The UKST H Survey - Scientific Background

The Astrophysics

Much of the interest in the survey thus hangs on questions of star formation rates and efficiencies, and the mechanisms by which star formation is triggered and/or propagated. To try to answer these questions, the first step, of course, is to collect the data with which to identify and map in detail the diverse types of star forming regions and their wider environments. Star formation might be expected to proceed in dense molecular clouds either because of instabilities within the clouds themselves, or because of triggering by external energetic events near the cloud -- ie. by sequential or self propagating star formation. Triggers for the star formation activity might include expanding ionisation regions around young star clusters (Elmegreen & Lada 1977), expanding stellar wind bubbles around clusters of O stars (Williams, Blitz & Stark 1995), supersonic protostellar winds or supernova shocks (McCray & Kafatos 1987). To understand the nature of star formation we therefore also need to identify and study regions where such triggering can occur. Since massive star formation leads to the production of HII regions, regions of star formation are readily located via their line emission, in particular that at H which thus provides one of the most direct optical indicators of star formation.

One example of how we might proceed is given by our extant CO surveys and the opportunities for further specific, high resolution CO survey observations. (See for example, the contribution on current -Orionis work by Lang & Masheder at this meeting). Given the velocity resolution of the CO data, and the generally well ordered velocity field in the molecular ISM, we will have immediate access to distance estimates and hence three dimensional positions for the star forming regions within the Milky Way disc.

Questions of Star Formation

Some major questions we might seek to investigate fall basically into one of two areas.
1. How does star formation (SF) proceed through a molecular cloud or cloud complex? Does SF occur sporadically throughout a cloud independent of the SF history, or does it spread systematically. Especially in the latter case, is the SF via collapse of cloud cores/clumps triggered by previous activity of the types discussed in the previous paragraphs. (See Williams et al. (1995) for an excellent recent discussion of these problems with particular regard to the Rosette Molecular Cloud and neighbouring Rosette Nebula.) Once we have a catalogue of star forming regions we can study their physical make up by making comparisons between the distributions of the molecular gas (eg. Oliver, Masheder & Thaddeus 1996, Oliver 1996) the ionised gas, as seen in H, and other indicators of interstellar gas and/or star formation activity, in particular neutral hydrogen (eg. Burton 1985 Stark et al. 1992), or IRAS far infra-red flux (Wouterlout & Brand 1989). Structures that would be of especial interest include shells of H, HI or CO around SF regions, or H seen outside CO clouds due to background ionisation.
2. Which parameters within or external to a molecular cloud determine its star formation efficiency (SFE), that is the fraction of the available mass which is turned into stars? Internal parameters obviously include mass, velocity dispersion and density, and the radial density profile, all obtainable from the CO meaurements. The external influences include neighbouring, or even relatively distant, ionising sources, visible in H, which may alter the ionisation temperature (Elmegreen & Lada 1977), and the interaction with surrounding HI envelopes (Wannier et al. 1991), since the ram pressure is comparable to the self gravity of a typical cloud. An important point in favour of external influences would be regular gradients in SFE across complexes (Williams et al. 1995). We can obviously attempt to use the ratio of H to CO emission as a measure of the mass of young stars compared to the mass of molecular material and hence of the SFE. Although precise quantitative measurements may be difficult because of extinction in the clouds, the observed ratio should certainly provide good qualitative and relative estimates of SFE.

© Copyright Astronomical Society of Australia 1997