Methods

The Large Magellanic Cloud (LMC) offers a broad range of HII regions of varying luminosities and morphologies, and for over a dozen of these, a complete, classified census is available for the hottest, most massive stars (e.g., Massey etal. 1995; Oey 1996). Likewise, a catalog of uniform nebular photometry is available for the H luminosities of the HII\ regions (Kennicutt & Hodge 1986). With the aid of stellar atmosphere models, it is possible to estimate the Lyman continuum (Lyc) photon emission rate for the individual stars, and obtain a predicted total nebular H\ luminosity, to compare directly with the observed values.

Table 1 shows the results of this comparison for 12 objects. Column 1 identifies the nebula by its designation in the Davies, Elliott & Meaburn (1976; DEM) H catalog, and column 2 identifies the corresponding OB association from the Lucke & Hodge (1970; LH) catalog. In the third column, we list the number of O stars () identified spectroscopically from the reference shown in the last column. We have considered the ionizing contributions of only O stars, but we identify those associations containing WR stars with an asterisk on . Column 4 shows the ratio of the observed H\ luminosity () to that predicted () from the observed O stars, using the stellar Lyc emission rates of Schaerer & deKoter (1997). is adjusted for the observed extinction, derived from the reddenings found in the stellar observations. Further details regarding the measurement of and computation of may be found in Oey & Kennicutt (1997). In column 5, we classify the nebular morphology as diffuse HII region (D), superbubble (S), or composite (C).

Table 1: Ratio of Observed and Predicted HII Region Luminosities

	DEM	LH			Morph	Reference
	10B	2	7	0.81	D	J. Wm. Parker, unpublished
	31	6	6	0.40	S	Oey (1996)
	34	9, 10	44	0.66	C	Parker etal. (1991)
	106	38	8	0.61	S	Oey (1996)
	152, 156	47, 48	35	0.66	C	Oey & Massey (1995)
	192	51, 54	25	0.83	S	Oey & Smedley (1997)
	199	58	22	1.22	C	Garmany etal. (1994)
	226	73	4	0.93	S	Oey (1996)
	243	83	11	0.54	D	Oey (1996)
	293	110	1	1.09	D	Conti etal. (1986)
	301	114	7	0.20	S	Oey (1996)
	323, 326	117, 118	20	1.13	D	Massey etal. (1989)

Not including DEM 152A.

The median is 0.74, suggesting that many HII regions are significantly density-bounded. However, there are many sources of substantial uncertainty, which are discussed in detail by Oey & Kennicutt (1997). These include uncounted O and WR stars; the effect of B stars, whose ionizing fluxes are poorly known; accounting for LMC metallicity; observational errors in extinction; and error in the LMC distance. However, the modeled ionizing fluxes of O stars are by far the dominant source of uncertainty, contributing about % to the overall median . We therefore estimate that , or equivalently, that a median fraction of ionizing radiation that escapes the local nebulae is in the range 0 - 51%.

© Copyright Astronomical Society of Australia 1997