Parallel Sessions: ISM and Star Formation

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Parallel Sessions ISM and Star Formation
Ive been asked to report on the parallel
sessions on the interstellar medium and star
formation. For me the high point of the meeting
was yesterday afternoon's session. It was kicked
off by Philippe Andre who gave the most concise
description I had heard of the direction that
infrared and submillimeter astronomy should be
heading He asked, What determines the Initial
Mass Function, IMF? What generates pre-stellar
cores? What are the time scales? Why and how do
protostellar regions collapse? What makes them
fragment? How is turbulence involved? The mass
distribution of pre-stellar condensations
resembles the IMF, which is at least in part
determined by cloud fragmentation. But what
generates pre-stellar condensations in the ISM?
Philippe then described a program with Herschel
and ALMA to at least partially answer these
questions.
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Pre-Stellar Cores
The next speaker was Ted Bergin, who sketched the
nature of pre-stellar coresinterstellar cloud
cores not yet complicated by the burdens of
collapse and, therefore, sufficiently simple to
be credibly modeled with means at hand today.
B68 is such a cloud. Its column density can be
judged, as Alves, Lada Lada (2001) first
showed, by its extinction of background sources.
From this density profile and radiative transfer
calculations, we can obtain a physico-chemical
model. We assume that CO, CN and CS are
tracers of the clouds surface composition, and
that NH3 and N2H are tracers of the bulk of the
cloud. We then model the chemical composition
and physical characteristics of the cloud as a
function of radial distance inward from the
surface, provided we can correctly estimate the
strength of the ambient surface radiation field,
thus bounding physical conditions. The cloud is
cold in its center, warm at the surface.
Herschel and ALMA, as Ted promised, should fill
in the anatomical details.
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The Promise of Spectroscopy
For a long time, we have been telling ourselves
that spectroscopy will lead to greater insight.
Pepe Cernicharo showed us how true this can be.
Using archival ISO data he showed us how he and
his colleagues extracted perhaps the most
beautiful FIR /SMM astronomical spectrum I have
seen (Gonzales-Alfonso et al., ApJ 613, 247,
2004). It portrays the ultraluminous infrared
galaxy Arp 220 in the lines of H2O, OH, NH, CH,
C II, NH3 and O I, in the wavelength range 50
180 ?m. This shows how much more can be
achieved if one uses interactive techniques and
knows how to proceed spectroscopically. None
of the machines we are constructing are simple.
This makes it difficult to extract the most
complete information from raw data and this is
likely to remain true both of Herschel and ALMA.
Pepe has shown us what we will need to do to take
full advantage of these two missions.
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Data Banks
Pepe, Eric Herbst and Karl Menten all stressed
the needs for greatly enlarged spectroscopic data
banks. We need to know atomic, molecular and
ionic level structures, bond strengths, and set
up compendia of collision cross sections and
reaction rates with likely ambient constituents.
Eric stressed that these need to be available
for insertion into plausible chemical models that
might take the form of master equations involving
gas/grain-surface reactions, or Monte Carlo
models, and take into account relevant
hydrodynamics and radiative transfer. None of
this is simple but thats the way it
is! Fortunately, as the posters showed, a number
of groups are already setting up various data
banks (Dubernet et al., Kerschbaum et al.,
Wakelam et al.), while others are providing
urgently needed laboratory data (Bruneleau, et
al., Chihara, et al., Hornekaer, et al., Koike et
al., and Suto, et al.)
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Laboratory Studies
A lack of laboratory studies is likely to
eventually impede interpretation of data that
will become available with Herschel and ALMA.
Though many colleagues, notably Xander Tielens,
have been actively seeking a greater
community-wide effort, much remains to be
done. It is, therefore, gratifying that
Kerschbaums group in Vienna has been conducting
laboratory studies of likely solids of
astrophysical interest, at low temperatures, in
the far-infrared, where we know little.
Forsterite, fayalite, frozen methanol,
carbonates, and graphite, all have absorption/
emission features in the Herschel/PACS spectral
band, providing us with new analytical means for
estimating ambient physical and chemical
conditions. Wil van Breugel spoke of experiments
he is conducting with high energy particle
bombardment. These suggest that interstellar
cosmic rays are responsible for amorphosizing
originally crystalline grains.
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Theoretical Chemistry
Eric Herbst gave a beautiful talk on progress in
chemistry, emphasizing how much remains to be
done if we are to derive full benefit from the
funds expended on Herschel and ALMA. Some
chemical reactions and physical characteristics
are too difficult to measure directly and need to
be calculated. The posters showed considerable
efforts underway by Faure et al., Juvela and
Padoan, Giuliano et al., Morata and Herbst,
Giacomo et al., Pulecka et al., Rapacioli et al.,
Staeuber et al., Talbi and Chandler, Vaidya and
Anandarao, Wickramasinghe and Wickramasinghe, and
Woitke. Unfortunately, there still is
insufficient funding for both laboratory studies
and theoretical efforts, and we are likely to end
up with observations on Herschel, SOFIA and ALMA
that we are unable to adequately place in
context.
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Deuterated Species
A number of speakers, and several posters spoke
of the. fractionation of deuterated species,
H2D, D2H, N2D, HDO, D2CO, CH2DO, CD2OH, ND, .
. . Poster by Lis et al., and Vastel et al.
showed that, by now, ND2H and ND3 have both been
detected, in abundance, though the precise ratio
of the two is not yet firmly established.
Presumably this will happen soon. The
fractionation is attributed to the reaction
H3 HD -gt H2D H2 where the H3 is
believed to arise in the diffuse interstellar
medium through the action of cosmic rays. By
successive deuteration molecular species like ND3
come into being. The expected destruction of H3
by CO should also be checked observationally.
With as much attention as has been paid to
deuteration in recent years, the reactions
involved may soon be reliably understood.
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Beautiful Images and Spectra
We saw some beautiful images, that drive home
with astounding clarity how complex some
star-forming regions are, and yet how much more
we know for seeing the pictures of them. Most of
the images came from the recent first release of
Spitzer data, which Tom Soifer presented. It is
hard to over-estimate how much we will learn not
only about Galactic star formation from these
pictures, but also the closely related processes
in nearby galaxies, and in the giant mergers that
abound at red shifts z1 to 2. The spectra that
Spitzer is providing, with the discovery of new
PAH features, and the sensitivity to reach out to
great distances across the Universe should also
contribute to our understanding of the local
interstellar medium and how it gives rise to
stars.
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Multispecies Spectra of Different Clouds or
portions of Clouds
We also saw beautiful spectra of individual
regions of complex clouds, showing the emission
and/or absorption features of different chemical
species. This is clearly a sensible approach.
However, we should be prepared to accept that
such multispecies investigations will have to be
applied to many thousands of different regions,
before we see a significant return. These
spectra are usually going to be difficult to
disentangle, since any given line of sight is
likely to cut through a region that is just as
complex along that sight-line as along any line
that would transversely cut across the maps we
now see Spitzer producing.
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New Odin Results
Ake Hjalmarson showed us a number of recently
compiled submillimeter spectra obtained with the
Odin satellite. Odin is a 1.1 meter telescope
and, like the smaller SWAS telescope that
preceded it, it has carried out pioneering
studies of water vapor and other species at
kilometer-per-second spectral resolving powers.
Unlike SWAS, Odin has also been able to obtain
spectra over sizeable wavelength ranges. Ake
showed us a portion of such a spectrum of the
Orion region from 550 to 557.5 GHz. He was
disappointed that he was able to explain every
one of the many spectral features the plot
exhibits. He had hoped for new features that
might reveal more. However, as Karl Menten
pointed out in his talk, the submillimeter range
is expected to swamp us with unexplainable data
from Herschel/HIFI and ALMA. So, perhaps we
should be happy to have found at least one
spectrum we understand.
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The Outflow from AGB Stars (I)
This morning we had exciting talks by Thibaut
Lebertre, Hans Olofsson and Tom Millar on the
outflow from AGB stars and their successors, the
proto-planetary nebulae. Among AGB stars only
IRC 10216 currently exhibits a useful number of
spectral features to permit detailed analysis of
physical conditions in the outflow. Herschel and
ALMA are likely to provide us with a much longer
list of chemical species, with sufficient numbers
of individual features for many of these to
permit reasonable modeling. With better data,
however, we will also need greatly improved
analytical tools so that the outflow may be
properly understood. Current mass loss estimates
for many of these stars are wildly divergent,
depend on a wide variety of different
assumptions, and prevent us from satisfactorily
estimating the rate at which heavy elements from
the outflows are enriching the Milky Way
interstellar medium.
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The Outflow from AGB Stars (II)
In this context, Id like to call attention to a
striking poster by Pulecka, Schmidt and
Szerba. Until about five years ago, it was
acceptable to fit the spectrum of a dust-shrouded
evolved star by postulating the superposition of
a variety of dust species, PAHs, very small
grains, and big grains, at different temperatures
and radial distances from the star. Though this
procedure provided good-looking fits, it often
violated laws of physics. Only slightly better,
was a fit that took radiative transfer into
account, but postulated a distribution of
circumstellar dust that again was arbitrary
except in yielding a good fit. About five
years ago, Moshe Elitzur and coworkers provided a
DUSTY code to couple radiative transfer and
dynamics, thus providing a physically more
plausible outflow. Pulecka et al. have now
added two more steps. They take into account
both gravity and chemical reactions in the stars
atmosphere and outflow. This, finally, is a
credible model, and should yield correspondingly
satisfactory insights.
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High-Mass Star Formation
Frederic Schuller and Vincent Minnier spoke
about very different approaches for finding sites
of high-mass star formation -- about which we
still know comparatively little. Minniers talk
reminded me of an old paper,Infrared and Radio
Appearance of Cocoon Stars, ApJ 148, 443, 1967,
that Kris Davidson and I wrote nearly forty years
ago. At the time, we were just beginning to fly
liquid helium-cooled telescopes with far-infrared
detectors on rockets and wondering what a newborn
star might look like before it had blown away the
cocoon of dust within which it was born? Kris,
then a first year graduate student at Cornell,
stopped by my office one day, and we decided wed
look into it. Clearly, the star would heat the
dust and wed see the far infrared radiation.
And Kris figured that a sufficiently massive star
would ionize the gas immediately around it and
emit free-free radiation. So we wrote this up and
sent it off. It is nice to see that a FIR
source associated with an ultra-compact H II
region still fits the prescription today. Even
simple ideas often take many decades to check out.
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Summary
The lessons I will be taking home with me from
this workshop are that the incredibly powerful
observatories we are now building and will soon
be using, are going to yield such enormous
amounts of data, that matters will become
considerably more complex before we can pick our
way through the trove of data and re-establish a
level of simplicity. It will be hard work but
also exhilarating. And when it is all over,
well understand the interstellar medium and star
formation in considerably greater detail than we
do today. Speakers after speaker seemed to agree
that ALMA and Herschel will force us to be far
more systematic in approaching our handling of
data than ever before. They will make us set up
vastly more comprehensive compendia of chemical
reaction chains and other properties of matter,
and will require us to install complex
computational modeling tools. Herschel and ALMA
will drastically change how we work!