Chemical abundances in the inner 5 kpc of the Galactic disk

High-resolution, high signal-to-noise spectral data are presented for four young B-type stars lying towards the Galactic Centre. Determination of their atmospheric parameters from their absorption line profiles, and uvby photometric measurement of the continua indicate that they are massive objects lying slightly out of the plane, and were probably born in the disk between 2.5-5 kpc from the Centre. We have carried out a detailed absolute and differential line-by-line abundance analyses of the four stars compared to two stars with very similar atmospheric parameters in the solar neighbourhood. The stars appear to be rich in all the well sampled chemical elements (C, N, Si, Mg, S, Al), except for oxygen. Oxygen abundances derived in the atmospheres of these four stars are very similar to that in the solar neighbourhood. If the photospheric composition of these young stars is reflective of the gaseous ISM in the inner Galaxy, then the values derived for the enhanced metals are in excellent agreement with the extrapolation of the Galactic abundance gradients previously derived by Rolleston et al. (2000) and others. However, the data for oxygen suggests that the inner Galaxy may not be richer than normal in this element, and the physical reasons for such a scenario are unclear.

Exploration of the Kuiper Belt by High-Precision Photometric Stellar Occultations: First Results

We report here the first detection of hectometer-size objects by the method of serendipitous stellar occultation. This method consists of recording the diffraction shadow created when an object crosses the observer's line of sight and occults the disk of a background star. One of our detections is most consistent with an object between Saturn and Uranus. The two other diffraction patterns detected are caused by Kuiper Belt objects beyond 100 AU from the Sun and hence are the farthest known objects in the solar system. These detections show that the Kuiper Belt is much more extended than previously believed and that the outer part of the disk could be composed of smaller objects than the inner part. This gives critical clues to understanding the problem of the formation of the outer planets of the solar system.

Phase-resolved emission spectroscopy of a hydrogen rf discharge for the determination of quenching coefficients

Collisional effects can have strong influences on the population densities of excited states in gas discharges at elevated pressure. The knowledge of the pertinent collisional coefficient describing the depopulation of a specific level (quenching coefficient) is, therefore, important for plasma diagnostics and simulations. Phase resolved optical emission spectroscopy (PROES) applied to a capacitively coupled rf discharge excited with a frequency of 13.56 MHz in hydrogen allows the measurement of quenching coefficients for emitting states of various species, particularly of noble gases, with molecular hydrogen as a collision partner. Quenching coefficients can be determined subsequent to electron-impact excitation during the short field reversal phase within the sheath region from the time behavior of the fluorescence. The PROES technique based on electron-impact excitation is not limited â?? in contrast to laser techniques â?? by optical selection rules and the energy gap between the ground state and the upper level of the observed transition. Measurements of quenching coefficients and natural fluorescence lifetimes are presented for several helium (3 1S,4 1S,3 3S,3 3P,4 3S), neon (2p1 ,2p2 ,2p4 ,2p6), argon (3d2 ,3d4 ,3d18 and 3d3), and krypton (2p1 ,2p5) states as well as for some states of the triplet system of molecular hydrogen.

Determination of the degree of dissociation in an inductively coupled hydrogen plasma using optical emission spectroscopy and laser diagnostics

Gas temperature is of major importance in plasma based surface treatment, since the surface processes are strongly temperature sensitive. The spatial distribution of reactive species responsible for surface modification is also influenced by the gas temperature. Industrial applications of RF plasma reactors require a high degree of homogeneity of the plasma in contact with the substrate. Reliable measurements of spatially resolved gas temperatures are, therefore, of great importance. The gas temperature can be obtained, e.g. by optical emission spectroscopy (OES). Common methods of OES to obtain gas temperatures from analysis of rotational distributions in excited states do not include the population dynamics influenced by cascading processes from higher electronic states. A model was developed to evaluate this effect on the apparent rotational temperature that is observed. Phase resolved OES confirmed the validity of this model. It was found that cascading leads to higher apparent temperatures, but the deviation (~25 K) is relatively small and can be ignored in most cases. This analysis is applied to investigate axially and radially resolved temperature profiles in an inductively coupled hydrogen RF discharge.

Determination of quenching coefficients in a hydrogen RF discharge by time-resolved optical emission spectroscopy

In gas discharges at elevated pressure, radiation-less collisional de-excitation (quenching) has a strong influence on the population of excited states. The knowledge of quenching coefficients is therefore important for plasma diagnostics and simulations. A novel time-resolved optical emission spectroscopic (OES) technique allows the measurement of quenching coefficients for emission lines of various species, particularly of noble gases, with molecular hydrogen as collision partner. The technique exploits the short electron impact excitation during the field reversal phase within the sheath region of a hydrogen capacitively coupled RF discharge at 13.56 MHz. Quenching coefficients can be determined subsequent to this excitation from the effective lifetime of the fluorescence decay at various hydrogen pressures. The measured quenching coefficients agree very well with results obtained by means of laser excitation. The time-resolved OES technique based on electron impact excitation is not limited - in contrast to laser techniques - by optical selection rules and the energy gap between the ground state and the observed excited level.