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1995PASP..107...90M - Publ. Astron. Soc. Pac., 107, 90-90 (1995/January-0)
Observations of photoevaporing interstellar clouds. (Dissertation summary).
McCULLOUGH P.R.
Abstract (from CDS):
In the second part, we present images of the Orion Trapezium made with a unique adaptive optics system that uses either starlight or Rayleigh-backscattered laser light to correct for atmospheric wavefront distortion. The H-alpha and I band images reveal the region around the bright Trapezium stars, which has been studied extensively at radio wavelengths. Approximately one half of the stars in this region are positionally associated with knots of ionized gas, known as pigs, which are interpreted as photoevaporating envelopes of low mass stars. Disks are not observed directly but are inferred from the relatively low extinction to the stars inside the pigs. The comet-like morphology of the pigs is the result of an equilibrium between photoionization, recombination, and shadowing; interaction with the wind from theta-1C ∼Ori is not required. The sizes of the ionized component of the envelopes increase with distance from the source of ionization, theta-1C Ori. The size-distance relationship suggests that the mass loss rates from the pigs are all approximately the same. The actual value of the mass loss rate is model dependent, but is ∼1.2 x 10^-7 M_☉yr^-1.
In the third and last part, we extend the laser beacon concept to a polychromatic beacon in order to infer the wavefront tilt by measuring the differential tilt caused by dispersion. The general concept of a polychromatic beacon is to make the atmosphere produce a second beacon of a different color at altitude. Two mechanisms capable of producing the color conversion are Raman scattering in the troposphere, and resonant florescence in the mesosphere. Because the differential tilt is quite small, high precision is required and thus also high photon return rates. Our analysis indicates that existing lasers may satisfy the theoretical requirements; however, the installation and operation of such lasers may be prohibitively expensive using current technology.
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