LOW MASS STAR FORMATION IN THE ORION MOLECULAR CLOUD (OMC1) ?

Observations show that violent outflows of material accompany the birth of stars, especially massive stars as in Orion. Outflows may collide with gas nearby the star, causing shock waves to pass through the medium, strongly compressing the gas. Gravitationally unstable clumps may form, leading to a new generation of stars. It has long been thought that this scenario could provide the feedback to accelerate star-formation, but observational evidence was lacking. Recent observations from Laurence Vannier (Observatoire de Paris) and her colleagues provide this evidence and show that shock-triggered star formation may indeed take place.At a distance of about 450 pc (ie 1500 light years), the Orion molecular cloud (OMC1) is one of the nearest star forming region. OMC1 contains a large number of young massive O and B stars, the so-called Trapezium stars, low mass stars and proto-stellar objects like IRc2 and the Becklin Neugebauer (BN) object seen . Outflows from the IRc2/BN region excite strongly the surrounding gas. It is possible to observe molecular hydrogen in several ro-vibrational lines in the infrared K band. Trapezium stars, emitting an intense ultraviolet radiation field, may also play a role in the excitation and the subsequent emission of the hydrogen molecule. OMC1 observation in Halpha emission line. (OHP 1.2m telescope Image). The Trapezium stars are indicated, and the size of the image is about 5.7 arcmin on each side. North is up, and East is left. Observations of OMC1 small scale structure thanks to Adaptive Optics. OMC1 has been observed in the near infra-red using the 3.6m ESO telescope and the CFHT, both equipped with adaptive optics (AO) systems. AO allows observations to be performed removing the distorting effects of the turbulent atmosphere of the Earth. With AO, it is possible to achieve a spatial resolution of between 0.1 and 0.15 arcseconds, similar to that achieved by the Hubble Space Telescope (HST). shows a mosaic of fields observed at 2.121 microns in the S(1) v=1-0 rovibrational emission line of H2. OMC1 observation at 2.121 microns with a 0.15" spatial resolution (ESO-ADONIS and CFHT-PUEO images). The Trapezium stars are at the bottom (Ori A, B, C...). The top size of the image is 36 arcsecond. North is up, and East is left. The high spatial resolution affords a view of very small structures in OMC1 and allows identification of clumpy structures of sizes down to a few x 10-4 pc ( 50 AU, an astronomical unit AU = Earth-Sun distance). It is these and larger clumps which are generated through the action of shocks and which may be sites of future star formation. May the observed clumps be gravitationally unstable ? Gravitational stability of a cloud of gas is determined by a combination of size, density and temperature, magnetic fields and turbulence apart. The physical conditions in the observed clumps can be determined using models of shocks and comparing the predicted brightness of H2 emission with that observed at several different wavelengths. The most important parameter determined from the analysis is the gas density after the passage of the shock. The density is found to be 3.5 107 cm-3 in brighter regions, an increase in density of more than a factor of 30 due to the action of the shock. The temperature rapidly falls to 10K after the shock has passed by. Detailed analysis of images of H2 reveals that the structure of shocked regions is not fractal, contrary to most molecular clouds in the Galaxy, but has a preferred scale of 4 mpc (800 AU). With the density and temperature mentioned above, this scale is of the order of the Jeans length, suggesting that some clumps should be gravitationally unstable. Low mass stars ( 0.1 solar mass) may form in the future from among the denser clumps of gas observed, showing how a burst of star-formation may propagate through a molecular cloud triggered in OMC1 by outflows from massive stars.