Cool Stars 14 - Submitted Abstract # 324
This version created on 05 October 2006

Cool Stars in Hot Places:  The Role of Environment in the
Formation of Low Mass Stars and Planetary Systems


S. Tom Megeath, University of Toledo

It is well established that cool stars form in a range of
environments. In contrast to young hot stars, which are almost
always found in dense clusters, cool stars form both in relative
isolation within cold dark clouds as well as in dense clusters
with hundreds of low and high mass stars. In all of these
environments, cool stars are thought to dominate in both number
and total stellar mass. This range of environments raises
interesting questions regarding the formation of cool stars. How
does the environment affect the properties of the nascent stars?
Does environment influence the initial conditions of planet
formation (or prevent planet formation) by perturbing primordial
disks around cool stars? Is there a "typical" environment for
cool star formation? In what environment did our sun form in?

There are good reasons to believe that environment plays a
significant role in both the formation of low mass star and in
the subsequent formation of planets around these stars.
Numerical simulations of the formation of clusters predict that
dynamical interactions between protostars are important as the
protostars compete to accrete gas from the surrounding cloud.
During the pre-main sequence phase, dynamical interactions may
influence planet formation by stripping disks around young
stars. Radiation from hot stars may play an even more important
role. HST and Spitzer observations have shown cool stars forming
in pillars of molecular gas sculpted by UV radation from
neighboring OB stars. Once these stars are enveloped by the HII
region, HST observations and theoretical analyses have shown
that the UV radiation can evaporate the circumstellar disks,
eroding away the outer disks. The effect of hot massive stars on
cool star and planet formation is given particular importance by
recent analysis of short-live radionuclides in meteorites. These
results provide strong evidence that the Sun and Solar System
formed in close proximity to a massive star that went supernova
very near the time of the Sun's formation.

The role of environment is becoming an essential topic for any
discussion on cool star formation. Recent progress on this topic
is being driven in large part by observations from Spitzer and
the other Great Observatories. Spitzer has completed surveys of
ten molecular clouds in the nearest 1 kpc, and can identify both
clustered and isolated young stars by the presence of infrared
excesses from circumstellar disks and/or envelopes. These
observations are being complemented by Chandra observations
which identify young cool stars through their elevated X-ray
emission. With their ability to identify isolated stars, these
two observatories are providing information on the demographics
of cool star formation, which, unlike previous near-IR surveys,
are not biased towards dense clusters. These results are
complemented by a decade of HST observations which show the
impact of hot stars on cool star formation and early evolution.
The goal of this splinter session is to discuss our current
understanding of the demographics of cool star formation, the
influence of environment on the star and planet formation
implied by these demographics, and the most likely formation
environment of the sun. The hope is to move toward a consensus
on the relative importance of isolated vs clustered star
formation (at least at the solar circle in the current epoch),
and how much the typical low mass star is affected by its
environment (e.g., what are the typical fluxes of UV radiation
impinging on young stars and forming solar systems, and what
effects do they have). 

Invited contributions:

Star Formation in Orion and Perseus: Lessons from the Nearest Common Environments
John Bally, University of Colorado, Boulder

Observations demonstrate that most stars are born in short-lived 
clusters or multiple star systems. Many form within a few parsecs 
of massive stars which produce intense UV radiation and die in 
supernova explosions. Thus, during their first few million years, 
forming planetary systems are likely to experience dynamical 
perturbations from passing sibling stars, become exposed to intense 
UV radiation fields, and be peppered by supernova ejecta containing 
short-lived radioactive species. I will discuss some recent 
observations that put new constraints on massive star formation in 
Orion, explore the impacts of nearby massive stars on the circumstellar 
environments and disks surrounding low-mass stars, and the importance 
of protostellar feedback in star formation. 



The influence of the star forming environment on planet formation: Theory
Fred Adams, University of Michigan

This talk outlines posssible effects of the cluster environment on solar 
systems forming within them. The first task is to understand the dynamical 
evolution of the cluster themselves. This evolution, in turn, depends on the 
pre-cluster initial conditions, the star formation efficiency, and the gas 
removal history. For clusters of intermediate size, with N = 100 -- 3000, the 
dynamics are highly chaotic and multiple realizations of equivalent cases 
(typically 100 simulations per initial condition) must be carried out in order 
to build up a robust statistical description of the results, e.g., the 
probability distribution of closest approaches, the mass profiles, and the 
probability distribution for the radial locations of cluster members. These 
results provide a framework from which to assess the effects of groups/clusters 
on star and planet formation. The distributions of radial positions can be used 
in conjunction with the probability distributions of the expected FUV 
luminosities to determine the radiation exposure of circumstellar disks. These 
radiation levels can then be used in conjunction with photoevaporation models 
to assess the damage inflicted on circumstellar disks. The distributions of 
closest approaches can be used in conjunction with scattering cross sections to 
determine the probability of disruption for newly formed solar systems. This 
talk discusses how the interaction rates, radiation levels, and corresponding 
odds of disruption vary with cluster size N. 


The Birth Environment of the Sun:  Low-mass Star Formation Near Massive Stars
Jeff Hester, Arizona State University

Recent studies of the decay products of short-lived radionuclides in 
meteorites, in particular the confirmation of the presence of live Fe-60 in the 
early Solar System, provides unambiguous evidence that the Sun and Solar System 
formed near a massive star that went supernova very early in the process of 
planet formation. This environment is very different from that typically 
assumed for the young Sun. We consider the question of the formation of 
low-mass stars in environments modified by massive stars, presenting a scenario 
that links observational phases in the early lives of such objects into a 
proposed evolutionary sequence. This sequence begins with the likely triggering 
of low-mass star formation in gas compressed by and expanding HII region, and 
ends with truncated protoplanetary disks exposed to intense radiation from 
nearby massive stars, and possibly to the effects of nearby supernovae. We also 
discuss some implications of the astrophysical environment of the young Sun on 
the properties of our planetary system. 


Spitzer surveys of molecular clouds and the formation environment of low mass stars
S. T. Megeath, University of Toledo

The Spitzer space telescope is a powerful tool for studying the birth 
environment of young stars. With its capability to rapidly survey large areas 
and detect mid-infrared emission from dusty disks and envelopes around young 
stars, Spitzer can efficiently map the distribution of young stars with disks 
and protostars in molecular clouds. This talk reports on Spitzer observations 
of a sample of molecular clouds within 1 kpc of the Sun. The first topic will 
be the relative importance of clustered and isolated star formation. From an 
examination of three molecular clouds, Orion A, Orion B, and Ophiuchus, we find 
that approximately 60% of the stars with disks and protostars are found in 
large clusters, 15% are found in smaller clusters and groups, and 25% are found 
in relative isolation. The assumptions and uncertainties in this analysis will 
be addressed. The talk will then focus on the role environment may play in star 
and planet formation. We find that less than 50% of the stars with disks in our 
sample of molecular clouds are found in HII regions where they are exposed to 
strong far UV radiation. Furthermore, the stellar densities in even the densest 
clusters are insufficient for dynamical interactions to disturb a large 
fraction of the disks and influence planet formation, at least on a < 100 AU 
scale. However, the densities of protostars in some regions is sufficient for 
interactions between protostars. We present preliminary evidence that the 
luminosity of protostars increases in crowded regions. 



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