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Session 5 - Accretion and Outflows in YSOs.
Display session, Wednesday, January 07
Exhibit Hall,

[5.04] The Orion Nebula Proplyds: Properties and Survival

C. R. O'Dell (Rice University)

Some 45 of the 150 known proplyds in the Orion Nebula have been analyzed using public archive high resolution images made with HST's WFPC2. The images were calibrated into emission line surface brightnesses in H\alpha by means of calibration constants derived from comparison of Orion fields with calibrated groundbased images and correction for contamination of the f656n signal by the nearby [N II] lines and the underlying continuum. This analysis indicates that their outer bright cusps are ionization boundaries, thus excluding models where the inner disks are heated by the entire UV radiation field. The objects increase with size as the 1/2 power of the true distance from the photoionizing star, \theta^1C Ori.

It is frequently assumed that the material freely flows from the proplyds through their ionization fronts, producing mass loss rates of about 10^-7 solar masses per year. Since the masses of these disks are about 0.01 solar masses, this leads to expected lifetimes of only about 10^5 years, which is likely to be shorter than the age of the Trapezium Cluster. Moreover, there is no evidence of completed destruction of the proplyds closest to \theta^1C Ori, arguing that the mass loss rates are overestimated by the models assuming free flow.

Because of this quandary, the brightness distribution in the ionized atmospheres of a set of proplyds was determined. These objects were compared with the expectations of models employing intrinsic r^-3 distributions (as expected for freely flowing gas) and exponential distributions (as expected for a gas in hydrostatic equilibrium with a constant force). The comparison took into consideration the smearing caused by the instrumental PSF, which is important in the critical innermost region. The observations best match an exponential distribution. If this indicates that a quasi-equilibrium exists, then the mass loss rate must be much lower than has previously been assumed and it is much more likely that the disk material may survive long enough to allow planet formation.

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