Over the past few decades, participant researchers have contributed significantly
to our understanding of violent star formation (VSF) on a wide range of
spatial scales, using both
theoretical and observational approaches. The following, thoroughly discussed objectives
are precisely those where
an increased synergy will combine all of our existing strengths and
provide the impetus for major steps forward in our understanding of the very
process of VSF, clearly underscoring the need for a combined effort.
Although the objectives cover a very wide area, we hope to be able to advance substantially
around these topics during the workshop. We also envisage that the workshop will strengthen
collaborations for years to come.
A. Interactions, feedback, non-linear effects.
A.1: Understanding the radiative, chemical, and
energetic drivers of VSF. Massive and intermediate-mass stars, as well as
supernovae (SNe), are responsible for the bulk production of radiation,
chemical elements and mechanical energy released in the process of VSF.
Network researchers are among the world-leading experts in stellar evolution,
nucleosynthesis, and stellar atmospheres. We will develop new and improve
existing models of stellar structure and evolution by including the effects of
the transport of angular momentum and chemical species by rotation, gravity
waves, and magnetic fields, which are as yet poorly understood. These will be
tested rigorously in the coming months by direct comparison to observations - with 4-10m-class
telescopes, to some of which we have preferred access - of accurate stellar
parameters, surface abundances, rotational velocities, etc. The resulting
state-of-the-art models will be included in and made available to the
community through the ``Legacy Tool'', described in Topic E.
A.2: Star Formation Feedback on Galactic Scales.
Stellar feedback on galactic scales, such as traced by galactic winds and
recurring SF in gas-poor environments, can act positively, i.e. through the
triggering of additional SF on larger scales, as well as negatively, by
self-regulating or even suppressing further SF. At present, it is unclear
under what conditions which of the competing processes dominate in galactic
environments, and to what extent the associated feedback controls the
evolution of galaxies. To understand these large-scale feedback effects is
essential for our understanding of the early formation of galaxies and their
subsequent evolution. We will begin to undertake a detailed study of
the feedback efficiency for different morphological types of galaxies in
relation to their environment and the dominant mode of SF.
A.3: The Parametrisation of Stellar Feedback. To
quantify how stellar feedback regulates SF and chemical enrichment, a key
issue in contemporary astrophysics, we need to understand the detailed physics
of massive stars and their interactions with their environments. Among the participants to the workshop,
some have individually played pioneering roles in defining the stellar
wind parameters of massive stars and their evolutionary paths as a function of
metallicity. We will pool this expertise to develop accurate
evolutionary synthesis models which predict stellar wind feedback parameters
as a function of evolutionary state and metallicity (A.1). We will
then test these new models against observations of the interactions with their
environments of individual stars, and of young massive clusters in nearby and
distant galaxies. The overall strategic objective will be to quantify the
efficiency of the stellar wind feedback process as a function of metallicity
and thereby provide a key input parameter for cosmological models.
A.4: Follow energy pathways from young clusters to
the ISM. Massive stars and star clusters have a very strong impact on their
environment. As the ISM is the reservoir from which subsequent generations of
stars may form, it is crucial to understand how, when and where stellar energy
is released, and how the ISM reacts. A large variety of observational
approaches will have to be used to assemble a consistent description of the
impact of massive stars on the ISM. The workshop brings together scientists
highly versed in modeling gas and dust physical processes, and thus creates
the capacity to draw the most from the observed diagnostics. It is a very
timely goal to organise such a workshop and the collaborations that no doubt will
follow, since soon we will be flooded with
much more data from new ground and space-based observatories. Our results will
be placed in a larger perspective to understand the impact of star formation
on galactic scales (A.2), and consequently the possibility for a
galaxy to sustain extended episodes of VSF. This will be a major piece in the
construction of a scenario for the first episodes of SF in primordial
galaxies.
B. Stellar population synthesis models, stellar evolution
and stellar atmospheres
including W-R and cold stars at high resolution.
B.1: Chemo-dynamical and spectral synthesis models
of galaxy evolution. Possible participants are among the world leaders in the
development of integrated spectral synthesis models and chemo-dynamical models
of galaxy evolution, which are keys to the interpretation of the star
formation (SF) and abundance patterns observed on scales of galaxies and
galaxy clusters, at any redshift. We will provide much needed updates and
crucial improvements to the input physics of these models, in particular
related to the interaction processes between SN ejecta and the surrounding
medium, by including the realistic treatment of thermal conduction and
turbulent mixing in 3D codes. By including high-resolution spectral synthesis
models, combining the state of the art of stellar evolution and stellar
atmosphere models, nebular emission and radiative transfer within dusty media,
we will ensure major advances in the accuracy of our modeling techniques.
High-resolution spectral synthesis models are fundamental to constrain the
extinction properties of young populations, and of the age mix of composite
populations, and thus to obtain a correct determination of recent SF
histories. These models will form part of the Legacy Tool (Topic E).
B.2: The Stellar Atmospheres Laboratory
The creation of a center devoted to the calculation of model stellar atmospheres and to
the analysis of stellar spectra (the Stellar Atmospheres Laboratory)
has two main goals: the addition of new numerical techniques and physical phenomena
(e.g equation of state and treatment of convection), and to
create updated stellar tools for the analysis of stellar populations.
The activities of the Laboratory will therefore be naturally linked to
B.1 and E (the Legacy tool).
The Laboratory has already installed and running several relevant codes
(Kurucz ATLAS9 AND SYNTHE, Lanz and Hubeny TLUSTY, and the LTE codes by Auer) and
is developing extended calculations to widen the parameter range (e.g. the opacities and the wavelength reach).
The goal now is to abandone some of the ``classical approximations'' and to incorporate
new codes through existing or new collaborations
in order to be able to compute realistic models and spectra
of early-type stars with strong winds and of late-type stars with extended atmospheres,
arbitrary abundances, cool stars, non-static atmospheres, etc.
The incremented computer power that the Laboratory will need,
in common with the Legacy Tool, will also be available to other groups
with similar needs in our Institute.
C. Initial Mass Function and HR diagrams, modelling of star
formation in clusters
and as individual stars.
C.1: The Initial Mass Function in VSF Regions. The
proportion of low to high-mass stars at the epoch of SF (the Initial Mass
Function; IMF) appears to be almost universal in quiescent galactic disk
regions. However, an increasing body of observational evidence suggests that
the IMF in VSF environments may be deficient in low-mass stars. Our
proposed participants were among the first to address this issue robustly in young
(``super'') star clusters. Preliminary results indicate that the IMF may
indeed be dependent on the environment and on the mode of SF. If so, this will
have far-reaching consequences for our understanding of the very process of SF
itself and for the evolution and assembly histories of galaxies. We will
investigate the shape of the IMF in VSF regions, by means of high-resolution
spectroscopy on 8m-class telescopes, to establish any environmental dependence
of the IMF robustly and conclusively, thus providing an important impetus to
improve significantly our understanding of the poorly understood process of SF
itself.
C.2: Galactic star formation histories from
resolved stellar populations. We will attempt to study the SF histories
of nearby dwarf galaxies in great detail, using (synthetic vs. observational)
colour-magnitude analyses, based on Hubble Space Telescope (HST) and Very
Large Telescope (VLT) observations. This technique is significantly more
accurate than the use of integrated spectra (due to the age-metallicity
degeneracy) and allows for a better quantification of both the stellar
evolutionary life-times and the shape of the IMF in quiescent galactic disks
(cf. C.1). The spatial dependence of the SF episodes within a given
galaxy (and thus of the SF efficiency on the environment; see D.1)
will be derived in order to test the predictions of chemo-dynamical models
(B.1). Observational evidence suggests that significant populations
of, predominantly, proto-globular clusters may form in violently interacting
galaxies. Studies of the relative ages of the globular clusters in our own
Galaxy suggest that they may be divided into a group of metal-poor clusters
that was likely formed around the time of the formation of the Galaxy, and one
of slightly younger ``thick disk'' clusters. We will study the connection (if
any) between this second group and the thick disk by analysing their chemical
composition and age, and will begin to address the question of whether the population
of thick disk globular clusters in the Galaxy may have originated in the same
violent phases that created the thick disk itself, leading to the temporary
suspension of SF in the Galaxy, i.e., before the formation of the thin disk.
D. Triggering cluster formation, environment, super-massive nuclear starclusters.
D.1: Star Formation in Isolated vs. Interacting
Galaxies. One of the most fundamental questions yet to be resolved thoroughly
is whether SF is a universal process, scaled in intensity and/or efficiency
from the continuous SF in spiral galaxies to starbursts in dwarf/irregular
galaxies and - by up to another 2 orders of magnitude in efficiency - to SF
in massive interacting galaxies, with Ultraluminous IR Galaxies (ULIRGs) being
the most extreme cases, or - alternatively - whether a discontinuity between
``normal'' and ``violent'' SF exists instead. The very reasons for the
differences in SF efficiency are yet to be fully understood, i.e., whether
they are due to the difference in total mass, metallicity, or related to the
dynamics of galactic interactions that give rise to large-scale compression,
gas flows and shocks. A concerted effort is required to observe, model and
interpret the variety of SF modes in a robust and consistent way. The relative
amount of SF going into compact star clusters, as opposed to more diffuse SF,
was recently shown to hold a decisive clue by some of our colaborators, and this
Topic is therefore closely related to C.2. In the early
Universe, starbursts appear to have been the rule rather than the exception.
Understanding VSF on galaxy-wide scales hence is a crucial prerequisite for
cosmological structure formation models.
D.2: Violent Star Formation versus Nuclear
Activity. Observational evidence suggests both direct and indirect
connections between VSF and nuclear activity. However, many crucial open
questions remain, including the frequency and connection between interactions,
starbursts and AGN activity, the order of these events, whether ULIRGs are an
evolutionary phase prior to the QSO phase, whether there is a relationship
between starbursts and black hole properties, and whether the AGN will evolve
in parallel to the starburst. Elucidating the relationship between intense
star formation and the AGN phenomenon is crucial to our understanding of the
formation of galaxies and the supermassive black holes in the early Universe.
Our strategic goal is to understand the sequence of events that lead to the
formation of QSO activity, and to place galaxies with nuclear activity in the
context of galaxy formation/evolution.
E. A `Legacy Tool' with modules of databases, models,
data mining,
machine learning, statistics and visualization.
E.1: Production of a Legacy Tool for evolution
modeling. The Legacy Tool, as a meta-objective, will act as a strongly federative
object as it will collect models built and data gathered and it will deliver the results back to the workshop
for the participants
to validate them. The plan is to provide the astronomical community with a long overdue
open-source software tool incorporating theoretical
high-resolution synthesis models of stellar populations and atmospheres, chemical evolution,
abundance determination, chemo-dynamical and hydrodynamical models (i.e.,
including the results from A.1, B.1 and B.2). The tool will consist of
modules covering basic data and computational components (theoretical models,
observational data access, and machine learning - including new, highly
efficient optimisation and pattern recognition algorithms, thus allowing us to
adequately deal with large databases), analysis components (statistics, data
mining), and a user-friendly graphical interface. This will - for the first
time - allow a comprehensive and self-consistent evaluation of the physics of
VSF over a large range of wavelengths, spatial and temporal scales, as well as
of the effects of simplifying assumptions and of feedback from the
interstellar medium (ISM). The various models developed by the different
participants only address a limited number of these topics. The
increased synergy of the workshop is imperative to obtain a consistent solution
to the overall problem of the modeling of stellar populations. This Topic
and E.2 are the ones in which our Computer Science colleagues will have the strongest input.
They will also be those with which we will be able to reach the Community as a whole both academicaly
and as an exercise of Public Outreach.
E.2: Visualization of the results obtained with
the Legacy Tool. This Topic will help us to visually understand the results,
and at the same time will constitute a powerful medium of communication and teaching.