A New Class of Compact H II regions in the Magellanic Clouds
1er octobre 2006
A study carried out at the Paris Observatory shows the presence of a new class of compact H II regions, dubbed Low-excitation Blobs (LEBs), in the Magellanic Clouds. These objects represent the low excitation counterpart of HEBs previously discovered in our neighboring galaxies. It is important to study them not only because they are a genuine and sofar overlooked component of the interstellar medium, but also because they should yield information on the initial mass function of massive stars, more particulary on its lower part.
The ionized content of the Magellanic Clouds is not limited to giant and supergiant H II regions or to a relatively small number of supernova remnants. Detection of high-excitation blobs (HEBs) showed the presence of an unknown H II component lying mainly adjacent to or toward the typical giant H II regions. In contrast to the ordinary H II regions of the Magellanic Clouds, which are extended structures spanning several arcminutes on the sky (> 50 pc) and are powered by a large number of hot stars, HEBs are very dense small regions usually 4" to 10" in diameter (1 to 3 pc) and affected by local dust. The study pertains to a sample of 29 H II regions which have the same physical size. The spectroscopic observations conducted at the European Southern Observatory (ESO) allow the astronomers to obtain, among other things, the luminosity in Hβ, L(Hβ), and the excitation, represented by the line ratio [O III/Hβ], for these objects. When the excitation is plotted against the luminosity, the data show a linear relationship. The linear fits are not surprising. In fact, current photoionization models of spherically symmetric H II regions suggest that the [O III]/Hβ ratio depends on the effective temperature of the star(s), the ionization parameter, and the gas metallicity. Models by Stasinska (1990) show a linear correlation between [O III]/Hβ and the Hβ luminosity for H II regions with the same size, the same gas density and the same number of exciting stars, but with increasing effective temperatures. The reason for this relationship is that as the temperature increases, producing higher [O III]/Hβ ratios, the ionization parameter increases as well, increasing the Hβ luminosity. The models also show that lower metallicity environments favor higher [O III]/Hβ ratios, but the metallicity dependence can be outweighed by the ionization parameter. Two groups of objects show up in both galaxies. Those populating the upper right parts of the plots are known to be HEBs. These are LMC N160A1, N11A, N159-5, N83B and SMC N88A, N81 They were called so because of their small size, compactness, and higher [O III]/Hβ ratio compared to common giant H II regions in the Magellanic Clouds. Here, in light of new observations, it is possible to provide a more precise definition taking all the data into account. In the LMC sample, an HEB is an object that stands above the linear fit in the [O III]/Hβ-Log L(Hβ) space and that has a luminosity of Log L(Hβ) 30.0 W or Log L(Hβ) 3.4 L(sun). This means that an LMC HEB should at the same time have an [O III]/Hβ ratio higher than 4.0. If we use the flatter fit, N105A-IR can also qualify as an HEB. As regards the SMC HEBs, the "classical" members, N88A and N81, fulfill the two criteria specified above. They have at the same time higher excitation compared to the LMC HEBs. Consequently, as a working hypothesis, we call LEBs all the other objects that do not meet the above requirements for excitation and luminosity. Of course one can also consider an intermediate group between the two extreme cases. The pile-up behavior of the HEBs in the LMC plot is not clearly understood at this stage. Why is such feature not seen for the SMC HEBs ? We note that only two HEBs are present in the SMC plot, implying that, even if this feature existed for the SMC objects, it cannot be shown by the present data. Tentative explanations could be given by taking possible variations in the ionization parameter into account. For example, ionization-boundedness in the HEBs, which are powered by hotter exciting sources or by more numerous sources with the same temperature. Since the HEBs have more or less the same size and, consequently, comparable amounts of matter, the larger Lyman continuum photon fluxes cannot correspondingly create more luminous H II regions. If this explanation is valid, we need to know the physical conditions that give rise to hotter stars or to a larger number of exciting stars, while the available quantity of gas around them remains unchanged. Contact M. Heydari-Malayeri (Observatoire de Paris, LERMA, et CNRS)
Dernière modification le 4 mars 2013