We present a general procedure for obtaining the present density fluctuation probability distribution given the statistics of the initial conditions. The main difficulties faced with regard to this problem are those related to the nonlinear evolution of the density fluctuations and those posed by the fact that the fields we are interested in are

We show that SBS1150+599A is most probably a planetary nebula located in the Galactic halo (becoming PN G135.9+55.9). Our photoionization models indicate that its oxygen abundance is lower than 1/100 solar.

Planetary nebulae (PNe) have been used to trace the chemical evolution of galaxies, not only in the Milky Way (Esteban & Peimbert 1995 and references therein) but also in nearby galaxies (Dopita et al. 1997; Richer et al. 1999; Jacoby & Ciardullo 1999). Extragalactic PNe have also been used as secondary distance indicators by means of the planetary

The existence of an extended component of diffuse ionized gas (DIG) is a well-known property of spiral galaxies. The integrated ensuremath {{H}alpha } luminosity of the DIG is very high (accounting for ~ 50% of the total ensuremath {{H}alpha } emission in a galaxy), which makes it important to understand the origin of this diffuse component.

We have compared direct estimates of the ionizing photon flux from the stars observed within the H II region NGC 346 in the Small Magellanic Cloud with its observed H alpha luminosity using a series of photoionization models of the region. The tests were based on the use of the photoionization code Cloudy (Ferland 2000), aided by H, He, N, O, Ne, S

Available from the Minor Planet Center.