Young stellar clusters and star formation throughout the Galaxy (original) (raw)
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The structure and evolution of young stellar clusters
2006
We examine the properties of embedded clusters within 1 kiloparsec using new data from the Spitzer Space Telescope, as well as recent results from 2MASS and other ground-based near-infrared surveys. We use surveys of entire molecular clouds to understand the range and distribution of cluster membership, size and surface density. The Spitzer data demonstrate clearly that there is a continuum of star-forming environments, from relative isolation to dense clusters. The number of members of a cluster is correlated with the cluster radius, such that the average surface density of clusters having a few to a thousand members varies by a factor of only a few. The spatial distributions of Spitzer-identified young stellar objects frequently show elongation, low density halos, and sub-clustering. The spatial distributions of protostars resemble the distribution of dense molecular gas, suggesting that their morphologies result directly from the fragmentation of the natal gas. We also examine the effects of the cluster environments on star and planet formation. Although Far-UV and Extreme-UV radiation from massive stars can truncate disks in a few million years, fewer than half of the young stars in our sample (embedded clusters within 1 kpc) are found in regions of strong FUV and EUV fields. Typical volume densities and lifetimes of the observed clusters suggest that dynamical interactions are not an important mechanism for truncating disks on solar system size scales.
The Impact of Massive Stars on the Formation of Young Stellar Clusters
2001
Massive OB stars play an important role in the evolution of molecular clouds and star forming regions. The OB stars both photo--ionize molecular gas as well as sweep up and compress interstellar gas through winds, ionization fronts, and supernovae. In this contribution, we examine the effect massive stars have on the formation of young stellar clusters. We first discuss the
Monthly Notices of the Royal Astronomical Society, 2013
We present a survey of 130 Galactic and extragalactic young massive clusters (YMCs, 10 4 < M/M ⊙ < 10 8 , 10 < t/Myr < 1000) with integrated spectroscopy or resolved stellar photometry (40 presented here and 90 from the literature) and use the sample to search for evidence of ongoing star-formation within the clusters. Such episodes of secondary (or continuous) star-formation are predicted by models that attempt to explain the observed chemical and photometric anomalies observed in globular clusters as being due to the formation of a second stellar population within an existing first population. Additionally, studies that have claimed extended star-formation histories within LMC/SMC intermediate age clusters (1-2 Gyr), also imply that many young massive clusters should show ongoing star-formation. Based on visual inspection of the spectra and/or the colour-magnitude diagrams, we do not find evidence for ongoing star-formation within any of the clusters, and use this to place constraints on the above models. Models of continuous star-formation within clusters, lasting for hundreds of Myr, are ruled out at high significance (unless stellar IMF variations are invoked). Models for the (nearly instantaneous) formation of a secondary population within an existing first generation are not favoured, but are not formally discounted due to the finite sampling of age/mass-space.
Early Evolution of Stellar Groups and Clusters: Environmental Effects on Forming Planetary Systems
The Astrophysical Journal, 2006
This paper studies the dynamical evolution of young groups/clusters, with N = 100 − 1000 members, from their embedded stage out to ages of ∼ 10 Myr. We use N -body simulations to explore how their evolution depends on the system size N and the initial conditions. Motivated by recent observations suggesting that stellar groups begin their evolution with subvirial speeds, this study compares subvirial starting states with virial starting states. Multiple realizations of equivalent cases (100 simulations per initial condition) are used to build up a robust statistical description of these systems, e.g., the probability distribution of closest approaches, the mass profiles, and the probability distribution for the radial location of cluster members. These results provide a framework from which to assess the effects of groups/clusters on the processes of star and planet formation, and to study cluster evolution. The distributions of radial positions are used in conjunction with the probability distributions of the expected FUV luminosities (calculated here as a function of cluster size N ) to determine the radiation exposure of circumstellar disks. The distributions of closest approaches are used in conjunction with scattering cross sections (calculated here as a function of stellar mass using ∼ 10 5 Monte Carlo scattering experiments) to determine the probability of disruption for newly formed solar systems. We use the nearby cluster NGC 1333 as a test case in this investigation. The main conclusion of this study is that clusters in this size range have only a modest effect on forming planetary systems. The interaction rates are low so that the typical solar system experiences a single encounter with closest approach distance b ∼ 1000 AU. The radiation exposure is also low, with median FUV flux G 0 ∼ 900 (1.4 erg s −1 cm −2 ), so that photoevaporation of circumstellar disks is only important beyond 30 AU. Given the low interaction rates and modest radiation levels, we suggest that solar system disruption is a rare event in these clusters.
2010
We present a global study of low mass, young stellar object (YSO) surface densities in nearby (< 500 pc) star forming regions based on a comprehensive collection of Spitzer Space Telescope surveys. We show that the distribution of YSO surface densities in the solar neighbourhood is a smooth distribution, being adequately described by a lognormal function from a few to 10^3 YSOs per pc^2, with a peak at 22 stars/pc^2 and a dispersion of 0.85. We do not find evidence for multiple discrete modes of star-formation (e.g. clustered and distributed). Comparing the observed surface density distribution to previously reported surface density threshold definitions of clusters, we find that the fraction of stars in clusters is crucially dependent on the adopted definitions, ranging from 40 to 90%. However, we find that only a low fraction (< 26%) of stars are formed in dense environments where their formation/evolution (along with their circumstellar disks and/or planets) may be affected by the close proximity of their low-mass neighbours.
Monthly Notices of the Royal Astronomical Society: Letters, 2010
We present a global study of low-mass, young stellar object (YSO) surface densities ( ) in nearby (<500 pc) star-forming regions based on a comprehensive collection of Spitzer Space Telescope surveys. We show that the distribution of YSO surface densities in the solar neighbourhood is a smooth distribution, being adequately described by a lognormal function from a few to 10 3 YSOs pc −2 , with a peak at ∼22 stars pc −2 and a dispersion of σ log 10 ∼ 0.85. We do not find evidence for multiple discrete modes of star formation (e.g. clustered and distributed). Comparing the observed surface density distribution to previously reported surface density threshold definitions of clusters, we find that the fraction of stars in clusters is crucially dependent on the adopted definitions, ranging from 40 to 90 per cent. However, we find that only a low fraction (<26 per cent) of stars are formed in dense environments where their formation/evolution (along with their circumstellar discs and/or planets) may be affected by the close proximity of their low-mass neighbours.
Understanding Our Origins: Star Formation in H II Region Environments
Recent analysis of the decay products of short-lived radiounclides (SLRs) in meteorites, in particular the confirmation of the presence of live 60 Fe in the early Solar System, provides unambiguous evidence that the Sun and Solar System formed near a massive star. We consider the question of the formation of low-mass stars in the environments of massive stars, presenting a scenario for the evolution of a star and its disk as it is overrun by the ionization front at the edge of an expanding H ii region. The stages in this scenario include: (1) compression of molecular gas around the edge of the H ii region; (2) induced low-mass star formation in this compressed gas; (3) an "EGG" phase when a dense star-forming clump is overrun by the ionization front; (4) a "proplyd" phase during which the disk is truncated by photoevaporation; (5) a long-lasting phase during which a young star and its truncated disk evolve in the hot, tenuous interior of an H ii region; and (6) a phase when the ejecta from one or more nearby supernova explosions overruns the disk, injecting SLRs including 26 Al and 60 Fe. Most of these stages can be observed directly. The exceptions are stage (2), which must be inferred from the localization of low-mass protostars in compressed molecular gas near ionization fronts, and stage (6), which is an unavoidable consequence of the presence of low-mass protostars seen near massive stars that will go supernova within a few million years. (This differs from models in which the same supernova is responsible for both triggering the formation of a star and injecting SLRs.) This mode of star formation may be more characteristic of how most low-mass stars form than is the mode of star formation seen in regions such as the Taurus-Auriga molecular cloud. We discuss the implications of this scenario for our understanding of star formation, including the possible role of photoionization in limiting the masses of stars. We also discuss the implications of the young Sun's astrophysical environment for our understanding of the formation and evolution of the Solar System. These include the effects of intense UV radiation from nearby massive stars on the structure and chemistry of the disk, dynamical effects due to close encounters of the Solar System with other cluster members, and the role of the decay of SLRs in the evolution of the Solar System. We conclude that low-mass stars and their accompanying disks form and evolve differently near massive stars than they do in regions like Taurus-Auriga, and that these differences have profound implications for our understanding of our origins.
Young open clusters in the Galactic star forming region NGC 6357
Astronomy & Astrophysics, 2014
Context. NGC 6357 is an active star forming region with very young massive open clusters. These clusters contain some of the most massive stars in the Galaxy and strongly interact with nearby giant molecular clouds. Aims. We study the young stellar populations of the region and of the open cluster Pismis 24, focusing on their relationship with the nearby giant molecular clouds. We seek evidence of triggered star formation "propagating" from the clusters. Methods. We used new deep JHK s photometry, along with unpublished deep Spitzer/IRAC mid-infrared photometry, complemented with optical HST/WFPC2 high spatial resolution photometry and X-ray Chandra observations, to constrain age, initial mass function, and star formation modes in progress. We carefully examine and discuss all sources of bias (saturation, confusion, different sensitivities, extinction). Results. NGC 6357 hosts three large young stellar clusters, of which Pismis 24 is the most prominent. We found that Pismis 24 is a very young (∼1-3 Myr) open cluster with a Salpeter-like initial mass function and a few thousand members. A comparison between optical and infrared photometry indicates that the fraction of members with a near-infrared excess (i.e., with a circumstellar disk) is in the range 0.3-0.6, consistent with its photometrically derived age. We also find that Pismis 24 is likely subdivided into a few different subclusters, one of which contains almost all the massive members. There are indications of current star formation triggered by these massive stars, but clear age trends could not be derived (although the fraction of stars with a near-infrared excess does increase towards the Hii region associated with the cluster). The gas out of which Pismis 24 formed must have been distributed in dense clumps within a cloud of less dense gas ∼1 pc in radius. Conclusions. Our findings provide some new insight into how young stellar populations and massive stars emerge, and evolve in the first few Myr after birth, from a giant molecular cloud complex.
Protostars and Planets IV OBSERVATIONS AND THEORY OF STAR CLUSTER FORMATION
Young stars form on a wide range of scales, producing aggregates and clusters with various degrees of gravitational self-binding. The loose aggregates have a hierarchical structure in both space and time that resembles interstellar turbulence, suggesting that these stars form in only a few turbulent crossing times with positions that map out the previous gas distribution. Dense clusters, on the other hand, are often well mixed, as if self-gravitational motion has erased the initial fine structure. Nevertheless, some of the youngest dense clusters also show sub-clumping, so it may be that all stellar clustering is related to turbulence. Some of the densest clusters may also be triggered. The evidence for mass segregation of the stars inside clusters is reviewed, along with various explanations for this effect. Other aspects of the theory of cluster formation are reviewed as well, including many specific proposals for cluster formation mechanisms. The conditions for the formation of b...
Young Stars and Planet Formation
2016
Author(s): Haney, Laura | Advisor(s): Zuckerman, Ben | Abstract: Young stars represent important laboratories for studying stellar and planet formation. Furthermore, low-mass stars are not only the most common product of star formation, but also provide appealing conditions for the direct imaging of exoplanets. Determining youth in solar-type stars and low-mass stars can be challenging. I have examined young stars and the planet formation processes that occur around them from several different perspectives. First, I examined one of the most commonly-used age determination methods for solar-type stars - chromospheric activity. I compiled a large (~2,800 star) sample of stars with known chromospheric activity levels, calculated their ages, and searched for evidence of an infrared excess (indicative of a circumstellar debris disk) using data from the Wide Field Infrared Survey Explorer (WISE). I found that, while the distribution of stars with a strong infrared excess peaks at young ag...