Astron. Astrophys. 337, 832-846 (1998)

1. Introduction

A large fraction of early-type main sequence stars (Wood & Churchwell 1989) are associated with ultracompact HII -Regions (UCHII s). These are characterized by large electron densities , sizes  pc and temperatures  K. The overpressure in these regions should lead to expansion and dissipation on time scales of a few thousand years. Considering the expected lifetimes of massive stars of several million years, however, the high abundance of observed UCHII s translates into UCHII mean lifetimes of several years. This contradiction can be resolved by: 1) the UCHII s could be constrained by high pressure in their vicinity, or 2) by gravitationally infalling material (Reid et al. 1981), or 3) there could exist a process which continuously "feeds" the UCHII s with matter. High pressures certainly can be expected in the highly turbulent molecular cloud cores, which are the birthplaces of young massive stars (De Pree et al. 1995, García-Segura & Franco 1996, Xie et al. 1996). Still, it is not clear how turbulence in a cold clumpy medium can contain the warm ( K), high density ionized material for extended periods of time - many of the technical details of this proposal need to be worked out.

The photoevaporating disk model proposed by Hollenbach et al. (1993) and Yorke & Welz (1993) offers an attractive alternative. A circumstellar disk around a luminous OB star is continuously photoionized by the central source. The existence of a powerful stellar wind can modify the quantitative details of this model, but the basic result remains the same. Long-lived UCHII s are the necessary consequence of disks around hydrogen-ionizing sources. In a subsequent paper by Hollenbach et al. (1994) the quasi-steady state structure of disks around ionizing sources with winds has been calculated (semi-) analytically and in Yorke (1995), Yorke & Welz (1996, hereafter Paper I), and Richling & Yorke (1997, hereafter Paper II) the evolution of such circumstellar disks has been followed numerically under a variety of conditions.

In Paper I it has been stressed that the phenomenon of disks in the process of photoionization is not restricted to the (presumably highly symmetrical) case of circumstellar disks around OB stars. Disk formation is a common by-product of the star formation process. Because OB stars seldom form in isolation, close companions with disks to a powerful source of ionizing UV radiation and a stellar wind should be common. Strongly asymmetric UCHII s should result.

Wood & Churchwell (1989) observed 75 UCHII s at  cm and 6 cm with spatial resolution of 04 using the VLA telescope and classified them by their spatial morphological structure into several types:

• cometary shaped (20%),

• core-halo (16%),

• shell type (4%),

• irregular or multiply peaked (17%) and

• spherical or unresolved (43%).

In order to interpret these observations in light of the photoionized disk models, further work must be done in refining the hydrodynamical models for the asymmetric morphological configurations expected when a disk is ionized by external sources. Diagnostic radiation transfer calculations of these numerical models are necessary for a quantitative comparison.

Goal of the present investigation is to determine spectral characteristics and to calculate the expected isophote maps of the symmetrical UCHII s which result from circumstellar disks around OB stars. We are restricted by the limited number of star/disk configurations which have been considered to date. We discuss in detail the physical (Sect. 2) and numerical (Sect. 3) models of radiation transfer which we employed. The results for selected hydrodynamical models from Papers I and II are discussed in Sect. 4 and compared to observations of specific sources in Sect. 5. We summarize our main conclusions in Sect. 6.

© European Southern Observatory (ESO) 1998

Online publication: August 27, 1998
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