Astron. Astrophys. 340, 508-520 (1998)

1. Introduction

The existence of circumstellar disks around low-mass stars was confirmed by HST observations in the Orion Nebula (O`Dell et al. 1993, O'Dell & Wen 1994). Disks well in front of the HII region appear as dark silhouettes against the bright nebula background. These direct disk images allow comparison with simple disk models which in turn yields a lower limit for the disk masses of the order (McCaughrean & O'Dell 1996, Hayward & McCaughrean 1997, McCaughrean et al. 1998).

Disks close to ionizing sources are called proplyds (O`Dell et al. 1993). They directly interact with the external radiation field and appear as elongated emission-line objects with tails pointing away from the two major ionization sources Ori A and C (O'Dell & Wong 1996, McCullough et al. 1995). That the observed objects contain young stars is supported by optical and infrared continuum observations indicating the presence of low mass stars within proplyds (McCaughrean & Stauffer 1994) and the molecular hydrogen emission of silhouettes within teardrop shaped proplyds (Chen et al. 1998).

Proplyds were previously detected at radio wavelengths (Garay et al. 1987, Churchwell et al. 1987, Felli et al. 1993a). The deduced electron densities are of order and typical for ultracompact HII regions. Stecklum et al. (1998) proposed to use ultracompact HII regions close to early type stars as tracers for proplyds in more distant star formation regions and found a possible candidate in the Lagoon Nebula. A large fraction of ultracompact HII regions "fed" by externally ionized disks would be another proof for the common occurrence of circumstellar disks.

Henney et al. (1996) and Johnstone et al. (1998) investigate the photoevaporation of disks by an external radiation field analytically. Henney et al. (1996) model the bow-like structure of the proplyds as the result of the interaction between an external stellar wind with the photoevaporated wind from the circumstellar disk. This two-wind model can successfully reproduce the general head-tail feature when comparing theoretical emission maps with those of the Orion proplyds. But the predicted O[III ] line profiles show little agreement with the observed kinematic properties (Henney et al. 1997). The model of Johnstone et al. (1998) includes the effects of Lyman continuum EUV photons ( 13.6 eV) and of FUV photons (6 eV 13.6 eV). FUV photons heat the region between the embedded disk and the ionization front and the resulting neutral flow influences the size of the ionized envelope. This model assumes radially symmetric neutral winds from spherical clumps and neglects the effects of a backward redirected evaporation flow (see also Johnstone & Bertoldi 1998).

By contrast we begin our radiation hydrodynamical simulations with a thin disk possessing a finite scale height. We calculate the reaction of the disk to the sudden power on of an external source and follow the evolution of both the evaporative flow and the internal structure of the disk. In addition, we consider the transport of diffuse UV photons which allow an appropriate modeling of the regions shadowed by the disk. In this first investigation we only consider EUV radiation and defer the effects of FUV photons to a subsequent study. With our numerical work we hope to find answers to the questions: How long can disks survive in an asymmetrical external radiation field? How does the structure of the disk develop? In Sect. 2 we briefly describe the physics and numerical methods used. Sect. 3 introduces the starting models and the parameters which characterize the individual simulations. The numerical results are presented in Sect. 4. In Sect. 5 we discuss the results of diagnostic 3D radiation transfer calculations. We summarize our results in the light of observations in Sect. 6.

© European Southern Observatory (ESO) 1998

Online publication: November 9, 1998
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