2,341 research outputs found
Reconstructing the Arches I: Constraining the Initial Conditions
We have performed a series of N-body simulations to model the Arches cluster.
Our aim is to find the best fitting model for the Arches cluster by comparing
our simulations with observational data and to constrain the parameters for the
initial conditions of the cluster. By neglecting the Galactic potential and
stellar evolution, we are able to efficiently search through a large parameter
space to determine e.g. the IMF, size, and mass of the cluster. We find, that
the cluster's observed present-day mass function can be well explained with an
initial Salpeter IMF. The lower mass-limit of the IMF cannot be well
constrained from our models. In our best models, the total mass and the virial
radius of the cluster are initially (5.1 +/- 0.8) 10^4 Msun and 0.76 +/- 0.12
pc, respectively. The concentration parameter of the initial King model is w0 =
3-5.Comment: 12 pages, 14 Figures, revised and accepted for publication in MNRA
Disks in the Arches cluster -- survival in a starburst environment
Deep Keck/NIRC2 HK'L' observations of the Arches cluster near the Galactic
center reveal a significant population of near-infrared excess sources. We
combine the L'-band excess observations with K'-band proper motions, to confirm
cluster membership of excess sources in a starburst cluster for the first time.
The robust removal of field contamination provides a reliable disk fraction
down to our completeness limit of H=19 mag, or about 5 Msun at the distance of
the Arches. Of the 24 identified sources with K'-L' > 2.0 mag, 21 have reliable
proper motion measurements, all of which are proper motion members of the
Arches cluster. VLT/SINFONI K'-band spectroscopy of three excess sources
reveals strong CO bandhead emission, which we interpret as the signature of
dense circumstellar disks. The detection of strong disk emission from the
Arches stars is surprising in view of the high mass of the B-type main sequence
host stars of the disks and the intense starburst environment. We find a disk
fraction of 6 +/- 2% among B-type stars in the Arches cluster. A radial
increase in the disk fraction from 3 to 10% suggests rapid disk destruction in
the immediate vicinity of numerous O-type stars in the cluster core. A
comparison between the Arches and other high- and low-mass star-forming regions
provides strong indication that disk depletion is significantly more rapid in
compact starburst clusters than in moderate star-forming environments.Comment: 51 pages preprint2 style, 22 figures, accepted by Ap
Deep near-infrared imaging of W3 Main: constraints on stellar cluster formation
Embedded clusters like W3 Main are complex and dynamically evolving systems
that represent an important phase of the star formation process. We aim at the
characterization of the entire stellar content of W3 Main in a statistical
sense to identify possible differences in evolutionary phase of the stellar
populations and find clues about the formation mechanism of this massive
embedded cluster. Methods. Deep JHKs imaging is used to derive the disk
fraction, Ks-band luminosity functions and mass functions for several
subregions in W3 Main. A two dimensional completeness analysis using artificial
star experiments is applied as a crucial ingredient to assess realistic
completeness limits for our photometry. We find an overall disk fraction of 7.7
2.3%, radially varying from 9.4 3.0 % in the central 1 pc to 5.6
2.2 % in the outer parts of W3 Main. The mass functions derived for three
subregions are consistent with a Kroupa and Chabrier mass function. The mass
function of IRSN3 is complete down to 0.14 Msun and shows a break at M
0.5 Msun. We interpret the higher disk fraction in the center as evidence for a
younger age of the cluster center. We find that the evolutionary sequence
observed in the low-mass stellar population is consistent with the observed age
spread among the massive stars. An analysis of the mass function variations
does not show evidence for mass segregation. W3 Main is currently still
actively forming stars, showing that the ionizing feedback of OB stars is
confined to small areas ( 0.5 pc). The FUV feedback might be influencing
large regions of the cluster as suggested by the low overall disk fraction.Comment: 15 pages, 13 figures, accepted by A&
Age spread in Galactic star forming region W3 Main
We present near-infrared JHKs imaging as well as K-band multi-object
spectroscopy of the massive stellar content of W3 Main using LUCI at the LBT.
We confirm 13 OB stars by their absorption line spectra in W3 Main and spectral
types between O5V and B4V have been found. Three massive Young Stellar Objects
are identified by their emission line spectra and near-infrared excess. From
our spectrophotometric analysis of the massive stars and the nature of their
surrounding HII regions we derive the evolutionary sequence of W3 Main and we
find an age spread of 2-3 Myr.Comment: 4 pages, 2 figures, To appear in conference proceedings of "370 years
of Astronomy in Utrecht
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
Multiple episodes of star formation in the CN15/16/17 molecular complex
We have started a campaign to identify massive star clusters inside bright
molecular bubbles towards the Galactic Center. The CN15/16/17 molecular complex
is the first example of our study. The region is characterized by the presence
of two young clusters, DB10 and DB11, visible in the NIR, an ultra-compact HII
region identified in the radio, several young stellar objects visible in the
MIR, a bright diffuse nebulosity at 8\mu m coming from PAHs and sub-mm
continuum emission revealing the presence of cold dust. Given its position on
the sky (l=0.58, b=-0.85) and its kinematic distance of ~7.5 kpc, the region
was thought to be a very massive site of star formation in proximity of the
CMZ. The cluster DB11 was estimated to be as massive as 10^4 M_sun. However the
region's properties were known only through photometry and its kinematic
distance was very uncertain given its location at the tangential point. We
aimed at better characterizing the region and assess whether it could be a site
of massive star formation located close to the Galactic Center. We have
obtained NTT/SofI JHKs photometry and long slit K band spectroscopy of the
brightest members. We have additionally collected data in the radio, sub-mm and
mid infrared, resulting in a quite different picture of the region. We have
confirmed the presence of massive early B type stars and have derived a
spectro-photometric distance of ~1.2 kpc, much smaller than the kinematic
distance. Adopting this distance we obtain clusters masses of M(DB10) ~ 170
M_sun and M(DB11) ~ 275 M_sun. This is consistent with the absence of any O
star, confirmed by the excitation/ionization status of the nebula. No HeI
diffuse emission is detected in our spectroscopic observations at 2.113\mu m,
which would be expected if the region was hosting more massive stars. Radio
continuum measurements are also consistent with the region hosting at most
early B stars.Comment: Accepted for publication in Astronomy and Astrophysics. Fig. 1 and 3
presented in reduced resolutio
Measurements and quasi-quantum modeling of the steric asymmetry and parity propensities in state-to-state rotationally inelastic scattering of NO (2?1/2) with D2.
Relative integrated cross sections are measured for spin-orbit-conserving, rotationally inelastic scattering of NO
K-shell x-ray spectroscopy of atomic nitrogen
Absolute {\it K}-shell photoionization cross sections for atomic nitrogen
have been obtained from both experiment and state-of-the-art theoretical
techniques. Due to the difficulty of creating a target of neutral atomic
nitrogen, no high-resolution {\it K}-edge spectroscopy measurements have been
reported for this important atom. Interplay between theory and experiment
enabled identification and characterization of the strong
resonance features throughout the threshold region. An experimental value
of 409.64 0.02 eV was determined for the {\it K}-shell binding energy.Comment: 4 pages, 2 graphs, 1 tabl
HST/NICMOS Paschen-alpha Survey of the Galactic Center: Overview
We have recently carried out the first wide-field hydrogen Paschen-alpha line
imaging survey of the Galactic Center (GC), using the NICMOS instrument aboard
the Hubble Space Telescope. The survey maps out a region of 2253 pc^2 around
the central supermassive black hole (Sgr A*) in the 1.87 and 1.90 Micron narrow
bands with a spatial resolution of 0.01 pc at a distance of 8 kpc. Here we
present an overview of the observations, data reduction, preliminary results,
and potential scientific implications, as well as a description of the
rationale and design of the survey. We have produced mosaic maps of the
Paschen-alpha line and continuum emission, giving an unprecedentedly high
resolution and high sensitivity panoramic view of stars and photo-ionized gas
in the nuclear environment of the Galaxy. We detect a significant number of
previously undetected stars with Paschen-alpha in emission. They are most
likely massive stars with strong winds, as confirmed by our initial follow-up
spectroscopic observations. About half of the newly detected massive stars are
found outside the known clusters (Arches, Quintuplet, and Central). Many
previously known diffuse thermal features are now resolved into arrays of
intriguingly fine linear filaments indicating a profound role of magnetic
fields in sculpting the gas. The bright spiral-like Paschen-alpha emission
around Sgr A* is seen to be well confined within the known dusty torus. In the
directions roughly perpendicular to it, we further detect faint, diffuse
Paschen-alpha emission features, which, like earlier radio images, suggest an
outflow from the structure. In addition, we detect various compact
Paschen-alpha nebulae, probably tracing the accretion and/or ejection of stars
at various evolutionary stages.Comment: accepted for publication in MNRAS; a version of higher resolution
images may be found at http://www.astro.umass.edu/~wqd/papers/hst/paper1.pd
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