Showing papers by "M. S. Oey published in 1997"

The superbubble size distribution in the interstellar medium of galaxies

Abstract: We use the standard, adiabatic shell evolution to predict the differential size distribution N (R) for populations of OB superbubb1es in a uniform interstellar medium (ISM). Assuming that shell growth stalls upon pressure equilibrium with the ambient ISM, we derive N (R) for simple cases of superbubble creation rate and mechanical luminosity function (MLF). For constant creation and an MLF ¢(L)ocL-P, we find that N(R)ocRl - 2P for R Re, where the characteristic radius Re '" 1300 pc for typical ISM parameters. For R Re it is dominated by growing objects. The relation N (R) ocR 1-2P appears to be quite robust, and also results from momentum-conserving shell evolution. We predict a peak in N (R) corresponding to individual supernova remnants (SNRs), and suggest that the contribution of Type Ia SNRs should be apparent in the observed form ofN(R). We present expressions for the porosity parameters, Q20 and Q30' derived from our analysis. Q20 is dominated by the largest superbubb1es for /3 2, whereas Q30 is normally dominated by the few largest shells. We examine evolutionary effects on the H II region luminosity function (H II LF), in order to estimate /3. We find that for a nebular luminosity fading with time t, .fE oc t - ~, there is a minimum observed slope amin for the H II LFs. Empirical measurements all show a > amin, therefore implying that usually we may take /3 =a. We also find that if nebular luminosity is instantaneously extinguished at some given age, rather than continuously fading, no amin will be observed. Comparison with the largely complete H I hole catalogue for the SMC shows surprising agreement in the predicted and observed slope of N (R). This suggests that no other fundamental process is needed to explain the size distribution of shells in the SMC. Further comparison with largely incomplete H I data for M31, M33 and Holmberg II also shows agreement in the slopes, but perhaps hinting at systematic differences between spiral and 1m galaxies. We estimate porosities that are substantially < 1 for all of the galaxies except Holmberg II, for which we obtain values ~ 1. Most of these galaxies therefore may not be strongly dominated by a hot interstellar component. However, porosity results for the Gal~ remain inconclusive with the available data.

Comparison of H II region luminosities with observed stellar ionizing sources in the Large Magellanic Cloud

Abstract: We estimate the total predicted Lyc emission rates of OB associations for which the complete census of O star spectral types exists. The results are compared to the observed H-alpha luminosities of the host H II regions. We find evidence for substantial leakage of ionizing photons from some H II regions, while others appear to be radiation bounded. We estimate that overall for the LMC, 0-51% of the ionizing radiation escapes the local nebulae, and would be available to ionize the diffuse, warm, ionized medium (WIM) in that galaxy. This range of values is consistent with the observed 35% fraction of \Ha\ luminosity emitted by the WIM in the LMC, as well as the corresponding fractions observed in other nearby galaxies. It is therefore possible that photoionization by O stars is indeed the dominant ionization mechanism for the WIM.