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The photoevaporation of interstellar clouds. II - Equilibrium cometary clouds
Bertoldi, Frank; McKee, Christopher F.
AA(California, University, Berkeley), AB(California, University, Berkeley)
Astrophysical Journal, Part 1 (ISSN 0004-637X), vol. 354, May 10, 1990, p. 529-548. (ApJ Homepage)
Publication Date:
NASA/STI Keywords:
Comets, H Ii Regions, Interstellar Magnetic Fields, Interstellar Matter, Nebulae, Star Formation, Astronomical Models, Continuous Radiation, Early Stars, Gravitational Effects, Stellar Evolution
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An approximate analytic theory for the evolution of an interstellar cloud which is exposed to the ionizing radiation of a newly formed star is developed. This paper will follow the evolution of a photoevaporating cloud beyond its initial radiation-driven implosion. Under the realistic assumption of axisymmetry, the imploded cloud is shown to assume a steady, cometary shaped configuration in which the embracing ionization front balances the pressure of the neutral gas. The steady cloud configuration is determined solely by the cloud mass, the incident Lyman continuum flux, and the adiabatic index of the gas. Because of the rocket effect of the evaporating gas, the cloud is accelerated away from the ionizing star. Over a wide range of parameters, the cloud shape, its mass-loss rate, lifetime, and acceleration are computed. The characteristic rocket velocity is typically 10 km/s, but can be as low as 5 km/s, which is significantly lower than previous estimates. Magnetic fields are taken into account in an approximate way and are shown to be a significant factor for the cloud pressure support and gravitational stability. The theory is applied to the photoevaporating globules in the Rosette and Gum Nebulae.

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