type stars. The difference in PMS and
main-sequence binary frequencies is apparently not an evolutionary effect;
recent attention has focussed on correlations between binary frequency and
stellar density or cloud temperatures. Accretion disks are common among young
binary stars. Binaries with separations between 1 AU and 100 AU have
substantially less submillimeter emission than closer or wider binaries,
suggesting that they have truncated their disks. Evidence of dynamical clearing
has been seen in several binaries. Remarkably, PMS binaries of all separations
show evidence of circumstellar disks and continued accretion. This suggests
that the circumstellar disks are replenished from circumbinary disks or
envelopes. The frequent presence of disks suggests that planet formation can
occur in binary environments, and formation of planets in wide binaries is
already established by their discovery. Circumbinary disk masses around very
short period binaries are ample to form planetary systems such as our own. The
nature of planetary systems among the most common binaries, with separations
between 10 AU and 100 AU, is less clear given the observed reduction in disk
mass, though they may have disk masses adequate for the formation of
terrestrial-like planets.Comment: 32 pages, including 6 Postscript figures (TeX, uses psfig.sty); to
appear in "Protostars & Planets IV". Gif figures with captions and high-res
Postscript color figure available at
http://hven.swarthmore.edu/~jensen/preprints/ppiv.htm
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
Mapping the Outer Edge of the Young Stellar Cluster in the Galactic Center
We present new near-infrared spectroscopic observations of the outer edges of
the young stellar cluster around the supermassive black hole at the Galactic
center. The observations show a break in the surface-density profile of young
stars at approximately 13 arcsec (0.52 pc). These observations
spectroscopically confirm previous suggestions of a break based on photometry.
Using Gemini North's Near-Infrared Integral Field Spectrometer (NIFS) we are
able to detect and separate early- and late-type stars with a 75% completeness
at Ks = 15.5. We sample a region with radii between 7" to 23" (0.28 pc to 0.92
pc) from Sgr A*, and present new spectral classifications of 144 stars brighter
than Ks = 15.5, where 140 stars are late-type (> 1 Gyr) and only four stars are
early-type (young, 4-6 Myr). A broken power-law fit of the early-type
surface-density matches well with our data and previously published values. The
projected surface-density of late-type stars is also measured and found to be
consistent with previous results. We find that the observed early-type
surface-density profile is inconsistent with the theory of the young stars
originating from a tightly bound infalling cluster, as no significant trail of
young stars is found at radii above 13". We also note that either a simple disk
instability criterion or a cloud-cloud collision could explain the location of
the outer edge, though we lack information to make conclusive remarks on either
alternative. If this break in surface-density represents an edge to the young
stellar cluster it would set an important scale for the most recent episode of
star formation at the Galactic center.Comment: 17 pages, 11 figures, 3 tables, ApJ accepte
The orbital motion of the Quintuplet cluster - a common origin for the Arches and Quintuplet clusters?
We investigate the orbital motion of the Quintuplet cluster near the Galactic
center with the aim of constraining formation scenarios of young, massive star
clusters in nuclear environments. Three epochs of adaptive optics high-angular
resolution imaging with Keck/NIRC2 and VLT/NACO were obtained over a time
baseline of 5.8 years, delivering an astrometric accuracy of 0.5-1 mas/yr.
Proper motions were derived in the cluster reference frame and were used to
distinguish cluster members from the majority of field stars. Fitting the
cluster and field proper motion distributions with 2D gaussian models, we
derive the orbital motion of the cluster for the first time. The Quintuplet is
moving with a 2D velocity of 132 +/- 15 km/s with respect to the field along
the Galactic plane, which yields a 3D orbital velocity of 167 +/- 15 km/s when
combined with the previously known radial velocity. From a sample of 119 stars
measured in three epochs, we derive an upper limit to the velocity dispersion
in the core of the Quintuplet cluster of sigma_1D < 10 km/s. Knowledge of the
three velocity components of the Quintuplet allows us to model the cluster
orbit in the potential of the inner Galaxy. Comparing the Quintuplet's orbit
with the Arches orbit, we discuss the possibility that both clusters originated
in the same area of the central molecular zone. [abridged]Comment: 40 pages, 12 figures, accepted for publication in Ap
Properties of the Remnant Clockwise Disk of Young Stars in the Galactic Center
We present new kinematic measurements and modeling of a sample of 116 young
stars in the central parsec of the Galaxy in order to investigate the
properties of the young stellar disk. The measurements were derived from a
combination of speckle and laser guide star adaptive optics imaging and
integral field spectroscopy from the Keck telescopes. Compared to earlier disk
studies, the most important kinematic measurement improvement is in the
precision of the accelerations in the plane of the sky, which have a factor of
six smaller uncertainties (~10 uas/yr/yr). We have also added the first radial
velocity measurements for 8 young stars, increasing the sample at the largest
radii (6"-12") by 25%. We derive the ensemble properties of the observed stars
using Monte-Carlo simulations of mock data. There is one highly significant
kinematic feature (~20 sigma), corresponding to the well-known clockwise disk,
and no significant feature is detected at the location of the previously
claimed counterclockwise disk. The true disk fraction is estimated to be ~20%,
a factor of ~2.5 lower than previous claims, suggesting that we may be
observing the remnant of what used to be a more densely populated stellar disk.
The similarity in the kinematic properties of the B stars and the O/WR stars
suggests a common star formation event. The intrinsic eccentricity distribution
of the disk stars is unimodal, with an average value of = 0.27 +/- 0.07,
which we show can be achieved through dynamical relaxation in an initially
circular disk with a moderately top-heavy mass function.Comment: 65 pages, 22 figures, 8 tables, submitted to Ap
Objects and primordial black holes
We suggest that " objects" recently discovered in the Galactic Center may
be clouds of gas bound by the gravitational field of stellar-mass black holes
produced in the interactions of sublunar primordial black holes with neutron
stars. If dark matter is composed of primordial black holes with masses
, these black holes can be captured by neutron
stars in the Galactic Center, where the dark matter density is high. After the
capture, the neutron star is consumed by the black hole, resulting in a
population of black holes. These stellar-mass black holes,
accompanied by gaseous atmospheres, can account for the observed properties of
the objects, including their resilience to tidal disruption by the
supermassive black hole in the Galactic Center while also producing emission
consistent with inferred luminosities.Comment: 8 pages, 2 figures, Accepted into Phys. Rev.
Recent Results and Perspectives for Precision Astrometry and Photometry with Adaptive Optics
Large ground-based telescopes equipped with adaptive optics (AO) systems have
ushered in a new era of high-resolution infrared photometry and astrometry.
Relative astrometric accuracies of <0.2 mas have already been demonstrated from
infrared images with spatial resolutions of 55-95 mas resolution over 10-20''
fields of view. Relative photometric accuracies of 3% and absolute photometric
accuracies of 5%-20% are also possible. I will review improvements and current
limitations in astrometry and photometry with adaptive optics of crowded
stellar fields. These capabilities enable experiments such as measuring orbits
for brown dwarfs and exoplanets, studying our Galaxy's supermassive black hole
and its environment, and identifying individual stars in young star clusters,
which can be used test the universality of the initial mass function.Comment: SPIE Conference Proceedin
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