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VITI, WILLIAMS: MEETING REPORT
Recent advances in
star-formation studies
S
tar formation is one of the most active
areas of astronomy today. It is a key to
understanding not only the evolution of the
Milky Way, but also the immense luminosities
of starburst and merging galaxies and the emission from high-redshift galaxies. Inevitably,
most work on star formation focuses on the
Milky Way, as here we have a grandstand view.
Star formation is a wide-ranging subject. It is
observationally driven, but requires a detailed
understanding of the formation and destruction
of the tracers by which we follow the evolutionary process. Numerical simulation, too, has the
capability to illuminate the large-scale behaviour in star-forming regions. All these topics
were covered in a well-attended RAS Discussion
meeting on 8 October 2004.
Presentations were given on observations of
low and high-mass star-forming regions, on the
chemical/dynamical modelling of such observations, and on numerical simulations of hydrodynamic and magnetohydrodynamic fluids. The
discussions generated by these presentations
were often challenging and confrontational.
This is evidently one of those areas of science in
which individual attitudes still matter, and challenges to those attitudes raise high passion.
Wrong results right
Star formation occurs in molecular clouds. By
processes that are still a matter of intense
debate, relatively low-density molecular gas
(~103 H2 cm–3) is converted into small dense
clumps in which the density is at least a hundred
times greater. Josep-Miquel Girart (CSIC
Barcelona) presented a detailed observational
study of one particular molecular cloud known
as L673. His single-dish observations showed
the cloud in low resolution to have a segregation of the CS and NH3 tracers, with the NH3
contours much more confined than those of CS
(the “wrong” result, based on a consideration
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David A Williams and Serena Viti
survey the contributions made
to this fast-moving field at a
recent RAS Discussion Meeting.
The resulting debates were
often both challenging and
confrontational.
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1: Contour maps of the emission in HCO+ (1–0) and CS (2–1) transitions from part of the molecular
cloud L673. The figures on the right show single-dish (FCRAO) and array (BIMA) maps and their
composite maps. It is evident that although the BIMA maps detect only around one tenth of the total
emission, the structure revealed in the BIMA maps is recovered and emphasized in the composite
maps (Morata, Estalella and Girart 2004). We conclude that this cloud is clumpy, on a small scale.
of the emission properties of the two molecules).
This anomaly was interpreted by Taylor,
Morata and Williams a decade ago as caused by
time-dependent chemistry occurring in unresolved transient clumpy structure within the
cloud. Girart presented recent results from the
radio interferometer BIMA of a portion of the
same cloud showing – for the first time – the
existence of the predicted, previously unresolved, structure. This structure is necessarily
transient, and has a size scale less than ~0.08 pc.
A more recent study by Morata, Estalella and
Girart (2004) combines both single-dish and
array results and confirms this picture (figure 1).
Evidently, this cloud is a dynamical region filled
with clumps that are formed and destroyed on a
time scale ~1 My. The most massive of the
clumps in the spectrum of masses deduced by
Girart et al. is comparable to its virial mass, suggesting that this population may merge with the
population of objects capable of further collapse
to form pre-stellar cores, though this connection
remains to be firmly established. Nevertheless,
the demonstration that this cloud is dynamic on
these temporal and spatial scales must surely be
relevant to the onset of star formation. It is sur-
prising that few other observers have followed
Girart and collaborators in these high-resolution studies that reveal so much about regions
of low-mass star formation.
The mechanism by which low-density gas is
converted into high-density cores is still controversial. Some authors rely on an ill-defined
hydrodynamical turbulence. More recently,
however, MHD-related effects have been
explored. Falle and Hartquist (2002) showed
that where magnetic pressure dominates gas
pressure (so-called low β cases) the passage of
slow shocks can create substantial density
enhancements (figure 2). In one of the most
important theoretical advances presented at the
meeting, Andrew Lim (Leeds) showed that these
low β cases (and the consequent inhomogeneities) arise naturally as disturbances pass
through a magnetized medium. In 2-D simulations (Lim, Falle and Hartquist 2004) Lim
showed that structures such as giant molecular
clouds (GMCs) should arise from low-density
magnetized gas, and that denser cores would
similarly appear within the subclouds of the
GMC. This result enables a somewhat more
consistent dynamical/chemical modelling of
A&G • February 2005 • Vol. 46
VITI, WILLIAMS: MEETING REPORT
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star-forming regions to be made.
Garrod’s chemical simulations on these lines
were presented by Jonathan Rawlings (UCL),
who showed that these recover many of the
small-scale features and their properties detected
by Girart and collaborators. In contrast to these
theoretical speculations, it was especially
refreshing for everyone at the meeting to hear
Derek Ward-Thompson’s (Cardiff) review of the
observed initial conditions for low-mass star formation. This brought a useful sense of the physical reality into the discussion. These conditions
are fairly constrained, and present a well-posed
challenge to theory of low-mass star formation.
Hot core chemistry
The formation of massive stars (as opposed to
stars like the Sun) is rare and dramatic. It can be
studied through the short-lived remnants of the
core not incorporated into the massive star.
These so-called hot cores contain within them
the evaporated material of ices deposited during
the collapse, and observations presented by
Helen Thompson (Hertfordshire) show a very
interesting chemical structure. It is a goal of
astrochemistry to be able to use this structure to
infer the physical conditions during the collapse.
A further useful tool is to study the formation of
deuterated species in such objects, as Mark
Roberts (Manchester) described. The fundamental D/H ratio in the galaxy is about 10–5, yet
in hot cores D can replace H in some molecules
to such an extent that – for example –
NH2D/NH3 can be ~0.1, a “fractionation” by a
factor of some ten thousand. To find such results
in hot cores where the temperature is raised by
the presence of the nearby hot star was a surprise, and the implication for massive star formation is that the high fractionation reflects a
long period at low temperature in the pre-stellar
A&G • February 2005 • Vol. 46
temperature
2: An axisymmetric simulation of a spherical cloud that
has collapsed under a higher external pressure. β is the
plasma parameter (gas pressure/magnetic pressure),
initially unity in the cloud. Axis scales are in parsecs, run
time in Myrs and the colour scale refers to the
temperature panel. Due to the magnetic field, the fast
magnetosonic speed in the cloud is highest in the
direction perpendicular to the field lines. This allows a
pressure-driven shock to propagate much faster in this
direction and leads to a slightly (and somewhat counterintuitive) prolate shape during the collapse, despite the
main flow onto the cloud being confined by the magnetic
field to be roughly “vertical” with respect to the field lines
(see the velocity arrow plot, top right). A combination of
the “horizontal” compression of the cloud and atomic
cooling leads to the interior of the cloud becoming
strongly magnetically dominated during the collapse
(β~0.01). The temperature and density in the cloud are
such that the gas is thermally unstable at this point. At
later times β increases again due to equalization of the
gas pressure along the field lines.
phase, during which the freeze-out of heavy molecules (such as CO) is nearly complete, so that
the fractionation-enhancing reactions dominate.
Stellar discs are expected to develop around
most newly formed stars. These are the sites of
planet formation and the focus of much current
observational attention. Glenn White (Kent)
mapped methanol emission around one particular disc and found the emission to be exceptionally extended in space. He interpreted this as a
consequence of selective desorption of that molecule from clathrates, as proposed by Scott and
Allamandola a decade ago. This work, and the
work on hot cores, is a powerful reminder that
progress in understanding observations of star
formation can only be made with the aid of
appropriate laboratory studies of the microscopic processes occurring in the gas and ices (as
has been emphasized by the recent work of Martin McCoustra and colleagues (Nottingham) on
desorption from low-temperature mixed ices).
In principle, numerical hydrodynamic or magnetohydrodynamic simulations should throw
light on the entire process of star formation from
diffuse gas to the inference of the initial mass
function of stars. In practice, however, these calculations are fraught with difficulties. The two
main numerical approaches that are commonly
used are the adaptive mesh refinement (AMR)
and the smoothed particle hydrodynamics (SPH)
techniques. Richard Klein (Lawrence Livermore
Laboratories and University of California at
Berkeley), an exponent of the AMR methods,
explored in some detail the limitations of SPH
by comparison to his own calculations. Matthew
The formation of massive stars
can be studied through the shortlived remnants of the core not
incorporated into the massive star
Bate (Exeter), who uses the SPH approach,
robustly defended his methods, and a spirited
debate ensued that was much enjoyed by those
who had not invested much intellectual capital
in such work. Perhaps the message that most
people took away was the important conclusion
that one should look sceptically at all large
hydrodynamic or MHD simulations. For example, one might note that much of the relevant
physics has as yet to be generally included in any
of these huge, numerically complex, and very
time-consuming computations. One might also
note that the calculations of Lim, Falle and
Hartquist, which do attempt to be more realistic
in terms of the microphysics, suggest that the
gross macroscopic behaviour of the gas is greatly
modified by its microscopic properties. The
safest conclusion at present seems to be that we
have not yet arrived at that happy state when we
can safely rely on the results of these truly massive calculations to infer the nature of star formation. Some may have left the meeting with
concerns about one or both of the AMR or SPH
techniques, but perhaps the most encouraging
sight at the end of the meeting was that of Klein
and Bate sitting down together, deep in conversation, planning test calculations that may
resolve the differences of opinion between them.
In summary, it was a compelling and interesting day in which many important physical and
technical issues were raised – and which showed
that forthright debate still has its place in science. There is no doubt that star formation is
one of the most exciting areas of astronomy
today and – melding as it does the macroscopic
and microscopic worlds – one of the most challenging and important to astronomy. ●
David A Williams and Serena Viti, Dept of Physics
and Astronomy, University College London.
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