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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 1.30 FCRAO 240 FCRAO+BIMA 120 60 0 0 –120 –60 –240 240 BIMA HCO+ 1–0 120 Δδ (″) 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. CS 2–1 120 0 –120 –240 240 120 0 –120 –240 60 0 –60 60 0 –60 60 0 –60 Δα (″) 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 density 4.00 2.75 1.50 0.25 –1.0 121.9 beta 60.9 0 0 61.0 0.5 5.00 0.38 4.25 0.25 3.50 0.12 2.75 0.0 2.00 121.9 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. 1.31