The nature of star formation in the Trapezium Cluster

Abstract

The Trapezium Cluster stellar population is studied in detail using near-infrared and optical means in order to probe the clustered mode of low and high mass star formation. We determine fundamental stellar parameters such as the spectral types, ages, masses, extinctions and dust excesses for a significant number of cluster stars. Various techniques are applied to deredden the stars in the color-magnitude diagram and hence compare intrinsic positions with theoretical evolutionary tracks. Through these means, we estimate properties of the low mass stellar population to greater accuracy than has previously been achieved. Near-infrared photometry of Trapezium Cluster stars provides an initial evaluation of the nature of the cluster population. This evaluation is improved upon using optical spectroscopy to measure spectral types of a large number of Trapezium Cluster stars for the first time. We find our sample of Trapezium Cluster stars to have a mean spectral type of mid-K, in agreement with :findings for the low mass stars in the vicinity of, and external to, the central cluster. The stars are dereddened on the color-magnitude diagram using our acquired spectral types. Their intrinsic positions provide the most accurate determination for the cluster age obtained to date, -10/6 yr, confirming the pre-main sequence nature of the population. This age estimate is extended to the infrared cluster population of more than 550 stars revealed by infrared-array images. The K luminosity function for the infrared cluster is used in combination with the cluster age to derive the stellar mass distribution. The slope of the mass function obtained here is found to be comparable with the slopes of field initial mass functions. A mean stellar mass of, -0.9 Mo is estimated for the low mass stars. Our determinations for the masses, ages, and spectral types of Trapezium Cluster stars shows that they are a similar stellar population to the more extended Orion Nebula Cluster population, except in density of stars. The mass density of the Trapezium system of low mass and high mass (01 Ori) stars is found to be -4690 Mo pc-3 , approximately 1.5 times greater than previous estimates based on optical studies. The stellar mass derived for the low mass cluster is also used to calculate the star formation efficiency in the region to first order, -72%. This is similar to, but higher than, the star formation efficiencies determined in other regions of embedded cluster formation. The mean extinction estimated for the low mass cluster stars in our sample place the stars at approximately the same depth into the molecular cloud as the Trapezium OB stars, at the near-face of the cloud. Our sample is biased towards optical members of the cluster, suggesting that a significant number of the low mass stars may be embedded more deeply in the molecular cloud than the OB stars. However, using the K luminosity function for the infrared cluster we determine that the low mass cluster is most probably not spread through the whole cloud, but is pre-dominantly located close to the near-face of the cloud. Dust excess determinations show that the Trapezium Cluster stars sampled here contain a typical proportion of classical (dust-excess) T Tauri stars compared with naked (no dust-excess) T Tauri stars for a young stellar population. Approximately one-third of our sample have insignificant dust excesses. Calcium II IR triplet emission is observed in members of our Trapezium Cluster sample. We judge that the strengths of the triplet features imply a circumstellar disk origin for the emission. The frequency of calcium triplet emitting stars is estimated for our sample. We compare this estimate with the proportion of triplet emitters in a sample of Chamaeleon pre-main sequence stars. We find that 20-30 % of classical T Tauri stars in the two populations exhibit triplet emission; the frequency of triplet emission in the Trapezium Cluster sample is found to be comparable with that in Chamaeleon. We perform an approximate dynamical analysis of the Trapezium Cluster star-forming region using our estimates for stellar mass and age. The low mass cluster is found to be at an early stage in its dynamical evolution, and has not had time to completely relax as a system and lose its initial characteristics. It is too young, therefore, to exhibit mass segregation, and the observed isothermality of the stars is proposed here to arise from the distribution of the clumps from which the stars have formed. The high mass stars considered separately are determined to be old enough to have relaxed as a system. We find that, if the 01 Ori stars are located centrally with respect to the low mass cluster, then they are most likely to have formed in their present locations rather than have arrived there from larger radii through dynamical friction processes. In addition, the binding energies of the two high mass binary systems are found to be almost forty times the energy of the low mass cluster. It is suggested that the binary energies must have been acquired through their formation processes instead of through dynamical interactions with the low mass cluster. Thus, present characteristics of the stellar population of the Trapezium Cluster directly relate to the conditions under which the cluster formed, and are not due to dynamical processes among the stars themselves. The total stellar mass determined for the low mass cluster is estimated to be sufficient to bind the cluster at this time. This remains true even with future removal of gas from the region. However, continued dynamical interactions may lead to the eventual dissipation of the low mass cluster. Show more