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Star Formation current science issues

Star formation is one of the current hot topics in astrophysics, with particular emphasis on the implications for the occurrence of planets in the Milky Way and for cosmology (e.g. via the SN rate). More massive stars evolve faster and produce more violent winds and radiation which may prevent planet formation. Many YSOs and SFR in the Milky Way have been studied well enough to adopt chatacteristics for the timescale of star formation and [InitialMassFunction][IMF]], but these have some limitations:

• There are ambiguities between the age of a YSO and its mass;
• The metallicity and environmental effects are not systematically understood;
• Few or no investigations have dealt with all available data, but restrict themselves to one or two regions of the e-m spectrum.
• Star-forming regions are often very extended (arcmin, even degrees) with structure on all scales from the whole parent cloud down to individual protoplanets, and very dusty and obscured.
• Hence the properties of a given region can be described accurately in a statistical sense but there are usually many individual objects which are anomalous or unknown.

Cross-ID of YSO characteristics is often based on resolutions of several arcsec or worse. The ISO 15 um beam of 6" represents ~2500 au (0.01pc) at the distance of Orion. In the absence of high-res velocity or absorption information the ambiguity along the line of sight can be even greater. There are ~50 young stars within a 60" diameter region around the Trapezium, so the chances of two or more sources lying within the ISO beam is very significant. Convincingly systematic correlations of YSO properties do exist on such scales but this may be at least partly due to the clustering, reflecting the tendency of low-mass slow evolving and high-mass fast evolving YSO to form together. For example, methanol masers were intially found associated with IRAS hot cores. However blind surveys showed the masers also occur in much darker regions and high resolution studies show that there is often a large offset, suggesting that the maser and IR sources are separate, in which case these methanol masers can represent an earlier stage of evolution. This does not invalidate the association of methanol, IR cores and star formation, but it prevents a single precise characterisation of the object.