103 research outputs found
Embedded Clusters in Molecular Clouds
Stellar clusters are born embedded within giant molecular clouds (GMCs) and
during their formation and early evolution are often only visible at infrared
wavelengths, being heavily obscured by dust. Over the last 15 years advances in
infrared detection capabilities have enabled the first systematic studies of
embedded clusters in galactic molecular clouds. In this article we review the
current state of empirical knowledge concerning these extremely young
protocluster systems. From a survey of the literature we compile the first
extensive catalog of galactic embedded cluster properties. We use the catalog
to construct the mass function and estimate the birthrate for embedded clusters
within roughly 2 Kpc of the Sun. We find that the embedded cluster birthrate
exceeds that of visible open clusters by an order of magnitude or more
indicating a high infant mortality rate for protocluster systems. Less than
4-7% of embedded clusters survive emergence from molecular clouds to become
bound clusters of Pleiades age. The vast majority (90%) of stars that form in
embedded clusters form in rich clusters of 100 or more members with masses in
excess of 50 solar masses. We review the role of embedded clusters in
investigating the nature of the IMF which, in one nearby example, has been
measured over the entire range of stellar and substellar mass, from OB stars to
subsellar objects near the deuterium burning limit. We also review the role
embedded clusters play in the investigation of circumstellar disk evolution and
the important constraints they provide for understanding the origin of
planetary systems. Finally, we discuss current ideas concerning the origin and
dynamical evolution of embedded clusters and the implications for the formation
of bound open clusters.Comment: To appear in Annual Reviews of Astronomy and Astrophysics v41.
Manuscript consists of 73 pages with 14 figures. Due to space limitations 5
figures are included as separate low quality jpeg figures. A complete
manuscript with full resolution figures can be downloaded from
http://cfa-www.harvard.edu/~clada/preprints.htm
A Near-Infrared L Band Survey of the Young Embedded Cluster NGC 2024
We present the results of the first sensitive L band (3.4 micron) imaging
study of the nearby young embedded cluster NGC 2024. Two separate surveys of
the cluster were acquired in order to obtain a census of the circumstellar disk
fraction in the cluster. From an analysis of the JHKL colors of all sources in
our largest area, we find an infrared excess fraction of > 86%. The JHKL colors
suggest that the infrared excesses arise in circumstellar disks, indicating
that the majority of the sources which formed in the NGC 2024 cluster are
currently surrounded by, and likely formed with circumstellar disks. The excess
fractions remain very high, within the errors, even at the faintest L
magnitudes from our deeper surveys suggesting that disks form around the
majority of the stars in very young clusters such as NGC 2024 independent of
mass. From comparison with published JHKL observations of Taurus, we find the K
- L excess fraction in NGC 2024 to be consistent with a high initial incidence
of circumstellar disks in both NGC 2024 and Taurus. Because NGC 2024 represents
a region of much higher stellar density than Taurus, this suggests that disks
may form around most of the YSOs in star forming regions independent of
environment. We find a relatively constant JHKL excess fraction with increasing
cluster radius, indicating that the disk fraction is independent of location in
the cluster. In contrast, the JHK excess fraction increases rapidly toward the
central region of the cluster, and is most likely due to contamination of the K
band measurements by bright nebulosity in the central regions of the cluster.
We identify 45 candidate protostellar sources in the central regions of the NGC
2024 cluster, and find a lower limit on the protostellar phase of early stellar
evolution of 0.4 - 1.4 X 10^5 yr, similar to that in Taurus.Comment: 37 pages, 8 figures, 3 tables, To appear in the Astronomical Journa
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
Hunting Galaxies to (and for) Extinction
In studies of star-forming regions, near-infrared excess (NIRX)
sources--objects with intrinsic colors redder than normal stars--constitute
both signal (young stars) and noise (e.g. background galaxies). We hunt down
(identify) galaxies using near-infrared observations in the Perseus
star-forming region by combining structural information, colors, and number
density estimates. Galaxies at moderate redshifts (z = 0.1 - 0.5) have colors
similar to young stellar objects (YSOs) at both near- and mid-infrared (e.g.
Spitzer) wavelengths, which limits our ability to identify YSOs from colors
alone. Structural information from high-quality near-infrared observations
allows us to better separate YSOs from galaxies, rejecting 2/5 of the YSO
candidates identified from Spitzer observations of our regions and potentially
extending the YSO luminosity function below K of 15 magnitudes where galaxy
contamination dominates. Once they are identified we use galaxies as valuable
extra signal for making extinction maps of molecular clouds. Our new iterative
procedure: the Galaxies Near Infrared Color Excess method Revisited (GNICER),
uses the mean colors of galaxies as a function of magnitude to include them in
extinction maps in an unbiased way. GNICER increases the number of background
sources used to probe the structure of a cloud, decreasing the noise and
increasing the resolution of extinction maps made far from the galactic plane.Comment: 16 pages and 16 figures. Accepted for publication in ApJ. Full
resolution version at
http://www.cfa.harvard.edu/COMPLETE/papers/Foster_HuntingGalaxies.pd
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