DENSE CLUMPS OF IONIZED-GAS NEAR PI-SCORPII, AS REVEALED BY THE FINE-STRUCTURE EXCITATION OF N-II

Absorption lines at 1085 angstrom from three fine-structure levels in the N II ground state were recorded in the spectrum of pi-Sco by the IMAPS sounding rocket payload. The two excited levels are populated by electron collisions in the denser concentrations of ionized gas within the H II region created by this B1 V + B2 V binary star. After a brief review of similar, but generally less accurate, measurements of the column densities of N II, N II*, and N II** toward other stars by the Copernicus satellite, we concentrate on the results for pi-Sco and show that most of the material is concentrated within a length scale of 0.025 pc, with a mean ionized gas density of 40 cm-3. As a possible interpretation, we model the observed clumped gas as evaporation flows off condensed, neutral objects that are tightly clustered around pi-Sco. By comparing the observations to predictions from a maximum likelihood analysis and Monte Carlo simulations, we find that the compact objects are approximately 10(15) cm in diameter and fill a volume with a dimension of at least 0.02 pc at a density of several times 10(5) objects per cubic parsec. A cluster of "partially ionized globules" (PIGs) with almost identical characteristics has previously been observed surrounding theta(1) Ori C, the brightest member of the Orion Trapezium cluster. After considering the mass loss and rocket effect for the pi-Sco and theta(1) Ori C globules, we conclude that in both cases the globules cannot be neutral, condensed clouds, but are probably photoevaporating circumstellar disks surrounding low-mass stars within a gravitationally bound cluster. The ionized photoevaporation flows off the predicted globules around pi-Sco should be observable in the radio continuum and in H-alpha line emission; we predict sizes of order 1" at peak emission measures for the ionized gas of order 10(5) cm-6 pc.