arXiv subjects
Publications and source records attributed to Antonio Peimbert.
A few results that indicate the presence of temperature variations in gaseous nebulae are reviewed. The evidence is based on: a) temperatures derived from different methods, and b) on comparisons of abundances predicted by models of galactic chemical evolution with abundances derived from observations.
We study the effect that the temperature structure has on the determination of the primordial helium abundance, Yp. We provide an equation linking T(OIII), the temperature derived from the [OIII] lines, and T(HeII), the temperature of the He I lines, both for HII regions with O++ only and for HII regions where a fraction of O+ is present. By means of T(HeII), which is always smaller than T(OIII), we derive the helium abundances of 5 objects with low and very low metallicity (NGC 346, NGC 2363, Haro29, SBS 0335-052, and I Zw 18); these objects were selected from the literature because they include the 3 low metallicity objects with the best line determinations and the 2 objects with the lowest metallicity. From these abundances we obtain that Yp(nHc)=0.2356+-0.0020, a value 0.0088 lower than that derived by using T(OIII). We call this determination Yp(nHc) because the collisional contribution to the Balmer line intensities has not been taken into account. All the recent Yp determinations in the literature have not taken into account the collisional contribution to the Balmer line intensities. By considering the collisional contribution to the Balmer line intensities of these five objects we derive that Yp(+Hc)=0.2384+-0.0025.
Spectrophotometry in the 3400-7400 range is presented for 13 areas of the brightest H II region in the SMC: NGC 346. The observations were obtained at CTIO with the 4-m telescope. Based on these observations its chemical composition is derived. The helium and oxygen abundances by mass are given by: Y(SMC)=0.2405+-0.0018 and O(SMC)=0.00171+-0.00025. From models and observations of irregular and blue compact galaxies it is found that dY/dO=3.5+-0.9 and consequently that the primordial helium abundance by mass is given by: Yp=0.2345+-0.0026 (1-sigma). This result is compared with values derived from Big Bang nucleosynthesis, and with other determinations of Yp.
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.