Antonio Peimbert

Molecular Hydrogen in the Direction of ζ Orionis A

Lyman and Werner band absorption features from interstellar molecular hydrogen in rotational levels J = 0, 1, 2, 3 and 5 were observed in the spectrum of zeta Ori A by the Interstellar Medium Absorption Profile Spectrograph (IMAPS). Most of the molecules are found in two distinct velocity components, with one of them showing profile shapes that become broader and show small displacements toward more negative velocities as J increases. This change in profile shapes, along with evidence from atomic lines at high negative velocities, supports an interpretation that the molecules are initially created by associative detachment of H- in a partially ionized, warm gas that is cooling and recombining behind a stationary shock front. The front probably arises from an interaction of high speed gas from the Orion stars and the ambient medium. These molecules are most conspicuous in the high J levels. Later, when the gas becomes much cooler, mostly neutral, and more compressed as it comes nearly to a halt, the H2 is primarily created by reactions on the surfaces of dust grains and is more easily seen in the lowest J levels.

ON THE O/H, Mg/H, Si/H, AND Fe/H GAS AND DUST ABUNDANCE RATIOS IN GALACTIC AND EXTRAGALACTIC H II REGIONS*

We derive the Mg/H ratio in the Orion nebula and in 30 Doradus. We also derive the O/H and the Fe/O ratios in the extremely metal-poor galaxy SBS 0335-052 E. We estimate the dust depletions of Mg, Si, and Fe in Galactic and extragalactic H II regions. From these depletions we estimate the fraction of O atoms embedded in dust as a function of the O/H ratio. We find an increasing depletion of O with increasing O/H. The O depletion increases from about 0.08 dex, for the metal poorest H II regions known, to about 0.12 dex, for metal-rich H II regions. This depletion has to be considered to compare nebular with stellar abundances.

Densities, Temperatures, Pressures, and Abundances Derived from O II Recombination Lines in H II Regions and their Implications

Based on high-quality observations of multiplet V1 of O II and the NLTE atomic computations of O II, we study the density and temperature of a sample of H II regions. We find that the signature for oxygen-rich clumps of high density and low temperature is absent in all objects of our sample: one extragalactic and eight Galactic H II regions. The temperatures derived from (1) recombination lines (RLs) of O II, and (2) RLs of H I together with Balmer continua are lower than those derived from forbidden lines, while the densities derived from RLs of O II are similar or smaller than densities derived from forbidden lines. Electron pressures derived from collisionally excited lines are about two times larger than those derived from RLs. These results imply that the proper abundances are those derived from RLs and suggest that other processes in addition to direct photoionization, such as dissipation of turbulent energy in shocks, magnetic reconnection, and shadowed regions, might be responsible for the large abundance discrepancy factor and t 2 values observed in H II regions.

Nebular Spectroscopy: A Guide on H ii Regions and Planetary Nebulae

We present a tutorial on the determination of the physical conditions and chemical abundances in gaseous nebulae. We also include a brief review of recent results on the study of gaseous nebulae, their relevance for the study of stellar evolution, galactic chemical evolution, and the evolution of the universe. One of the most important problems in abundance determinations is the existence of a discrepancy between the abundances determined with collisionally excited lines and those determined by recombination lines, this is called the ADF (abundance discrepancy factor) problem; we review results related to this problem. Finally, we discuss possible reasons for the large t

Chemical abundances of the Galactic HII region NGC 3576 derived from VLT echelle spectrophotometry

^2

Planetary nebulae in our Galaxy and beyond

values observed in gaseous nebulae.

THE PRIMORDIAL HELIUM ABUNDANCE AND THE NUMBER OF NEUTRINO FAMILIES

We present echelle spectrophotometry of the Galactic H II region NGC 3576. The data have been taken with the VLT UVES echelle spectrograph in the 3100 to 10400 angstroms range. We have measured the intensities of 458 emission lines, 344 are permitted lines of H0, He0, C+, N0, N+, N++, O0, O+, Ne+, S++, Si0, Si+, Ar0 and Ar+; some of them are produced by recombination and others mainly by fluorescence. Electron temperatures and densities have been determined using different continuum and line intensity ratios. We have derived He+, C++, O+, O++ and Ne++ ionic abundances from pure recombination lines. We have also derived abundances from collisionally excited lines for a large number of ions of different elements. Remarkably consistent estimations of t2 have been obtained by comparing Balmer and Paschen to [O III] temperatures, and O++ and Ne++ ionic abundances obtained from collisionally excited and recombination lines. The chemical composition of NGC 3576 is compared with those of other Galactic H II regions and with the one from the Sun. A first approach to the gas-phase Galactic radial abundance gradient of C as well as of the C/O ratio has been made.