11,383 research outputs found
Two 2MASS-Selected Young Stellar Clusters: Photometry, Spectroscopy, and the IMF
We present near-infrared J, H, and K_s images and K-band spectroscopy of two
newly discovered stellar clusters at different stages of evolution. Our spectra
suggest the presence of massive YSOs in the heavily embedded cluster in the
star-forming region near radio source G353.4-0.4 and an O5-O6V star in the
cluster near radio source G305+00.2. We determine a K-band luminosity function
(KLF) for both clusters and an initial mass function (IMF) for the cluster near
G305+00.2. The derived IMF slope is -1.5 if the KLF is used to derive the IMF
and is -0.98 if the color-magnitude diagram and spectra are used. The more
reliable CMD-based slope is flatter than the Salpeter value usually found for
stellar clusters. We find that using the KLF alone to derive an IMF is likely
to produce an overly steep slope in stellar clusters subject to variable
extinction.Comment: 16 pages, 18 figures, accepted to A
Modeling the Near-Infrared Luminosity Functions of Young Stellar Clusters
We present the results of numerical experiments designed to evaluate the
usefulness of near-infrared luminosity functions for constraining the Initial
Mass Function (IMF) of young stellar populations. From this numerical modeling,
we find that the luminosity function of a young stellar population is
considerably more sensitive to variations in the underlying initial mass
function than to either variations in the star forming history or assumed
pre-main-sequence (PMS) mass-to-luminosity relation. To illustrate the
potential effectiveness of using the KLF of a young cluster to constrain its
IMF, we model the observed K band luminosity function of the nearby Trapezium
cluster. Our derived mass function for the Trapezium spans two orders of
magnitude in stellar mass (5 Msun to 0.02 Msun), has a peak near the hydrogen
burning limit, and has an IMF for Brown Dwarfs which steadily decreases with
decreasing mass.Comment: To appear in ApJ (1 April 2000). 37 pages including 11 figures, AAS:
ver 5.
Inhomogeneous quenches in a fermionic chain: exact results
We consider the non-equilibrium physics induced by joining together two tight
binding fermionic chains to form a single chain. Before being joined, each
chain is in a many-fermion ground state. The fillings (densities) in the two
chains might be the same or different. We present a number of exact results for
the correlation functions in the non-interacting case. We present a short-time
expansion, which can sometimes be fully resummed, and which reproduces the
so-called `light cone' effect or wavefront behavior of the correlators. For
large times, we show how all interesting physical regimes may be obtained by
stationary phase approximation techniques. In particular, we derive
semiclassical formulas in the case when both time and positions are large, and
show that these are exact in the thermodynamic limit. We present subleading
corrections to the large-time behavior, including the corrections near the
edges of the wavefront. We also provide results for the return probability or
Loschmidt echo. In the maximally inhomogeneous limit, we prove that it is
exactly gaussian at all times. The effects of interactions on the Loschmidt
echo are also discussed.Comment: 5 pages+14 pages supplementary material+9 figure
The Luminosity & Mass Function of the Trapezium Cluster: From B stars to the Deuterium Burning Limit
We use the results of a new, multi-epoch, multi-wavelength, near-infrared
census of the Trapezium Cluster in Orion to construct and to analyze the
structure of its infrared (K band) luminosity function. Specifically, we employ
an improved set of model luminosity functions to derive this cluster's
underlying Initial Mass Function (IMF) across the entire range of mass from OB
stars to sub-stellar objects down to near the deuterium burning limit. We
derive an IMF for the Trapezium Cluster that rises with decreasing mass, having
a Salpeter-like IMF slope until near ~0.6 M_sun where the IMF flattens and
forms a broad peak extending to the hydrogen burning limit, below which the IMF
declines into the sub-stellar regime. Independent of the details, we find that
sub-stellar objects account for no more than ~22% of the total number of likely
cluster members. Further, the sub-stellar Trapezium IMF breaks from a steady
power-law decline and forms a significant secondary peak at the lowest masses
(10-20 times the mass of Jupiter). This secondary peak may contain as many as
\~30% of the sub-stellar objects in the cluster. Below this sub-stellar IMF
peak, our KLF modeling requires a subsequent sharp decline toward the planetary
mass regime. Lastly, we investigate the robustness of pre-main sequence
luminosity evolution as predicted by current evolutionary models, and we
discuss possible origins for the IMF of brown dwarfs.Comment: 74 pages, 30 figures, AASTeX5.0. To be published in the 01 July 2002
ApJ. For color version of figure 1 and online data table see
http://www.astro.ufl.edu/~muench/PUB/publications.htm
The S-Star Cluster at the Center of the Milky Way: On the nature of diffuse NIR emission in the inner tenth of a parsec
Sagittarius A*, the super-massive black hole at the center of the Milky Way,
is surrounded by a small cluster of high velocity stars, known as the S-stars.
We aim to constrain the amount and nature of stellar and dark mass associated
with the cluster in the immediate vicinity of Sagittarius A*. We use
near-infrared imaging to determine the -band luminosity function
of the S-star cluster members, and the distribution of the diffuse background
emission and the stellar number density counts around the central black hole.
This allows us to determine the stellar light and mass contribution expected
from the faint members of the cluster. We then use post-Newtonian N-body
techniques to investigate the effect of stellar perturbations on the motion of
S2, as a means of detecting the number and masses of the perturbers. We find
that the stellar mass derived from the -band luminosity
extrapolation is much smaller than the amount of mass that might be present
considering the uncertainties in the orbital motion of the star S2. Also the
amount of light from the fainter S-cluster members is below the amount of
residual light at the position of the S-star cluster after removing the bright
cluster members. If the distribution of stars and stellar remnants is strongly
enough peaked near Sagittarius A*, observed changes in the orbital elements of
S2 can be used to constrain both their masses and numbers. Based on simulations
of the cluster of high velocity stars we find that at a wavelength of 2.2
m close to the confusion level for 8 m class telescopes blend stars will
occur (preferentially near the position of Sagittarius A*) that last for
typically 3 years before they dissolve due to proper motions.Comment: 14 pages, 11 figures, minor changes to match the published version in
Astronomy & Astrophysic
Near-Infrared Microlensing of Stars by the Super-Massive Black Hole in the Galactic Center
We investigate microlensing amplification of faint stars in the dense stellar
cluster in the Galactic Center (GC) by the super-massive black hole (BH). Such
events would appear very close to the position of the radio source SgrA*, which
is thought to coincide with the BH, and could be observed during the monitoring
of stellar motions in the GC. We use the observed K-band (2.2 um) luminosity
function (KLF) in the GC and in Baade's Window, as well as stellar population
synthesis computations, to construct KLF models for the inner 300 pc of the
Galaxy. These, and the observed dynamical properties of this region, are used
to compute the rates of microlensing events, which amplify stars above
specified detection thresholds. We present computations of the lensing rates
and amplifications as functions of the event durations (weeks to years), for a
range of detection thresholds. We find that short events dominate the total
rate and that long events tend to have large amplifications. For the current
detection limit of K=17 mag, the total microlensing rate is 0.003 1/yr, and the
rate of events with durations >1 yr is 0.001 1/yr. Recent GC proper motion
studies have revealed the possible presence of one or two variable K-band
sources very close to SgrA* (Genzel et al 97; Ghez et al 98). These sources may
have attained peak brightnesses of K~15 mag, about 1.5-2 mag above the
observational detection limits, and appear to have varied on a timescale of ~1
yr. This behavior is consistent with long-duration microlensing of faint stars
by the BH. However, we estimate that the probability that such an event could
have been detected during the course of the recent proper motion studies is
\~0.5%. A ten-fold improvement in the detection limit and 10 yr of monthly
monitoring would increase the total detection probability to ~20%. (Abridged)Comment: 29 p. with 5 figs. To appear in ApJ. Changed to reflect published
version. Short discussions of solar metallicity luminosity function and
star-star microlensing adde
Deep Near-Infrared Imaging af an Embedded Cluster in the Extreme Outer Galaxy: Census of Supernovae Triggered Star Formation
While conducting a near-infrared (NIR) survey of ``Digel Clouds'', which are
thought to be located in the extreme outer Galaxy (EOG), Kobayashi & Tokunaga
found star formation activity in ``Cloud 2'', a giant molecular cloud at the
Galactic radius of ~ 20 kpc. Additional infrared imaging showed two embedded
young clusters at the densest regions of the molecular cloud. Because the
molecular cloud is located in the vicinity of a supernova remnant (SNR) HI
shell, GSH 138-01-94, it was suggested that the star formation activity in
Cloud 2 was triggered by this expanding HI shell. We obtained deep J (1.25 um),
H (1.65 um) and K (2.2 um) images of one of the embedded clusters in Cloud 2
with high spatial resolution (FWHM ~0".3) and high sensitivity (K ~ 20 mag, 10
sigma). We identified 52 cluster members. The estimated stellar density (~ 10
pc^{-2}) suggests that the cluster is a T-association. This is the deepest NIR
imaging of an embedded cluster in the EOG. The observed K-band luminosity
function (KLF) suggests that the underlying initial mass function (IMF) of the
cluster down to the detection limit of ~ 0.1 M_sun is not significantly
different from the typical IMFs in the field and in the near-by star clusters.
The overall characteristics of this cluster appears to be similar to those of
other embedded clusters in the far outer Galaxy. The estimated age of the
cluster from the KLF, which is less than 1 Myr, is consistent with the view
that the star formation was triggered by the HI shell whose age was estimated
at 4.3 Myr (Stil & Irwin). The 3-dimensional geometry of SNR shell, molecular
cloud and the embedded cluster, which is inferred from our data, as well as the
cluster age strongly suggest that the star formation in Cloud 2 was triggered
by the SNR shell.Comment: 19pages, 8 figures, 1 table, accepted to ApJ. Full paper (pdf) with
high resolution figures available at
http://www.ioa.s.u-tokyo.ac.jp/~ck_yasui/papers/Cloud2N_1.pd
The IMF and Star Formation History of the Stellar Clusters in the Vela D Cloud
We present the results of a Near-Infrared deep photometric survey of a sample
of six embedded star clusters in the Vela-D molecular cloud, all associated
with luminous (~10^3 Lsun) IRAS sources. The clusters are unlikely to be older
than a few 10^6 yrs, since all are still associated with molecular gas. We
employed the fact that all clusters lie at the same distance and were observed
with the same instrumental setting to derive their properties in a consistent
way, being affected by the same instrumental and observational biases. We
extracted the clusters' K Luminosity Functions (KLF) and developed a simple
method to correct them for extinction, based on colour-magnitude diagrams. The
reliability of the method has been tested by constructing synthetic clusters
from theoretical tracks for pre-main sequence stars and a standard Initial Mass
Function (IMF). The clusters' IMFs have been derived from the dereddened KLFs
by adopting a set of pre-main sequence evolutionary tracks and assuming coeval
star formation. All clusters are small (~100 members) and compact (radius
\~0.1-0.2 pc); their most massive stars are intermediate-mass (~2-10 Msun)
ones. The dereddened KLFs are likely to arise from the same distribution,
suggesting that the selected clusters have quite similar IMFs and star
formation histories. The IMFs are consistent with those derived for field stars
and clusters. Adding them together we found that the ``global'' IMF appears
steeper at the high-mass end and exhibits a drop-off at ~10 Msun. In fact, a
standard IMF would predict a star with M>22.5 Msun within one of the clusters,
which is not found. Hence, either high-mass stars need larger clusters to be
formed, or the IMF of the single clusters is steeper at the high-mass end
because of the physical conditions in the parental gas.Comment: 17 pages, 14 figures, to be published in Astronomy & Astrophysic
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