Photoevaporating Stellar Envelopes Observed with Rayleigh Beacon Adaptive Optics
Session 32 -- Circumstellar Environments
Oral presentation, Wednesday, January 12, 2:15-3:45, Salon VI Room (Crystal Gateway)

## [32.02] Photoevaporating Stellar Envelopes Observed with Rayleigh Beacon Adaptive Optics

P. R. McCullough (Astronomy Depts., UC Berkeley and UI Urbana-Champaign), R. Q. Fugate, B. L. Ellerbroek, C. H. Higgins, J. C. Christou (Starfire Optical Range, Phillips Laboratory), J. M. Spinhirne (Rockwell Power Systems), J. F. Moroney (Adaptive Optics Associates; and Psychology Dept., Carnegie Mellon University), R. A. Cleis (the Optical Sciences Company; and Rockwell Power Systems)

We present H$\alpha$~and I band images of a ${\sim}1^\prime$~diameter field centered on $\theta^1$~Orionis made with a unique adaptive optics system that uses either starlight or Rayleigh-backscattered laser light to correct for atmospheric wavefront distortion. Approximately one half of the stars in this region are positionally associated with knots of ionized gas, which are interpreted as photoevaporating envelopes of low mass stars. The acronyms PIGs, EIDERs, or ProPlyDs all refer to these same knots. The H$\alpha$~fluxes of the PIGs are proportional to their 2-cm radio continuum flux densities, and for nearly all the ionized knots, the 2-cm brightness temperatures are consistent with $\theta^1$~Ori as the primary source of ionization. The thermal pressure in the ionized knots is much greater than the ram pressure of the wind from an O-type star, so the comet-like morphology of the nebulosities cannot be caused by a hydrodynamical interaction such as a bow-shock. Instead, it is the result of an equilibrium between photoionization, recombination, and shadowing.

The positions of the stars associated with the PIGs in the observational HR diagram indicate they are pre-main-sequence stars with masses less than ${\sim}3 M_\odot$, with ${\sim}1 M_\odot$ being typical. Also, the radii of the ionized component of the PIGs grow with distance from $\theta^1$~Ori, which we interpret as evidence that the mass loss rates from the PIGs are all the same, within a factor of two.