261 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
The shapes of column density PDFs - The importance of the last closed contour
The probability distribution function of column density (PDF) has become the
tool of choice for cloud structure analysis and star formation studies. Its
simplicity is attractive, and the PDF could offer access to cloud physical
parameters otherwise difficult to measure, but there has been some confusion in
the literature on the definition of its completeness limit and shape at the low
column density end. In this Letter we use the natural definition of the
completeness limit of a column density PDF, the last closed column-density
contour inside a surveyed region, and apply it to a set of large-scale maps of
nearby molecular clouds. We conclude that there is no observational evidence
for log-normal PDFs in these objects. We find that all studied molecular clouds
have PDFs well described by power-laws, including the diffuse cloud Polaris.
Our results call for a new physical interpretation for the shape of the column
density PDFs. We find that the slope of a cloud PDF is invariant to distance
but not to the spatial arrangement of cloud material, and as such it is still a
useful tool to investigate cloud structure.Comment: A&A Letter, accepted. Comments welcom
Molecular clouds have power-law probability distribution functions
In this Letter we investigate the shape of the probability distribution of
column densities (PDF) in molecular clouds. Through the use of low-noise,
extinction-calibrated \textit{Herschel}/\textit{Planck} emission data for eight
molecular clouds, we demonstrate that, contrary to common belief, the PDFs of
molecular clouds are not described well by log-normal functions, but are
instead power laws with exponents close to two and with breaks between and , so close to the CO self-shielding limit
and not far from the transition between molecular and atomic gas. Additionally,
we argue that the intrinsic functional form of the PDF cannot be securely
determined below , limiting our ability to
investigate more complex models for the shape of the cloud PDF.Comment: Letter to the Editor, to appear in A&
A new method to unveil embedded stellar clusters
In this paper we present a novel method to identify and characterize stellar
clusters deeply embedded in a dark molecular cloud. The method is based on
measuring stellar surface density in wide-field infrared images using star
counting techniques. It takes advantage of the differing -band luminosity
functions (HLFs) of field stars and young stellar populations and is able to
statistically associate each star in an image as a member of either the
background stellar population or a young stellar population projected on or
near the cloud. Moreover, the technique corrects for the effects of
differential extinction toward each individual star. We have tested this method
against simulations as well as observations. In particular, we have applied the
method to 2MASS point sources observed in the Orion A and B complexes, and the
results obtained compare very well with those obtained from deep Spitzer and
Chandra observations where presence of infrared excess or X-ray emission
directly determines membership status for every star. Additionally, our method
also identifies unobscured clusters and a low resolution version of the Orion
stellar surface density map shows clearly the relatively unobscured and diffuse
OB 1a and 1b sub-groups and provides useful insights on their spatial
distribution.Comment: A&A, in press; 13 pages, multi-layer figures can be displayed with
Adobe Acrobat Reade
On the Star Formation Rates in Molecular Clouds
In this paper we investigate the level of star formation activity within
nearby molecular clouds. We employ a uniform set of infrared extinction maps to
provide accurate assessments of cloud mass and structure and compare these with
inventories of young stellar objects within the clouds. We present evidence
indicating that both the yield and rate of star formation can vary considerably
in local clouds, independent of their mass and size. We find that the surface
density structure of such clouds appears to be important in controlling both
these factors. In particular, we find that the star formation rate (SFR) in
molecular clouds is linearly proportional to the cloud mass (M_{0.8}) above an
extinction threshold of A_K approximately equal to 0.8 magnitudes,
corresponding to a gas surface density threshold of approximaely 116 solar
masses per square pc. We argue that this surface density threshold corresponds
to a gas volume density threshold which we estimate to be n(H_2) approximately
equal to 10^4\cc. Specifically we find SFR (solar masses per yr) = 4.6 +/- 2.6
x 10^{-8} M_{0.8} (solar masses) for the clouds in our sample. This relation
between the rate of star formation and the amount of dense gas in molecular
clouds appears to be in excellent agreement with previous observations of both
galactic and extragalactic star forming activity. It is likely the underlying
physical relationship or empirical law that most directly connects star
formation activity with interstellar gas over many spatial scales within and
between individual galaxies. These results suggest that the key to obtaining a
predictive understanding of the star formation rates in molecular clouds and
galaxies is to understand those physical factors which give rise to the dense
components of these clouds.Comment: accepted for publicaton in the Astrophysical Journal; 22 pages, 4
figure
First Resolved Dust Continuum Measurements of Individual Giant Molecular Clouds in the Andromeda Galaxy
© 2020 The American Astronomical Society.In our local Galactic neighborhood, molecular clouds are best studied using a combination of dust measurements, to determine robust masses, sizes, and internal structures of the clouds, and molecular-line observations to determine cloud kinematics and chemistry. We present here the first results of a program designed to extend such studies to nearby galaxies beyond the Magellanic Clouds. Utilizing the wideband upgrade of the Submillimeter Array (SMA) at 230 GHz, we have obtained the first continuum detections of the thermal dust emission on sub-GMC scales (∼15 pc) within the Andromeda galaxy (M31). These include the first resolved continuum detections of dust emission from individual giant molecular clouds (GMCs) beyond the Magellanic Clouds. Utilizing a powerful capability of the SMA, we simultaneously recorded CO(2-1) emission with identical (u, v) coverage, astrometry, and calibration, enabling the first measurements of the CO conversion factor, α CO(2-1), toward individual GMCs across an external galaxy. Our direct measurement yields an average CO-to-dust mass conversion factor of α' CO-dust = 0.042 ± 0.018 M o (K km s -1 pc 2) -1 for the J = 2-1 transition. This value does not appear to vary with galactocentric radius. Assuming a constant gas-to-dust ratio of 136, the resulting α CO = 5.7 ± 2.4 M o (K km s -1 pc 2) -1 for the 2-1 transition is in excellent agreement with that of GMCs in the Milky Way, given the uncertainties. Finally, using the same analysis techniques, we compare our results with observations of the local Orion molecular clouds, placed at the distance of M31 and simulated to appear as they would if observed by the SMA.Peer reviewedFinal Published versio
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