Reduction and analysis techniques for infrared imaging data

Infrared detector arrays are becoming increasingly available to the astronomy community, with a number of array cameras already in use at national observatories, and others under development at many institutions. As the detector technology and imaging instruments grow more sophisticated, more attention is focussed on the business of turning raw data into scientifically significant information. Turning pictures into papers, or equivalently, astronomy into astrophysics, both accurately and efficiently, is discussed. Also discussed are some of the factors that can be considered at each of three major stages; acquisition, reduction, and analysis, concentrating in particular on several of the questions most relevant to the techniques currently applied to near infrared imaging

Binary stars in young clusters: models versus observations of the Trapezium Cluster

The frequency of low-mass pre-main sequence binary systems is significantly lower in the Trapezium Cluster than in Taurus-Auriga. We investigate if this difference can be explained through stellar encounters in dense clusters. To this effect, a range of possible models of the well observed Trapezium Cluster are calculated using Aarseth's direct N-body code, which treats binaries accurately. The results are confronted with observational constraints. The range of models include clusters in virial equilibrium, expanding clusters as a result of instantaneous mass loss, as well as collapsing clusters. In all cases the primordial binary proportion is larger than 50 per cent, with initial period distributions as observed in Taurus-Auriga and the Galactic field. It is found that the expanding model, with an initial binary population as in the Galactic field, is most consistent with the observational constraints. This raises the possibility that the primordial group of OB stars may have expelled the cluster gas roughly 50 000 yr ago. The cluster's bulk expansion rate is thus a key observable that needs to be determined. The other models demonstrate that the rapidly decreasing binary proportion, its radial dependence and the form of the period distribution, together with structural and kinematical data, are very useful diagnostics on the present and past dynamical state of a young cluster. In particular, kinematical cooling from the disruption of wide binaries is seen for the first time.Comment: 34 pages, 13 figures, LaTeX, uses own_sngl.sty, accepted by New

The OMC-1 molecular hydrogen outflow as a fragmented stellar wind bubble

We present new images of the OMC-1 molecular hydrogen outflow, made using long-slit spectroscopy in order to accurately subtract the underlying continuum emission. These images reveal an extremely clumpy, quasi-spherical inner shell that breaks up at larger radii into bow-shocks and trailing wakes in the north-west, as originally described by Allen & Burton (1993); a fainter counter-finger to the south-east is newly discovered in the present data. While the outflow appears to be broadly bipolar, this is probably due to an interaction between an initially spherical wind from the source and a large-scale density enhancement surrounding it, rather than direct collimation imposed close to the source. The clumpy appearance of the inner shell confirms the prediction of the recent model of Stone, Xu, & Mundy (1995), in which a spherical and time-varying wind fragments a swept-up shell, producing high-velocity shrapnel, which in turn drives bow-shocks into the surrounding gas, resulting in the observed "fingers". As an alternative to the single varying source proposed by Stone et al., we speculate that several young sources in the BN-KL cluster may have been responsible for first sweeping up the shell and then fragmenting it.Comment: 21 pages, 5 figures, in press (Astron. J., 1997 January), uses aasms4.sty, also available at http://www.mpia-hd.mpg.de/MPIA/Projects/THEORY/preprints.html#maclo

Interpreting the Mean Surface Density of Companions in Star-Forming Regions

We study the interpretation of the mean surface density of stellar companions as a function of separation (or the two point correlation function of stars) in star-forming regions. First, we consider the form of the functions for various simple stellar distributions (binaries, global density profiles, clusters, and fractals) and the effects of survey boundaries. Following this, we study the dependencies of the separation at which a transition from the binary to the large-scale clustering regime occurs. Larson (1995) found that the mean surface density of companions follows different power-law functions of separation in the two regimes. He identified the transition separation with the typical Jeans length in the molecular cloud. However, we show that this is valid only for special cases. In general, the transition separation depends on the volume density of stars, the depth of the star-forming region, the volume-filling nature of the stellar distribution, and on the parameters of the binaries. Furthermore, the transition separation evolves with time. We then apply these results to the Taurus-Auriga, Ophiuchus, and Orion Trapezium star-forming regions. We find that while the transition separation in the Taurus-Auriga star-forming region may indicate a typical Jeans length, this is not true of the Orion Trapezium Cluster. We also show that there is no evidence for sub-clustering or fractal structure in the stars of the Orion Trapezium Cluster. This is consistent with the fact that, if such structure were present when the stars formed, it would have been erased by the current age of the cluster due to the stellar velocity dispersion. [Abstract abridged]Comment: Accepted by MNRAS. 21 pages, 22 figures. Also available at http://www.mpia-hd.mpg.de/theory/bate